This work proposes an optically controllable beam-steering device, fabricated using cholesteric liquid crystals (CLCs) that are doped with azobenzene. The trans-cis photoisomerization of azobenzene changes the pitch of the CLC fingerprint structure and shifts the diffraction angle. The diffraction angle increases when the cell is irradiated with UV light, and restored when it is irradiated with green light. Combining the photoisomerization effect with electrical effect, the CLC beam-steering device provides a steering angle of ~19°.The tuning is continuous and could be completed within a few seconds.
© 2010 OSA
A non-mechanical beam steering diffraction device is an important component in many optical systems, such as optical interconnects , optical communications , projection displays , and optical data storage . Liquid crystal (LC) devices which provide efficient electro-optical modulation are excellent candidate beam-steering devices because of their low driving voltage and ease of fabrication.
Several LC-based beam-steering devices have been developed [5–12]. The most common configuration is based on a prism-type phase profile [6,7,11,12]. The required blazed-grating phase profile in the LC layer can be achieved using a structure of multiple electrodes onto which a periodically varying potential is applied. The beam steering angle can be varied by changing the periodic voltage profile. The main shortcomings of such devices are the complexity of their fabrication and operation. Cholesteric liquid crystal (CLC) films with a fingerprint structure [13,14] are highly promising for use in beam-steering devices. They are easily fabricated because of the self-assembly characteristic of CLC. The special feature of a CLC fingerprint grating is its tunable pitch. Subacius et al.  demonstrated a cholesteric grating with a field-controlled period. The diffraction angle can be shifted 15° by varying the applied voltage. Fuh et al.  produced an optically controlled cholesteric grating by laser heating and doing with a dichotic guest-host dye.
This study demonstrates an optically switchable beam steering device that is based on an azobenzene-doped CLC fingerprint texture. The photoisomerization of azobenzene varies the pitch of CLC [15,16] and shifts the diffraction angle of the fingerprint texture. Its tuning is fast and reversible under illumination by light of different wavelengths. The electrical tuning effect is also demonstrated and compared with the optical effect. Combining optical and electrical effects, the present CLC beam-steering device provides a steering range of ~19°
2. Experiments, results, and discussions
In this experiment, the nematic liquid crystal E7 (Merck) has a positive dielectric anisotropy △ε = 13.8 at f = 1 kHz, and refractive indices n0 = 1.52 and ne = 1.75 at 25 °C. The nematic liquid crystal was doped with 3% chiral agent S811 (Merck), which has a helical twist power of ~-11μm −1 at 25 °C, to yield a cholesteric LC with a pitch length of ~1.75 μm. Then ~5% azobenzene derivative 4-pentyloxy-phenyl-4-methoxy-phenyl-diazene (azoC5) (home-synthesized)  was added to the cholesteric LC mixture. After homogeneous mixing, an empty cell that was made from two glass plates coated with indium-tin-oxide was filled with the mixture. The glass plate was spin-coated with a polyimide AL-1426B (Daily Polymer Corporation) layer and rubbed to cause homogeneous alignment. The cell gap was 3.7μm and the cell thickness to pitch ratio (d/p) of the sample was ~2. Figure 1(a) schematically depicts the experimental setup. When a voltage of ~1.5 V (AC 1 kHz) was applied to the sample, it formed a cholesteric grating with a fingerprint structure. Figure 1(b) presents the fingerprint structure under a cross-polarizing optical microscope. The CLC molecules twisted continuously in a manner that was determined by the d/p ratio and the surface alignment directions of the two substrates . Also, the direction of the LC molecules in the middle layer determined the orientation plane, and thereby the strip direction. Under the conditions applied herein, the strips are oriented in the rubbing direction when a suitable voltage was applied to the cell. To observe the beam-steering effect, a UV light source (365 nm) and a diode-pumped solid-state (DPSS) laser (532 nm) were used as pump beams. A He-Ne laser (632.8 nm) with linear polarization in the strip direction was used as a probe beam.
Figure 2(a) shows the dynamic change in the diffraction pattern as the cell was irradiated with UV light with an intensity of 34 mW/cm2. Integer orders ( ± 1) of diffraction correspond to the pitch length P in the bulk, and half-integer orders ( ± 1/2) are caused by a period of 2P near the substrate surfaces associated with the boundary of the substrates [19,20]. The doped AzoC5 molecules undergo a transformation from the trans- to the cis-isomers when irradiated with UV light, and from the cis- to the trans-isomers when irradiated by green wavelengths. Azobenzene derivatives typically exist in the relatively stable trans state. The pitch length of the cholesteric LC cell declined upon the photo-isomerization of the azobenzene derivatives from the trans- to cis-isomers, for the following reason . The rod-like trans-azobenzene molecule favors the stabilization of the cholesteric phase, but the bent cis-azobenzene molecule tends to disorganize the molecular orientations of the liquid crystal phase of the host, changing its geometrical structure and reducing the pitch length. Accordingly, the decrease in the pitch length increases the diffraction angle.
