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Two electrically controllable holographic polarization gratings are spatially multiplexed to serve as an image splitter for further stereogram application. An external AC voltage is used to modulate the diffraction efficiency of the fabricated holographic image splitter for the switching of a 3D stereogram and a 2D plane image. The polarization grating was generated by two writing beams with orthogonal circular-polarization state in dye-doped liquid crystal films. With proper experimental arrangements, a quasi single order diffraction was obtained and the other non-zero order diffractions were suppressed in this presented polarization grating. This characteristic leads to a good image contrast ratio for a stereogram. Finally, a switchable stereogram system using circular-polarization beam as backlighting was demonstrated successfully.
©2013 Optical Society of America
1. Introduction
Liquid crystal (LC) based gratings have received a significant amount of attention due to their promising applications in display technology, photonics and optical communications [1–3]. Their high birefringence, high sensitivity to external electric field, and the effect of surface anchoring forces allow us to develop several optical devices with particular characteristic. Among these LC-based gratings, polarization holograms [4–15] are very interesting and useful for highly functionalized optical devices owing to the possibility to control the beam propagation direction and the polarization state of diffractions. Among various LC materials, dye-doped liquid crystal (DDLC) is a very promising recording material for polarization hologram application. Recently, our group had reported a polarization holographic grating generated by two orthogonal linear-polarized writing beams in DDLC, and the + 1 order diffraction efficiency was enhanced with unequal intensity of two writing beams [15]. In this paper, we used the same DDLC to generate particular polarization gratings with two orthogonal circular-polarized writing beams. Without any surface rubbing treatment for the DDLC films, only a strong + 1 order diffraction and a very weak −1 order diffraction were observed in these presented gratings. The other non-zero order diffractions of the same polarization grating were suppressed. The quasi single order diffraction makes these gratings become attractive for stereogram applications.
The basic principle of stereoscopic displays is deliver two different images to the left eye and the right eye for an observer separately. Nowadays, the common ways to generate image splitters for auto- stereogram on a liquid crystal display (LCD) panel focus on technology of parallax barrier [16], and lenticular lens array [17]. In addition to these methods, our group has also presented one new type image splitter for auto-stereogram which is implemented with a special holographic optical element (HOE) [18]. Basically, this HOE is a composition of two holographic gratings, and these two holographic gratings are spatially multiplexed to serve as the image splitter. Among these methods, one extended application of using lenticular lens array for stereoscopic display is that the display function can be switched between 3D and 2D vision mode by applying an external electric field when a LC-based lenticular lens array is used [19].
Accordingly, as shown in Fig. 1, we also extend the application of the proposed HOE and presented a switchable holographic image splitter for a stereogram application. Unlike our previous study, the passive HOE without switchable function is generated by two recording beams with the same linear polarization state [18]. In this study, the switchable holographic image splitter was generated with two spatial-multiplexed holographic polarization gratings. Each holographic polarization grating was written by two orthogonal circular-polarization recording beams in DDLC films. The diffraction efficiency of the presented polarization gratings can be modulated by applying an external AC voltage. This characteristic could be regarding as the switch function of the image splitter, and the switchable function leads to two different operation modes for displaying a 3D stereogram and a 2D plane image. In section 2, the properties of polarization holographic gratings generated within the recording materials are analyzed. Based on these properties, the fabrication method of the proposed image splitter is designed and is illustrated in section 3. In section 4, the experimental results of the switchable image splitter are demonstrated.
2. Materials
The dye-doped liquid crystal films were used as the holographic material in this experiment. The DDLC films were fabricated with nematic liquid crystal (E7) and Methyl Red (MR) with mixing ratio of 97:3 wt. %. The alignment layers were formed by spin coating with homogenous alignment polyimide (AL-58, PI) film on indium-tin-oxide-coated glass substrates. After that, two spin-coated substrates without any further surface rubbing treatment were used to fabricate an empty cell for the injection of the mixture. As shown in Fig. 2, cell configurations were prepared by sandwiching two substrates together with the alignment layers facing each other with dimension of 2cm × 3cm, and a cell gap of 3μm was maintained by plastic spacers. The homogeneously mixed compound was infiltrated into the empty cell by capillary action to form a dye-doped liquid crystal sample.
Figure 3(a) shows the experimental system for recording a holographic polarization grating in dye-doped liquid crystal. A diode-pump-solid state laser with 532nm wavelength was used as the light source for the holographic experiment. The laser beam was collimated and then divided by a polarization beam splitter (PBS). A half wave plate (HWP) placed in the front of collimated system was used to modulate the beam intensity of these two divided beams. Two quarter wave plates (QWP) were used to generate right circular polarization state or left circular polarization state for these two separated beams. In our arrangement, the normal incident beam was a right circular-polarized wave, and the other beam was a left circular-polarized wave. The angle between two writing waves was 2 degree. The intensity for each writing beam was and exposure time was around 10min. Finally, the DDLC was recorded as holographic polarization gratings by these two orthogonal circular polarization waves. The period of the grating is therefore around 15μm.
