Tunable photonic crystals with liquid crystals 1-D –Electrical & thermal tuning of filters (porous silicon and Si/SiO 2 ) –Electrically tunable lasing 2-D –Thermal tuning of PBG of porous silicon and III-V structures –Electrically tunable photonic crystal laser 3-D –Electrical & thermal tuning of inverse opals & opals –Thermal tuning of porous silicon Photonic crystal fiber
Liquid crystal tuning no field E applied E field ELECTRIC FIELD “cold” (nematic) “hot” (isotropic) TEMPERATURE 5 Å 2 nm T c ~ 58°C E7 liquid crystal: n o ~ 1.5, n e ~ 1.7 For positive anisotropy LC
Porous silicon 1-D photonic crystals Electrochemical etching of porous silicon Infiltration of E7 LC in vacuum Confined geometry: LC in 20 nm pore 200nm
Porous silicon 1-D photonic crystals Electrochemical etching of porous silicon Infiltration of E7 LC in vacuum Confined geometry: LC in 20 nm pore
Porous silicon 1-D photonic crystals n LC ~ 0.02 Q ~ 400 with LC 25°C61°C Reflectance (%) Wavelength (nm) 14 dB attenuation Constricted geometry limits liquid crystal rotation and, hence, effective birefringence
Porous silicon 1-D photonic crystals Resonance red shift (nm) Temperature, C ZLI-4788 liquid crystals Resonance red shift (nm) Temperature, C BL087 liquid crystals Resonance red shift (nm) Temperature, C E7 liquid crystals (nematic) (isotropic) Resonance red shift (nm) Temperature, C 5CB liquid crystal macropore mesopore
Porous silicon 1-D photonic crystals ~ Resonance red shift (nm) Voltage (V rms ) mesopore macropore Resonance red shift (nm) Voltage (V rms ) mesopore Negative anisotropy LC 10 dB 5 dB
Si/SiO 2 photonic crystals G. Pucker et al., J. Appl. Phys. 95, 767 (2004) Fabrication by “chip bonding” with 950 nm gap between multilayer mirrors Poly-Si/SiO 2 mirrors formed by LPCVD
Si/SiO 2 photonic crystals E7 and Merck 6608 ( <0) LC free to rotate in 950 nm cavity Homogeneous LC alignment achieved by rubbing with lens paper Homeotropic LC alignment achieved by treatment with monolayer of lecithin Infiltration by heating above clearing point
Si/SiO 2 photonic crystals E7 and Merck 6608 ( <0) LC free to rotate in 950 nm cavity Homogeneous LC alignment achieved by rubbing with lens paper Homeotropic LC alignment achieved by treatment with monolayer of lecithin Infiltration by heating above clearing point nono nene
1-D photonic crystal laser Electrical tuning of E47 dye- doped LC Polyimide coating and unidirectional rubbing set LC alignment 2 micron spacer sets gap for LCs R. Ozaki et al., Appl. Phys. Lett. 82, 3593 (2003)
1-D photonic crystal laser Optically pumped dye-doped LC laser using 2 nd harmonic of Nd:YAG (wavelength tuning accomplished electrically)
Porous silicon 2-D photonic crystals Formed by lithographic prestructuring and electrochemical etching Pitch = 1.58 m, pore diameter ~ 1.38 m E7 LC infiltrated by heating to isotropic phase (strong capillary action of pores draw up LC) S.W. Leonard et al., Phys. Rev. B 61, R2389 (2000)
Porous silicon 2-D photonic crystals No LC LC (nematic & isotropic phase) H-pol (electric field perpendicular to pores) LC absorption lines near 3 m and above 6 m Air band edge shifts 70nm when LC heated to isotropic phase Dielectric band edge unchanged
Porous silicon 2-D photonic crystals LC likely take on axial alignment (LC director parallel to pore axis) or escaped radial alignment (LC director anchored homeotropically at pore wall and escapes to third dimension at the center of the pore) measured simulation assuming axial alignment of LC
Porous silicon 2-D photonic crystals LC alignment can be more accurately determined by using 2 H-NMR Uses quadripolar splitting of 2 H-NMR signal: v is angle between director and magnetic field parallelplanar polarescaped radial G. Mertens et al., Appl. Phys. Lett. 83, 3036 (2003)
Porous silicon 2-D photonic crystals LC alignment can be more accurately determined by using 2 H-NMR Uses quadripolar splitting of 2 H-NMR signal: v is angle between director and magnetic field parallelplanar polarescaped radial G. Mertens et al., Appl. Phys. Lett. 83, 3036 (2003)
