A tri-color composite volume holographic polymer dispersed liquid crystal (H-PDLC) grating and its application to 3-dimensional (3D) color autostereoscopic display are reported in this paper. The composite volume H-PDLC grating consists of three different period volume H-PDLC sub-gratings. The longer period diffracts red light, the medium period diffracts the green light, and the shorter period diffracts the blue light. To record three different period gratings simultaneously, two photoinitiators are employed. The first initiator consists of methylene blue and p-toluenesulfonic acid and the second initiator is composed of Rose Bengal and N-phenyglycine. In this case, the holographic recording medium is sensitive to entire visible wavelengths, including red, green, and blue so that the tri-color composite grating can be written simultaneously by harnessing three different color laser beams. In the experiment, the red beam comes from a He-Ne laser with an output wavelength of 632.8 nm, the green beam comes from a Verdi solid state laser with an output wavelength of 532 nm, and the blue beam comes from a He-Cd laser with an output wavelength of 441.6 nm. The experimental results show that diffraction efficiencies corresponding to red, green, and blue colors are 57%, 75% and 33%, respectively. Although this diffraction efficiency is not perfect, it is high enough to demonstrate the effect of 3D color autostereoscopic display.
© 2015 Optical Society of America
In recent years, there is an increased effort to develop naked eye 3-dimensional (3D) autostereoscopic display due to the rapid growth of mobile display devices such as display for smart phones . By replacing the current 2-dimensional (2D) display with 3-D display, a more realistic 3D view can be realized. A core element in naked eye 3D display is an image splitter, which separates left and right images.
Switchable volume holographic polymer-dispersed liquid crystal (H-PDLC) grating is a promising image splitter due to the following reasons. First, it has a high diffraction efficiency. Second, only one resonant Bragg wavelength diffracts due to the high wavelength selectivity of volume grating. Although H-PDLC has been successfully used for different applications, including electro-optic devices [2–5], optical switches [6, 7], electro-optic filters [8, 9], display systems [10, 11], distributed feedback laser [12, 13], and photonic crystals [14, 15], one cannot realize color 3D display by employing a single H-PDLC grating. At least three volume gratings are needed. For example, the first one diffracts red beam, the second one diffracts green beam, and the third one diffracts blue beam.
To realize this different color diffraction effect, the most convenient way is to record the grating by different color laser beams. In PDLC, different photoinitiators are needed to record at different wavelengths. For example, Ingacure 1173 was used to record at UV , Rhodamine 6G dye and Ingacure 784 were used to record at green and blue wavelength region , and Methylene Blue (MB) and coinitiator – p-toluenesulfonic acid were used to record at red [18, 19]. However, since above previous works contained only a single type of photoinitiator, one could not realize tri-color (including all three red, green, and blue colors) recording. To overcome this fundamental limitation, in this paper, we report a new type PDLC that contains two groups of photoinitiators, which enables tri-color recording so that a tri-color composite H-PDLC grating can be realized. Furthermore, the paper also discusses the application of this unique composite tri-color H-PDLC grating to the application of 3D color autostereoscopic display. In comparison to other approaches, including spatial-multiplexed stereoscopic method [20–23], lenticular lens , and parallax barrier , the tri-color composite H-PDLC grating offers the advantage of avoiding diffraction angle-shift with no need of changing exposure angle.
2. Preparation of PDLC sample with two groups of photoinitiators
PDLC samples with two groups of photonitiators were prepared by mixing (1) 44.71 wt% acrylic monomer: ebecryl 8301 acrylated urethane purchased from UCB Inc., (2) 34.76 wt% nematic liquid crystal (LC): TEB50 purchased from Beijing Tsinghua Yawang Liquid Crystal Material Co., (3) 9.94 wt% cross-linking monomer: 1-Vingyl-2-pyrrolidinone purchased from Aldrich Inc., (4) 0.15 wt% photoinitiator: Rose Bengal (RB) purchased from Aldrich Inc., (5) 0.4 wt% coinitiator: N-phenylglycine purchased from Aldrich Inc., (6) 0.15 wt% photoinitiator: Methylene Blue (MB) purchased from Aladdin Industrial Inc., (7) 0.4 wt% coinitiator: P-toluenesulfonic acid purchased from Alddin Industrial Inc., and (8) 9.94 wt% surfactant: S-271 purchased from Chemistry Inc. To ensure a uniform mixing, first, the mixture was heated and stirred in an ultrasonic cleaner in dark condition. Then, the mixture stayed quietly for 24-48 hrs so that a homogeneous mixture could be achieved.
