- Email: [email protected]
Optically addressed electroclinic liquid crystal spatial light modulator with an a-Si:H photodiode
162
Journal of Non-CrystallineSolids 115 (1989) 162-164 North-Holland
Section 9: Modulators, memories and detectors
OPTICALLY ADDRESSED ELECTROCLINIC LIQUID CRYSTAL SPATIAL LIGHT MODULATOR WITH AN a-Si:H PHOTODIODE
I. ABDULHALIM, G. MODDEL, K. M. JOHNSON, AND C. M. WALKER Department of Electrical and Computer Engineering and Center for Optoelectronic Computing Systems, University of Colorado, Boulder, CO 80309-0425 USA
A fast optically addressed spatial light modulator (OASLM) is demonstrated using a p-i-n photodiode of hydrogenated amorphous silicon (a-Si:H) and the electroclinic effect in chiral smectic A liquid crystals. The response time depends on the material and the temperature. We used a material with a response time of 40 ~s at 29°C and 4 ~s at 51K2. This work shows that there is an excellent match between a-Si:H photodiodes as photosensors and high speed liquid crystals as electrooptic modulators. The optical response varies with the field and write-light intensity, allowing for grey level applications. 1. INTRODUCTION A spatial light modulator is a device which can modulate two dimensional optical data, addressed by either electrical or optical signals. OASLMs are advantageous because two dimensional information can be imprinted onto the device in parallel. They have potential applications in areas such as optical computing, optical displays, and optical signal processing. In an OASLM an incident write-beam is absorbed in a photosensor producing spatial variations in the modulating layer, which modulates a read beam. High performance of the OASLM relies on a suitable choice for the photosensor and the electrooptic modulator. For the nematic liquid crystal (LC), which has a response time in the milliseconds range, photosensors of CdTe, CdS and c-Si have been used. 1 For the ferroelectric LC, which can switch with a response time of ~100 ps, a need arises for a higher speed photosensor. This was achieved recently using an a-Si:H p-i-n photodiode as the photosensor, since its response time is approximately 100 ns, and it allows for high resolution of over 40 l p / m m ? The electroclinic effect exists in chiral smectic A (SmA') LC and is usually obtained by heating the LC from the chiral smectic C (SmC') phase which is ferroelectric. The effect results in faster switching
0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
depending on the LC mixture and temperature. The response is continuous with the applied electric field, allowing for grey level applications. An electrically addressed electroclinic spatial light modulator was reported recently? An optically addressed electroclinic device was also reported 4 using bismuth silicon oxide crystal as the photosensor, which limited the response time. In this paper we incorporate an a-Si:H p-i-n photodiode as the photosensor for the higher speed electroclinic OASLM and report the resulting device performance. 2. EXPERIMENTAL The OASLM is composed of an a-Si:H p-i-n photosensor and LC layer sandwiched between two glass substrates coated with transparent electrodes. The a-Si:H photodiode with a p-i-n structure is deposited on one of the SnO 2 coated glass substrates and forms the photosensor. It was grown at ~250°C in a loadlocked, high-vacuum, rf plasma enhanced chemical vapor deposition (PECVD) system. After an approximately 100/~ doped p÷ layer using I% B2HJSiH 4 gas mixture, an intrinsic region -2 pan thick is deposited using 100% Sill 4 followed by a phosphorus doped n ÷ layer of -100/~ thick using approximately 900 p p m PHa/SiH 4. The sheet resistance of the n ÷ layer is p -10 l° f2/sq.
