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SPIE Photonics West 2010 · spie.org/pw584 Return to Contents

Contents7597: Physics and Simulation of Optoelectronic Devices XVIII . . . . . .585

7598: Optical Components and Materials VII . . . . . . . . . . . . . . . . . . . . .605

7599: Organic Photonic Materials and Devices XII . . . . . . . . . . . . . . . .622

7600: Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIV . . . . . . . . . . . . . . . . . . . . . . . . . . . . .636

7601: Terahertz Technology and Applications III . . . . . . . . . . . . . . . . . .653

7602: Gallium Nitride Materials and Devices V . . . . . . . . . . . . . . . . . . . .658

7603: Oxide-based Materials and Devices . . . . . . . . . . . . . . . . . . . . . . .684

7604: Integrated Optics: Devices, Materials, and Technologies XIV . .697

7605: Optoelectronic Integrated Circuits XII . . . . . . . . . . . . . . . . . . . . . .709

7606: Silicon Photonics V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .715

7607: Optoelectronic Interconnects and Component Integration X . . .730

7608: Quantum Sensing and Nanophotonic Devices VII . . . . . . . . . . . .741

7609: Photonic and Phononic Crystal Materials and Devices IX . . . . .761

7610: Quantum Dots and Nanostructures: Synthesis, Characterization, and Modeling VII . . . . . . . . . . . . . . . . . . . . . . . .776

7611: Advances in Photonics of Quantum Computing, Memory, and Communication III . . . . . . . . . . . . . . . . . . . . . . . . . .785

7612: Advances in Slow and Fast Light III . . . . . . . . . . . . . . . . . . . . . . . .790

7613: Complex Light and Optical Forces IV . . . . . . . . . . . . . . . . . . . . . .794

7614: Laser Refrigeration of Solids III . . . . . . . . . . . . . . . . . . . . . . . . . . .801

7615: Vertical-Cavity Surface-Emitting Lasers XIV . . . . . . . . . . . . . . . .804

7616: Novel In-Plane Semiconductor Lasers IX . . . . . . . . . . . . . . . . . . .810

7617: Light-Emitting Diodes: Materials, Devices, and Applications for Solid State Lighting XIV . . . . . . . . . . . . . . . . . . .825

7618: Emerging Liquid Crystal Technologies V . . . . . . . . . . . . . . . . . . . .840

7619: Practical Holography XXIV: Materials and Applications . . . . . . .850

7620: Broadband Access Communication Technologies IV . . . . . . . . .859

7621: Optical Metro Networks and Short-Haul Systems II . . . . . . . . . .862

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Conference 7597: Physics and Simulation of Optoelectronic Devices XVIIIMonday-Thursday 25-28 January 2010 • Part of Proceedings of SPIE Vol. 7597Physics and Simulation of Optoelectronic Devices XVIII

7597-01, Session 1

Microscopic theory and numerical simulation of quantum well solar cellsU. Aeberhard, Forschungszentrum Jülich GmbH (Germany)

With the increasing utilization of quantum confinement effects in photovoltaic devices through the use of low-dimensional structures such as quantum wells, wires or dots, microscopic theories start to be required for an accurate analysis and simulation of the device characteristics. These quantum photovoltaic devices are optoelectronic systems where optimization of both the

optical transitions and the transport properties are essential for an ideal device performance. One of the most advanced theoretical approaches able to describe both quantum optics and quantum transport in nanoscale devices is based on the Keldysh or non-equilibrium Green’s function (NEGF) formalism. In the present paper, the NEGF approach is adapted to cover the photovoltaic regime of operation of bipolar quantum well diodes as prototypes of quantum photovoltaic devices. A microscopic theory is developed that describes the essential photovoltaic processes of photogeneration, carrier relaxation, radiative recombination and transport to carrier selective contacts. The theory is numerically implemented using band structure models ranging from single-band effective mass to multiband tight-binding approaches. Interactions with photons and phonons are described perturbatively via corresponding self-energies on the level of the self-consistent Born approximation. Carrier injection is considered through contact self energies representing open-system boundary conditions. The optical properties follow from the dielectric function derived within the same theoretical framework via the calculation of the transverse interband polarization function. To illustrate the capabilities of the approach, a variety of microscopic and macroscopic device properties is determined from the numerically computed Green’s functions for different configurations of single and multiple quantum wells, such as the local density of states, charge carrier density, dark- and photocurrent as well as spectra for absorption and emission, besides key photovoltaic characteristics like I-V curves and quantum efficiency.

7597-02, Session 1

Modelling and performance of quantum well solar cellsP. Kailuweit, R. Kellenbenz, F. Dimroth, Fraunhofer-Institut für Solare Energiesysteme (Germany)

Embedding quantum wells into the depletion zone of the p-n-junction of a III/V solar cell offers an elegant way to optimize the cell performance by extending the absorption range of a solar cell beyond the bandgap of the host solar cell. A predictive theoretical modelling is an important prerequisite for solar cell design to save development cost and to understand the physical processes of such a solar cell. Such a model, based on a commercial TCAD system, will be presented for a quantum well solar cell. We calculated the external quantum efficiency of a GaAs solar cell with a stack of quantum wells embedded into it’s intrinsic region. The model was then calibrated to the measurements of a real quantum well solar cell to determine material dependent fitting parameters. After this calibration the predictive capability of the model was proved by simulation of a different sample, leaving all fit parameters fix. Results of different parameter variations will be presented which reveal the physical behaviour of such a solar cell and define the frame conditions of quantum well solar cells for use in applications.

7597-03, Session 1

Zonal efficiency limit calculation for nanostructured solar cellsJ. Kupec, ETH Zürich (Switzerland); S. Yu, B. Witzigmann, Univ. Kassel (Germany)

The famous Shockley-Queisser detailed balance calculation for determining the efficiency limit of a multi-junction solar cell with respect to the bandgap energies does not address the strong local deviations of the optical power absorption for the case of a nanostructured cell. Furthermore, the assumption of perfect absorption of all incident light exceeding the bandgap energies cannot be justified. We present a modified Shockley-Queisser efficiency limit calculation for nanostructured photovoltaic devices. It incorporates a rigorous wave optics calculation and spatially resolved generation of electron-hole pairs.

We apply this method to a single-junction and dual-junction InAs/InP nanowire array for the use in concentrator solar cells. We investigate the efficiency limits regarding the arrangement of the active regions within the wire. We evaluate the efficiency limit of various radial and axial junction designs and highlight the influence of the electromagnetic modal characteristics on the local generation rate as well as the interplay between nanowire geometry, arrangement of the active regions and the bandgap energies.

Our results indicate that in a nanowire array solar cell with low volume fill factor the efficiency limit can approach the values of planar thin-film devices. This observation indicates the occurrence of micro-concentration and underlines the necessity of a wave optics approach. The spatially and spectrally resolved analysis shows that generation on the surface of the nanowires is considerable particularly with regard to high energy photons. Therefore, it is necessary to efficiently extract those carriers for highest efficiency designs.

7597-04, Session 1

Higher limiting efficiencies for nanostructured solar cellsJ. Adams, Imperial College London (United Kingdom); G. Hill, J. S. Roberts, The Univ. of Sheffield (United Kingdom); K. W. Barnham, N. J. Ekins-Daukes, Imperial College London (United Kingdom)

New higher limiting efficiencies have been predicted for solar cells with low-dimensional absorbers. The strain-balanced quantum well solar cell is a GaAs p-i-n cell containing compressively-strained InGaAs quantum well (QW) layers. At the operating bias, the predominant loss mechanism is via radiative recombination from the QWs, however it has been found that the QWs have a fundamental efficiency advantage over bulk material as they radiate anisotropically. The effects of quantum confinement and strain in the QWs lift the degeneracy of the heavy hole (HH) and light hole (LH) valence bands. The HH band couples to emission perpendicular to the QWs, and the LH band couples predominantly to emission in the plane of the QWs. In compressively-strained QWs, the HH transition becomes more probable, so radiative emission is suppressed in the plane of the QWs. Fundamentally, emission need only occur in the same direction as absorption, a situation which can be engineered by adjusting the energetic valence band alignment via strain. This leads to a higher detailed-balance limiting efficiency than in previous calculations where emission was assumed to be isotropic. We will present the results of detailed-balanced efficiency modelling in the radiative limit for isotropic and restricted emission, including the effects of back-reflectors such as the angularly-dependent distributed Bragg reflector. We will discuss what occurs for a more realistic scenario where radiative recombination is not


Conference 7597: Physics and Simulation of Optoelectronic Devices XVIII

the only loss mechanism in the cell. We will also present experimental evidence indicating anisotropic emission in devices with highly-strained QWs.

