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

Nonlinear directional coupler • Refractive index changes with light intensity • Splitting ratio changes with light intensity

Material requirement • Low switching power: High n 2 ,  (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 • n 2 = 1.1e-14 cm 2 /W • I max • T g > 16 GW/cm = 163  C 2

Experimental Result Waveguide 2 Waveguide 1 • Length = 1/3 beat length (0.67 cm) • Switching at 5.5 GW/cm 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 Pump Signal In Signal Out Mirrors: Reflectivity > 95% Nonlinear medium • A wavelength selective device • Wavelength of the output signal depends on refractive index of the middle medium

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/cm 2 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 Device A 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:

P

2    0

L

2 (

P

 ) 2

Nonlinear material Periodic structure Linear material

Organic crystal + Semiconductor • Nonlinear material: mNA (organic crystal grown on the grating) • Linear material: SiN (grating)

Performance  (2) = 2*d 33 50 nm • mNA: d 33 = 20 pm/V • Period = 7  m • Length = 5 mm • Wavelength = 1.06  m 3  m • Conversion efficiency = 0.16% /W/cm 2 5 mm

An all-polymer one • Nonlinear polymer: diazo-dye-substituted • Linear polymer: UV curable epoxy resin

Fabrication Serial grafting technique: Photolithography RIE

Experimental Results 2 um 6  m • The nonlinear polymer: d 33 = 15 pm/V (after poled at 35 MV/m at 140  C) • Loss = 1.2 dB/cm • Period = 32  m • Wavelength = 1550 nm • Conversion efficiency = 0.5%/W/cm 2 5 mm

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 C 60 in toluene AlClPc in methanol InClPc in toluene • Length = 1 cm • Wavelength = 532 nm • Pulse width = 8 ns

Fullerenes (Bucky balls) • All-carbon cluster • Abundance of C=C gives plenty delocalizeable electrons • C 60 , C 70 , 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 = r 33 = 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 The polymers are spin-coated on the laser-array-existing semiconductor substrate Polymer waveguide

Features and applications • Loss < 1 dB/cm • Good polymer adhesion to the substrate • Applications: – Wavelength multiplexer/demultiplexer – MW-O-CDMA transmitter

Conclusions Polymers 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 Si GaAs • Pulse width = 25 ps, wavelength = 1060 nm