plasmonics sem report

8/6/2019 Plasmonics Sem Report

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PLASMONICSS. KRISHNA SANKER

B Tech EE S7M G College of Engineering

Abstract--- Currently, two of the most daunting

problems preventing significant increases in processor

speed are thermal and signal delay issues associated with

electronic interconnection. Optical interconnects, on the

other hand, possess an almost unimaginably large data

carrying capacity, and may offer interesting new solutions

for circumventing these problems. Unfortunately, their

implementation is hampered by the large size mismatch

between electronic and dielectric photonic components.

Dielectric photonic devices are limited in size by the

fundamental laws of diffraction to about half a wavelength

of light and tend to be at least one or two orders of

magnitude larger than their nanoscale electronic

counterparts. This obvious size mismatch betweenelectronic and photonic components presents a major

challenge for interfacing these technologies. Further

progress will require the development of a radically new

chip- scale device technology that can facilitate

information transport between nanoscale devices at optical

frequencies and bridge the gap between the world of

nanoscale electronics and microscale photonics.

We discuss a candidate technology that has

recently emerged and has been termed plasmonics. This

device technology exploits the unique optical properties of

nanoscale metallic structures to route and manipulate

light at the nanoscale. By integrating plasmonic,

electronic, and conventional photonic devices on the same

chip, it would be possible to take advantage of thestrengths of each technology. We present some of the

recent studies on plasmonic structures and conclude by

providing an assessment of the potential opportunities and

limitations for Si chip-scale plasmonics.

I. INTRODUCTION

Todays state-of-the-art microprocessors use ultrafasttransistors with dimensions on the order of 50 nm. Although itis now routine to produce fast transistors, there is a majorproblem in carrying digital information to the other end of amicroprocessor that may be a few centimeters away. Whereascopper wire interconnects carry digital information,

interconnect scaling has been insufficient to provide thenecessary connections required by an exponentially growingtransistor count. Unlike transistors, for which performanceimproves with scaling, the delay of interconnects increasesand becomes a substantial limitation to the speed of digitalcircuits . This limitation has become more evident over the

past 1 to 2 years, as the annual increase rate of the clock speedof microprocessors slowed greatly. Optical interconnects suchas fiber optic cables can carry digital data with a capacity91000 times that of electronic interconnects. Unfortunately,fiber optic cables are 1000 times larger compared withelectronic components, and the two technologies are difficultto combine on the same circuit. External optical interconnectsthat can connect different parts of the electronic chips via airor fiber cables have also been proposed. However, theresulting bulky configuration has limited the implementationof this idea. The ideal solution would be to have a circuit withnanoscale features that can carry optical signals and electriccurrents. One such proposal is surface plasmons, which are

electromagnetic waves that propagate along the surface of aconductor. The interaction of light with matter innanostructured metallic structures has led to a new branch of photonics called plasmonics. Plasmonic circuits offer thepotential to carry optical signals and electric currents throughthe same thin metal circuitry, thereby creating the ability tocombine the superior technical advantages of photonics andelectronics on the same chip.

II PLASMONICS AS A NEW DEVICETECHNOLOGY:

Metal nanostructures may possess exactly the right

combination of electronic and optical properties to tackle theissues outlined above and realize the dream of significantlyfaster processing speeds. The metals commonly used inelectrical interconnection such as Cu and Al allow theexcitation of surface plasmon- polaritons (SPPs). SPPs areelectromagnetic waves that propagate along a metal-dielectricinterface and are coupled to the free electrons in the metal.

The metallic interconnects that support such waves thusserve as tiny optical waveguides termed plasmonicwaveguides. The notion that the optical mode (light beam)diameter normal to the metal interface can be significantlysmaller than the wavelength of light has generated significant

excitement and sparked the dream that one day we will be ableto interface nanoscale electronics with similarly sized optical(plasmonic) devices.

It is important to realize that, with the latest advances inelectromagnetic simulations and current complementary


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Fi 1 A SPP propagati g along a metal

ielectric interface.These waves are transverse magnetic in nat re. From anengineering standpoint an SPP can be viewed as a specialt pe oflight wave propagating along the metal surface

metal oxide semiconductor (CM S)-compatible fabricationtechni ues, a variet of functional plasmonic structures can bedesigned and fabricated in a Si foundry right now. Current Si-based integrated circuit technology already uses nanoscale

metallic structures, such as Cu and Alinterconnects, to routeelectronic signals between transistors on a chip. This matureprocessing technology can thus be used to our advantage inintegrating plasmonic devices with their electronic anddielectric photonic counterparts. In some cases, plasmonicwaveguides may even perform a dual function andsimultaneously carry both optical and electrical signals, givingrise to exciting new capabilities.

