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We demonstrate a dual-operation-mode liquid crystal (LC) lens, which is fabricated with the silica nanoparticle-doped (SND) hybrid-aligned nematic (HAN) LC cell. With AC voltage, the cell behaves as a conventional LC lens. The response time of the SND HAN LC lens is faster than that of the conventional LC lens, which is fabricated using the pristine HAN LC cell. This is because that the doped silica nanoparticles may decrease the dielectric relaxation time constant of the cell. The addition of the silica nanoparticles also increases the viscosity of the LC host, suppresses the backflow motion of the LCs and then decreases the response time of the SND LC lens. With DC voltage, the electrophoretic motion of the doped silica nanoparticles and the agglomerate silica networks on the substrate surface cause the SND HAN LC cell to function as a bistable LC lens.
©2009 Optical Society of America
1. Introduction
Liquid crystal (LC) lenses with tunable focal length are currently topics of broad interest. Because there are no moving parts, the LC lenses are smaller, lighter, and cheaper than the conventional lens systems, which are fabricated with the glass substrates. Various approaches, such as polymer stabilization [1], modal control [2], hole-patterned electrode [3,4], lenticular-patterned electrode [5], surface relief profile [6] and dielectric LC droplet [7], have been proposed to fabricate the LC lenses. Among them, those with hole-patterned electrode structures are simple and are generally of high optical quality. The hole-patterned electrodes make the LC lens easy to fabricate lens arrays, which are widely used in optical imaging, optical memory and optical communication. The drawback is that most of the LC lenses are not bistable.
In the past few years, a lot of research has been devoted to the LC-silica nanoparticle dispersions [8–10]. The hydrophilic nanoparticles covered with hydroxyl groups can form, due to hydrogen bonding, agglomerate networks in the LC host. The hydroxyl groups on the silica surface and the polar nature of the LCs yield a homeotropic LC alignment on the silica surface, while the LCs in the void volume are parallel with the silica strands [8,11]. Recently, electrophoretically controlled nematic-silica dispersion has been reported [12]. The charged silica nanoparticles create a polarity-controlled multistable switching in the conventional hybrid-aligned nematic (HAN) cell [12]. Based on the concept of the electrophoretic motion, we have demonstrated a dual-operation-mode LC display device, which is fabricated with the silica nanoparticle-doped (SND) HAN LC cell [13]. With DC voltage, the charged nanoparticles move toward the planar side of the cell. The accumulated nanoparticles create agglomerate networks to stabilize the LCs in the homeotropic state. The homeotropic LC state is retained after the DC voltage is turned off, and hence results in a multistable LC device. On the contrary, with AC voltage, the electrophoretic motion of the silica nanoparticles is suppressed. The cell functions as a conventional LC display, returns to its homeoplanar state after the AC voltage is removed.
In this paper, we demonstrate a dual-operation-mode LC lens, which is made of the silica nanoparticle-doped HAN cell. With AC voltage, the demonstrated cell is a conventionally dynamic LC lens with fast response time; with DC voltage, the demonstrated cell becomes a bistable LC lens. The possible mechanisms are discussed.
2. Operation principle
Figure 1 presents the operation principle of the proposed dual-operation-mode LC lens. As shown in Fig. 1(a), a polarized light incidents to a HAN LC cell with hole-patterned electrode. The polarization direction of the incident light is parallel with the rubbing direction of the LC cell. When a DC voltage is supplied to the cell, the hole-patterned electrode structure creates a gradient electric field distribution, which is small in the hole-patterned area and is large outside the hole-patterned area. The LC molecules outside the hole-patterned area reorient in the homeotropic state; the associated refractive index experienced by the incident light is small. On the contrary, the LC molecules in the hole-patterned area are almost in the hybrid-aligned state; the associated refractive index experienced by the incident light is large. The incident light experiences a phase shift with a bell-like profile, and hence the hole-patterned HAN LC cell behaves as an optical lens [4]. Meanwhile, with DC voltage, the charged silica nanoparticles outside the hole-patterned area move toward the planar side (the top substrate) of the cell electrophoretically, create agglomerate networks to stabilize the LCs in the homeotropic state after the DC voltage is turned off. However, the silica nanoparticles in the hole-patterned area cannot move toward the planar side (the top substrate) of the cell, owing to the low fringe electric field. Consequently, the optical lens effect is retained after switching off the DC voltage. A voltage-pulse with opposite polarity repulses the accumulated silica nanoparticles away from the planar side of the cell, disrupts the agglomerate networks and then erases the memory focus state of the LC lens. On the contrary, as shown in Fig. 1(b), when an AC square-wave voltage is applied to the cell, the electrophoretic motion of the doped silica nanoparticles is suppressed. The SND HAN LC lens behaves as a conventional LC lens; i.e., the optical lens effect disappears after the AC voltage is turned off.