Figure 2(b) plots the variation of the first-order diffraction angle of a CLC fingerprint grating with the duration of UV irradiation. As is seen, the first-order diffraction angle is ~39° before UV irradiation, corresponding an equilibrium pitch of p = 2.01 μm. Following UV irradiation, the first-order diffraction angle changes to ~45°, and the corresponding equilibrium pitch is p = 1.79 μm. The result is consistent with the afore mentioned result that the pitch length of the cholesteric LC cell declines upon photo-isomerization of the azobenzene derivatives from trans- to cis-isomers . In the initial design, the pitch of the CLC cell without doping AzoC5 is 1.75 μm, revealing that doping trans-state azobenzenes into the sample and applying a voltage increases the pitch of the CLC to 1.99 μm. The trans-to-cis photo-isomerization of azobenzenes reduces the pitch of the CLC, and increases the diffraction angle of the CLC grating. Notably, this optical-tuning effect in a CLC finger-print grating is the opposite of the effect induced by the electrical field as well as thermal effects that increase the pitch of the CLC and thereby reduce the diffraction angle [5,8]. The optical-tuning angle range here is ~6°, and the tuning process is taking ~2s and continuous. Furthermore, it is possible to reduce the response time significantly by increasing the pump light intensity and using a fast-response azobenzene .
Figure 3(a) plots the stability of a CLC grating that is kept in the dark at room temperature after UV irradiation is turned off. The diffraction angle slowly returns to its initial value because of the low rate of the thermally induced cis-trans back isomerization . It takes a few hours to return completely to the initial diffraction angle. Therefore, the angle of the diffraction beam after tuning can be fixed for many minutes. The cis-trans back isomerization can be accelerated with the application of 532nm light. Applying a DPSS laser (93 mW/cm2) can complete the reverse-tuning within 4 s, as shown in Fig. 3(b). Accordingly, for the considered sample, the CLC pitch can be efficiently optically reduced by irradiating the sample with UV light, and then reversed increased by applying a green light.
The electrical tuning properties of the present CLC grating were also studied for comparison. Figure 4(a) plots the first-order diffraction angle versus applied voltage in electrical tuning. As the applied voltage is increased from 1.5V to 3.2V, the first-order diffraction angle is shifted from 39° to 26°, corresponding to a variation in CLC pitch from 2.01 μm to 2.89 μm. When the applied electric field is perpendicular to the helical axis of the cholesteric LC with positive electric anisotropy, the helical pitch is increased by the electric field . Notably, 1.5V is the minimum voltage that is required to stabilize the fingerprint structure. The maximum electrically achievable diffraction angle of the presented CLC grating is 39°. As shown in Fig. 2(b), the diffraction angle can be further increased optically to 45° with applying a voltage of 1.5 V to the sample. Therefore, the electrical and optical tuning methods can be combined to yield a ~19° beam-steering range, as presented in Fig. 4(b). Figure 4(b) also reveals that the diffraction beam spot is broadened as the applied voltage is increased, because of the field-induced distortion of the fingerprint structure. Consequently, the optical-tuning method can increase the beam-steering angle to beyond the limit of electrical tuning, while maintaining the uniform structure of the grating.
Figure 5 displays the tuning process of a CLC grating by optical and electrical methods. UV irradiation rapidly reduces the period of the grating. The grating period can remain almost constant for minutes after the UV light is turned off. Reverse tuning can be accelerated by irradiating the sample with a green light. Unlike in optical tuning, the electrical field increases the period of the CLC grating. Furthermore, the tuning ranges of the two mechanisms do not overlap. Hence, the tuning range can be expanded by combining the electrical and optical tuning methods.
In conclusion, this study demonstrates an optically controllable beam-steering device that is based on an azobenzene doped CLC gating. The sample can be irradiated with UV light to tune the first-order diffraction angle over a range of width ~6°. The tuning is continuous and could be completed within a few seconds. After the UV irradiation is turned off, the period of the grating remains almost constant for minutes. Yet, this tuning could also be reversed within 4s by irradiating the sample with 532nm green light. This optical tuning method is much faster and more stable than the thermal method of laser heating. Additionally, the grating structure is more uniform than that obtained by electrical tuning, and the optical tuning range does not overlap that range of tuning by the electrical method. Therefore, the two mechanisms can be combined to maximize the beam-steering angle. The present CLC beam-steering device provides a steering range of ~19°
The authors would like to thank the National Science Council (NSC) of the Republic of China (Taiwan) for financially supporting this research under Grant No. NSC 98-2112-M-006-001-MY3 and NSC 96-2112-M-110-015-MY3. Additionally, this work is partially supported by Advanced Optoelectronic Technology Center of the National Cheng Kung University. Ted Knoy is appreciated for his editorial assistance.
References and links
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