III IMAGING SPPs' with a PHOTON SCANNINGTUNNELING MICROSCOPE :

In order to study the propagation of SPPs, a photonscanning tunneling microscope was constructed (PSTM) bymodifying a commercially available scanning near-fieldoptical microscope. PSTMs are the tool of choice forcharacteri ing SPP propagation along extended films aswellas metal stri pe waveguide. Figure 2a shows how amicroscope objective atthe heart of our PSTM can be used tofocus a laser beam onto a metal film at a well-defined angleand thereby launch a SPP along the top metal surface

A sharp, metal-coated pyramidaltip (Figure 2b and 2c) isused to tap into the guided SPP wave locally and scatterlighttoward a far-field detector. These particular ti ps have a

nanoscale aperture at the top of the pyramid through whichlight can be collected. The scattered lightis then detected witha photomultiplier tube. The signal provides a measure of thelocal lightintensity right underneath the tip and, by scanningthe tip overthe metal surface, the propagation of SPPs can beimaged The operation of the PSTM can be illustrated by

investigating the propagation of SPPs on a patterned Au film(Figure 2d). Here, a focused ion beam (FIB) was used todefine a series of parallel grooves, which serve as a Bragggrating to reflect SPP waves. Figure 2e shows a PSTMimageof a SPP wave excited with a 780 nm wavelength laser anddirected toward the Bragg grating. The back reflection oftheSPP from the grating results in the standing wave interferencepattern observed in the image. From this type of experiment

the wavelength of SPPs can be determined in astraightforward manner and compared to theory.

IV EXPERIMENTS and SIMULATIONS onPLASMONIC WAVEGUIDES :

The valuable information about plasmonic structuresprovided by PSTM measurements allows us to evaluate theutility of plasmonics for interconnection. Plasmonic stripewaveguides provide a natural starting point forthis discussionas such stri pes very closely resemble conventional metal

interconnects.

Electron beam lithography has been used to generate 55nm thick Au stri pes on a SiO2 glass slide with stri pe widthsranging from 5 m to 50 nm. Au stri pes are ideal forfundamental waveguide transport studies as they are easy tofabricate, do not oxidi e, and exhibit a qualitatively similarplasmonic response to Cu and Al. Figure 3a shows an opticalmicrograph of a typical device consisting of a large Au areafrom which SPPs can be launched onto varying width metalstri pes. A scanning electron microscopy (SEM) image of a250 nm wide stripe is shown as an inset. The red arrow showshow lightis launched from a focused laser spotinto 1 m

wide strip

Figures 3b, 3c, and 3d show PSTM images of SPPsexcited at = 780 nm and propagating along 3.0 m, 1.5 m,and 0.5 m wide Au stripes, respectively. The 3.0 m widestri pe can be used to propagate signals over several tens ofmicrons. Similarto previous far-field measurements along Agstripes, it is clear that the propagation distance of SPPsdecreases with decreasing stripe width. A better understandingof this behavior can be obtained from full-field simulationsand a recently developed, intuitive ray optics picture forplasmon waveguides. A selection ofthese simulation results ispresented next, followed by a discussion ofthe potential usesforthese relatively short propagation distance waveguides.

This type of knowledge presented on the propagationbehavior of plasmonic interconnects (mode si e, propagationlength, and cutoff) is essential for chip-designers and processengineers. Itis clearthatthe short propagation distances found


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for plasmonic waveguides preclude direct competition withlow-loss dielectric waveguide components. However,

Fig 2. (a) Schematic representation of the operation of a PSTM that enables the study of SPP propagation along metal filmsurfaces. The red arrow shows how a SPP is launched from an excitation spot onto a metal film surface using a high numericalaperture microscope objective. (b) Scanning electron microscopy (SEM) image ofthe near-field optical cantilever probe used inthe experiments. The tip consists of a microfabricated; hollow glass pyramid coated with an optically thick layer of Al. Lightcan be collected or emitted through a 50 nm hole fabricated in the Al film on the top ofthe pyramid. (c) A cross-sectionalview ofthe same hollow pyramidaltip after a large section was cut out ofthe sidewall with a focused ion beam (FIB). In closeproximity to the surface, the pyramidaltip can tap into the propagating SPP and scatter out a little bit oflightthrough the 50nm hole (shown pictorially). The scattered lightis detected in the far-field, providing a measure ofthe local field intensity rightunderneath the tip. By scanning the tip overthe sample and measuring the intensity at each tip position, images of propagatingSPPs can be created. (d) SEMimage of an Au film into which a Bragg grating has been fabricated using a FIB. (e) PSTMimageof an SPP wave launched along the metal film toward the Bragg grating. The back reflection ofthe SPP from the Bragg gratingresults in the observation of a standing wave interference pattern

.


