Organic Nonlinear Optical Devices and Integrated Optics

Outline

• Directional Coupler

• Nonlinear Fabry-Perot Interferometer

• Frequency Converter

• Optical Limiter

• Integrated Optics

• Conclusions

Signal Switching I:Directional Coupler

Directional Coupler

• Interaction length and refractive index difference of the cores control the splitting ratio

Fluorine doped polyimide

• Fluorine content controls the refractive index of polyimide

• Core and cladding layer can be made from the same polymer---polyimide.

Fabrication

• To make multi-layer patterned structure, only need: spin coating, photolithography and RIE

mask

Nonlinear directional coupler

• Refractive index changes with light intensity

• Splitting ratio changes with light intensity

Material requirement

• Low switching power: High n2 , (2)

• Fast switching: Low response time

• Low propagation loss: Low absorption

• High optical damage threshold

• High thermal stability

A candidate: DPOP-PPV

• A side chain substituted PPV

• Loss = 0.4 dB/cm at 920 nm

• n2 = 1.1e-14 cm2/W

• Imax > 16 GW/cm2

• Tg = 163C

Experimental Result

• Length = 1/3 beat length (0.67 cm)

• Switching at 5.5 GW/cm2

Waveguide 1

Waveguide 2

Advantages and applications

Advantages:

• All optical switching

• Bar state splitting: 90/10

• Cross state splitting: 33/67

• Polymer: Easy processing

Applications:

• Beam splitter, Wavelength Add-Drop Multiplexer, Cross/Bar Switch

Signal Switching II:Fabry-Perot Interferometer

Nonlinear Fabry-Perot Device

• A wavelength selective device• Wavelength of the output signal depends on refractive index

of the middle medium

Signal In Signal Out

Nonlinear medium

Mirrors:

Reflectivity > 95%

Pump

Operation

• Nonlinear middle medium: poly-1,6-dicarbazoly 1-2,4-hexadyne (DCHC)

• Signal range: 700 - 900 nm

• Pump range: 637 - 645 nm

• Pump light changes the index of the middle medium and changes the wavelength selection at the output.

Experimental Results

Performance

• Pump: 2 GW/cm2 at 641 nm for 0.8 ps

• Turn on time: 0.33 ps

• Recovery time: 3 ps

• Can switch at 333 GHz

• All optical switching

• Very simple structure, easy processing

Frequency Conversion:Second Harmonic Generation

DeviceA waveguide-type with periodic

structure

Waveguide-type periodic structure

• Waveguide-type: compact, easy coupling to fibre/laser

• Periodic alternations of nonlinearities in the waveguide: enable phase-matching for light at and 2.

• Conversion:

220

2 )( PLP

Periodic structure

Nonlinear material

Linear material

Organic crystal + Semiconductor

• Nonlinear material: mNA (organic crystal grown on the grating)

• Linear material: SiN (grating)

Performance

• mNA: d33 = 20 pm/V

• Period = 7 m

• Length = 5 mm

• Wavelength = 1.06 m

• Conversion efficiency = 0.16% /W/cm2

5 mm

3 m

50 nm(2) = 2*d33

An all-polymer one

• Nonlinear polymer: diazo-dye-substituted

• Linear polymer: UV curable epoxy resin

FabricationSerial grafting technique:

Photolithography

RIE

Experimental Results

• The nonlinear polymer: d33 = 15 pm/V (after poled at 35 MV/m at 140C)

• Loss = 1.2 dB/cm

• Period = 32 m

• Wavelength = 1550 nm

• Conversion efficiency = 0.5%/W/cm2

5 mm

6 m

2 um

Signal Processing:Optical Limiter

Operation of Optical Limiter• Low fluence:

Linear transmittance

• High fluence: Clamped output level

Reverse saturable absorption

• Low intensity: Molecule is in low absorption state. Linear transmittance

• High intensity: Molecule is in photoinduced absorbing state. The material becomes highly absorptive.

• Candidate material: – Metallo-Phthalocyanines– Fullerenes

Metallo-Phthalocyanines

• Very weak ground state absorption

• Strong excited state absorption

Experimental Results

• Length = 1 cm

• Wavelength = 532 nm

• Pulse width = 8 ns

C60 in toluene

AlClPc in methanol

InClPc in toluene

Fullerenes (Bucky balls)

• All-carbon cluster

• Abundance of C=C gives plenty delocalizeable electrons

• C60, C70, C 76, ...

