Wavelength Tuning of the Photonic Band Gap in Chiral Liquid Crystals using Electrically

Commanded Surfaces

A. CONCEPT

Further reading: 1) L. Komitov, B. Helgee, J. Felix, A. Matharu, Appl. Phys. Lett. 86, 023502 (2005) 2) S.S. Choi, S. M. Morris, W.T.S. Huck, H.J. Coles, Appl. Phys. Lett, 91 , pp.231110(1)-231110(3), (2007). 3) S.S. Choi, S.M. Morris, H. J. Coles, W.T.S. Huck, Soft Matter, 5, 354-362, (2009)

FIGURE 1. Schematic of the cell structure and the wavelength tuning mechanism using electrically commanded surface switching.

1) Electrical control of the photonic band gap (PBG) in chiral nematic liquid crystals (N*LCs) is highly desirable. However, direct switching of the N*LC can result in the disruption of the PBG.

2) In order to maintain the optical quality of the PBG an indirect switching method based on electrically commanded surface switching using ferroelectric liquid crystal (FLCs) was employed.

3) Due to the in-plane rotation of the FLC at the surface, a torque is experienced by the N*LC molecules at the interface with the FLC. As a result, the PBG is observed to blue-shift in accordance with a contraction of the pitch.

D. TUNING MECHANISM

B. COMMAND SURFACES

0.0 0.5 1.0 1.5 2.00

20

40

60

80

1 : 0.6 Vm-1

2 : 1.1 Vm-1

3 : 1.6 Vm-1

4 : 2.4 Vm-1

5 : 12 Vm-1

6 : 20 Vm-1

Electric field signal

6

5

4

3

2

1

Time (ms)

Switc

hing

ang

le (D

eg)

0 1 2 3 4 520

30

40

Ave

rage

sw

itchi

ng a

ngle

, av

(Deg

)

Frequency (kHz)

(a)

0.0 0.5 1.0 1.5 2.00

20

40

60

80

Switc

hing

ang

le (D

eg)

Time (ms)

1 kHZ 5 kHz

0 5 10 15 20 250

10

20

30

40

Ave

rage

sw

itchi

ng a

ngle

, av

(Deg

)

Electric Field (Vm-1)

(b) (c)

(d)

0.0 0.5 1.0 1.5 2.00

20

40

60

80

1 : 0.6 Vm-1

2 : 1.1 Vm-1

3 : 1.6 Vm-1

4 : 2.4 Vm-1

5 : 12 Vm-1

6 : 20 Vm-1

Electric field signal

6

5

4

3

2

1

Time (ms)

Switc

hing

ang

le (D

eg)

0 1 2 3 4 520

30

40

Ave

rage

sw

itchi

ng a

ngle

, av

(Deg

)

Frequency (kHz)

(a)

0.0 0.5 1.0 1.5 2.00

20

40

60

80

Switc

hing

ang

le (D

eg)

Time (ms)

1 kHZ 5 kHz

0 5 10 15 20 250

10

20

30

40

Ave

rage

sw

itchi

ng a

ngle

, av

(Deg

)

Electric Field (Vm-1)

(b) (c)

0.0 0.5 1.0 1.5 2.00

20

40

60

80

Switc

hing

ang

le (D

eg)

Time (ms)

1 kHZ 5 kHz

0 5 10 15 20 250

10

20

30

40

Ave

rage

sw

itchi

ng a

ngle

, av

(Deg

)

Electric Field (Vm-1)

(b) (c)

(d)FIGURE 2. The switching angle of the FLC compound (R1809) as a function of the electric field amplitude and frequency. (a) The increase in the switching angle as the electric field amplitude at 1 kHz is increased, (b) average switching angle as a function of electric field amplitude at 1 kHz during 2ms, (c) a comparison of the switching angle at 5 kHz and 1 kHz for a constant amplitude of 13.7Vμm-1, (d) average switching as a function of frequency with a constant amplitude of 13.7 V μm-1 during 2ms.

2) The following results were recorded for the FLC compound sandwiched between glass substrates.

C. PHOTONIC BAND GAP TUNING

500 550 600 65040

50

60

70

80

90

100

2 13

Tra

nsm

itta

nce

(%)

Wavelength (nm)

1 ON:No FLC surface 2 ON:single FLC surface 3 ON:Dual FLC surfaces

0 5 10 15 20590

595

600

605

610

615

Lon

g-P

BE

(nm

)

Electric Field (Vm-1)

1 kHz 2 kHz 3 kHz 4 kHz 5 kHz

580 600 62040

50

60

70

80

90

100

Tra

nsm

itta

nce

(%)

Wavelength (nm)

0 Vm-1

7.8 Vm-1

11.0 Vm-1

14.1 Vm-1

15.7 Vm-1

17.2 Vm-1

18.1Vm-1

19.5 Vm-1

20.8 Vm-1

(c)(a) (b)

580 600 62040

50

60

70

80

90

100

Tra

nsm

itta

nce

(%)

Wavelength (nm)