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Fig 3.(a) Optical microscopy image of a SiO2 substrate with an array of Au stripes attached to a large launchpad generated by

electron beam lithography. The red arrow illustrates the launching of an SPP into a 1 m wide stripe. (b, c, and d) PSTMimagesof SPPs excited at = 780 nm and propagating along 3.0 m, 1.5 m, and 0.5 m wide Au stri pes, respectively

Fig 6. Plot of(a) SPP propagation length and (b) spatial extent ofthe SPP modes as a function ofthe center-layer thickness forMIM and IMI plasmonic waveguides. The insets illustrate plotted terms. The reflection pole method was used for = 1.55 mwith Au as the metal and air as the insulator.

plasmonic structures can add new types of functionality tochips that cannot be obtained with dielectric photonics. One

category of structures offering unique capabilities is activeplasmonic interconnects. Such interconnects may offer newcapabilities by introducing nonlinear optical or electricalmaterials into otherwise passive plasmonic waveguides. Ifthenonlinearities are strong enough, these devices can be madesmall compared with the characteristic decay lengths of SPPs

and their performance parameters should not suffer from theunavoidable resistive losses in the metals.While we have shown that weakly guided stri pe waveguides

cannot achieve deep sub wavelength confinement, there existalternative strongly guiding geometries that can providemarkedly better confinement. This category of structures is ofgreat interest for novel interconnection schemes Waveguidesconsisting of two closely spaced metals also combinepropagation distances of a few microns with deep-


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subwavelength confinement. Figure 6a shows a comparison ofpropagation lengths (where the exponential decay in |E

z| falls

to the 1/e point) for planar metal/insulator/metal (MIM)waveguides and waveguides consisting of a metal filmsandwiched between two insulators (IMI waveguides). Thesecalculations were performed using the well-establishedreflectionpole method at the important telecommunicationswavelength of 1.55 m.

For a sufficiently large center-layer thickness, thepropagation lengths for these two types of waveguidesconverge to the propagation length found for a single interface(dashed line in Figure 6a). This is reasonable since the SPPmodes on the two metal surfaces decouple when the spacingbetween them becomes large. As the center layer thicknessdecreases and the SPPs at the two interfaces start to interact,the propagation length along MIM structures decreases whileitincreases along IMI structures. In fact, IMI waveguides canreach centimeter propagation distances for very thin metalfilms. For obvious reasons, these are termed long-range SPPmodes. These large propagation distances can be understoodby realizing that the spatial extent of the modes becomes as

large as 10 m at these extremely thin metallic filmthicknesses (Figure 6b). In this case, the SPP waves are almostentirely propagating in the very low loss air region with verylittle field intensity inside the lossy (resistive) metal.

The MIM modes exhibit a continuous decrease in thepropagation length as the center insulating layer thickness isreduced. However, Figure 6b shows that by pushing themetals closer together it is feasible to realize deepsubwavelength mode diameters without running into problemswith cutoff. For example, the spatial extent decreases to about100 nm (


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groove should be smallerthan a critical angle. For V groovesmade from silver with a vacuum wavelength of 0.6328 mm,this critical wedge angle is found to be 102-. The measuredlateral localization of a structure with a 40- wedge angle is300 nm, which is superior to the nanoparticle-basedplasmonic waveguides. However, the reported experimentaland theoretical decay lengths for the same V grooveshapedplasmonic waveguide are 1.5 mm and 2.25 mm, respectively,

which are obviously too short for any application of theseplasmonic structures. The propagation distance performanceof the V grooveshaped SP waveguides has been recentlyextended to 250 mm (11). By using focused ion-beam millingtechniques, 460-mm-long V grooveshaped plasmonicwaveguides were fabricated on gold layers that were depositedon a substrate of fused silica. Scanning near-field opticalmicroscope measurements of these structures were made atoptical communication wavelengths (1425 to 1620 nm). For astructure with a 0.6-mm-wide and 1.0-mm-deep groove wedge(corresponding to a 17- wedge angle), the SP propagationlengths were measured to be within 90 to 250 mm. The modewas well confined along the lateral direction, and themeasured mode width was 1.1 mm. Thus there is a basic

trade-offin all Plasmon waveguide geometries between modesize and propagation loss. One can have a low propagationloss atthe expense of a large mode size, or a high propagationloss with highly confined light. A hybrid approach, whereboth plasmonic and dielectric waveguides are used, has beensuggested as a solution to this tradeoff.These waveguides aredesigned for 1500-nm operation and exhibit losses on theorder of 1.2 dB/mm, and they can guide light around 0.5-mm bends. Light can also be efficiently coupled between moreconventional silicon waveguides, where these Plasmonwaveguides with compact couplers and surface plasmonoptical device scan be constructed by using planar circuitfabrication techniques. Introducing gain to the plasmonicwaveguides can also bring a solution to the limited