To record the holographic grating in the mixture, the mixture was sandwiched by a pair of indium-tin-oxide (ITO) coated glasses to form a PDLC cell. The thickness of cell was controlled by spacers. In the experiment, the thickness of cell was 20μm.
3. Fabrication and analysis of tri-color composite H-PDLC grating
Figure 1 shows the experimental setup for writing tri-color composite H-PDLC grating. Three laser sources with different color output wavelengths 632.8 nm (red), 532 nm (green), and 441.6 nm (blue) were used to write the tri-color composite grating. After the expansion, three laser sources had the same 18 mm diameter beam size. Each beam was divided into two coherent beams, which were recombined by a pair of mirrors at the location of PDLC so that the holographic grating could be written. In the experiment, the half-angle between two recombined beams was 11.3°. The exposure light intensity for red, green, and blue beams were 4.99, 16.79, and 4.48 mW/cm2, respectively. The recording was conducted at the ambient room temperature ~25°C. The exposure times of achieving maximum diffraction efficiency for red, green, and blue laser beams were 20 min, 1 min, and 20 min, respectively.
The PDLC material and corresponding tri-color composite H-PDLC grating were also quantitatively analyzed. First, the transmittance of photoinitiators RB and MB were measured by an Ocean Optics USB4000 spectrometer. For the purpose of comparison, the absorption of pure photoinitiator was also measured by solving RB and MB in pure water. Figure 2(a) shows the measured transmittance of pure photoinitiator, in which the pink and blue curves correspond to the transmittance of RB and MB, respectively. One can clearly see that the main absorption of RB is at 560 nm and the main absorption of MB is at 600 nm, which are consistent with the previously reported data. Figure 2(b) shows the measured transmittance of photoinitiator within PDLC. It can be seen that the main absorption peaks almost maintain at the same location. Furthermore, to enhance the absorption, a little bit 0.05% silver nanoparticles (NPs) were also added in the PDLC. The transmittance of silver NP doped PDLC is shown by the brown curve in Fig. 2(b). In addition to the absorption peaks located at green line and red line, there are also broad absorptions at blue line. Thus, one can write tri-color composite H-PDLC grating in this unique PDLC.
Second, the period and surface morphology of the H-PDLC grating were analyzed. According to Bragg’s law , the period of H-PDLC grating, , can be expressed asEq. (1), the calculated grating periods written by red (632.8 nm), green (532 nm), and blue (441.6 nm) laser beams were 1615nm, 1358nm and 1127nm, respectively. To compare the theoretical data with experimental result, grating periods of recorded H-PDLC gratings were measured by atomic force microscope (AFM), as shown in Fig. 3. It can be seen that the grating periods written by red, green, and blue lasers are 1523 nm, 1219 nm and 1078 nm, respectively, which are just slightly smaller than the theoretical values. It is speculated that this is due to the polymer grating shrinkage after the exposure. At any rate, this experimental result confirmed that RGB tri-color composite grating could be written by three RGB color lasers simultaneously in this unique double photoinitiator PDLC, which was a quick and cost-effective approach.
Third, the diffraction efficiencies as a function of exposure time and bias electric voltage were measured. Figure 4(a) shows the measured positive first-order diffraction efficiency as a function of exposure time. A probe beam (0.3mW, 532nm) was incident on the grating at the expected Bragg angle. A timer blocking the writing beams was switched on and recorded the exposure time. The diffraction efficiency is defined as , where and represent the positive first-order diffracted and transmitted light intensities, which can be measured by two individual optical powermeters. It can be seen that the diffraction efficiency increases as the exposure time increases until it reaches the maximum value. The maximum diffraction efficiencies for red, green and blue gratings are 57%, 75% and 33%, respectively. The corresponding exposure times are 20 min, 2 min, and 20 min, respectively. The green light has the highest sensitivity. Figure 4(b) shows the measured diffraction efficiency as a function of biasing voltage, which is studied by increasing the external electric field applied on the gratings gradually, ranging up to ~100V at a fixed frequency of 50Hz. Here, the threshold driving voltage is defined as the voltage value when the diffraction efficiency is equal to 90% of the initial value of diffraction efficiency. It can be seen that the diffraction efficiency maintains almost the same value as long as the biasing voltage is less than the threshold voltage ~60V. Since the thickness of our sample was 20μm, the corresponding threshold electric field was around 3V/μm
4. Application of tri-color composite H-PDLC grating to 3D color autostereoscopic display
To illustrate the application of tri-color composite H-PDLC grating to 3D autostereoscopic display, we assumed that there were two adjacent color pixels. Each color pixel consisted of three red, green, and blue sub-pixels. To ensure that the light from left color pixel reached left eye and the light from right color pixel reached right eye, two tri-color composite H-PDLC gratings with different diffraction directions were employed, as illustrated in Fig. 5. In other words, the diffraction direction of tri-color composite H-PDLC grating of left color pixel was designed to diffract incoming light beam to the left eye while the diffraction direction of tri-color composite H-PDLC grating of right color pixel was designed to diffract incoming light beam to the right eye. Since tri-color composite H-PDLC grating was composed of three RGB volume sub-gratings, 3D color autostereoscopic display could be realized.