7597-05, Session 1

FDTD simulation of metallic gratings for enhancement of electromagnetic field by surface plasmon resonanceH. P. Paudel, South Dakota State Univ. (United States); K. Bayat, Univ. of Waterloo (Canada); M. F. Baroughi, South Dakota State Univ. (United States); S. May, Univ. of South Dakota (United States); D. W. Galipeau, South Dakota State Univ. (United States)

Enhancement of electromagnetic field by engineered metallic nanostructure on the metal surface by exciting surface plasmon polaritons is investigated. We employed 3D Finite-Difference Time Domain (FDTD) method for simulation of gratings in visible and NIR radiations. Surface plasmon resonance has potential application in photonic circuit miniaturization, optical signal processing, optical signal switching, optical data storage, biosensors and absorption enhancement in solar cell. Electromagnetic near-field enhancement by two-dimensional array of metallic gratings on metal surface and their characterization by size, shape, period, lattice types, radiation angles and wavelength were not investigated previously. Rectangular and cylindrical protrusions of two dimensional gold (and silver) arrays in square and triangular lattice on gold (and silver) surface interfaced with dielectric material of refractive index 1.49 (PMMA) were simulated. We found that the cylindrical grating in square lattice has the maximum enhancement by 10 times and 50 times at sharp edges, which is predominantly due to excitation of surface plasmons by grating’s localized plasmons. We showed that grating parameters such as grating period, aspect ratio and grating height can be tuned for excitation of surface plasmon polaritons for particular frequencies of interest. Results also showed that triangular lattice grating has the wider enhancement bandwidth than square lattice gratings and grating height is the most sensitive parameters of field enhancement. We got the identical result from another FDTD simulation tool ‘Meep’. The same amount of enhancement is found when the measured reflectance is substituted in analytical expression of field enhancement derived from conservation of energy. The investigation explored the novel way of enhancing field above metal surface by tuning the geometric sizes of gratings.

7597-06, Session 2

Multi-exciton generation in solar cells (Keynote Presentation)A. Zunger, National Renewable Energy Lab. (United States)

Abstract not yet in.

7597-07, Session 2

Vitual single-crystalline GaAs photovoltaics on flexible metal substratesA. Freundlich, V. Selvamanickam, Univ. of Houston (United States)

Integration of III-V semiconductors with inexpensive flexible substrates is a desirable feature to many opto-electronic applications. In particular combining the unsurpassed performance of GaAs based multi-junction technologies with conventional roll to roll processing standards of thin film industry could lead to paradigm-shifting reduction of the cost of solar electricity.

Here we report the fabrication of single crystalline GaAs epilayers

on thin (50 microns) flexible polycrystalline metallic substrates by molecular beam epitaxy (MBE). The flexible poly-crystalline Ni-based substrates were coated with an oxide-ceramic epitaxial buffer, adapted from a previously developed structure for high Tc superconductor wire technology, followed by a very thin (


contribute considerably to the optimization of these devices. In this work we present an electrical model to simulate DSCs based on a Finite Element Method as an extension of TiberCAD code. The CAD allows to calculate steady-state properties and ideal IV characteristics of the cell shaping 1,2 and 3D meshes of the device. The model handles all the charge carriers (cation, iodide, triiodide, electron) coupled to Poisson equation on the same footing within drift-diffusion equations. Recombination processes using mass law equation at the oxide/electrolyte interface and photogeneration of electrons in the oxide are included in the model. The aim of this work is to investigate the different performances of the cell by changing not only the topology of the oxide region, but also the position of the electrodes, cathode and photoanode, in order to optimize the cell characteristics.

7597-10, Session 3

Simulation and design of core-shell GaN nanowire LEDsB. J. Connors, M. Povolotskyi, R. Hicks, B. Klein, Georgia Institute of Technology (United States)

The high crystalline quality, large junction surface area, and insensitivity to c-axis oriented polarization fields make core-shell doped GaN nanowire p-n junctions exciting prospects for use as LEDs. The LED external efficiency depends upon the spatial distribution of optical recombination within the device, which may be controlled through the use of radial heterojunctions, such as quantum wells and electron blocking layers. In this work, we explore the impact of an axially varying doping profile on the spatial distribution of optical recombination in a GaN nanowire LED.

The numerical simulation of the nanowire LED is carried out using the TiberCAD simulation package. This package provides a finite-element-based solution to the drift-diffusion model of a nanowire. Simulations of core-shell nanowire LEDs are performed with various doping profiles to produce variations in the optical recombination distribution throughout the device.

In a core-shell device with a uniformly doped n-type core, the current density tends to travel primarily along the core, as the mobility of electrons is much greater than that of holes in GaN devices. The optical recombination is concentrated beneath the p-contact, where most of the current crosses the p-n junction. By properly setting a tapered doping profile in the n-type core, it is possible to increase the uniformity of the optical recombination along the junction. In certain geometries this will increase the extraction efficiency of the nanowire LED.

7597-11, Session 3

Modelling of AlN/GaN superlattices for integration in near-UV distributed Bragg reflectorsA. Zorila, J. Jacquet, Supélec LMOPS (France); A. Ougazzaden, Georgia Institute of Technology (France); F. Genty, Supélec LMOPS (France)

One of the main problem for the realization of high reflectivity GaN-based Bragg mirrors operating in the UV and near-UV wavelength range is to limit the crack formation due to the lattice mismatch between the different nitride compounds while keeping a large refractive index contrast. In this system, the highest index contrast is obtain between AlN and GaN but these compounds exhibit a lattice mismatch as high as 2.4 %. Recent works have demonstrated that the introduction of several AlN/GaN superlattices (SLs) in a classical AlN/GaN quarterwavelength layers stacking strongly improved the crystalline quality and thus the optical properties of such a mirror. In this work, we have studied several AlN/GaN SLs for their use directly as low index material in a Bragg mirror. Such a configuration should allow to combine the limitation of cracks by SLs with the improvement of the index contrast. First, the band structure

of different AlN/GaN SLs was modelled and gap energies lower than 3 eV could be found for several pseudo-alloys by choosing the right relative thicknesses of AlN and GaN. Then, the optical properties of the (AlN/GaN SLs)/GaN Bragg mirrors, centered at 400 nm and including these low gap SLs, were simulated showing that high reflectivities (>99%) can be waited from these structures with a relatively low number of pairs (< 20).

7597-12, Session 3

Optoelectronic and transport properties of nanocolumnar InGaN/GaN quantum disk LEDsF. Sacconi, G. Penazzi, M. Auf der Maur, A. Pecchia, A. Di Carlo, Univ. degli Studi di Roma Tor Vergata (Italy)

Recent interest in III-nitride nanocolumn LEDs is due to their promising features, such as a nearly dislocation-free growth. Moreover, nanocolumns with a controlled variation of indium content in the active region, the InGaN quantum disk (QD), are believed to provide an high efficiency emission in the whole visible spectrum. In this work we use the multi-scale software tool TiberCAD [1] to study the transport and optical properties of a InGaN QD in a GaN nanocolumn p-i-n diode structure, both with a macroscopical and an atomistic approach. The aim of TiberCAD project is a device simulator able to treat several different physical problems at the relevant length scale, by applying each time the proper physical models, ranging from macroscopical to atomistic representations. TiberCAD simulator includes a mixed environment for FEM and atomistic calculations. In such an environment, it is possible to perform self-consistent FEM/tight-binding multiscale calculations.

First, the strain and the drift diffusion equations for charge transport are solved in the whole device, together with a k.p multiband approximation for the computation of the quantum states in the QD and its optical properties.

Then, in a multiscale paradigm, the atomistic structure of the active region is generated, based on geometry and mesh informations. Finally, the total (electric and polarization) potential is projected on the on-site elements of the sp3s*d5 empirical tight-binding (ETB) Hamiltonian. On the other hand, strain is included by applying displacements to the crystal lattice and by scaling the tight-binding parameters accordingly.

Quantum states, electron and hole densities, and optical recombination spectrum are computed within the InGaN QD by means of ETB calculations; ETB particle densities are then projected up to the macroscopic scale for a selfconsistent solution of the drift-diffusion equations.

[1] M. Auf der Maur, M. Povolotskyi, F. Sacconi, and A. Di Carlo, Superlatt. and Microstruct., 41, 5-6, (2007), p. 381-385

7597-13, Session 3

Numerical optimization of light-emitting diodes for high-efficiency operationO. Heikkilä, J. Tulkki, Helsinki Univ. of Technology (Finland)

We investigate how efficient the LEDs can ultimately be, how the efficiency of LEDs is determined and how much of the efficiency is sacrificed when high optical intensities are sought for using extensive numerical simulations. The operation of LEDs is simulated using a model that accounts for the macroscopic carrier transport, photon extraction and heat exchange. The junction temperature is solved self consistently along with the carrier transport equations. The model is currently employed to simulate simple bulk AlGaAs-GaAs double heterostructure LEDs which are expected to be currently capable of the most efficient operation. The model is used to calculate e.g. the external quantum efficiency (EQE) and the wall plug efficiency to find the optimal operating range. Also comparative calculations are made using simplified analytical models. It will be shown that the optimal operating point is not necessarily found at the maximum of EQE because of the



contribution of lattice heat to the photon energy. Also the plausibility of electro-luminescent cooling in realistic LED structures is discussed. The efficiency droop limiting the high power operation is analyzed to find the underlying mechanisms in double heterojunction structures. The model is also used to predict a limiting current at which an insufficiently cooled LED becomes thermally unstable. Simple measures to increase the efficiency, reduce the power consumption and lower the junction temperature of LEDs are introduced based on the results, and experimental setups for measuring various material and device parameters are proposed.