propagation distances. This situation is theoreticallyinvestigated by considering the propagation of SPs on metallicwaveguides adjacent to a gain medium The analytic analysisand numeric simulation results show that the gain mediumassists the SP propagation by compensating for the metallosses, making it possible to propagate SPs with little or noloss on metal boundaries and guides. The calculated gainrequirements suggest that lossless, gain-assisted surfaceplasmon propagation can be achieved in practice for infraredwavelengths. Recently, a new kind of SP geometry has beensuggested to solve theoretically the issue of confinementversus propagation length The new mechanism for confiningmuch more field in the low-index region rather than in theadjacent high-index region is based on the relative dispersive

characteristics of different surface plasmon modes that arepresent in these structures. The structures have asubwavelength modal size and very slow group velocity overan unusually large frequency bandwidth. Simulations showthatthe structures exhibit absorption losses limited only by theintrinsic loss ofthe metal. Currently, there is no experimentaldata that supports these simulations. However, the new

suggested SP structure is quite promising and deservesattention from the experimental research groups that areworking on plasmonic waveguides.

Plasmonic chi ps will have optical input and output ports,and these ports will be optically connected to conventionaldiffraction-limited photonic devices by plasmonic couplers.The couplers should have high conversion efficiency, along

with a transmission length that is longer than the opticalwavelength to avoid the direct coupling of the propagatingfar-field light to the nanophotonic devices inside theplasmonic chip. A promising candidate forthis feature can befabricated by combining hemispherical metallic nanoparticlesthat work as a plasmonic condenser and a nanodot-basedplasmonic waveguide . When the focused plasmons move intothe coupler, the transmission length through the coupleris 4.0mm. Nanodots can also be used for focusing SPs into a spot ofhigh near-field intensity having a subwavelength width .Figure 2A shows the SEMimage of such a sample containing19 200-nm through-holes arranged on a quarter circle with a5-mm radius. The SPs originating from these nanodots arecoupled to a metal nanostrip waveguide. A near-field scanning

optical microscopy (NSOM) image ofthis structure was takenat 532-nm incident wavelength with horizontal polarization.The near-field image (Fig. 2B) shows that the focused SPspropagate along the subwavelength metal guide, where theypartially penetrate into the 100-nm-wide

Fig. 1. (A) FDTD simulations show the electric fieldproduced within the plasmon waveguide structure. (B) Aplasmon waveguide consists of nanoscale gold dots on asilicon-on-insulator surface.


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Fig. 2. (A) SEMimage of a nanodot focusing array coupled to

a 250-nm-wide Ag stri p guide. (B) NSOM image of the SPintensity showing subwavelength focusing.

bifurcation at the end of the guide, thus overcoming thediffraction limit of conventional optics. The measuredpropagation distance is limited to 2 mm, and the propagationdistances are expected to be much longer with improvedfabrication processes and by using properly designed metal-dielectric hybrid structures. The combination of focusingarrays and nanowaveguides may serve as a basic element inplanar plasmonic circuits.

Active control of plasmons is needed to achieve plasmonicmodulators and switches. Plasmonic signals in a metal-on-dielectric waveguide containing a gallium section a fewmicrons long can be effectively controlled by switching thestructural phase of gallium . The switching can be achieved byeither changing the waveguide temperature or by external

optical excitation. The signal modulation depth can exceed80%, and switching times are expected to be in the picosecondtime scale. The realization of an active plasmonic device bycombining thin polymer films containing molecularchromophores with thin silver film has also been reported .The molecular plasmonic device consists of two polymerlayers, one containing donor chromophore molecules and theother containing acceptor fluorophore molecules. Coupled SPs

are shown to provide an effective transfer of excitation energyfrom donor molecules to acceptor molecules on opposite sidesof metal film up to 120 nanometers thick. The donors absorbincident light and transfer this excitation energy by dipole-dipole interactions to the acceptors. The acceptors then emittheir characteristic fluorescence. These results are preliminarydemonstrations for active control of plasmonic propagation,and future research should focus on the investigation ofelectro-optic, all-optical, and piezoelectric modulation ofsubwavelength plasmon waveguide transmission. Extensiveresearch efforts are being put forth in order to achieve an all-plasmonic chip. In the nearterm, plasmonic interconnects may be used to address the capacity problem in digital circuitsincluding microprocessors. Conventional electronic

interconnects may be used to transfer the digital data amongthe local arrays of electronic transistors. But, when a lot ofdata need to travel from one section of a chip to anotherremote section of the chi p, electronic information could beconverted to plasmonic information, sent along a plasmonicwire, and converted back to electronic information at thedestination. Unfortunately, the current performance ofplasmonic waveguides is insufficient for this kind ofapplication, and there is an urgent need for more work in thisarea. If plasmonic components can be successfullyimplemented as digital highways into electronic circuits, thiswill be one ofthe killer applications of plasmonics.