Experimental Results

• Solvent used plays an important role

Linear + Nonlinear:Integrated Optics

Advantages of polymer

• Low loss: 0.1 dB/cm at 1550 nm

• Controllable nonlinearities by doping/poling

• Low cost: only need spin-coating, photolithography and RIE

• Mechanical properties: rugged, flexible

• Precise control of refractive index: conveniently done by doping

• Convenient thickness control: spin-coating

Example 1: All polymer waveguide and MZ

• All polymer 3-D structures

• Achieve multi-level interconnections

Material

• UV15LV: low loss polymer as waveguide

• Polyurethane with tricyano chromophores: Active polymer with electro-optic coefficient = r33= 12 pm/V

• Waveguide loss = 0.5 dB/cm

Phase modulator

• Upper level: EO modulator• Lower level: waveguide

Example 2: Optical Transceiver

Characteristics

• Integrate polymer waveguide into semiconductor system

• Use polymer for waveguide and splitter

• Easy fabrication of polymer Y-branch structure

Example 3: Laser array and beam combiner

Laser array

Polymer beam combiner

Material

Polymer waveguide

The polymers are spin-coated on the laser-array-existing semiconductor substrate

Features and applications

• Loss < 1 dB/cm

• Good polymer adhesion to the substrate

• Applications: – Wavelength multiplexer/demultiplexer– MW-O-CDMA transmitter

ConclusionsPolymers are good for:• waveguide structure: low loss• EO or nonlinear operation: high and controllable

nonlinearities• Multi-level structure (3D): result of easy processing

Hybrid semiconductor/polymer structures or all polymer structures give rise to ample opportunities

Reference 1 • Polymer Directional Coupler

– J. Kobayashi et al., “Directional Couplers Using Fluorinated Polyimide Waveguides,” Journal of Lightwave Technology, Vol.16, No. 4, pp. 610-613, 1998.

– T. Gabler et al., “Application of the polyconjugated main chain polymer DPOP-PPV for ultrafast all-optical switching in a nonlinear directional coupler,” Journal of Chemical Physics, Vol. 245, pp. 507-516, 1999.

• Polymer Fabry-Perot Device– M. Bakarezos et al., “Ultrafast nonlinear refraction in an integrated Fabry-

Perot etalon containing polydiacetylene,” Proc. CLEO ‘99, CWF12, pp. 258, 1999.

Reference 2• Polymer waveguide second harmonic generation devices

– T. Suhara et al., “Optical Second-Harmonic Generation by Quasi-Phase Matching in Channel Waveguide Structure Using Organic Molecular Crystal,” IEEE Photonic Technology Letters, Vol. 5, No. 8, pp. 934-936, 1993.

– Y. Shuto et al., “Quasi-Phase Matched Second-Harmonic Generation in Diazo-Dye-Substitued Polymer Channel Waveguides,” IEEE Journal of Quantum Electronics, Vol. 33, No. 3 pp. 349-357, 1997.

• Optical limiter– Y. Sun et al., “Organic and inorganic optical limiting materials. From

fullerenes to nanoparticles,” International Reviews in Physical Chemistry, Vol. 18, No. 1, pp. 43-90, 1999.

• Integrated Optics– S. M. Garner et al., “Three-Dimensional Integrated Optics Using Polymers,”

IEEE Journal of Quantum Electronics, Vol. 35, No. 8 pp. 1146-1155, 1999.– N. Bouadma et al., “Monolithic Integration of a Laser Diode with a Polymer-

Based Waveguide for Photonic Integrated Circuits,” 1994.– T. Ido et al., “A simple low-cost polymer PLC platform for hybrid integrated

transceiver modules,” 2000

Appendix A

Semiconductor NLDC

• Based on MQW SC laser

• Operate at the transparency point

Properties

• Good nonlinearity

• Fast response

• Lower switching power

• Complicated structure (e.g. MQW)

• Need current injection (120 mA)

• Loss = 25 dB/cm at 879 nm

Other SC structures[Villenevue, 1992]

• no current injection is required

• still need MQW

• splitting ratio and switching power are comparable to the nonlinear polymer ones.

• Semiconductor Directional coupler– S. G. Lee et al., “Subpicosecond switching in a current injected GaAs/AlGaAs multiple-

quantum-well nonlinear directional coupler,” Applied Physics Letters,Vol. 64, pp. 454-456, 1994.

– A. Villeneuve et al., “Ultrafast all-optical switching in semiconductor nonlinear directional couplers at half the band gap,” Applied Physics Letters, Vol. 61, pp. 147-149, 1992.

Appendix B

Carrier generation through nonlinear optical process

• Direct bandgap material:– 2PA– intensity dependent: effective for ultrashort

pulse (ps to sub-ps)

• Indirect bandgap material:– linear indirect absorption – fluence dependent: good for ps to 100s ns

Experimental Results

• Pulse width = 25 ps, wavelength = 1060 nm

Si

GaAs