7597-15, Session 4

Superconducting optoelectronicsI. Suemune, Hokkaido Univ. (Japan)

Optoelectonics is expanding its application fields, such as optical-fiber communications, displays, solid-state lightening and so on. Superconductivity has been regarded as a basic research field for some time but now it is also expanding the application fields to mass-transport, superconducting magnet for NMR and MRI, and highly sensitive SQUID magnetic-field sensors and so on. However up to very recently, the two fields have very few communications with each other. Now new trend is coming up: quantum information communication and processing. Several candidates have been proposed for quantum information processing of “Qubits”, and one prominent candidate is superconducting qubits. They have relatively long coherence time due to macroscopic quantum coherence and have an advantage for future solid-state integration. These “processing qubits” have to be converted into “messenger qubits” to form networks for quantum information. The latter will rely on the present optical-communication technology. For this purpose, development of the interface technologies for the alliance of photonics and superconductivity will open the new possibility. The authors have proposed a photon-emitting LED combined with superconducting electrodes, SQLED, which is expected to be a key device to interface the two fields. This device is also expected to generate on-demand entangled photon pairs. The main mechanism is based on the coherent spatial extension of the Cooper-pair states into the photon emitting active layer, which is expected to enhance the oscillator strength of the radiative recombination processes by the Cooper-pair involvement in the recombination processes. In this talk, the fundamental principle of the SQLED operations and new findings of LED light output enhancement and shortening of radiative recombination lifetimes will be presented together with its future prospect.

7597-16, Session 4

Properties of n-InAsSbP/n-InAs interfaceB. A. Matveev, A. Ankundinov, N. Zotova, K. Sergey, T. L’vova, M. Remennyy, A. Rybal’chenko, N. Stus’, Ioffe Physico-Technical Institute (Russian Federation)

InAsSbP/InAs based heterostructures are quite commonly used for LEDs and photodiodes (PDs) operating in the mid-IR spectral range mostly at wavelengths around 3300 nm. The latter wavelength is important for optical analyzers of the C - H gases, e.g. of the methane gas or other hydrocarbons that have strong fundamental absorption band at 3300 nm. Band gap discontinuities at an InAsSbP/InAs interface are mentioned to affect the light source performance and particularly the stimulated emission at low temperatures. However no study of the band discontinuity values in the InAsSbP/InAs structure was undertaken so far. In this work we present experimental results permitting evaluation of the n-InAsSbP/n-InAs interface discontinuities and show their impact on the performance of double heterostructure n-InAsSbP/n-InAs/p-InAsSbP LEDs and photodiodes operating at room temperature at the wavelength of 3300 nm.

The report will focus on IR - image analysis, current - and capacity - voltage integral characteristics, scanning Kelvin probe microscopy (SKPM) measurements and surface potential distribution as a function

of forward and reverse bias performed in flip-chip or (110) - cleaved devices. The above measurements together with surface photovoltage distribution analysis enabled us determining positions and values of the potential barriers in n-InAsSbP/n-InAs/p-InAsSbP heterostructures.

The obtained data will be discussed with the reference to designing mid-IR LEDs with uniform current/emission distribution over the active area and PDs with reduced capacity.

7597-17, Session 4

Band structure calculation of dilute-As GaNAs by first principleX. Li, H. Tong, H. Zhao, N. Tansu, Lehigh Univ. (United States)

Nitride semiconductors play important roles for light-emitting diodes (LEDs) and lasers in visible spectrum. High-efficiency LEDs and low-threshold lasers based on InGaN quantum wells in green regime are challenging due to charge separation, high strain misfit dislocation, and possible large interband Auger recombination.

Here, we investigate the band structure of the dilute-As GaNAs alloys (0% up to 8% As) by employing First-Principle method, for exploring new active materials in visible spectrum. Significant studies have been carried out on understanding the band structures of dilute-nitride GaNAs (up to 5% N) for near IR emission. In contrast to that, very few studies have been carried out on analyzing the band structure of the dilute-As GaNAs alloy, thus its important parameter such as band offsets between individual conduction bands and/or valence bands at the gamma point of the Brillouin zone (BZ) remain unknown. The properties of the band structures for the GaNAs alloys have fundamental impacts on the development of these materials into advanced device applications. The band structures of the dilute-As GaNAs were calculated by the Density-Functional Theory that adopts the Generalized Gradient Approximation, and the calculation routine is along high-symmetrical k-points in the BZ. The incorporation of dilute-As into the GaNAs alloy leads to significant decrease in the bandgap of the material, which allows direct band gap transition covering from 2.8eV (As-content of 3%) down to 1.8eV (As-content of 7%). The First-Principle shows that the dilute-As GaNAs alloy as a candidate for excellent active material for LEDs and lasers in visible regime.

7597-18, Session 4

Lasing and gain characteristics in Ga(NAsP) heterostructures on SiC. Lange, S. Chatterjee, W. W. Rühle, Philipps-Univ. Marburg (Germany); N. Koukourakis, N. Gerhardt, M. Hofmann, Ruhr-Univ. Bochum (Germany); S. Liebich, M. Zimprich, R. Fritz, B. Kunert, K. Volz, W. Stolz, Philipps-Univ. Marburg (Germany)

The creation of a direct band-gap material on silicon is envisioned to enable advanced optoelectronic integrated circuits with drastically improved performance combining the advantage of optical data processing with the well-established silicon processing technology. However, the realization of a laser diode based on Si remains a challenge to date due to the indirect nature of its band structure. One possible solution is the integration of standard direct band gap III/V laser material such as GaAs or InP on Si-substrates. Still, the large lattice mismatch of these materials to Si causes high densities of threading dislocations in the epitaxial films, preventing any long-term stable lasing operation of integrated III/V laser diodes. In order to avoid dislocation formation, the novel direct band gap Ga(NAsP)/GaP-material system has been introduced which exhibits lasing up to room temperature for epitaxial growth on GaP-substrates for both optical and electrical injection.

Here, we investigate the gain and lasing characteristics of a Ga(NAsP)/(BGa)(AsP) multiple quantum-well laser structure grown by metal-organic vapour-phase epitaxy on an exactly-oriented (001) Si substrate. We observe lasing action after pulsed optical excitation for various samples.



The emission power shows a clear threshold behaviour as function of excitation density at temperatures ranging from 10 K up to 125 K. Clear mode spectra are observed and an analysis with the Hakki-Paoli method yields a lower limit for maximal modal gain of 5 cm-1.

7597-19, Session 4

Influence of disorder on photoluminescence dynamics of Ga(AsBi)A. Chernikov, S. Chatterjee, C. Bückers, S. W. Koch, Philipps-Univ. Marburg (Germany); S. Imhoff, A. Thränhardt, Technische Univ. Chemnitz (Germany); X. Lu, The Univ. of British Columbia (Canada); T. Tiedje, Univ. of Victoria (Canada); S. Johnson, Arizona State Univ. (United States)

The design new long wavelength material on GaAs substrates with the ability to produce telecom or even longer wavelengths on GaAs substrates is a major task in todays development of novel alloyed semiconductor materials. Recently, dilute Ga(AsBi) has been proposed as a potential candidate for an such an emitter material due to its giant bandgap bowing.

Here, we present time-resolved photoluminescence experiments on a 30nm Ga(AsBi) epilayers as function of excitation flux and lattice temperature. The sample contains a Bi fraction of about 4.5% and was grown by molecular beam epitaxy on semi-insulating 100 GaAs substrate. A 80MHz, 100fs Ti:sapphire oscillator centred at 1.55eV and 1.35eV is used for excitation above and below the GaAs substrate band gap energy; a standard spectrometer and streak-camera setup with a water-cooled S1 cathode yielding a time-resolution of 2ps is used for detection. The peak of the near band edge emission shifts from 1.13eV to 1.23eV as the excitation flux is increased be more than two orders of magnitude at a lattice temperature of 10K. This blue shift is a clear sign of disorder present in the system. This is also supported by the low-energy tail of the emission spectrum which is spectrally more flat than the high-energy flank. The decay times increase across the emission band indicating a complex interplay between carrier relaxation and recombination. A pronounced S-shape is found for the emission peak as function of lattice temperature yielding a disorder potential of several 10’s of meV.

7597-20, Session 5

Carrier dynamics in ZnMgO studied by time-resolved photoluminescenceA. Chernikov, M. Koch, S. Chatterjee, K. Volz, B. Pasenow, S. W. Koch, Philipps-Univ. Marburg (Germany); P. J. Klar, M. Eickhoff, B. K. Meyer, Justus-Liebig-Univ. Giessen (Germany); B. Laumer, T. A. Wassner, M. Stutzmann, Technische Univ. München (Germany)

A major step towards the development of ZnO-based UV emitters is the development of quantum well structures. Typically, Zn1-xMgxO is used as barrier material due to its similar in-plane lattice parameter and considerable band offsets. Here, we investigate the properties of different wurtzite Zn1-xMgxO epilayers with varying Mg contents. All samples were grown by plasma-assisted molecular beam epitaxy on c-plane sapphire substrates, using a MgO/ZnMgO buffer .

We perform time-resolved photoluminescence experiments as function of excitation flux and lattice temperature. A frequency-tripled 80MHz, 100fs Ti:sapphire oscillator centred at 4.3eV is used for excitation; a standard streak camera setup yielding a time-resolution of 800fs is used for detection.

