Highly photorefractive hybrid liquid crystal device for a video-rate holographic display

In this paper, we demonstrate a dynamic holographic display in a quantum dot (ZnS/InP) doped liquid crystal device, where one of the interior cell surfaces is covered by a ZnSe layer. Such a hybrid device shows substantially improved photorefractive sensitivity of 2.2 cm/J, which is almost 300 times larger than that in ZnS/InP doped liquid crystal device without the ZnSe layer. The holographic grating can form at intensities as low as ~0.8 mW/cm, and exhibit a fast optical response of several to tens of milliseconds. Exploiting the superior performances of photosensitivity and fast response of this device, we obtain dynamic holographic videos of red, green, and blue colors, as well as a reconstructed image of high grayscale fidelity. ©2016 Optical Society of America OCIS codes: (090.5694) Real-time holography; (160.3710) Liquid crystals; (160.5320) Photorefractive materials; (160.6000) Semiconductor materials; (190.4400) Nonlinear optics, materials. References and links 1. D. 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Introduction
Holographic display is a true three-dimensional (3D) technique since it is capable of providing all the depth cues of an object or a scene, thus permits viewers to see the world in three dimensions with no fatigue [1,2].Optical holographic display based on organic photorefractive (PR) holographic materials is one of the effective approaches to realize the true 3D display [3,4].Moreover, these materials can be readily scalable for manufacturing large-size panels.Many research groups have thus been working on exploiting new materials such as photorefractive polymers, photorefractive liquid crystal, etc [3][4][5][6][7][8][9][10].
Photorefractive polymer exhibits high diffraction efficiency (DE) and high spatial resolutions [3,4,6].Nevertheless, a high applied electrical field of up to ~50 V/μm is usually required to produce sufficient refraction index change.Photorefractive liquid crystals (LCs) can attain a fast nonlinear photorefractive response with a low applied field owing to the large birefringence of the liquid crystals [11,12], and thus are a promising candidate for holographic displays.In our previous work, we proposed and demonstrated quantum dot (QD) doped LCs with a high diffraction efficiency and a fast response, using an applied electrical field of 1 V/µm [5].However, due to the low photorefractive sensitivity, a high recording intensity of hundreds of mW/cm 2 was still required to achieve a rapid response of several milliseconds.Thus, doped LCs with higher photorefractive sensitivity is favorable for dynamic holographic displays.Many researchers used organic/inorganic semiconductive layers to enhance photorefractive response of the doped LCs [10,[13][14][15][16][17][18], where ZnSe is a well-known semiconductive material for its large charge carrier mobility, photoconductivity, high quality and low cost [10,16,19].Due to the high photoconductivity of the ZnSe layer, an illumination around 460 nm can lead to an effective photogeneration of charge carrier in the cell using relatively low laser intensity, resulting in a larger refractive index changes and faster transport speed compared with the one without ZnSe layer [16,19].
Here we propose a hybrid LC cell that combines ZnS/InP doped LC with a ZnSe surface layer of the cell.Due to this combination, a diffraction efficiency of ~12% and response time of ~6 ms are obtained at a recording intensity as low as ~0.8 mW/cm 2 , thus the nonlinear index coefficient n 2 is as high as 2.1 cm 2 /W.The photorefractive sensitivity S of the hybrid LC device is up to 2.2 cm 3 /J, which is about 300 times larger than that in ZnS/InP doped LC without the ZnSe surface layer.The dependences of the diffraction efficiency and response time on recording laser powers are also investigated experimentally.The structure of the hybrid LC device is schematically presented in Fig. 1.Mylar slips with a thickness of about 30 µm are sandwiched between two transparent indium tin oxide (ITO) substrates to maintain the cell gap.One of the ITO substrates is deposited with a 200-nm-ZnSe film using resistive heating evaporation technique with a deposition rate of 1 nm/s.The other substrate is covered with polyimide (PI) so that the LC is aligned homeotropically.The material is prepared by mixing quantum dots (ZnS/InP) solution in chloroform (5 mg/ml) with a nematic LC (5CB), and then dried at 70 °C for 10 hours in a vacuum drying oven to evaporate the chloroform.The weight percent of the ZnS/InP is 0.05%.The mass ratio of QD and 5CB is 0.5:1000.The cell is placed under a polarizing optical microscope for light transmission observation.The LCs are adjusted to be aligned well perpendicular to the substrates.The transmission spectrum of the LC device is measured in the wavelength range of 350-800 nm using a UV-3100 spectrophotometer as shown in Fig. 2. One can see that the transmission of the hybrid device is around 37% at 460 nm, and the absorption is 170 cm −1 .