Fig. 1 Schematic demonstration of the dual-operation-mode HAN LC lens with (a) DC voltage-pulse, and (b) AC voltage-pulse.
3. Experimental
To fabricate the dual-operation-mode LC lens, a hole-patterned HAN LC cell was filled with the mixture comprised the nematic E7 (from Merck) and the silica nanoparticles R812 (primary particle size 7 nm, from Degussa-Huls). The concentration of the doped silica nanoparticles was 1 wt%. Figure 1 displayed the cell configuration, which was assembled by a homeotropic-treated indium tin oxide (ITO) glass substrate on the bottom and a planar-treated ITO glass substrate on the top. The thickness of the cell was 75 μm. The etched diameter (the hole-patterned area) on the top substrate of the cell was ~2 mm. In this experiment, because R812 was a negative charged silica nanoparticle [10], when the positive sign of the DC voltage was connected to the planar side of the cell, the cell was defined to be subject to a positive DC voltage excitation [11].
An expanded He-Ne laser (632.8 nm) with a diameter of ~3 mm was employed to measure the focal length of the LC lens, which was placed after a linear polarizer. The rubbing direction of the cell was parallel with the polarization direction of the incident light. A photodetector was placed after the LC lens to measure the transmitted intensity of the incident light. The active diameter of the photodetector was 400 μm. The focal length of the LC lens was the distance between the LC lens and the position at which the measured transmitted light intensity was a maximum. The focal lengths of the SND HAN LC lens at various AC voltages (1 kHz, square-wave) were measured. Notably, the doped silica nanoparticles scattered the incident light and then decreased the transmitted intensity of the LC lens slightly. A CCD camera was placed ~20 cm after the LC lens to capture the focused image. The response time of the HAN LC lens was also measured. When an AC voltage was supplied to the LC lens suddenly, the incident light beam focused; the transmitted intensity measured by a photodetector placed at the focus of the LC lens increased suddenly. The rise time of the LC lens was the time taken for the transmitted intensity to reach from 10% to 90% when an AC voltage was suddenly supplied to the LC lens. Similarly, the fall time was the time for the transmitted intensity to reach from 90% to 10% when the supplied AC voltage was suddenly removed from the LC lens.
4. Results and discussion
Figure 2 shows the captured images of incident polarized light through the SND HAN LC lens. Figure 2(a) displays the initially un-focused image with zero voltage. With 160 V AC voltage, the incident light through the LC lens is focused to a small spot, as displayed in Fig. 2(b), which returns to the initially un-focused image after the supplied AC voltage is removed, as shown in Fig. 2(c). The obtained results indicate that the LC lens under AC voltage operation behaves as a conventional LC lens. The measured focal lengths of the SND HAN LC lens at various AC voltages are shown in Fig. 3 . When the voltage is low, the focal length of the SND HAN LC lens slightly decreases with increasing voltage, because the LC molecules outside the hole-patterned area gradually rotate toward the homeotropic state, but those in the hole-patterned area are still in the hybrid-aligned state. As the voltage increases further, the focal length of the LC lens increases monotonically, because the LC molecules outside the hole-patterned area are in the homeotropic state, but those in the hole-patterned area begin to rotate toward the homeotropic state, owing to the increased fringe field.
Fig. 2 Images of the incident polarized light through the 1 wt% SND HAN LC lens: (a) initially un-focused image with zero voltage; (b) focused image with 160 V AC voltage; (c) un-focused image after turning off the 160 V AC voltage.
Figure 4 shows the measured optical response curves of the pristine HAN LC lens and the SND HAN LC lens with 100 V AC voltage-pulse. As shown in the figure, the doped silica nanoparticles simultaneously decrease the rise time and fall time of the cell. Both the rise time and fall time measured in the SND HAN LC lens are ~15% of those measured in the pristine HAN LC Lens. The fast switching mechanism for the SND HAN LC cell may be attributed to the interfacial polarization effect between the nematic host and the doped silica nanoparticles [14,15]. As reported, the addition of the silica nanoparticles may decrease the dielectric relaxation time of the LC host. Furthermore, the doped silica nanoparticles also increase the effective viscosity of the LC host. The increased viscosity suppresses the backflow motion of the LC molecules, and then decreases the response time of the cell. Notably, when the doped nanoparticle concentration exceeds 1.5 wt%, the response time of the SND HAN LC lens will increase, due to the markedly increased viscosity of the LC host.