The peak of the near band edge emission shifts from 3.38eV to 4.05eV as the Mg concentration is increased from x=0 to x=0.35 at a lattice temperature of 10K. The decay times increase by more than one order of magnitude as the Mg concentration increases indicating stronger

localization. This is further supported by an increase of the respective spectral linewidths.

Typically, a series of phonon replica is observed for all samples. At low temperatures, the sample with x=0 (clearly) shows replica of the near band-edge emission while the replica signatures from bound excitons become more important as the Mg concentration is increased.

The bound excitons are ionized as the lattice temperature is increased revealing the zero phonon line as well as phonon replica associated with the free exciton emission line similar to the well-known ZnO material system.

7597-21, Session 5

Frequency modulation response of two-section quantum cascade lasersE. Luzhansky, F. Choa, Univ. of Maryland Baltimore County (United States)

Abstract: we describe the theory of frequency modulation (FM) response of Quantum Cascade Lasers (DFB QCL). It includes cascading effect on QCL’s maximum modulation frequency. Theory is enhanced with description of FM response of two section Distributed Feedback Laser (DFB) QCL. It is shown, that, in contrast to laser diodes, the FM response of two section QCLs is independent of the optical power. It can be optimized by correlation of two sections’ effective lengths and by controlling the relative difference of their linewidth enhancement factors, . Utilization of model for the single section DFB QCL and agreement with previous results is shown as well.

7597-22, Session 5

Physical-random number generation using laser diodes’ inherent noisesH. Nishimura, K. Doi, T. Ushiki, T. Sato, M. Ohkawa, Y. Odaira, Niigata Univ. (Japan)

Random numbers can be classified as either pseudo- or physical-random in character. This work demonstrates how laser diodes’ inherent noise can be exploited for use in generating physical-random numbers in the field of cryptography.

Pseudo-random numbers’ periodicity renders them inappropriate for use in cryptographic applications, but, naturally-generated, physical-random numbers have no calculable periodicity, thereby making them ideally-suited to the task. Laser diodes naturally produce a wideband “noise” signal that is believed to have tremendous capacity and great promise, for the rapid generation of physical-random numbers for use in cryptographic applications.

Because the character and shape of the laser diode’s oscillation exert tremendous influence on the intensity and frequency noises, we need to determine which noise is best suited for the generation of fast physical-random numbers. To do this, we worked to identify the frequency noises by observing the transmitted light intensity through the frequency reference, and generated the physical-random numbers using an analog-digital (A/D) converter that produces, for example, 8-digits binary numbers from the detected laser intensity.

In the initial stages of the experiment, we measured a laser diode’s output, at a fast photo detector and generated physical-random numbers using laser diode’s intensity noises. We then identified and evaluated the binary-number-line’s statistical properties. Because the frequency noise has higher frequency components than the intensity noise has, our preliminary result shows that fast physical-random numbers are obtainable, using the laser diode’s frequency noise characteristic.



7597-23, Session 5

Complex low energy gain switching pulse processing using a highly nonlinear optical loop mirrorC. de Dios Fernandez, H. R. Lamela, Univ. Carlos III de Madrid (Spain)

When it comes to obtaining short optical pulses from diode lasers, Gain Switching (GS) shows as a direct and straightforward technique. It is a compact source for frequency tunable, high repetition rate, low-cost optical pulses in the picosecond range that can be applied to Commercial Off the Shelf (COST) diode lasers. Pulses from GS are widely used nowadays [1][2]. However, their application in new fields is at times limited. The pulses obtained have low power, an asymmetric shape and frequently present a trailing substructure that forms pedestals [3]. Depending on the laser technology considered, Fabry-Perot lasers, DFB [4] or VCSELs [5], the pulses generated vary from hundreds of picosecond to tens of picoseconds. Thus, improving the quality of GS pulses, due to their complex structure, low power and long temporal width, is a challenging task.

Nonlinear Optical Loop Mirrors (NOLM) are interferometric nonlinear devices based on an optical fiber Sagnac configuration, which are extensively used as part of optical communication systems [6] and as pulse compression and shaping devices. They are versatile and several studies have proposed different designs for these loops [7][8]. Hence, they make good candidates for the improvements of GS sources. However, as nonlinear devices, they exhibit a power dependant behavior. Then, the performance of NOLM is sensitive to the characteristics of the input pulse train. Also, long loops and high input energies are often needed to obtain compression or reshaping of the pulses. As a result, the capabilities of NOLMs cannot be used directly on low quality pulses, such as those obtained from GS diode lasers, without preprocessing stages [9].

In this work, we study the improvement of complex low quality pulses obtained from GS sources using a compact Highly Nonlinear Optical Loop Mirror (HNOLM) based on a high speed Nonlinear Semiconductor Optical Amplifier (NLSOA) and a Microstructured Optical Fiber (MOF). The loop is compact, containing 20 m of optical fiber and does not require any intermediate stage to process the pulses. The capability of the HNOLM to improve the pulse characteristics has been studied for several input energies in this work. Finally, the pulse quality has been evaluated too using the temporal pulse width, spectral width, pulse shape, pedestals height and pedestal width. Results show that HNOLM provides direct compression and pulse shaping for picosecond complex pulses obtained from a DFB COTS laser operating within the 1550 nm window.

References:

[1] H. Shams, A. Kaszubowska-Anandarajah, P. Perry, and L. Barry, “Demonstration and optimization of an optical impulse radio ultrawideband distribution system using a gain-switched laser transmitter,” Journal of Optical Networking, vol. 8, no. 2, pp. 179-187, 2008.

[2] H Liu, C. Gao, J. Tao, W. Zhao, and Y. Wang, “Compact tunable high power picosecond source based on Yb-doped fiber amplification of gain switch laser diode,” Optics Express, vol. 16, no. 11, 2008.

[3] P. Vasil’ev, I.H. White, and J. Gowar, “Fast phenomena in semiconductor lasers,” Rep. Prog. Phys., vol. 63, 2000.

[4] K. Otsubo et al., “Uncooled 25 Gbis/s direct modulation of semi-isulating buried-heterostuctured 1.3 um AlGaInAs quantum-well DFB lasers”,” Electronics Letters, vol. 44, pp. 631-633, 2008.

[5] M. Nakazawa, H. Hasegawa and Y. Oikawa, Student Member, IEEE “10-GHz 8.7-ps Pulse Generation From a Single-Mode Gain-Switched AlGaAs VCSEL at 850 nm” IEEE Photonics Technology Letters, vol. 19, no. 16, 2007.

[6] S. Boscolo, S. K. Turitsyn, and K. J. Blow, “Nonlinear loop mirror-based all-optical signal processing in fiber-optic communications,” Optical Fiber Technology, vol. 14, pp. 299-316, March 2008.

[7] S. Takamatsu, H. Watanabe, T. Matsuyama, H. Horinaka and K. Wada,

“Pulse-shaping of gain-switched pulse from multimode laser diode using fiber Sagnac interferometer,” Optics Express, vol. 16, no. 24, 2008.

[8] K. Smith, E.J. Greer, N.J. Doran, D.M. Bird, and K.H. Cameron, “Pulse amplification and shaping using a nonlinear loop mirror that incorporates a saturable gain,” Optics Express, vol. 17, no. 6, pp. 408-410, March 1992.

[9] Y. J. Chai, I. Y. Khrushchev, R. V. Penty, and I. H. White, “Interferometric Noise Suppression by Means of Dispersion-Imbalanced Loop Mirror Over a Wavelength Range of 28 nm,” IEEE Photonic Technology Letters, vol. 14, no. 3, 2002.

7597-24, Session 6

Quantum design and experimental realization of high-power VECSELsS. W. Koch, Philipps Univ. Marburg (Germany)

Our micsrocsopic quantum theory is used to design VECSELs for special emission wavelengths and/or high power output. The main ingredients of the quantum design scheme are reviewed and examples of realized structures and experimental results are shown.

This work is done in collaboration with J.V. Moloney, J. Hader, Y. Kaneda et al. Tucson, W. Stolz, B. Kunert, S. Chatterjee, et al., Marburg

7597-25, Session 6

Cavity design and heat management in vertical-external-cavity surface-emitting lasers (VECSELs)T. Lu, Y. Kaneda, M. J. Yarborough, College of Optical Sciences, The Univ. of Arizona (United States); J. Hader, Nonlinear Control Strategies, Inc. (United States); J. V. Moloney, College of Optical Sciences, The Univ. of Arizona (United States) and Nonlinear Control Strategies (United States); A. Chernikov, M. Koch, B. Kunert, W. Stolz, S. Chatterjee, C. Bückers, S. W. Koch, Philipps-Univ. Marburg (Germany)

Vertical-External-Cavity Surface-Emitting Lasers (VECSELs) or semiconductor disk lasers in the infrared spectral range display a highly desirable combination of high output power, good efficiency, and good beam quality out of a small package. Additionally, high-speed frequency-doubled semiconductor disk lasers have been realized covering large parts of the visible spectrum, e.g., for full-color laser projection or as laser guide-star systems. The realization of high-power devices such as the latter requires a careful interplay between cavity design and heat management.