Furthermore, a He-Ne laser with 632.8nm wavelength locating outside the absorption spectrum range of MR was employed as a probe beam. The collimated He-Ne laser beam was also passed through the PBS and then passed through the QWP. Accordingly, the diffraction of the polarization gratings was investigated by a normal incident probe beam with a right circular-polarization. The diffraction order was defined as shown in Fig. 3(b).
Owing to the negative torque effect [20], the nematic liquid crystals (NLC) within the sample will be reoriented and their direction will become perpendicular to the polarization direction of incident beam. In our system, when the DDLC was illuminated with the interference field generated by two orthogonal circular-polarization waves, the polarization pattern and direction of NLC were modulated as shown in Fig. 4(a). In this configuration, the NLC molecules were rotated along the transverse x direction to construct a particular polarization grating. This periodic structure has been analyzed numerically by Sarkissian et al. [1]. In such a polarization grating, we found that only a strong + 1 order and a very much weaker −1 order diffraction will appear with a right circular-polarized probe beam. The other order diffractions almost disappeared. Once the probe beam become a left circular-polarized beam, we found that only a strong −1 order and a much weaker + 1 order diffraction will appear. This phenomenon is the same with the experiments presented by Provenzano et al. [21]. According to the model described in Ref. 1, diffraction efficiency with 100% could be obtained if material thickness L fits the condition , where ne and no are index for extra-ordinary wave and ordinary wave, and is wavelength. In this condition, the only allowed diffraction orders turn out to be the + 1 and −1 orders. Furthermore, once the incident beam is a right-circularly polarized wave, only the + 1 order diffraction will be obtained. Similarly, a incident wave with left-circularly polarization only generates −1 order diffraction. And therefore, a diffraction phenomenon similar with Bragg diffraction is obtained. According to the equation, the required thickness for 100% diffraction efficiency is 1.4μm only. However, the material thickness we fabricated is 3μm owing to the spacer and therefore lower diffraction efficiency is obtained in our experiments. Higher diffraction efficiency can be obtained by tunings the material thickness [21].
Fig. 4 Configuration of polarization gratings (a) without applied AC voltage, and (b) with large applied AC voltage.
Figure 5 shows the diffraction efficiency of the grating probed with a right circular polarization beam. The diffraction efficiency is defined as diffracted power divided by the incident probe power. Moreover, we also measure variation on diffraction efficiency with the same probe beam when applying an external AC voltage from 0V to 10V. As shown in Fig. 5, we can observe the diffraction efficiency of + 1 order is much higher than −1order diffraction at 0V. The phenomena of a quasi single order diffraction is accordingly verified. After applying AC voltage, we can observe the + 1 order diffraction decreased rapidly from 1.3V to 2V, and then it was enhanced slightly from 2V to 3V. After that, the + 1 order diffraction decreased to less than 2% when the applying electric field was 10V finally. Meanwhile, the −1 order diffraction kept quite low and almost disappeared at higher applied voltage. Because of the positive dielectric anisotropy of E7, the application of an external electric field across the cell induces a reorientation of the LC molecules and leads to a spatially uniform configuration as shown in Fig. 4(b). We can find the period structure has been destroyed, and accordingly there were no diffraction anymore when a high AC voltage is applied. However, when the applied high voltage is blocked, the period structure of the LC molecules restored to its initial state and the + 1 order diffraction appear with the same diffraction efficiency again. Our experimental results showed that the repeatability of electrical switching is very reliable and the obtained alignment state performs high stability.
In addition, we also investigated the polarization state of the diffraction beam from the recorded polarization grating. As shown in Fig. 3(b), when we rotated the QWP in the x-y plane, we can change the polarization state of the probed beam. We defined 0° as the y-direction which is perpendicular to the incident plane. In our experimental arrangement, the probed beam will become right circular-polarized when the QWP was rotated to 45°. In Fig. 6, we can observe the DE of + 1 order diffraction will decrease first and then increase again with the increasing of rotation angle. On the contrary, −1 order diffraction will increase to its maximum value and then decrease again. Worth to be noticed is that when the QWP was rotated to 135° the + 1 order diffraction decrease to minimum and −1 order diffraction increase to maximum. In this condition, the probed beam has become left circular-polarized, and we find diffraction power was transferred from + 1 order to −1 order when the polarization state of probed beam was changed to be orthogonal to its initial state. This characteristic is the same with the experiments presented by Sarkissian et al. [22] and it is helpful for us to simplify the generation process of the holographic image splitter.