Fig. 4 Measured optical response curves of the pristine HAN LC lens and the 1 wt% SND HAN LC lens with 100 V AC voltage-pulse excitation.
Figure 5 shows the captured images of the incident polarized light through the SND HAN LC lens with DC voltage operation. Figure 5(a) shows the initially un-focused image with zero voltage. With a + 160 V DC voltage, the incident light focuses to a small spot, as shown in Fig. 5(b), which is retained after turning off the DC voltage, as shown in Fig. 5(c). However, because the supplied DC voltage forces the doped silica nanoparticles continually moving toward the substrate surface, disturbing the LC configuration and then causing the LC molecules in an unstable condition. Consequently, the focused image of the LC lens with DC voltage [Fig. 5(b)] is different from that of the LC lens after turning off the supplied DC voltage [Fig. 5(c)]. The image mismatch between Figs. 5(b) and 5(c) can be solved by applying a 1 kHz AC square-wave voltage to the cell at the state of Fig. 5(c). The results are shown in Figs. 5(d) and 5(e). This is because that with 1 kHz AC voltage, the LC configuration that creates the lens effect is remained, but the electrophoretic effect that moves the doped silica nanoparticles is suppressed. Consequently, the unstable LC configuration due to the moving of the silica nanoparticles disappears, and the focused images with and after DC voltage application become equivalent. The above results indicate that under DC voltage operation, the SND HAN cell becomes a bistable LC lens. Notably, the focus of the LC lens can be changed by varying the amplitude of the DC voltage, indicating that developing a multistable LC lens with adjustable focal length is possible. The memory focus image of Fig. 5(e) can be erased by applying a negative DC voltage to the LC lens. As shown in Fig. 6(a) , with a negative 160 V DC voltage, the doped silica nanoparticles are repulsed away from the substrate surface, the agglomerated silica networks that stabilize the LC molecules homeotropically are disrupted. The focused image expands. After turning off the negative 160 V DC voltage, the initially un-focused image appears, as shown in Fig. 6(b). Notably, a low frequency AC voltage-pulse is also effective in erasing the memory focus image of Fig. 5(e). This is because that the low frequency AC voltage rotates the LC molecules, disrupting the agglomerate silica networks. Figure 6(c) shows the focused image of the cell with 160 V, 10 Hz AC square-wave voltage, the focusing effect of the LC lens remains. When the low frequency AC voltage is removed, the cell returns to the initially un-focused image, as shown in Fig. 6(d).
Fig. 5 Images of the incident light through the 1 wt% SND HAN LC lens: (a) initially un-focused image with zero voltage; (b) focused image with + 160 V DC voltage, (c) focused image after turning off the + 160 V DC voltage (memory state); (d) focused image with 160 V, 1 kHz AC voltage; (e) focused image after turning off the 160 V, 1 kHz AC voltage (memory state).
Fig. 6 Images of the incident light through the SND HAN LC lens: (a) with – 160 V DC voltage; (b) after turning off the – 160 V DC voltage; (c) with 160 V, 10 Hz AC voltage; (d) after turning off the 160 V, 10 Hz AC voltage.
5. Conclusions
In conclusion, we have demonstrated a dual-operation-mode LC lens, which is fabricated with the SND HAN LC cell. With AC voltage, the cell functions as a conventional LC lens. The response time of the SND HAN LC lens is ~15% of the conventional HAN LC lens. This is because that doped silica nanoparticles may decrease the dielectric relaxation time constant of the LC host. The addition of the silica nanoparticles may also increase the effective viscosity of the LC host. The increased effective viscosity impedes the backflow motion of the LC molecules, and then decreases the response time of the LC lens. With DC voltage, the electrophoretic characteristics and the agglomerate networks of the silica nanoparticles cause the HAN LC lens to function as a bistable LC lens. The focus of the LC lens can be changed by applying the different DC voltage, indicating that developing a multistable LC lens with adjustable focal length is possible. However, the accumulated silica nanoparticles may scatter the incident light, degrading the optical performance of the LC lens. Further studies on optimization and application of the SND HAN LC lens are under way.
Acknowledgements
This work was financially supported by the National Science Council of the Republic of China, Taiwan, under Contract Nos. NSC 95-2112-M-018-005-MY3 and NSC 98-2112-M-018-002-MY3.
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