Materials and operation principle
As shown in Fig. 1(b), two coherent linearly polarized beams (λ = 460 nm) are incident onto the hybrid LC device, generating an interference intensity pattern.The spatially non-uniform light intensity further gives rise to a non-uniform distribution of the photo-induced charge carriers (electrons and holes).The cell is tilted at an angle of 45° with respect to the bisector of the two recording beams, the electrons and holes are thus separated and transported along the grating wave vector once a DC voltage is applied, resulting in a modulated photoconductivity and a non-uniform space charge field E sc .Consequently, a spatially modulated reorientation of LCs and a periodic refractive index modulation are formed [20,21].When the recording beams are turned off, the distribution of the photoinduced charges becomes uniform and the grating disappears.Both the build-up time and decay time of the grating depend on the efficiency of the charge generation, and are inversely proportional to the photoconductivity [22].In the proposed hybrid LC device, due to the assistance of the highly photosensitive ZnSe layer, the charge carriers can be excited more effectively than ZnS/InP solely doped LC cell.Thus, much lower recording intensity is needed.In order to verify the PR nature of the gratings, we carry out a photoconductivity measurement, where the resistance of device is recorded by a multimeter (Keithley, 2000) under illumination of uniform light with different laser powers.The laser used is an optically pumped semiconductor continuous-laser (Coherent, Genesis MX460-500) of 460 nm wavelength.From the measured resistance as a function of incident laser power, the photoconductivity through the hybrid LC device is calculated and shown in Fig. 3.The photoconductivity is highly nonlinear to the laser power.The photoconductivity first increases dramatically in a low laser power region, and then increases very slowly in a higher power region.

#258744
The photorefractive characteristics of the hybrid LC device are measured using pump-probe technique [23] at 460 nm wavelength, where two polarized recording beams with the same diameters of 2.5 mm are set to be s-polarization and intersected on the cell at an angle of about 2°.The interference region is probed by a linearly p-polarized He-Ne laser with a diameter of 3 mm and a power of 0.5 mW.
Figure 4 shows the dependence of first-order diffraction efficiency on recording laser power under a DC applied electrical field of 5 V/μm, with the inset showing a diffraction pattern.The real-time diffraction efficiency η is defined as the ratio of the diffracted probe power to the transmitted probe power.Multi-order diffraction is observed.The diffraction efficiency of the cell exhibits high sensitivity at laser powers of 0.2 µW to 15 µW, and at higher light intensities it keeps almost constant.The maximum diffraction efficiency is up to 12%.Since the formed grating is in the Raman-Nath region, the first order diffraction efficiency is proportional to , where Δn is the index grating amplitude, d is the thickness of the cell [24].Using d=30 μm, λ=460 nm, and recording laser power of 40 μW, we obtain Δn=1.7×10 −3 and thus the nonlinear index coefficient n 2 as high as 2.1 cm 2 /W using n 2 =Δn/I, where I is recording laser intensity of ~0.8 mW/cm 2 .We noticed an increase of firstorder diffraction efficiency for larger periods.Additionally, as the amplitude of index modulation Δn induced by the space-charge field E sc is proportional to the photoconductivity change of the materials [20,24], more effective dopant with high quantum efficiency can be used to improve the photorefractive effect.Figure 5 shows the dynamic behavior of grating formation with respect to different recording laser powers.The build-up time measures the time of DE ascending from 10% to 90%, while the decay time measures the duration of DE descending from 90% to 10%.Furthermore, the higher intensity of recording laser, the faster the build-up times are, where a build-up time of 3.6 ms can be obtained at a total laser power of 160 µW measured by an oscilloscope (Agilent, DSO-X 2012A).

Response time vs. recording laser power
Figure 6 shows the response time versus the recording laser power.The measured buildup time of the grating is inversely related to the photoconductivity change.During the buildup process, the photo-induced charges increase abruptly, causing a sharp increase of photoconductivity, and therefore a fast response is obtained [22,24].Additionally, the decay time is nearly a constant (~4 ms), especially in the range between 40 µW and 200 µW.It should be noted that the fast decay time is mainly attributed to the dielectric torque, irrespective of light intensity.Due to the combination of homeotropical alignment and positive dielectric anisotropy of host 5CB, after removing the exciting light, the applied DC field exerts a dielectric torque which forces the deformed 5CB molecules to return back to initial unperturbed homeotropic state.Thus, the effects of both dielectric torque and elastic restore torque accelerate the decay process [25].