We experimentally investigate a model high-power device. A 1040nm VECSEL is realized using the semiconductor chip and a spherical output coupler in a linear cavity design. First, the performance is investigated as function of the reflectance of the output coupler. Next, we investigate the influence of the pump spot profile on the lasers parameters. It is investigated by varying the pump optics and their geometry, e.g., angle under which the pump is incident and role of aberrations such as defocus or tilt, such that the performance is optimized.

Different heat spreading and heat removal concepts in materials such as copper and diamond are compared. The respective performance is monitored under comparable high power pumping and cavity conditions by recording the spectrally resolved emission and the input-output characteristics. Finally, different cavity geometries, their respective impact on the mode size at the active region and the consequences for power scalability are discussed.



7597-26, Session 6

Ultrafast circular polarization oscillations in spin-polarized vertical-cavity surface-emitting laser devicesN. C. Gerhardt, M. Li, H. Jaehme, H. Soldat, M. R. Hofmann, Ruhr-Univ. Bochum (Germany); T. Ackemann, Univ. of Strathclyde (United Kingdom)

The combination of spin injection with optical amplification in spin-lasers offers new encouraging possibilities for future devices. Here we investigate the room temperature spin, carrier and polarization dynamics of electrically pumped vertical-cavity surface-emitting lasers (VCSELs) on a picosecond time scale, after injection of a small degree of spin polarization for the electron band population. For this purpose, we apply a hybrid excitation technique combining continuous-wave unpolarized electrical and picosecond spin-polarized optical excitation of a commercial VCSEL device. The experimental results demonstrate ultrafast circular polarization oscillations due to spin injection with an oscillation frequency of 11.6 GHz. The polarization dynamics are faster than the intensity dynamics due to the relaxation oscillations. Even more interesting, the circular polarization oscillations persist for more than 5 ns after spin injection, which is much longer than the spin-relaxation time in this device. We compare the experimental results with theoretical calculations on the basis of rate equation models for spin-polarized lasers in order to analyze the complex interplay between birefringence, spin- and carrier relaxation as a reason for the long persisting circular polarization oscillations in spin-VCSELs. A detailed understanding of the spin-dynamic mechanism of the polarization oscillations offers the possibility for independent polarization and intensity modulation in future spin-VCSEL devices.

7597-27, Session 6

Electro-optically modulated VCSELs and RCLEDsJ. A. Lott, V. A. Shchukin, N. N. Ledentsov, VI Systems GmbH (Germany); T. D. Germann, A. Strittmatter, A. Mutig, D. Bimberg, Technische Univ. Berlin (Germany)

We present the design, simulation, operating principles, growth, fabrication, and measured performance results of 850 nm-range electro-optically modulated (EOM) vertical cavity surface emitting lasers (VCSELs) and resonant cavity light emitting diodes (RCLEDs). The device structures are grown in a well-controlled single monolithic epitaxial growth step by both molecular beam epitaxy and metal-organic vapor phase epitaxy and are produced via standard large-volume micrometer-scale-device processing techniques. Finally we discuss how to use this innovative approach for the realization of high speed (> 10 Gb/s) optical signal modulation to enable the next generation of low cost and extremely reliable short-reach optical data communication systems.

7597-28, Session 7

Micro-diffraction lenses with subwavelength structures designed by the genetic algorithmT. Shirakawa, Univ. of Tokyo (Japan); K. L. Ishikawa, RIKEN (Japan); S. Suzuki, Y. Yamada, Ricoh Co., Ltd. (Japan); H. Takahashi, Univ. of Tokyo (Japan)

Recent progress of nanotechnology has enabled the fabrication of diffractive optical elements (DOEs) with subwavelength structures (SWSs). However SWS DOEs can be designed neither by scalar diffraction theory nor by Fourier optics. To solve this difficulty, we have recently developed a design method based on the finite-difference time-domain (FDTD) method and the genetic algorithm (GA), called the

GA-FDTD method. In this study, we present the design of SWS micro diffraction lenses with a 4 μm radius.

Maxwell’s equations are solved by the FDTD for a system with axial symmetry with the perfectly matched layer absorbing boundary condition. The relief structure and height of the lens are optimized with the GA using binary coding. In consideration of actual fabricability, the grating width is 100 nm at least and the aspect ratio is 5 at most.

The optimized structure has more than twice as high focusing ability at a wavelength of 660 nm as that previously reported based on the binarization of the Fresnel lens, and turns out to be robust against fabrication error. Its grating height (400 nm) is different from that of the Fresnel lens (576 nm), and the relation between the widths of neighboring gratings cannot be expressed in a simple way. Furthermore, we have successfully designed a lens with equal focal length at three different wavelengths (660, 532, and 445 nm). The structures, which are not intuitive, are hard to deduce from experiences. These results indicate the effectiveness of the GA-FDTD method in the design of SWS DOEs.

7597-29, Session 7

LED integrated optical encoderL. Acevedo, M. P. Y. Desmulliez, Heriot-Watt Univ. (United Kingdom)

The incoherent wave front emitted from a LED can be modelled by analysing the diffraction produced by a 4 micron period grating attached to the refractive glass sitting on top of the LED. Such a configuration is used for optical encoding application [reference needed, paper by John Carr, ESTC 2008.

In our application, the 460 microns diameter LED with an epitaxy thickness of 3.066 um is used within a monolithically integrated optical encoder. The incident light source from the LED is impinging onto the index grating manufactured at the surface of the substrate of the LED and further transmitted to a scale grating with a diffraction order of +1 and -1. Surfaces to determine the incidence irradiance (power/unit-area) impinging into the photodetector placed 1 mm away, the emitted divergence radiance per solid angle (power/steradian), and the diffractive flux allow us to compare these optical outputs with those obtained experimentally. Experimental data have been obtained by recording the power incident on the tip of a 8 um core optical fiber as it scans across the LED. Moreover, the theoretical model and the measured Lambertian beam are compared to the output parameters of the LED (2013 Farfield LED) as shown in Figures (1,2) and simulated source in Figures (3, 4, 5) below.

The simulating environment determines the behaviour of the rays of the light source in a Lambertian wavefront profile. It also predicts the Talbot effect produced by the grating when the light is propagated to the gratings of the optical encoder. Therefore by analysing the geometric configuration of the system such as the gratings period and pitch, the distance between LED and photodetectors, can be optimised to reduce optical crosstalk and increase output throughput

References

Optical Encoder Readhead Chip 078-1-4244-2814-4/08/$25.00@2008 IEEE

7597-30, Session 7

Study of propagation modes of bent waveguides and micro-ring resonators by means of the aperiodic Fourier modal methodD. Bucci, B. Martin, A. Morand, Institut de Microélectronique Électromagnétisme et Photonique (France)

In the last years, several numerical methods have been studied and applied to the analysis of high index contrast bent waveguides.

Very often, the problem is treated by using a conformal mapping, which



translates the bending into an equivalent index profile.

In this article, we will discuss the implementation of a full vectorial 2D mode solver by means of the Aperiodic Fourier Modal Method, developed directly in cylindrical coordinates.

In the first part of our work, we will develop a shorthand notation and the mathematical rules useful to describe the problem in a matrix form. The search of propagation modes is then reconducted to the search of eigenvectors of a matrix. We will at first test our formulation in 1D with results given by the conformal mapping technique.

In a second time, we will use the complete 2D solver to the determination of the resonance frequencies and quality factors of micro-ring resonators made on silicon surrounded by silica. These characteristics are related to the real and imaginary part of the propagation constants. By comparison with 3D-FDTD analysis, we will show how our implementation can be used to accurately describe the behavior of micro-rings having a bending radius as low as 1 μm. This technique is general and can be applied on micro-rings having an arbitrary cross-section and a quality factor comprised between 100 and 10000.

Perspectives of this work include the study of the propagation, as well as the coupling between micro-ring resonators and waveguides.

7597-32, Session 7

Three-dimensional meshfree numerical method for optical structuresF. Alharbi, King Abdulaziz City for Science and Technology (Saudi Arabia) and IBM Almaden Research Ctr. (United States); J. C. Scott, IBM Almaden Research Ctr. (United States)

A new and efficient three dimensional multi-domain spectral method formulation is developed to simulate and analyze optical structures. The formulation is meshfree and functional expansion based where the unknown physical quantities are approximated by expansions by preselected basis sets. The expansion coefficients are satisfying the governing equations. This is the fundamental difference between spectral methods and the conventional finite difference and finite element methods, where the unknown quantities are approximated by their values at meshing points.

To enhance the performance of the method, the computational window is divided into domains where the structural functions are smooth. This allows avoiding the errors that are caused by the Gibbs phenomenon, which is associated with discontinuities. For bounded domains, Chebyshev polynomials and Fourier series are used and predefined exponential basis sets are used for half-bounded ones.

The formulation is very flexible where multiple coupled physical quantities can be obtained simultaneously. For example, in the case of electromagnetic field, the three electric field components are solved concurrently. Also, the boundary conditions are considered analytically without any approximation. Additionally and as common for spectral methods, the method is very accurate and fast. This is mainly because the approximation by expansion reduces the numerical time and memory requirements.

The method is used to study wave propagation and scattering and modal behavior of many optical structures. To demonstrate the validity and the accuracy of the presented method, it is compared with various published methods, where many optical structures are studied.