L Abdulhalim et al./Electroclinic liquid crystal spatial light modulator The LC layer of thickness 1.5 - 2.0 ~rn filled a gap between the a-Si:H thin film and another glass substrate coated with In:SnO 2. The LC was aligned using a standard polymer rubbing technique. We used the LC mixture M764E produced by BDH5 which undergoes the following phase sequence S 5_~C SmC" 28~C SmA* 73_~C N* 91~C I where S, N" and I designate solid, chiral nematic and isotropic, respectively. The smectic layers in our geometry are perpendicular to the substrate plane. The active area of the finished device is -1 cm 2. In the SmA" phase the molecular director (optic axis) is parallel to the normal to the layers. By applying an electric field E between the electrodes, the optic axis rotates due to the electroclinic coupling~ in the plane of the substrates to a direction which depends on the sign of the field. Let 0÷ be the induced tilt angle due to forward biasing the photodiode with a voltage V÷ and 0_ due to reverse biasing with V. We apply a square-wave voltage changing from V. = 28 V to V = -2 V between the electrodes inducing a rotation c~ = 0+ + 0. For the normally incident read light (830 nm) polarized along the optic axis in the V. period (OFF state), the maximum transmission change between crossed polarizers (ToN-ToFF)is proportional to sin2(2a). Here ToFF accounts for the nonzero minim u m level in the OFF state due to nonidealities in the LC. The write-beam (wavelength 514.5 nm) is incident at a small angle from the normal on the a-Si:H side and absorbed by the photodiode layer. For the optical response measurements the write beam pulse is produced using an acoustooptic modulator. A delay/ pulse generator is used to synchronize the write light and the applied voltage so that the write light is ON only for a portion (100 ps) of the period when the applied voltage is in the V_ period. 3. RESULTS AND DISCUSSION Figure I shows the optical signal modulation at two different temperatures, together with the applied
163
square-wave voltage and the write-light pulses. The modulation is OFF during V. period and it turns ON only when the write light is ON during the V. period. It turns OFF again when the applied voltage returns to V+. Above the transition temperature T c = 28°C the modulation is due to the electroclinic effect, as shown in Fig. l(b). This behavior indicates that the LC layer is charged during V+ period because the photodiode is conducting. During V_ the photodiode is not conducting, and the LC layer stays almost fully charged until the photogeneration of carriers during the write-light pulse deposits charge of the opposite polarity on the LC, causing switching. The voltage drop across the LC depends on the capacitances 2 of both the LC and the a-Si:H. In the transition from V. to V_, the difference is divided between the two elements. For the dimensions in our device, approximately 40% of Vpp is dropped across the LC, modulating it even without a write beam. However, because the transmittance change (ToN-ToFF) varies as sin2(2c0, this 40% of Vpp modulates the transmission by a much smaller fraction. (d) Applied
Voltoge
(c) Write Light --TON (b) T = 55 °C --ToFF --O - - TON
(o) T : 24°C
--ToFF --O
[
i
0
0.4
L
J
t
0.8
1.2
1.6
2.0
Time Scote (ms)
FIGURE 1 Oscilloscope traces showing the optical response of the OASLM both in the SmC" phase at T=24°C (a) and in the SmA'phase at T = 35°C (b). The write-light pulse with a width of 100 ~ and the applied voltage square wave (Vpp = 30 V, V_ = -2 V), are shown in (c) and (d) respectively. The write light intensity was 5.5 m W / c m 2 in (a) and 7.4 m W / c m 2 in (b). The vertical scale is arbitrary but in (b) is twice that in (a).
164
L AbdulhMim et al./ Electroclinic fiquid crystal spatial light modulator
The modulation, defined as M = (ToN- To~)/(Ton + ToFF) and the response time (measured from 0% to 90% of the transmittance change) both decrease with temperature as shown in Fig. 2. These are simply explained by the temperature dependence 3'6of the tilt angle as 0 ~ E/(T - Tc) and the characteristic time for the q motion as x - rl/(T - Tc), where rl is the viscosity which decreases also with temperature.
bility of the a-Si:H p-i-n photodiode as a photosensor for the microsecond switching times of electroclinic LCs at room and higher temperatures. Further investigation of the operating characteristics and optimization of the device are in progress. (o) 2 5 . 4 oC (b) 5 6 . 6 *C (c) 50,9%
-..... --
CURRENT (Amps)
io-sj
VOLTAGE (Volts)
I I
g 40
=
40
_10
-5
.