Photorefractive sensitivity
Photorefractive sensitivity is an important parameter for evaluating holographic devices.It describes the light induced change of refractive index by absorbed energy per unit volume.The sensitivity S is defined as [26,27]: where Δn is index grating amplitude, τ is build-up time, α is absorption coefficient, and I is recording laser intensity.In our experiment, the build-up time of the grating (τ=5.5 ms) and laser power of 40 µW (a diameter of 2.5 mm) lead to an estimated photorefractive sensitivity of 2.2 cm 3 /J.This value is remarkably high compared with some other photorefractive materials, which are usually in the order of 10 −2 to 10 −3 cm 3 /J [27,28].For comparison, we also fabricate two other kinds of cell configurations.These devices are studied with the same optical setup as the proposed hybrid LC device.For device_1, pure 5CB is contained inside a homeotropical cell with a ZnSe layer; For device_2, QD-doped 5CB is filled into a homeotropical cell without a ZnSe layer; Device_3 is the hybrid LC cell mentioned above.The specifications of each sample are listed in Table 1.The comparisons of the three devices are presented in Table 2. Compared with device_1 which is filled with pure 5CB, the photorefractive responses of device_2-3 filled with QD doped LCs are much better.And the sensitivity of device_3 is almost 300 times that of device_2.Therefore, the ZnSe layer-assisted QD doped LCs has the best photorefractive performances, indicating that the combination of QD and ZnSe layer is more effective to increase the photorefractive sensitivity.Figure 7 shows the schematic representation of dynamic holographic display to demonstrate the performance of the hybrid LC cell.Dynamic binary images at a refresh rate of 25 Hz generated by a personal computer are displayed on an amplitude spatial light modulator (SLM, Holoeye), and illuminated to the cell as the object beam.An optically pumped semiconductor laser at 460 nm wavelength is expanded to 4 cm and used to provide the coherent reference and object beams, which are both set to be s-polarization.The object-toreference beam ratio is 1:1 and the total intensity is about 1 mW/cm 2 .The reconstructed images are projected onto a white board.

Holographic display
A diffracted moving image of a "running horse" is observed as shown in Visualization 1, Visualization 2, Visualization 3. Figure 8(a) shows a series of snapshots captured from the diffracted video as the cell is illuminated by three different wavelengths of 632.8 nm, 532 nm, and 488 nm, respectively.Compared with the binary images, displaying gray-scale images means larger amount of data recorded in each hologram with the same number of pixels [29].In the experiment, we also load a high definition (HD) image of a "panther" to the SLM.The obtained diffracted images exhibit fairly good gray-scale fidelity, as shown in Fig. 8(b).

Conclusion
In conclusion, we have demonstrated a video-rate holographic display in a ZnSe layer-assisted QDs doped LCs device.Covering a ZnSe layer onto an interior cell surface of QDs doped LC cell is proved to substantially improve its photorefractive sensitivity.Even at a rather low light intensity of ~0.8 mW/cm 2 , the hybrid LC cell exhibits excellent photorefractive sensitivity and response speed.The photosensitivity is up to 2.2 cm 3 /J, which is 300-fold more than the uncovered one.The build-up time and decay time are both around 6 ms, which are sufficient for practical video-rate display applications.The tremendous increase in the photorefractive sensitivity should stem from the high photoconductivity of the ZnSe layer, that is sensitivity to low laser intensity.With the use of such a hybrid LC cell, we achieve a video-rate dynamic holographic display with high contrast ratio.We also retrieve a diffracted image with high gray-scale fidelity.Our experimental results imply that the combination of semiconductive layer with QDs doped LCs is an efficient method towards achieving a dynamic, color, and high efficiency holographic display.

Figure 1 .
Figure 1.(a) Structure of the hybrid LC device; (b) Geometry for recording holograms.

Figure 2 .
Figure 2. Transmission spectrum of the hybrid LC device.

Fig. 3 .
Fig. 3. Dependence of photoconductivity in the hybrid LC cell on the laser powers.

Figure 4 .
Figure 4. Dependence of the diffraction efficiency on the total recording laser powers.

Fig. 5 .
Fig. 5. First-order diffraction intensity versus response time at different recording laser powers.

Fig. 6 .
Fig. 6.Dependence of the response time on the total recording laser powers.

Figure 8 .
Figure 8.(a) Snapshots of the holographic videos reconstructed by three different wavelengths of 632.8 nm, 532 nm, and 488 nm at an applied electrical field of 5 V/μm (Visualization 1, Visualization 2, Visualization 3); (b) Reconstructed images of a "panther".