7597-33, Session 7

Synthesis of titanium indiffused LiNbO3 waveguides with desired modal fieldsE. K. Sharma, Univ. of Delhi (India); G. J. Saxena, Univ of Delhi (India)

In this paper we report a procedure based on variational approximation, to obtain the refractive index profile and in turn the process parameters

for fabrication of waveguide from the desired modal field. It has been shown that the best variational separable field for a channel waveguide corresponds to the solution of two equivalent planar waveguides in the x and y directions. We show that for a desired modal field X(x)Y(y), one can obtain the refractive index profile parameters , h and w. We have illustrated the use of the procedure in the design of waveguides for optimum coupling efficiency between a fiber and waveguide. Assuming the separable channel waveguide field it is possible to extract the optimal modal field parameters and hence, the refractive index profile parameters, for maximum coupling efficiency which in turn define the process parameters or fabrication conditions of the waveguide such as temperature and time of diffusion, width and thickness of the titanium strip. For a given strip width, each profile width w corresponds to a fixed profile depth h. In an attempt to maximize coupling efficiency along the x-direction to unity by appropriate choice of X(x) for a typical communication fiber, we realized that the corresponding w resulted in an h which reduced efficiency in the y-direction to a very small value. Hence, it was necessary to compromise on the choice of X(x); we restricted the choice so as to obtain an efficiency of above 90% in the x-direction. With this choice it is possible to find an appropriate h and n for maximum coupling efficiency. Once the three parameters h, w and are known, the strip thickness, time and temperature can be defined. The results have been validated by comparison with those obtained by simulation on the BPM CAD simulator.

7597-34, Session 8

InAs quantum dot-based devices for ultrafast photonic signal processingO. Wada, Kobe Univ. (Japan)

This paper discusses first the quantum dot (QD) growth technique based on molecular beam epitaxy (MBE), and then describes their application to photonic devices for ultrafast telecommunications such as SOAs and all-optical switches. An important issue in QD-SOAs is the polarization sensitivity which is primarily caused by the flattened shape of QDs. We have introduced our unique approach of columnar dot technique in which a number of QD layers are closely stacked to form a better geometrical isotropy. By optimizing the geometry and strain in InAs/InGaAs/InP QD system, polarization-insensitive SOA performance has been demonstrated for the first time at 1.55 mm. We also propose an alternative approach of using remotely stacked QD layers and demonstrate basic polarization-insensitive characteristics. As for all-optical switches, recent result of vertical structure QD-based all-optical switches is described. This switching device has a vertical cavity composed of a pair of asymmetric distributed Bragg reflectors (DBRs) incorporating a QD nonlinear medium. Through optimizing the design and fabrication of vertical structure all-optical devices, optical reflection-type switch response with a time constant as fast as 23 ps has been demonstrated. We thus illustrate the unique prospects of QD-based devices to future photonic communication and signal processing systems.

7597-35, Session 8

Thermal crosstalk reduction in IR thermo-electric photodetectors by lock-in method: 4D numerical simulations and experimental verificationW. Vandermeiren, J. Stiens, C. De Tandt, Vrije Univ. Brussel (Belgium); G. Shkerdin, V. Kotov, Institute of Radio Engineering and Electronics (Russian Federation); G. Borghs, IMEC (Belgium); P. Muys, Lambda Research Optics Europe (Belgium); R. Vounckx, Vrije Univ. Brussel (Belgium)

Laser induced temperature distributions inside doped semiconductor materials are used to derive laser beam profiles by means of the thermo-



electric Seebeck effect. Thermal diffusion will lead to a discrepancy between the optical intensity profile of the laser beam and the measured temperature distribution inside the semiconductor. An advanced numerical 4D finite element model describing the laser induced spatial temperature distribution in function of time in a layered GaAs based structure was developed in Comsol Multiphysics. Non-linearities as the temperature dependence of the absorption coefficient, the thermal conductivity and the Seebeck coefficient were taken into account. This model was used to investigate the optical chopper frequency dependence on the spatial thermal crosstalk level and the responsivity near the illuminated surface of the detector structure. It was shown that the frequency dependent crosstalk level can be reduced significantly by applying short chopping periods with respect to the thermal diffusion time constant. The thermal crosstalk is reduced to -10dB and -20dB for the first and second neighboring pixel respectively for a lockin frequency of 300 Hz. Experimental results of the spatial thermal crosstalk level and the responsivity were compared with simulations and satisfactory agreements between both were achieved. High power CO2 laser profile measurements obtained with our thermoelectric detector and a commercially available Primes detector were compared.

7597-37, Session 8

Ultrafast compact silicon-based ring resonator modulators using metal-insulator switching of vanadium dioxideJ. Nag, J. D. Ryckman, R. F. Haglund, Jr., S. M. Weiss, Vanderbilt Univ. (United States)

We describe an optical modulator based on a silicon ring resonator coated with vanadium-dioxide (VO2) motivated by the need for compact silicon-compatible optical switches operating at THz speeds. VO2 is a functional oxide undergoing metal-insulator transition (MIT) near 67C, accompanied by huge changes in electrical resistivity and near-infrared transmission. The MIT can be induced thermally, optically (by ultra-fast laser excitation in less than 100fs), and possibly with electric field. The ability to optically induce an ultrafast phase change in VO2 presents an excellent opportunity to realize high speed all-optical modulation in compact device structures. During transition, the large change in the refractive index (~1.96-3.25) in the near infrared, renders VO2 appropriate for integration with silicon-based ring resonators at telecommunication frequencies. VO2 is easily deposited on silicon and its ultrafast switching properties can be used to tune the effective index of ring resonators instead of depending on the weak electro-optic properties of silicon. The VO2-silicon ring resonator is thus expected to operate at speeds up to 10 THz utilizing low Q-factor, with shorter cavity lifetimes, thus enabling faster, more robust device response while maintaining compact device size. We are carrying out a proof-of-concept study using double-layer e-beam lithography to make ring resonator structures on SOI substrates with rings varying in diameter from 2- 22 um coupled to 98um-long nanotapered waveguide at a separation of 200nm. The rings will be coated with 50nm of VO2 by pulsed laser deposition. Our FDTD simulation results predict large modulation effects by switching the VO2 thermally.

7597-76, Session 8

Finite difference time domain analysis of ultra-broadband enhanced absorption of silicon surface with nanostructuresH. Wang, C. Chen, J. Wu, National Chung Cheng Univ. (Taiwan); S. Feng, National Univ. of Kaohsiung (Taiwan); S. Liu, National Chung Cheng Univ. (Taiwan)

We investigate the ultra-broadband enhanced absorption properties in a wavelength range of 0.3~1.2 μm for silicon surface with nanostructures by using finite difference time domain (FDTD) algorithm. Three different types of surface structures including cone-like, air-hole, and inverted Pyramids structures are simulated. We demonstrate absorptance enhancement of silicon surface with varying different geometric arrangements, structure sizes, and shapes. The best enhancement structure is cone-like type which can suppress the reflection of surface about 2.5%. We design a special arrangement for air-hole type surface that can increase the absorption of solar surface about 65%. In the inverted Pyramids structures, we find the U shape of structure will increase the absorption efficiency due to a gradual change of refractive index. The physics of such remarkable absorption for the structured silicon surfaces are discussed as well.

7597-40, Session 9

Random lasing in nanocrystalline ZnO powdersH. Kalt, J. Fallert, R. Dietz, J. Sartor, D. Schneider, C. F. Klingshirn, Univ. Karlsruhe (Germany)

We study the properties of random lasing modes in nanocrystalline ZnO powders by spatially resolved high-excitation photoluminescence spectroscopy. Both localized and extended modes are observed in the same spatial region of the powder. We find that the localized modes appear at much lower optical gain than the extended ones as predicted by theory.

7597-41, Session 9

Control random laser modes by local pumpingH. Cao, Yale Univ. (United States)

A difficulty concerning the practical application of random lasers is the lack of precise control over lasing mode properties. We show that the lasing modes can be modified by inhomogeneous pumping of random media. The spatial inhomogeneity of gain causes dramatic and complicated changes of the lasing modes even in the absence of gain saturation. Some lasing modes disappear, while new modes are created at various frequencies.



7597-42, Session 9

Visible-wavelength random lasing of (Zn,Cd,Mg)O quantum well structuresS. Kalusniak, S. Sadofev, J. Puls, H. Wuensche, F. Henneberger, Humboldt-Univ. zu Berlin (Germany)

Semiconductor lasers operating in the visible wavelength range have been a subject of extensive research during the past years. Ternary ZnCdO is a material with considerable potential in this regard, as it can cover in principle band-gaps from the ultraviolet to the near infrared spectral range.

We have systematically fabricated (Zn,Cd)O/ZnO single and multiple quantum well structures and elucidated their optical properties. In the low-density excitation regime, huge polarization-induced electric fields of some 108 V/m are signified by a strong red shift of the photoluminescence band with increasing well width as well as an increase of the lifetime from the ps- to the μs-time scale. Effective screening of these fields occurs already at moderate optical excitation in the 10 kW/cm² range and recovers practically the bare quantum-confined transition energies and short lifetimes. In the same excitation range laser action of specially designed multiple quantum well structures is observed. The low-temperature lasing threshold is only 25 kW/cm² and increases moderately up to room temperature (150 kW/cm²). The emission wavelength is systematically tuneable by structure design. The longest lasing wavelength achieved so far is 510 nm at room temperature.

