Wavelength Tuning of the Photonic Band Gap in Chiral Liquid Crystals using Electrically Commanded...
1
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 2.0 0 20 40 60 80 E lectric field signal 6 5 4 3 2 1 T im e (m s) Sw itching angle (D eg) 0 1 2 3 4 5 20 30 40 A verage sw itching angle, av (D eg) F req u en cy (kH z) (a) 0.0 0.5 1.0 1.5 2.0 0 20 40 60 80 Sw itching angle (D eg) T im e (m s) 1 kH Z 5 kH z 0 5 10 15 20 25 0 10 20 30 40 A verage sw itching angle, av (D eg) E lectric F ield (V m -1 ) (b) (c) (d) 0.0 0.5 1.0 1.5 2.0 0 20 40 60 80 1 :0.6 V m -1 2 :1.1 V m -1 3 :1.6 V m -1 4 :2.4 V m -1 5 :12 V m -1 6 :20 V m -1 E lectric field signal 6 5 4 3 2 1 T im e (m s) Sw itching angle (D eg) 0 1 2 3 4 5 20 30 40 A verage sw itching angle, av (D eg) F req u en cy (kH z) (a) 0.0 0.5 1.0 1.5 2.0 0 20 40 60 80 Sw itching angle (D eg) T im e (m s) 1 kH Z 5 kH z 0 5 10 15 20 25 0 10 20 30 40 A verage sw itching angle, av (D eg) E lectric F ield (V m -1 ) (b) (c) 0.0 0.5 1.0 1.5 2.0 0 20 40 60 80 Sw itching angle (D eg) T im e (m s) 1 kH Z 5 kH z 0 5 10 15 20 25 0 10 20 30 40 A verage sw itching angle, av (D eg) E lectric F ield (V m -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 500 550 600 650 40 50 60 70 80 90 100 2 1 3 T ransm ittance (% ) W avelen gth (n m ) 1 O N :N o FLC surface 2 O N :single F L C surface 3 O N :D ualFLC surfaces 0 5 10 15 20 590 595 600 605 610 615 L ong-P B E (nm ) E lectric F ield (V m -1 ) 1 kH z 2 kH z 3 kH z 4 kH z 5 kH z 580 600 620 40 50 60 70 80 90 100 T ransm ittance (% ) W avelen gth (n m ) 0 V m -1 7.8 V m -1 11.0 V m -1 14.1 V m -1 15.7 V m -1 17.2 V m -1 18.1V m -1 19.5 V m -1 20.8 V m -1 (c) (a) (b) 580 600 620 40 50 60 70 80 90 100 T ransm ittance (% ) W avelength (n m ) 1 kH z 2 kH z 3 kH z 4 kH z 5 kH z (d) 500 550 600 650 40 50 60 70 80 90 100 2 1 3 T ransm ittance (% ) W avelen gth (n m ) 1 O N :N o FLC surface 2 O N :single F L C surface 3 O N :D ualFLC surfaces 0 5 10 15 20 590 595 600 605 610 615 L ong-P B E (nm ) E lectric F ield (V m -1 ) 1 kH z 2 kH z 3 kH z 4 kH z 5 kH z 580 600 620 40 50 60 70 80 90 100 T ransm ittance (% ) W avelen gth (n m ) 0 V m -1 7.8 V m -1 11.0 V m -1 14.1 V m -1 15.7 V m -1 17.2 V m -1 18.1V m -1 19.5 V m -1 20.8 V m -1 (c) (a) (b) 580 600 620 40 50 60 70 80 90 100 T ransm ittance (% ) W avelength (n m ) 1 kH z 2 kH z 3 kH z 4 kH z 5 kH z (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 25 590 595 600 605 610 615 L ong-P B E (nm ) T im e (secon d s) E xperim ental(E-O n) Fitted curve (E -on) from eq (2) E xperim ential(E -O ff) Fitted curve (E -on) from eq (3) 0 5 10 15 20 25 0 10 20 30 40 0 10 20 30 40 E ffective surface rotaion ( ,D eg) A verage FL C rotaion ( ,D eg) E lectric field (V m -1 ) A verage sw itching angle of of F L C E stim ated effective surface rotation angle Fitted curve of surface rotation angle (a) (b) 0 5 10 15 20 25 590 595 600 605 610 615 L ong-P B E (nm ) T im e (secon d s) E xperim ental(E-O n) Fitted curve (E -on) from eq (2) E xperim ential(E -O ff) Fitted curve (E -on) from eq (3) 0 5 10 15 20 25 0 10 20 30 40 0 10 20 30 40 E ffective surface rotaion ( ,D eg) A verage FL C rotaion ( ,D eg) E lectric field (V m -1 ) A verage sw itching angle of of F L C E stim ated effective surface rotation angle Fitted curve of surface rotation angle 0 5 10 15 20 25 0 10 20 30 40 0 10 20 30 40 E ffective surface rotaion ( ,D eg) A verage FL C rotaion ( ,D eg) E lectric field (V m -1 ) A verage sw itching angle of of F L C E stim ated 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*E a . (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 1 wt off L i on L e t C (1 ) , 2 1 2 wt on L i off L e t C (2) 22 1 2 K n C e where 22 2 2 K n C e n e is the extraordinary refractive index , P is the pitch of N*LCs and γ is the rotational viscosity Su Seok Choi 1 , Stephen.M. Morris 1 , Wilhelm.T.S. Huck 2 , Harry.J. Coles 1 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-O FF E-O N Rubbing direction Rubbing direction ITO Polyimide Polyimide ITO G lass G lass FLC surface Bulk N *LC ( ε<0) FLC surface E-O FF E-O N Rubbing direction Rubbing direction ITO Polyimide Polyimide ITO G lass G lass FLC surface Bulk N *LC ( ε<0) FLC surface 1) Switching of the FLC compound with an applied electric field.
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Transcript of Wavelength Tuning of the Photonic Band Gap in Chiral Liquid Crystals using Electrically Commanded...
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.