VI PLASMONIC LIGHT SOURCES

The emerging field of plasmonics is not only limited to thepropagation of light in structures with subwavelengthdimensions. Plasmonics can also help to generate andmanipulate electromagnetic radiation in various wavelengthsfrom optics to microwaves. Since their introduction byNakamura in 1995 , InGaN-based semiconductor lightemitting diodes (LEDs) have become promising candidatesfor a variety of solid-state lightning applications . However,semiconductor-based LEDs are also notorious for their lowlight-emission efficiencies. Plasmonics can be used to solvethis efficiency problem . When InGaN/GaN quantum wells(QWs) are coated by nanometer silver or aluminum films, theresulting SPs increase the density of states and the

spontaneous emission rate in the semiconductor. This leads tothe enhancement oflight emission by SP-QW coupling, whichresults in large enhancements ofinternal quantum efficiencies.Time-resolved photoluminescence spectroscopymeasurements were used to achieve a 32-fold increase in thespontaneous emission rate of an InGaN/GaN QW at 440 nm


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.This enhancement of the emission rates and intensities resultsfrom the efficient energy transfer from electron-hole pairrecombination in the QW to electron vibrations of SPs at themetal-coated surface of the semiconductor heterostructure.This QW-SP coupling is expected to lead to a new class ofsuper bright and high-speed LEDs that offer realisticalternatives to conventional fluorescent tubes.

Similar promising results were obtained for organic LEDs

(OLEDs), which are now becoming popular as digitaldisplays. In an OLED, up to 40% of the power that can becoupled into air is lost due to quenching by SP modes. Aperiodic microstructure can be used to recover the power thatis normally lost to SPs. Using this approach, strong photoluminescence has been reported from a topemittingorganic light-emitting structure, where emission takes placethrough a thin silver film . The results indicate that theaddition of a nanopatterned dielectric overlayer to the cathodeof top-emitting OLEDs should increase light emission fromthese structures by two orders of magnitude over a similarplanar structure. The dielectric layer acts to couple the surfaceplasmon-polariton modes on the two metal surfaces, whereasits corrugated morphology allows the modes to scatter to light.

An OLED using a p-conjugated polymer emissive layersandwiched between two semitransparent electrodes was alsoreported . One of the electrodes was an optically thin gold filmanode, whereas the cathode was in the form of an opticallythick aluminum (Al) film with patterned periodicsubwavelength two-dimensional (2D) hole array that showedanomalous transmission in the spectral range of the polymer

photoluminescence band. At similar current densities, asevenfold electroluminescence efficiency enhancement wasobtained with the patterned Al device compared with a controldevice based on imperforated Al electrode, demonstrating thatthe method of patterning the electrodes into 2D hole arrays isefficient for this structure. Plasmonics can also be used toenhance the performance of lasers . A metal nano-aperturewas fabricated on top of a GaAs vertical cavity surface

emitting laser (VCSEL) for subwavelength optical near-fieldprobing. The optical near-field intensity and the signal voltageof nano-aperture VCSELs exhibit record high values becauseof the localized surface plasmons in metal nanostructures. Theenhancement factors of the optical near-field and voltagesignal are 1.8 and 2, respectively. Reducing the nano-aperturereduces the optical resolution of the VCSEL probe from 240nm to 130 nm. These results show that plasmon enhancementwill be helpful for realizing high-resolution optical near-fieldVCSEL probes. SPs also play a key role in the transmissionproperties of single apertures and the enhanced transmissionthrough subwavelength hole arrays . There has been intensecontroversy on the physical origin of the enhancedtransmission in these structures . Recent theoretical and

experimental analyses suggest that the enhanced transmissioncan be explained by diffraction assisted by the enhanced fieldsassociated with SPs . Although SPs are mostly studied atoptical frequencies, they can also be observed at themicrowave, millimeter-wave, and THz frequencies . Bytexturing the metallic surface with a subwavelength pattern,we can create SPs that are responsible for enhanced

Fig. 10. Future integrated plasmonic circuit, including: (0) incoupling structures; (1) color demultiplexing in a Z add/dropfilter; (2) bends and tapers in LR-SPP waveguides; (3) all-optical preprocessing logic; (4) integrated photodetection; (5) opticalclock incoupled from free space; (6) nano-optical subcircuit (on-chip integrated light source, electrooptic plasmostor, singlequantum dot devices, integrated photodetection); (7) collection of light via photon sorting; (8) integrated plasmonic colorfiltering; and (9) beam shaping of emitted light.