2o ~ 20
30 40 Temperature (°C)
50
FIGURE 2 Modulation and response time versus temperature. Same conditions as Fig. 1. The modulation against write-light intensity was also measured. In the SmC" phase there exists a threshold intensity because of the existence of ferroelectric domains. 7 In the SmA ° phase the behavior is linear for small intensities and then it saturates. This is explained according to the field dependence of the induced tilt angle which is linear at small fields and saturates as E l/a at high fields, a~ The saturation intensity itself increases with temperature. It starts from 5.5 m W / c m 2 in the SmC" phase and reaches 7.9 m W / cm 2 at 45~C. The photodiode I-V characteristics (Fig. 3) measured at different temperatures up to 51oC show that the current under OASLM operating voltages of several volts reverse bias is not a function of temperature; there is a small increase of the photocurrent in the negative bias range up to -2 volts. Although the characteristics are far from ideal, the low series resistance under forward bias and the high shunt resistance under reverse bias provide the essential properties for the OASLMs. These results confirm the suita-
FIGURE 3 The I-V characteristics for the p-i-n photodiode used under 1 m W / c m 2illumination with L = 6328/~ at (a) 25.4°C, Co) 36.6°C, and (c) 50.9°C. ACKNOWLEDGEMENTS This work was supported by the National Science Foundation Engineering Research Center Grant No. CDR-862236 and the Colorado Advanced Technology Institute. REFERENCES 1. U. Efron, J. Grinberg, P. O. Brantz, J. M. Little, P. g. Reif, R. N. Schwartz, J. Appl. Phys. 57, 1356 (1985). 2. G. Moddel, K. M. Johnson, W. Li, R. A. Rice, L. A. Pagano-Stauffer, M. A. Handschy, App1. Phys. Lett., August (1989). 3. G. Andersson, I. Dalai, P. Keller W. Kuczynski, S. T. Lagerwall, S. Skarp, B. Stebler, Appl. Phys. Lett. 51, 640 (1987). 4. N. Collings, W.A. Crossland, R.C. Chittick, M.F. Bone, Proc. SPIE 963, 46 (1988). British Drug House, Broom Road, Poole, BH12 UNN, England.
5.
BDH -
6.
S.
Garoff, IL B. Meyer, Phys. Rev. Lett. 38, 848 (1977).
7. N.A. Clark, S. Lagerwall, Appl. Phys. Lett. 36, 899 (1980).
Journal of Non-CrystallineSolids 115 (1989) 162-164 North-Holland
Section 9: Modulators, memories and detectors
OPTICALLY ADDRESSED ELECTROCLINIC LIQUID CRYSTAL SPATIAL LIGHT MODULATOR WITH AN a-Si:H PHOTODIODE
I. ABDULHALIM, G. MODDEL, K. M. JOHNSON, AND C. M. WALKER Department of Electrical and Computer Engineering and Center for Optoelectronic Computing Systems, University of Colorado, Boulder, CO 80309-0425 USA
A fast optically addressed spatial light modulator (OASLM) is demonstrated using a p-i-n photodiode of hydrogenated amorphous silicon (a-Si:H) and the electroclinic effect in chiral smectic A liquid crystals. The response time depends on the material and the temperature. We used a material with a response time of 40 ~s at 29°C and 4 ~s at 51K2. This work shows that there is an excellent match between a-Si:H photodiodes as photosensors and high speed liquid crystals as electrooptic modulators. The optical response varies with the field and write-light intensity, allowing for grey level applications. 1. INTRODUCTION A spatial light modulator is a device which can modulate two dimensional optical data, addressed by either electrical or optical signals. OASLMs are advantageous because two dimensional information can be imprinted onto the device in parallel. They have potential applications in areas such as optical computing, optical displays, and optical signal processing. In an OASLM an incident write-beam is absorbed in a photosensor producing spatial variations in the modulating layer, which modulates a read beam. High performance of the OASLM relies on a suitable choice for the photosensor and the electrooptic modulator. For the nematic liquid crystal (LC), which has a response time in the milliseconds range, photosensors of CdTe, CdS and c-Si have been used. 1 For the ferroelectric LC, which can switch with a response time of ~100 ps, a need arises for a higher speed photosensor. This was achieved recently using an a-Si:H p-i-n photodiode as the photosensor, since its response time is approximately 100 ns, and it allows for high resolution of over 40 l p / m m ? The electroclinic effect exists in chiral smectic A (SmA') LC and is usually obtained by heating the LC from the chiral smectic C (SmC') phase which is ferroelectric. The effect results in faster switching
0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