The laser action can be achieved without preparation of an optical cavity. Both spectral position and separation of the lasing modes are determined by the geometrical shape of excitation area and vary with the excitation power. A theoretical model assuming weak scattering reproduces the experimental finding reasonably well. For specially prepared micro-resonators, narrowing of the mode band and mode selection is demonstrated.

7597-43, Session 10

Lasing osicllation in a single quantum dot nanocavity system under strong/weak coupling regimeY. Arakawa, M. Nomura, S. Iwamoto, Y. Ota, N. Kumagai, The Univ. of Tokyo (Japan)

We dicuss recent advances in light-matter interaction in a single quantum dot embedded with photonic crystal nanocavity. We have successfully demonstrated a transition from strong coupling regime to lasing oscillation by the single dot gain.

7597-44, Session 10

Radiative efficiency of MOCVD grown quantum dot lasersL. J. Mawst, G. Tsvid, P. Dudley, J. Kirch, J. H. Park, Univ. of Wisconsin-Madison (United States)

Quantum dot lasers have been reported by MBE growth with high performance, although previous reports indicate that the overall radiative efficiency (i.e. Jspon/Jtotal) from such structures is surprisingly low (5-25%). Here, we study the gain and radiative efficiency of MOCVD grown InGaAs quantum dot lasers. Single-pass, multi-segmented amplified spontaneous emission measurements are used to obtain the gain, absorption, and spontaneous emission spectra in real units. Integration of the calibrated spontaneous emission spectra then allows for determining the overall radiative efficiency, which gives important insights into the role which nonradiative recombination plays in the active region under study. We use single pass, multi-segmented edge-emitting in which electrically

isolated segments allow to vary the length of a pumped region. In this study we used 8 section devices (the size of a segment is 50x300 μm) with only the first 5 segments used for varying the pump length. The remaining unpumped segments and scribed back facet minimize round trip feedback. Measured gain spectra for different pump currents allow for extraction of the peak gain vs. current density, which is fitted to a logarithmic dependence and directly compared to conventional cavity length analysis, (CLA). The extracted spontaneous emission spectrum is calibrated and integrated over all frequencies and modes to obtain total spontaneous radiation current density and radiative efficiency. We find radiative efficiency values of approximately 25% at RT for 5 stack QD active regions. By contrast, high performance InGaAs QW lasers exhibit radiative efficiency ~50% at RT.

7597-45, Session 10

Inhomogeneous quantum dot gain medium for improved spatial coherence in wide-aperture semiconductor lasersJ. Mukherjee, Tyndall National Institute (Ireland); J. G. McInerney, Univ. College Cork (Ireland)

Scaling the brightness (spatial coherence) along with output power has been a long-standing problem for semiconductor lasers. The difficulty arises due to complex spatio-temporal modal filamentation associated with light-matter interaction in the non-linear semiconductor lasing medium. We have done a detailed theoretical study of spatio-temporal dynamics in wide aperture semiconductor lasers in order to identify factors limiting spatial coherence in the high power regime, thereby finding routes to scale the brightness using the unique properties of quantum confined active regions. In this context, we have developed a detailed opto-electro-thermal model, based on Maxwell-Bloch formalism, to describe frequency-, carrier- and temperature-dependent gain and dispersion. First a steady-state electro-thermal model is developed to simulate the current and heat spreading in the laser. The thermal model is then used in conjunction with the Maxwell-Bloch based dynamical model to obtain pump-dependent modal intensity structure and spatial frequency spectral characteristics. Effects of both homogeneous and inhomogeneous gain broadening are analysed. It is shown, via linear stability analysis and high resolution space-time adaptive FEM simulations that crystal growth induced inhomogeneous gain broadening in quantum dot lasers enhances spatial coherence and leads to suppressed filamentation and stable far-fields in both thermal and non-thermal regimes even when the phase-amplitude coupling is comparable to that in quantum well gain medium. Strong spatial-mode competition in the inhomogeneously broadened gain medium under cw operation is also highlighted.

7597-46, Session 10

Bipolar self-consistent optoelectronic model for quantum dot lasersA. Martín-Mínguez, H. Odriozola, I. Esquivias, J. M. G. Tijero, Univ. Politécnica de Madrid (Spain); E. Pavelescu, J. P. Reithmaier, Univ. Kassel (Germany)

We present a simulator for Quantum Dot (QD) laser diodes. The simulator solves the complete bipolar semiconductor equations for the heterostructure. The special features of QD active layers are included through a multi-population non-equilibrium QD model. The interactions between the bulk material and Wetting Layer (WL), and between WL and each QD level and size are considered in terms of carrier capture/escape processes. The model includes inhomogeneous broadening due to the different QD size, homogeneous broadening in the gain, Shockley-Read-Hall, spontaneous and Auger recombination terms for bulk, WL and QD carriers.

The simulator is applied to 920 nm and 1060 nm lasers based on InGaAs/(Al)GaAs.



The influence of theoretical parameters, such as the homogeneous and inhomogeneous broadening, recombination parameters, capture times, band-offset, on the laser performance is analyzed. It is found that the maximum achievable gain depends on the symmetry of the QD electron and hole densities, which in turn depends on the band alignment. The simulation results are compared with experiments in broad area QD lasers. A high contribution of the bulk and/or WL non-radiative recombination terms has to be assumed to obtain a good agreement between the experimental and simulated threshold current densities and slope efficiencies. The measured temperature dependence of the emission wavelength for devices with different cavity lengths is reproduced in the simulations.

7597-47, Session 10

Bandwidth improvement by manipulating the high-frequency roll-off of an injection-locked QD laser operating at 1310 nmN. A. Naderi, M. C. Pochet, The Univ. of New Mexico (United States); V. Kovanis, Air Force Research Lab. (United States); L. F. Lester, The Univ. of New Mexico (United States)

The high-speed modulation characteristics of an injection-locked quantum dot Fabry-Perot semiconductor laser operating at 1310-nm under strong injection are investigated experimentally with a focus on the enhancement of the modulation bandwidth. The coupled system consists of a small-signal modulated quantum dot slave injected-locked by a CW DFB laser as the master. At particular injection strengths and frequency detuning levels between the master and slave laser, a unique modulation response is observed that differs from the typical modulation response observed in injection-locked systems. This unique response is characterized by a rapid low-frequency rise along with a 20 dB per decade high-frequency roll-off (typically 40 dB per decade) that enhances the 3-dB bandwidth of the locked system at the expense of losing modulation efficiency of about 20 dB at frequencies below 1 GHz. Such behavior has been previously observed both experimentally and theoretically in the high-frequency response characteristic of an injection-locked system using an externally-modulated master; however, the results shown here differ in that the slave laser is directly-modulated. The benefit of the observed response is that it takes advantage of the enhancement of the resonance frequency achieved through injection-locking without experiencing the low frequency dip that significantly limits the useful bandwidth in the conventional injection-locked response. The second benefit of this unique response is the improvement in the high frequency roll-off that extends the bandwidth. Finally a bandwidth improvement of ~7 times compared to the free-running slave laser has been achieved.

7597-48, Session 11

Modeling of photonic-crystal-based high-power high-brightness semiconductor lasersV. Shchukin, PBC Lasers GmbH (Germany); V. Kalosha, N. Ledentsov, D. Bimberg, Technical University of Berlin (Germany)

Modeling of semiconductor lasers based on multilayer epitaxial structures with ultrabroad vertical and ultrabroad lateral waveguides (WGs) suggests unique possibilities to fabricate high power high brightness lasers. Ultrabroad WGs are extremely advantageous as they allow extracting high power from a single chip, keeping low beam divergence and reducing undesirable non-linear effects in the gain medium, while one must ensure single mode lasing from a broad WG. Novel concepts addressing this challenge have been developed including vertical photonic crystal formed by epitaxial multilayer structure, lateral photonic crystal fabricated by multistripe processing with selective pumping of stripes, tilted wave laser based on phase matching effects in the propagation of optical modes in coupled resonators. The results have been employed in the fabrication of practical semiconductor lasers.

Experimental results on high brightness lasers are presented [1] for the wavelength regions of 635-660 nm, 850 nm, 980 nm, and 1060 nm.

