Tunable photonic crystals with liquid crystals 1-D –Electrical & thermal tuning of filters (porous silicon and Si/SiO 2 ) –Electrically tunable lasing

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


• Electrochemical etching of porous silicon

• Infiltration of E7 LC in vacuum

• Confined geometry: LC in 20 nm pore


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


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


~

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)


• 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)


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


• 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


• 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)


• 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


• 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)]



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


Q-factor < 100 (LOW!)9.5 nm shift ( ~ 1090 nm)

50 nm

(a ~ 300 nm)

Claims LC molecules aligned parallel to holes


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


• 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


• 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


• 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



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)


Results not great but demonstrate

the principle

ZLI1132 liquid crystal


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

Documents

Tunable photonic crystals with liquid crystals 1-D –Electrical & thermal tuning of filters (porous silicon and Si/SiO 2 ) –Electrically tunable lasing