depending on the LC mixture and temperature. The response is continuous with the applied electric field, allowing for grey level applications. An electrically addressed electroclinic spatial light modulator was reported recently? An optically addressed electroclinic device was also reported 4 using bismuth silicon oxide crystal as the photosensor, which limited the response time. In this paper we incorporate an a-Si:H p-i-n photodiode as the photosensor for the higher speed electroclinic OASLM and report the resulting device performance. 2. EXPERIMENTAL The OASLM is composed of an a-Si:H p-i-n photosensor and LC layer sandwiched between two glass substrates coated with transparent electrodes. The a-Si:H photodiode with a p-i-n structure is deposited on one of the SnO 2 coated glass substrates and forms the photosensor. It was grown at ~250°C in a loadlocked, high-vacuum, rf plasma enhanced chemical vapor deposition (PECVD) system. After an approximately 100/~ doped p÷ layer using I% B2HJSiH 4 gas mixture, an intrinsic region -2 pan thick is deposited using 100% Sill 4 followed by a phosphorus doped n ÷ layer of -100/~ thick using approximately 900 p p m PHa/SiH 4. The sheet resistance of the n ÷ layer is p -10 l° f2/sq.
L Abdulhalim et al./Electroclinic liquid crystal spatial light modulator The LC layer of thickness 1.5 - 2.0 ~rn filled a gap between the a-Si:H thin film and another glass substrate coated with In:SnO 2. The LC was aligned using a standard polymer rubbing technique. We used the LC mixture M764E produced by BDH5 which undergoes the following phase sequence S 5_~C SmC" 28~C SmA* 73_~C N* 91~C I where S, N" and I designate solid, chiral nematic and isotropic, respectively. The smectic layers in our geometry are perpendicular to the substrate plane. The active area of the finished device is -1 cm 2. In the SmA" phase the molecular director (optic axis) is parallel to the normal to the layers. By applying an electric field E between the electrodes, the optic axis rotates due to the electroclinic coupling~ in the plane of the substrates to a direction which depends on the sign of the field. Let 0÷ be the induced tilt angle due to forward biasing the photodiode with a voltage V÷ and 0_ due to reverse biasing with V. We apply a square-wave voltage changing from V. = 28 V to V = -2 V between the electrodes inducing a rotation c~ = 0+ + 0. For the normally incident read light (830 nm) polarized along the optic axis in the V. period (OFF state), the maximum transmission change between crossed polarizers (ToN-ToFF)is proportional to sin2(2a). Here ToFF accounts for the nonzero minim u m level in the OFF state due to nonidealities in the LC. The write-beam (wavelength 514.5 nm) is incident at a small angle from the normal on the a-Si:H side and absorbed by the photodiode layer. For the optical response measurements the write beam pulse is produced using an acoustooptic modulator. A delay/ pulse generator is used to synchronize the write light and the applied voltage so that the write light is ON only for a portion (100 ps) of the period when the applied voltage is in the V_ period. 3. RESULTS AND DISCUSSION Figure I shows the optical signal modulation at two different temperatures, together with the applied
163
square-wave voltage and the write-light pulses. The modulation is OFF during V. period and it turns ON only when the write light is ON during the V. period. It turns OFF again when the applied voltage returns to V+. Above the transition temperature T c = 28°C the modulation is due to the electroclinic effect, as shown in Fig. l(b). This behavior indicates that the LC layer is charged during V+ period because the photodiode is conducting. During V_ the photodiode is not conducting, and the LC layer stays almost fully charged until the photogeneration of carriers during the write-light pulse deposits charge of the opposite polarity on the LC, causing switching. The voltage drop across the LC depends on the capacitances 2 of both the LC and the a-Si:H. In the transition from V. to V_, the difference is divided between the two elements. For the dimensions in our device, approximately 40% of Vpp is dropped across the LC, modulating it even without a write beam. However, because the transmittance change (ToN-ToFF) varies as sin2(2c0, this 40% of Vpp modulates the transmission by a much smaller fraction. (d) Applied
Voltoge
(c) Write Light --TON (b) T = 55 °C --ToFF --O - - TON
(o) T : 24°C
--ToFF --O
[
i
0
0.4
L
J
t
0.8
1.2
1.6
2.0
Time Scote (ms)
FIGURE 1 Oscilloscope traces showing the optical response of the OASLM both in the SmC" phase at T=24°C (a) and in the SmA'phase at T = 35°C (b). The write-light pulse with a width of 100 ~ and the applied voltage square wave (Vpp = 30 V, V_ = -2 V), are shown in (c) and (d) respectively. The write light intensity was 5.5 m W / c m 2 in (a) and 7.4 m W / c m 2 in (b). The vertical scale is arbitrary but in (b) is twice that in (a).