Conclusion: LC alignment in pores is very sensitive to surface anchoring –Parallel alignment or escaped radial alignment with periodic array of defects Another paper claims planar polar alignment with no surface treatment and escaped radial or axial with defect when surface silanized [M. Haurylau et al., Phys. Stat. Sol. A 202, 1477 (2005)] Porous silicon 2-D photonic crystals
silanized untreated M. Haurylau et al., Phys. Stat. Sol. A 202, 1477 (2005)
III-V photonic crystals E-beam litho and RIE to define photonic crystal in MBE-grown AlGaAs/GaAs with GaInAs quantum dot layer as internal light source Infiltration with E7 LC –Sample mounted in flask and evacuated below 1mbar –LC injected through shot needle –Capillary forces draw in LC into 200nm air holes Ch. Schuller et al., Appl. Phys. Lett. 82, 2767 (2003)
III-V photonic crystals GaInAs quantum dot layer (internal light source) excited by 514nm line of Ar + ion laser Measure TE-pol Sample mounted on a copper heat sink coupled to a Peltier temperature controller
III-V photonic crystals Q-factor < 100 (LOW!) 9.5 nm shift ( ~ 1090 nm) 50 nm (a ~ 300 nm) Claims LC molecules aligned parallel to holes
III-V photonic crystals Q-factor < 100 (LOW!) 9.5 nm shift ( ~ 1090 nm) (a ~ 300 nm) Claims LC molecules aligned parallel to holes
Electrical tuning of photonic crystal laser Photonic crystal laser between two ITO glass plates –Laser defined within InGaAsP quantum well material –Fabricated using e-beam litho and RIE Infiltrated with nematic LC MLC-6815 (heated) B. Maune et al., Appl. Phys. Lett. 85, 360 (2004) a = 500 nm, r =165 nm, r defect =100 nm, slab thickness =320 nm. 2m2m
Electrical tuning of photonic crystal laser Electric field damped in holes due to screening by the conducting PC membrane Tuning of cavity achieved by changing refractive index in cladding (evanescent field) Pumped with semiconductor laser diode at 830 nm
Electrical tuning of photonic crystal laser Tuning range limited by small LC birefringence ( n=0.052) –Needed low LC index to maintain sufficient light confinement –If birefringence too large and LC disordered, scattering is a problem Surface anchoring and LC alignment also play role Q-switched LC photonic crystal laser has now been demonstrated
Porous silicon 3-D photonic crystals Formed by lithographic patterning and electrochemical etching with modulated current density Investigate photonic properties of light propagation along pore axis Pore diameter between 0.76 m and and 1.26 m Infiltrated with 5CB liquid crystal G. Mertens et al., Appl. Phys. Lett. 83, 3036 (2003)
Porous silicon 3-D photonic crystals LC band edge tuned by 140 nm as heated from 24°C to 40°C (T c ~35°C)
Opals – thermal tuning Formation by sedimentation of monodispersed silica spheres (300 and 550 nm diameters) Sandwich cells made of two glass plates with separation of 6, 25, and 50 m Infiltrated nematic LC ZLI1132 and smectic LC 1MC1EPOPB into interconected nanosized voids of opals K. Yoshino et al., Appl. Phys. Lett. 75, 932 (1999)
Opals – thermal tuning Results not great but demonstrate the principle ZLI1132 liquid crystal
Opals – thermal tuning Results not great but demonstrate the principle 1MC1EPOPB liquid crystal
Inverse opals – electrical tuning Fabricate polymer inverse opal using silica opal template (300nm spheres) Infiltrated 5CB nematic LC M. Ozaki et al., Adv. Mat.14, 514 (2002)
Inverse opals – electrical tuning 150 volt threshold!!! Large shift due to large filling fraction of LC in inverse opals
Inverse opals – electrical tuning Let’s look at response time when apply voltage and after turning off voltage… MEMORY EFFECTS! Not too bad for LCs
Photonic crystal fiber T. T. Larsen et al., Optics Express 11, 2589 (2003) LC introduced into the air holes (3.5mm) Merck MDA (cholesteric) and BDH TM216 (chiral Smectic A and cholesteric) Radial alignment of LC
Photonic crystal fiber THERMAL TUNING
Photonic crystal fiber 77°C89°C 91°C94°C MDA LC with T c =94°C
Photonic crystal fiber Thermo-optic fiber switch with extinction ratio of 60 dB based on temperature difference of 0.4°C near phase transition of Smectic A to nematic 974nm pump laser coupled into PC fiber