Figure 6 showed the experimental setup used for fabricating the tri-color composite H-PDLC used for 3D color autostereoscopic display. This setup was similar to the one as illustrated in Fig. 1 except that there were a rectangular pixel mask and the third beam splitter was put on a 360 rotating platform. The rectangular pixel mask was used to control the size and location of the grating so that it matched the location of pixel and the rotating stage was used to control the diffraction direction of the grating. Different diffraction directions were realized by rotating the third beam splitter. In the experiment, first, the tri-color composite H-PDLC grating for the left pixel was fabricated by setting rectangular pixel mask to the position of left pixel and making exposure for red, green, and blue laser beams. Then, the tri-color composite H-PDLC grating for the right pixel was fabricated by setting rectangular pixel mask to the position of right pixel, rotating the third beam splitter by 90 deg, and making exposure for red, green, and blue laser beams.
Three laser beams with wavelength of 632.8nm, 532nm and 441.6nm were mixed forming an approximately white beam and incident on the pattern with color filters. Then the six RGB optical beams carrying the information of pattern were incident on the corresponding sub-gratings to realize the beam splitter of left part and right part of the pattern. The distance of observation screen instead of people’s eyes from the grating was set to 25cm, distance between the diffracted left and right pixels was set to 7cm that is the same as the pupil distance, and the diffraction angle can be calculated according to geometrical relationship as 7.97°.
If the single-wavelength sensitive H-PDLC gratings are adopted to function as image splitter, color dispersion will let RGB hues transmit to the eyes with different directions. Taking a kind of H-PDLC grating that is only sensitive to the laser with wavelength of 532nm as an example, according to the equations below:
Figure 7(a) showed the pattern attached with color filters representing the original left and right color pixels. The shape of left pixel was a right half ring and consisted of three vertical RGB bars. The shape of right pixel was a left half ring and also consisted of three vertical RGB bars. Figure 7(b) showed the experimentally recorded diffracted left and right color pixels, which was taken in front of the observation screen. This experimental results confirmed that, first, we could clearly separate left and right color pixels. Thus, one could realize 3D autostereoscopic display by harnessing the described tri-color composite H-PDLC grating. Second, all three colors red, green, and blue were correctly displayed, which confirmed that this was color autostereoscopic display. Finally, there was negligible color dispersion due to the high wavelength selectivity of volume Bragg grating, which enabled 3D color autostereoscopic display.
In conclusion, a PDLC contained two photoinitiator systems: RB and MB was synthesized. It had the advantage of sensitive to all red, green, and blue wavelengths so that a tri-color composite volume H-PDLC grating could be conveniently and cost-effectively recorded by harnessing red (632.8 nm), green (532 nm), and blue (441.6 nm) laser beams. A 75% diffraction efficiency and a relatively high threshold electric field ~3V/μm were achieved. Such a diffraction efficiency was high enough to experimentally illustrate the application of this tri-color composite H-PDLC grating to 3D color autostereoscopic display. The experimental result was consistent with the theoretical prediction. Such kind of naked-eye 3D color display could be very useful in next general mobile device display because it was easy to use and cost-effective. In the future, we will further improve the diffraction efficiency by optimizing the material composition and holographic recording conditions.
This work is supported by Key Research Project from Shanghai Education Committee under Grant 14ZZ138, Shanghai Key Subject Construction funding under Grant s30502, and National Key Scientific Instrument Development Projects under Grant 2012YQ15008720.
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