164
L AbdulhMim et al./ Electroclinic fiquid crystal spatial light modulator
The modulation, defined as M = (ToN- To~)/(Ton + ToFF) and the response time (measured from 0% to 90% of the transmittance change) both decrease with temperature as shown in Fig. 2. These are simply explained by the temperature dependence 3'6of the tilt angle as 0 ~ E/(T - Tc) and the characteristic time for the q motion as x - rl/(T - Tc), where rl is the viscosity which decreases also with temperature.
bility of the a-Si:H p-i-n photodiode as a photosensor for the microsecond switching times of electroclinic LCs at room and higher temperatures. Further investigation of the operating characteristics and optimization of the device are in progress. (o) 2 5 . 4 oC (b) 5 6 . 6 *C (c) 50,9%
-..... --
CURRENT (Amps)
io-sj
VOLTAGE (Volts)
I I
g 40
=
40
_10
-5
.
2o ~ 20
30 40 Temperature (°C)
50
FIGURE 2 Modulation and response time versus temperature. Same conditions as Fig. 1. The modulation against write-light intensity was also measured. In the SmC" phase there exists a threshold intensity because of the existence of ferroelectric domains. 7 In the SmA ° phase the behavior is linear for small intensities and then it saturates. This is explained according to the field dependence of the induced tilt angle which is linear at small fields and saturates as E l/a at high fields, a~ The saturation intensity itself increases with temperature. It starts from 5.5 m W / c m 2 in the SmC" phase and reaches 7.9 m W / cm 2 at 45~C. The photodiode I-V characteristics (Fig. 3) measured at different temperatures up to 51oC show that the current under OASLM operating voltages of several volts reverse bias is not a function of temperature; there is a small increase of the photocurrent in the negative bias range up to -2 volts. Although the characteristics are far from ideal, the low series resistance under forward bias and the high shunt resistance under reverse bias provide the essential properties for the OASLMs. These results confirm the suita-
FIGURE 3 The I-V characteristics for the p-i-n photodiode used under 1 m W / c m 2illumination with L = 6328/~ at (a) 25.4°C, Co) 36.6°C, and (c) 50.9°C. ACKNOWLEDGEMENTS This work was supported by the National Science Foundation Engineering Research Center Grant No. CDR-862236 and the Colorado Advanced Technology Institute. REFERENCES 1. U. Efron, J. Grinberg, P. O. Brantz, J. M. Little, P. g. Reif, R. N. Schwartz, J. Appl. Phys. 57, 1356 (1985). 2. G. Moddel, K. M. Johnson, W. Li, R. A. Rice, L. A. Pagano-Stauffer, M. A. Handschy, App1. Phys. Lett., August (1989). 3. G. Andersson, I. Dalai, P. Keller W. Kuczynski, S. T. Lagerwall, S. Skarp, B. Stebler, Appl. Phys. Lett. 51, 640 (1987). 4. N. Collings, W.A. Crossland, R.C. Chittick, M.F. Bone, Proc. SPIE 963, 46 (1988). British Drug House, Broom Road, Poole, BH12 UNN, England.
5.
BDH -
6.
S.
Garoff, IL B. Meyer, Phys. Rev. Lett. 38, 848 (1977).
7. N.A. Clark, S. Lagerwall, Appl. Phys. Lett. 36, 899 (1980).