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transmission observed at microwave and millimeter wavefrequencies for 1D and 2D gratings with subwavelengthapertures . A subwavelength circular aperture with concentricperiodic grooves can be used to obtain enhanced microwavetransmission near the surface plasmon resonance frequency .These results show that enhanced transmission from asubwavelength circular annular aperture with a grating isassisted by the guided mode of the coaxial waveguide andcoupling to the surface plasmons. A 145-fold enhancementfactor is obtained with a subwavelength circular annularaperture surrounded by concentric periodic grooves. The samestructure also exhibits beaming properties that are similar to

the beaming effects observed from a subwavelength apertureat optical wavelengths . Figure 3 shows the electromagneticwaves from a subwavelength circular annular aperturesurrounded by concentric periodic grooves. The radiatedelectromagnetic waves have a very strong angularconfinement around the surface mode resonance frequency, inwhich the angular divergence of the beam is T3-. Enhancedtransmission at THz wavelengths is also reported for afreestanding metal foil perforated with periodic arrays ofsubwavelength apertures . The peak transmission at the lowestfrequency resonance is 0.6 for each aperture array, which isa factor of 5 larger than the fractional area occupied by theapertures. Doped semiconductors exhibit a behavior at THzfrequencies similar to that of metals at optical frequencies,

thus they constitute an optimal material for THz

plasmonics . Enhanced transmission of THz radiation isobserved by using arrays of subwavelength aperturesstructured in n-type silicon. This enhancement can be

explained by the resonant tunneling of SPs that can be excitedat THz wavelengths in doped semiconductors. Thetransmission increases markedly as the aperture size isaugmented and as the array thickness is reduced.

VII Si PLASMONICS: CHALLENGES ANDPROJECTIONS

As highlighted in this review, plasmonic components arerapidly evolving from discrete, passive structures towardintegrated active devices. In part, this progress has beenfacilitated by opportunities to dispersion engineermetallodielectric systems. Surface plasmons provide access to

an enormous phase space of refractive indices and propagationconstants that can be readily tuned through variation ofmaterial, dimension, or geometry. Via dispersion engineering, plasmonics has enabled a suite of passive and activesubwavelength optical components, including interconnects,negative index metamaterials, optoelectronic modulators, and plasmon-enhanced sources. Plasmonic geometries are also being explored for quantum optics, nonlinear optics,transformation optics, optical antennas, and nano-opticalcircuits. The ultrasmall mode size and high local fields of plasmonic architectures can combine the functionality of photonics and electronics on-chip. Still, a number ofchallenges should be addressed en-route to integrated plasmonics. These include: (i) investigating low-loss

plasmonic materials, with lower intrinsic absorption than Ag;(ii) developing a surface-plasmon laser that is both frequency-tunable and subwavelength in size; (iii) exciting and detectingsurface plasmons on-chip via electrical means; and (iv)demonstrating integrated plasmonic logic. Despite thesechallenges, Si-based plasmonic networks may be envisioned


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for the foreseeable future. Fig. 10 depicts an artistic renderingof a hybrid optoelectronic circuit, synthesizing a number ofplasmonic device concepts from the recent literature. On thefar left, light arrives from a dielectric waveguide which can bematched with low insertion loss to an LR-SPP waveguide .Wavelength demultiplexing is performed using Z dropfilters , while low-loss tapers and bends direct the LRSPPsinto the densely integrated section of the circuit . A compact,

lowpower all-optical plasmonic switch provides adynamically configurable interconnect system. Ultimately, theincident LR-SPPs are detected by waveguide-integratedphotodetectors . Separately, an optical clock signal arrives as afree space beam, which is captured by a grating anddistributed throughout the electronic processor by a branching plasmonic network. Utilizing a planar-compactMIMwaveguide, this signal could be carried to buriedlayers aswell as across the top surface . The electronic circuitis coupled to a nano-optical subsystem, which employs an on-chip optical train consisting of an integrated plasmonic lightsource , electrooptic plasmon modulators , and detectors.Here, the region of interest is represented as nanoguides,which address individual light emitting quantum dots . Other

exciting nano-optical systems on chip might include highlyintegrated chemical or biological analysis arrays that addressmultiple individual molecules. Finally, the module containsfunctionality for receiving and transmitting data by free spaceoptical communication. For detection, a small array of photodetectors is employed, which take advantage ofplasmonic collection of light. These concentric rings increasethe effective area of the detector, as well as integrating colorselectivity . On the output side, in conjunction with anintegrated broadband source, the wavelength of emitted lightis controlled dynamically by a tunable plasmonic color filter.Whether the output beams emanate from such a plasmoniccolor display or an alternative light source such as an array ofvertically emitting lasers, the output beams are conditioned to

