Tunable photonic crystals with liquid crystals
• 1-D– Electrical & thermal tuning of filters (porous silicon
and Si/SiO2)
– 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 tuningLiquid crystal tuning
no field
E
applied E field
ELECTRIC FIELD
“cold”(nematic)
“hot”(isotropic)
TEMPERATURE
5 Å
2 nmTc ~ 58°C
E7 liquid crystal: no ~ 1.5, ne ~ 1.7
For positive anisotropy LC
Porous silicon 1-D photonic crystalsPorous 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 crystalsPorous 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 crystalsPorous silicon 1-D photonic crystals
nLC ~ 0.02
Q ~ 400 with LC25°C 61°C
1400 1450 1500 1550
50
60
70
80
90
100
Re
flect
an
ce (
%)
Wavelength (nm)14 dB attenuation
Constricted geometry limits liquid crystal rotation and, hence, effective birefringence
Porous silicon 1-D photonic crystalsPorous silicon 1-D photonic crystals
30 40 50 60 70 80
0
1
2
3
4
5
6
7
Re
son
an
ce r
ed
sh
ift (
nm
)
Temperature, C
ZLI-4788 liquid crystals
30 40 50 60 70 80 90 100
-1
0
1
2
3
4
5
6
7
Re
son
an
ce r
ed
sh
ift (
nm
)
Temperature, C
BL087 liquid crystals
25 35 45 55 65 75
0
5
10
15
20
25
Re
son
an
ce r
ed
sh
ift (
nm
)
Temperature, C
E7 liquid crystals
(nematic)
(isotropic)
24 26 28 30 32 34 36 38 40 42
0
5
10
15
20
25
Re
son
an
ce r
ed
sh
ift (
nm
)
Temperature, C
5CB liquid crystal
macropore macropore
mesopore mesopore
mesopore mesopore
Porous silicon 1-D photonic crystalsPorous silicon 1-D photonic crystals
~
0 20 40 60 80
-6
-4
-2
0
Res
onan
ce r
ed s
hift
(nm
)
Voltage (Vrms)
mesopore
macropore
0 20 40 60 80 100 120 140
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Re
son
an
ce r
ed
sh
ift (
nm
)
Voltage (Vrms)
mesopore
Negative anisotropy LC
10 dB
5 dB
Si/SiOSi/SiO22 photonic crystals photonic crystalsG. Pucker et al., J. Appl. Phys. 95, 767 (2004)
Fabrication by “chip bonding” with 950 nm
gap between multilayer mirrors
Poly-Si/SiO2
mirrors formed by LPCVD
Si/SiOSi/SiO22 photonic crystals 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/SiOSi/SiO22 photonic crystals 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
none
1-D photonic crystal laser1-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 laser1-D photonic crystal laser
Optically pumped dye-doped LC laser using 2nd harmonic of Nd:YAG (wavelength tuning accomplished electrically)
Porous silicon 2-D photonic crystalsPorous silicon 2-D photonic crystals
• Formed by lithographic prestructuring and electrochemical etching
• Pitch = 1.58m, pore diameter ~ 1.38m
• 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 crystalsPorous silicon 2-D photonic crystals
No LC
LC (nematic
& isotropic phase)
• H-pol (electric field perpendicular to pores)
• LC absorption lines near 3m and above 6m
• Air band edge shifts 70nm when LC heated to isotropic phase
• Dielectric band edge unchanged
Porous silicon 2-D photonic crystalsPorous 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 crystalsPorous silicon 2-D photonic crystals
• LC alignment can be more accurately determined by using 2H-NMR
• Uses quadripolar splitting of 2H-NMR signal: v is angle between director and magnetic field
parallel planar polar escaped radial
G. Mertens et al., Appl. Phys. Lett. 83, 3036 (2003)
Porous silicon 2-D photonic crystalsPorous silicon 2-D photonic crystals
• LC alignment can be more accurately determined by using 2H-NMR
• Uses quadripolar splitting of 2H-NMR signal: v is angle between director and magnetic field
parallel planar polar escaped 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 crystalsPorous silicon 2-D photonic crystals
Porous silicon 2-D photonic crystalsPorous silicon 2-D photonic crystals
silanized
untreated
M. Haurylau et al., Phys. Stat. Sol. A 202, 1477 (2005)
III-V photonic crystalsIII-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 crystalsIII-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 crystalsIII-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 crystalsIII-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 laserElectrical 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, rdefect =100 nm, slab thickness =320 nm.
2m
Electrical tuning of photonic crystal laserElectrical 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 laserElectrical 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 crystalsPorous 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 0.76m and and 1.26m
• Infiltrated with 5CB liquid crystal
G. Mertens et al., Appl. Phys. Lett. 83, 3036 (2003)
Porous silicon 3-D photonic crystalsPorous silicon 3-D photonic crystals
LC band edge tuned by 140 nm as heated from 24°C to 40°C
(Tc~35°C)
Opals – thermal tuningOpals – 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 tuningOpals – thermal tuning
Results not great but demonstrate
the principle
ZLI1132 liquid crystal
Opals – thermal tuningOpals – thermal tuning
Results not great but demonstrate
the principle
1MC1EPOPB liquid crystal
Inverse opals – electrical tuningInverse 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 tuningInverse opals – electrical tuning
150 volt threshold!!!
Large shift due to large filling fraction of LC in
inverse opals
Inverse opals – electrical tuningInverse opals – electrical tuningLet’s look at response time when apply voltage
and after turning off voltage…
MEMORY EFFECTS!
Not too bad for LCs
Photonic crystal fiberPhotonic crystal fiberT. T. Larsen et al., Optics Express 11, 2589 (2003)
• LC introduced into the air holes (3.5mm)
• Merck MDA-00-1445 (cholesteric) and BDH TM216 (chiral Smectic A and cholesteric)
• Radial alignment of LC
Photonic crystal fiberPhotonic crystal fiber
THERMAL TUNING
Photonic crystal fiberPhotonic crystal fiber
77°C 89°C
91°C 94°C
MDA-00-1445 LC with Tc=94°C
Photonic crystal fiberPhotonic 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
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