1 kHz 2 kHz 3 kHz 4 kHz 5 kHz

(d)

500 550 600 65040

50

60

70

80

90

100

2 13

Tra

nsm

itta

nce

(%)

Wavelength (nm)

1 ON:No FLC surface 2 ON:single FLC surface 3 ON:Dual FLC surfaces

0 5 10 15 20590

595

600

605

610

615

Lon

g-P

BE

(nm

)

Electric Field (Vm-1)

1 kHz 2 kHz 3 kHz 4 kHz 5 kHz

580 600 62040

50

60

70

80

90

100

Tra

nsm

itta

nce

(%)

Wavelength (nm)

0 Vm-1

7.8 Vm-1

11.0 Vm-1

14.1 Vm-1

15.7 Vm-1

17.2 Vm-1

18.1Vm-1

19.5 Vm-1

20.8 Vm-1

(c)(a) (b)

580 600 62040

50

60

70

80

90

100

Tra

nsm

itta

nce

(%)

Wavelength (nm)

1 kHz 2 kHz 3 kHz 4 kHz 5 kHz

(d)FIGURE. 3. Wavelength change control of the PBG in a cell with FLC commanded surfaces. (a) blue-shifting of the long photonic band edge (L) toward shorter wavelengths as a function of electric field amplitude at a constant frequency of 1 kHz, (b) red-shift of L for different frequencies at a constant amplitude of 20.8 V μm-1, (c) the dependence of L on the electric field strength for 5 different frequencies ranging between 1 kHz and 5 kHz, (d) A comparison of the tuning range for three different cells: 1) No FLC surfaces, 2) One FLC command surface, 2) Dual FLC command surfaces.

1) The PBG was blue-shifted or red-shifted continuously as a function of the electric field strength or drive frequency.

2) An increase of the number of command surfaces resulted in an increase in the magnitude of the wavelength tuning range due to anti-phase rotation between the two surfaces.

(b)

0 5 10 15 20 25590

595

600

605

610

615

Lon

g-P

BE

(nm

)

Time (seconds)

Experimental (E-On) Fitted curve (E-on) from eq (2)

Experimential (E-Off) Fitted curve (E-on) from eq (3)

0 5 10 15 20 250

10

20

30

40

0

10

20

30

40

Eff

ectiv

e su

rfac

e ro

taio

n (

, D

eg)

Ave

rage

FL

C r

otai

on (

, D

eg)

Electric field (Vm-1)

Average switching angle of of FLC Estimated effective surface rotation angle

Fitted curve of surface rotation angle

(a)(b)

0 5 10 15 20 25590

595

600

605

610

615

Lon

g-P

BE

(nm

)

Time (seconds)

Experimental (E-On) Fitted curve (E-on) from eq (2)

Experimential (E-Off) Fitted curve (E-on) from eq (3)

0 5 10 15 20 250

10

20

30

40

0

10

20

30

40

Eff

ectiv

e su

rfac

e ro

taio

n (

, D

eg)

Ave

rage

FL

C r

otai

on (

, D

eg)

Electric field (Vm-1)

Average switching angle of of FLC Estimated effective surface rotation angle

Fitted curve of surface rotation angle

0 5 10 15 20 250

10

20

30

40

0

10

20

30

40

Eff

ectiv

e su

rfac

e ro

taio

n (

, D

eg)

Ave

rage

FL

C r

otai

on (

, D

eg)

Electric field (Vm-1)

Average switching angle of of FLC Estimated effective surface rotation angle

Fitted curve of surface rotation angle

(a)

Figure 4. (a) Comparison between the average FLC switching angle, , (closed square) and the estimated effective surface rotation angle, , (closed circle) as a function of the electric field strength at a constant frequency of 1 kHz. The blue line is the least mean squares fit of the effective surface rotation using the equation of f() = g*Ea. (b) The time dependence of the long photonic band edge for a single FLC command surface cell for an electric field of 20.8 Vμm-1 at 1 kHz was applied and removed, the curves represent fits to the data using Equations (2) and (3).

1)It was found that the change in the long wavelength band edge obeyed the form:

,2

1

1wt

offLionL etC

(1),2

1

2wt

onLioffL etC

(2)

22

1 2K

nC e

where

222 2

KnC e

ne is the extraordinary refractive index , P is the pitch of N*LCs and γ is the rotational viscosity

Su Seok Choi1, Stephen.M. Morris1, Wilhelm.T.S. Huck2, Harry.J. Coles1

1 Centre of Molecular Materials for Photonics and Electronics (CMMPE),Department of Engineering, University of Cambridge

2 Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge

E-OFF E-ON

Rubbing direction

Rubbing direction

ITO

Polyimide

Polyimide

ITO

Glass

Glass

FLC surface

Bulk N*LC(ε<0)

FLC surface

E-OFF E-ON

Rubbing direction

Rubbing direction

ITO

Polyimide

Polyimide

ITO

Glass

Glass

FLC surface

Bulk N*LC(ε<0)

FLC surface

1) Switching of the FLC compound with an applied electric field.