have small divergence, using an integrated plasmonic antennacollimator . As noted by Gordon Moore in his seminal 1965manuscript, Cramming more components onto integratedcircuits , the future of integrated electronics is the future ofelectronics itself. Perhaps the same can be said ofplasmonics. The promise of plasmonics for Si photonics liesin the ability to combine optical sources, interconnects,modulators, and detectors on the same chip, withinsubdiffraction-limited dimensions. Such an integrated photonic technology could revolutionize the bandwidth,speed, size, cost, and power requirements of moderncomputational networks, enabling more efficient solutions toincreasingly complex problems.

VIII LATEST DEVELOPMENTSThe possibility to confine light to the nanoscale and the

ability to tune the dispersion relation of light have evokedlarge interest and led to rapid growth of plasmonic research.The parallel development of nanoscale fabrication techniques

like electron beam lithography and focused-ionbeam millinghas opened up new ways to structure metals surfaces andcontrol surface plasmon polariton propagation and dispersionat the nanoscale.

In 2000, Mark L. Brongersma etal(and others) proposedthat EM energy could be transported below the diffractionlimit with high efficiency and group velocity greater than 0.1calong a wire of its characteristic length 0.1. In year 2002,

Maier et al experimentally observed the most efficientfrequency for transport to be 3.191015 rad/sec with acorresponding group velocity of 4.0x106 m/s for longitudinalmode of plasmon waveguide having an inter-particle distanceof 75 nm. The achieved bandwidth was calculated to be1.41014 rad/sec.

Dionne et al in year 2006 constructed slot waveguides.Slot waveguides can support both transverse electric andtransverse magnetic photonic polarisation. The loss in slotwaveguide can be minimised by using a low-refractive-indexmaterial; for example, a 100nm thick Ag/SiO2/Ag slabwaveguide sustains signal propagation up to 35 m atwavelength of 840 nm. In 2007, Feng et alobserved that fieldlocalisation could be improved by introducing the partial

dielectric filling of the metal slot waveguide, which alsoreduces propagation losses. The channel in metal surfacewaveguides supports surface plasmons at telecommunicationwavelength with very low loss (having propagation length of100 m) and well-confined guiding. In this experiment,surface plasmons are guided along a 0.6m wide and 1mdeep triangular groove in gold material.

Thin metallic strips can support long-range surface plasmonsa particular type of surface plasmon modecharacterised by electromagnetic fields mostly contained inthe region outside of the metal, i.e., in dielectric medium. Junget alin 2007 experimentally confirmed that long-range surfaceplasmons could transfer data signal as well as the carrier light.In a demonstration, a 10Gbps signal was transmitted over a

thin metallic strip (14nm thick, 2.5m wide and 4cm longgold strip). Furthermore, to reduce the propagation loss, JinTae Kim et al fabricated a low-loss, long-range surfacePlasmon polariton waveguide in an ultraviolet curable acrylate polymer having low refractive index and absorption loss. A14nm thick and 3m wide metallic strip cladded in acrylatepolymer material shows a loss of 1.72 dB/cm.

Rashid Zia et alobtained the numerical solution by usingthe full-vectorial magnetic field finite-difference method for55nm thick and 3.5nm wide strip on glass at a wavelength of800 nm and noted that surface plasmons are supported on bothsides of the strip and can propagate independently.

Alexandra et al in year 2008 suggested that triangularmetal wedge could guide surface plasmons at

telecommunication wavelength. It was experimentallyobserved that 1.43- 1.52m wavelength can propagate over adistance of about 120 m with confined-mode width of 1.3m along a 6m high and 70.5 angled triangular gold wedge.

IX FUTURE DIRECTIONS


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In the field of plasmonics, studying the way light interactswith metallic nanostructures will make it easierto design newoptical material devices. One primary goal of this field is todevelop new optical components and systems that are of thesame size as todays smallestintegrated circuits and that couldultimately be integrated with electronics on the same chip.The next step will be to integrate the components with an

electronic chip to demonstrate plasmonic data generation,transport and detection. Plasmon waves on metals behavemuch like light waves in glass. That means engineers can usetechniques like multiplexing or sending multiple waves.Plasmon sources, detectors and wires as well as splitters andeven plasmonsters can be developed. Applications mainlydepend on controlling the losses and the cost ofnanofabrication techniques. Enhanced and directed emissionof semiconductorluminescence (quantum dots) may well findcommercial application in plasmonassisted lighting in the nearfuture. Finally, plasmonic nanocircuits combine a highbandwidth with a high level of compaction and makeplasmonic components promising for all-optical circuits.Plasmonic wires will act as high-bandwidth freeways across

the busiest areas of the chi p. Plasmons can ferry data alongcomputer chi ps. Plasmonic switches required for this areunder development. Rotaxanes molecule is being used for thepurpose. Change in the shape ofthe molecule is the principleofthis molecular switch.