3. Fabrication of an image splitter
In the section 2, we have known we can use two orthogonal circularly polarized writing beams to generate a polarization grating in DDLC, in which a quasi + 1 order diffraction can be obtained. Furthermore, from result shown in Fig. 6, we can modulate the maximum diffraction order of a generated grating from + 1 order to −1 order by altering the polarization state of the probe beam. In other word, when the grating is generated again by using the same two writing beams but their polarization states have been altered with each other, the −1 order diffraction will become the maximum diffraction order if we still use the same polarized probe beam.
Accordingly, the fabrication process of the proposed image splitter is designed as shown in Fig. 7. First, a special mask was attached on the DDLC during the holographic exposure process. The mask is a one dimensional binary amplitude grating. The line width for transparent and opaque fringes is 500μm respectively. The whole dimension of the mask is 3cm x 2cm, and each pixel size of the mask is 500μm × 2cm. In the first step of fabrication process, two plane waves interfered with each other. The normal incident wave is a right-circularly polarized (RCP) wave and a left-circularly polarized (LCP) incident obliquely. This step will generate sub-holograms which diffract the image corresponding to left field of view of a stereogram. We can find these two beams only interfered on the unblocked locations of the recording material which is corresponding to the left column pixels of the panel. When a RCP probe beam was normal incident on sub-holograms for left field of view, the maximum diffraction occurs at + 1 order with LCP as shown in Fig. 7(b). In the second step of fabrication process, the mask was lateral translated with 500μm relative to the recording material. And then two plane waves interfered with each other again but their polarization states are contrast to the first recording step as shown in Fig. 7(c). This step will generate sub-holograms for right field of view of a stereogram. When a RCP probe beam was normal incident on sub-holograms for right field of view, the maximum diffraction occurs at −1 order with LCP as shown in Fig. 7(d). After completing these two recording processes, the mask was removed and the holographic image splitter based on DDLC has been fabricated. When a RCP probe beam was normal incident on the image splitter, image located on left column pixels will diffract to left side and image located at right column pixels will diffract to right side. The polarization states of both diffracted images are LCP. Though sub-holograms for left/right field of view within the fabricated holographic image splitter actually still generated very weak −1/+1 order diffraction, they are relatively very weak when comparing with the main diffraction signal.
4. System demonstration
Figure 8 shows the experimental setup for showing the result of a holographic image splitter. A special designed spatial-multiplexed image was prepared and carefully aligned with the generated image splitter based on DDLC. The special pattern is a composition of a right image “▽” on even column pixels and a left image “△” on odd column pixels. A collimated wave from He-Ne laser with 632.8nm wavelength and right circularly polarization state was normally incident on the pattern and then passed through the attached holographic image splitter.
In this setup, we can observe two images located on odd and even column pixels would be separated effectively. Figure 9(a) shows the diffracted results. Two images “△” and “▽” were separated successfully through our holographic beam splitter. It leaded a stereoscopic vision for observers located on the place of screen. There was no obvious cross talk noise in our practical experiments.
Fig. 9 Experimental results of image splitter by applying AC voltage with (a) 0V (b) 1.1V and (c) 10V on the holographic image splitter.
Furthermore, we also verified the effect of applied external AC voltage on diffraction efficiency of the holographic image splitter in order to demonstrate the switch function. As shown in Fig. 9, we show three diffraction images with different applied AC voltage at 0V, 1.1V, and 10.0V, respectively. Their actual corresponding diffraction efficiency can be obtained from Fig. 5. We can observe there are two obvious diffracted images propagating to right and left field of views through the fabricated HOE at 0V and 1.1V. However, when the applied AC voltage is 10.0V, these two images supposed to be separated by holographic image splitter disappeared. These observations indicate a significant result that we can easily perform the switch function for displaying a 3D stereogram or a 2D plane image by applying external AC voltage on the proposed holographic image splitter. In addition, the polarization state of the transmitted zero-order diffraction is still RCP in our experiments, but the polarization states of both diffracted right/left images are LCP. Since their polarization states are orthogonal to each other, the zero-order diffraction can be removed in practical application. The typical technique is attaching a quarter wave plate and a linear polarizer on the holographic image splitter for the transmitted and diffracted waves.
5. Conclusions
A polarization grating with quasi single order diffraction in dye doped liquid crystals was presented. The polarization grating was generated by two writing beams with orthogonal circular-polarization states. A switchable holographic image splitter based on the presented polarization grating in DDLC was demonstrated. The holographic image splitter was generated with two spatial-multiplexed holographic polarization gratings, which located on the right column pixels and left column pixels respectively. According to response of diffraction efficiency on applying the external AC voltage, we can easily perform switching function for a 3D or 2D vision mode electrically. The proposed technique can be improved by using a material with higher diffraction efficiency, and then it will enhance brightness performance for stereogram application. The proposed switchable holographic image splitter performs high potential as an alternative competing technology with switchable lenticular lens and switchable barrier stereo-displays.
Acknowledgments
This work is supported by the National Science Council of Taiwan under Contract No. NSC 101-2221-E-018-017.
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