[1] Dieter Bimberg et al, Photonics West 2010, OE120

7597-49, Session 11

Applying the joint Wigner time-frequency distribution to characterization of ultra-short optical pulses in the actively mode-locked semiconductor laser with an external single-mode fiber cavityA. S. Shcherbakov, P. Moreno Zarate, National Institute for Astrophysics, Optics and Electronics (Mexico); J. Campos Acosta, Consejo Superior de Investigaciones Científicas (Spain); Y. V. I. Il’n, I. S. Tarasov, A.F. Ioffe Physical-Technical Institute (Russian Federation)

An attempt is made to develop an approach to characterization of high-repetition-frequency trains including low-power picosecond optical pulses with an internal frequency modulation in both time and frequency domains in the case of exploiting semiconductor lasers matched effectively by an external single-mode fiber cavity. For these lasers, we analyze non-conventional regimes of the active mode-locking, which are connected with injecting various modulating signals and appearing specific composite states of a multi-pulse active mode-locking. Then, our approach uses the joint Wigner time-frequency distributions, which can be found due to involving the recently developed interferometric technique. In so doing, the modified scanning Michelson interferometer was exploited for shaping the field-strength auto-correlation functions peculiar to the above-mentioned types of light radiation. Theoretically, the Wigner distribution has an infinite resolution in time due to absence of averaging over any finite time interval. Moreover, for any finite lag length, it has an infinite frequency resolution. Together with this, the Wigner distribution being quadratic in nature is able to introduce various cross terms for a multi-component signal. For our analysis and creating the Wigner distributions, the data of experiments carried out with the InGaAsP/InP-heterolasers, operating at a wavelength of 1300 nm, had been used. When the optical signal consisted of contiguous ultra-short pulses with the repetition frequency up to 1.5 GHz, due to operating the semiconductor lasers in various active mode-locking regimes, typical pulse train-average auto-correlation functions had been characterized by temporal widths of about 3-15 ps. Additionally, stability of the chosen active mode-locking regimes had been followed and estimated.

7597-50, Session 11

Stability analysis of (Al,In)GaN laser diodes in an external cavityU. T. Schwarz, Fraunhofer-Institut für Angewandte Festkörperphysik (Germany); H. Braun, Regensburg Univ. (Germany); C. Raab, A. Able, F. Lison, Toptica Photonics AG (Germany)

We investigate experimentally and theoretically the spectral and dynamic properties of grating stabilized external cavity diode lasers (ECDL) with blue to UV (Al,In)GaN laser diodes. Operation in a single longitudinal mode of the external resonator results in extremely narrow linewidth and large coherence length. The frequency-selective feedback allows tuning of the laser wavelength and locking to an external reference. Here, a Littrow design is used for the grating stabilized ECDL employing commercial (Al,In)GaN laser diodes. The behaviour of the blue and UV laser diodes in combination with the grating is markedly different from red and IR laser diodes in an identical setup.

We use the rate equation model as described by Lang and Kobayashi to simulate the dynamics of the laser diode in a cavity with external feedback. This model provides linewidth and sidemode suppression.



Coherence collapse and the chaotic regime of laser dynamics are also well described. We take the input parameters from other measurements of (Al,In)GaN laser diodes, in particular optical gain, antiguiding factor, mirror reflectivity, carrier density, and carrier lifetime from Hakki-Paoli and streak camera measurements which were carried out in our laboratory. For comparison with red and IR laser diodes, we rely on parameter sets from literature.

The model reproduces the measured behaviour of the (Al,In)GaN ECDL extremely well. The dependency of the width of stability plateaus on injection current is in agreement with the experimental observation. Our preliminary result is that the Lang-Kobayashi model describes grating-stabilized ECDL in the blue and UV region as well as for red and IR ECDL. The experimentally observed difference between blue/UV and red/IR ECDL in terms of stability is reproduced by the simulation.

7597-51, Session 11

Semiconductor laser oscillation-frequency stabilization using the Faraday effectH. Arai, A. Sato, A. Sato, K. Nakano, T. Sato, M. Ohkawa, Niigata Univ. (Japan)

The semiconductor lasers in use today are on one hand, prized, and highly praised, for their small size, light weight, longevity and energy-efficiency, -and on the other, criticized for their susceptibility to frequency-fluctuations brought about by changes in temperature and drive current. Once this “wrinkle” is ironed out, semiconductor lasers will become the default light-sources, for satellites’ onboard interferometers.

Our studies have been directed at stabilizing oscillation to the atomic absorption line frequency reference, and using negative electrical feedback to the injection current. Frequency stabilization is accomplished, by either; a) applying direct modulation to the semiconductor laser’s drive current, or b) modulating the reference frequency, to obtain the error signal needed for stabilization. In this instance, Faraday effect-based stabilization was used. This indirect oscillation frequency stabilization has no discernable effect on spectra width, but, stability was no better than that observed in the system using the direct modulation.

When we compared Faraday effect- and direct modulation methods of stabilization, in order to uncover the root-cause of the discrepancy, sensors picked up system noise, the source of which was heat generated by the heavy current applied to a magnetic coil used to apply the Faraday effect. We also substituted a permanent magnet for the electromagnet.

7597-52, Session 11

Accurate source simulation in modern optical modeling and analysis softwareD. A. Jacobsen, E. R. Freniere, M. Gauvin, Lambda Research Corp. (United States)

Modern optical modeling and analysis programs allow users to create and analyze accurate optical and opto-mechanical systems in the software environment prior to building actual hardware based systems. The resultant accuracy of these models depends on the accuracy of the components that make up the model including the light source characteristics, surface and material properties, and the model geometry. In this paper we will consider factors that lead to improved modeling of the light source such as spectral and angular properties, the spatial distribution of light within the source, and the interaction of the light with the structure of the source. These factors are extremely important for near field modeling, especially for fiber and light pipe coupling. Several options will be discussed including simple source models such as point sources, ray files, surface properties that define optical parameters such as spectral and angular distribution, and detailed 3D solid models of the source. Simulated results for spectrum, angular, and spatial distribution will be compared to actual measurements for a variety of source types. Discussion will also include the appropriateness of each modeling approach with respect to different applications.

7597-67, Poster Session

Blue-emitting ZnSe random laserT. Takahashi, T. Nakamura, S. Adachi, Gunma University (Japan)

In contrast to the conventional semiconductor diode lasers, semiconductor random lasers can emit light without having Fabry-Perot resonators. Random laser action is known to be caused by strong multiple scattering of the emitted lights and light amplification in the disordered gain medium. Random laser shows unique characteristics, such as low threshold, spatial emission, and high coherency. It is clear that it is very important to develop random lasers emitting in the visible spectral region. However, most of the semiconductor random lasers (e.g., ZnO and GaN lasers) emit light in the near-UV region. ZnSe has an excitonic band gap of ~2.7 eV at room temperature and is, therefore, a promising material for realizing random laser in the visible spectral region. In this work, we report lasing action and characteristics of a blue-emitting ZnSe random laser at room temperature. Below the threshold excitation, we observe only a broad spontaneous emission peaking at ~470 nm. Above the threshold, several discrete lasing lines are observed to grow at the center of the spontaneous emission (~475 nm). The lasing line width is less than 0.4 nm which is found to be about 40 times smaller than the spontaneous emission.


All optical logic gates using active plasmonic device blockG. Oh, Chung-Ang Univ. (Korea, Republic of); D. Kim, Korea Photonics Technology Institute (Korea, Republic of); H. Kim, Y. Choi, Chung-Ang Univ. (Korea, Republic of)

Optical logic gates are essential requirement for achieving optical signal processing and optical computing systems. Though intensity-dependent all optical logic gates such as Mach-Zehnder interferometers and distributed-Bragg gratings have been researched, these methods based on nonlinear planer waveguide devices have disadvantages in size, switching time, and energy loss. In this work, we propose a novel all optical logic gates based on active plasmonics that may control the electron-photon coupling through an external effect. The phenomenon of surface plasmon resonance is basically appeared on attenuated total reflection mirror block. Under this resonance condition, the incident light gets highly absorbed and loses a fair amount of its energy, resulting surface plasmon polaritons wave (SPW) at the metal surface. The SPW is the propagation of the bound oscillation resulted by the electron-photon coupling, and can be controlled by external light. In this paper, we study a simple block for all-optical logic gate by using active plasmonics with specific metal layer configuration such as photonic crystal. In our structure, it is possible to control the propagating light in the prism by an external light under resonance condition. We also optimize this configuration by finite-difference time-domain method for metal layer configuration. Finally, we consider an optical NAND-gate using active plasmonics based on double device block. More detailed results and discussions will be presented.


Formulation of differential transfer matrix method in cylindrical geometryM. Jiani, M. Vesal, S. Khorasani, B. Rashidian, Sharif Univ. of Technology (Iran, Islamic Republic of)

Transfer and scattering matrix methods are widely in use for description of the propagation of waves in multilayered media. When the profile of refractive index is continuous, however, a modified formulation of transfer matrices does exist, which provides a complete analytical solution of the wave phenomena in such structures. For this purpose, we have previously formulated and reported the Differential Transfer Matrix



Method (DTMM) [1-4].

Previously reported variations of the so-called DTMM had been limited to Cartesian geometry where layered media form one-dimensional structures and plane waves are used as basis functions. In this work, we extend the formalism to cylindrical geometry with radial symmetry, in which Bessel functions need to be employed as basis functions. Hence, complete analytical formulation of the DTMM under radial and axial symmetry is described and derived.

[1] S. Khorasani, and K. Mehrany, “Analytical Solution of Wave Equation for Arbitrary Non-homogeneous Media,” Proceedings of SPIE, vol. 4772, pp. 25-36, Seattle (2002).

[2] S. Khorasani, and K. Mehrany, “Differential Transfer Matrix Method for Solution of One-dimensional Linear Non-homogeneous Optical Structures,” Journal of Optical Society of America B, vol. 20, no. 1, pp. 91-96 (2003).

[3] K. Mehrany, and S. K

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