X CHALLENGES REMAINING

Despite many advances in the field of plasmonics, severalimportant open questions and problems remain. For example,how can plasmons be efficiently excited with nanoscaleresolution resolution? Surface plasmon polaritons are usuallyexcited using far-field opticaltechniques, which have a higher

resolution than plasmonic phenomena under investigation.However, for true nanoscale plasmonic studies, asurfaceplasmon- polariton point source with nanoscaledimensions is required. What are the fundamental processesthat determine the losses of surface plasmon polaritons? isanotherimportant question. Practically, Plasmon experimentsare performed on poly-crystalline surfaces, and the limits tothe losses due to surface roughness, grain boundaries, etc arenot known. Surface plasmons propagate along the chain ofnanoparticles, but the losses are high. On the other hand,propagation losses are low in the case of nanowires, whichleaves open the possibility of surface-plasmon optical devices.the dream of making all-plasmonic devices requires furtherresearch. In orderto realise advanced active circuits,

there is a need for active modulator and switching componentsoperating at ultra-high bandwidth and lowpower utilisation.To manipulate surface Plasmon polaritons on a surface,reflectors are needed. So far, macroscopic Bragg reflectorsstructured into the surface have been used. For true nanoscaleintegration, nanoscale surface Plasmon polariton mirrors are

required. Once these are realised, nanoscale cavities to confinesurface plasmon polaritons can also be designed. The limits tothe mode volume and quality factor of plasmonic cavities arenot yet known. finally, the use of a particle beam ratherthan alight beam to excite surface plasmon polaritons raisesquestions and novel opportunities regarding the selectivitywith which surface plasmon modes with different symmetrycan be excited.

CONCLUSION

Plasmonics has the potential to play a unique and importantrole in enhancing the processing speed of future integratedcircuits. The field has witnessed an explosive growth overthelast few years and our knowledge base in plasmonics israpidly expanding. As a result, the role of plasmonic deviceson a chip is also becoming more well-defined and is capturedin Figure 8. This graph shows the operating speeds and criticaldimensions of different chip-scale device technologies. Inthe past, devices were relatively slow and bulky. Thesemiconductorindustry has performed an incrediblejob in

Fig. 2: Operating regimesapplicable size and speed scalefor plasmonic and other devices

scaling electronic devices to nanoscale dimensions.Unfortunately, interconnect delay time issues providesignificant challenges toward the realization of purelyelectronic circuits operating above 10 GHz. In starkcontrast, photonic devices possess an enormous data-carryingcapacity (bandwidth). Unfortunately, dielectric photoniccomponents are limited in their size by the laws of diffraction,

preventing the same scaling as in electronics. Finally,plasmonics offers precisely what electronics and photonics donot have: the size of electronics and the speed of photonics.Plasmonic devices, therefore, might interface naturally withsimilar speed photonic devices and similar size electroniccomponents. Forthese reasons, plasmonics may well serve as


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the missing link between the two device technologies thatcurrently have a difficult time communicating. By increasingthe synergy between these technologies, plasmonics may beable to unleash the full potential of nanoscale functionalityand become the next wave of chip-scale technology

REFERENCES

y . http://www.sciencedirect.comy . http://www.plasmonifocus.comy . http://appliedplasmonics.comy . http://newscientist.comy Plasmonics promises Faster Communication Dr

Verma, Mr Singh www. e f ymag . com

y Silicon-Based Plasmonics for On-Chip PhotonicsJennifer A. Dionne, Luke A. Sweatlock, Matthew T.

Sheldon, A. Paul Alivisatos, and Harry A. Atwatery Plasmonics: Merging Photonics and Electronics at

Nanoscale DimensionsScience 311, 189 (2006);

Ekmel Ozbay DOI: 10.1126/science.1114849

y PLASMONICS USING METALNANOPARTICLESTammy K. Lee and Parama Pal

ECE 580 Nano-Electro-Opto-Bio March 29th, 2007

y Plasmonics: Localization and guiding ofelectromagnetic energy in metal/dielectric structures

Stefan A. Maiera_ and Harry A. AtwaterThomas J.

Watson Laboratories of Applied Physics, California

Institute ofTechnology, Pasadena,California 91125


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plasmonics sem report

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