A real-time holographic display using doped liquid crystals
Using dyes and quantum dots to dope liquid crystal films achieves displays with reconstructed holographic videos at a refresh rate of 25Hz.
Developments in 3D display technology have drawn worldwide attention in recent years, most notably with movies such as Avatar. There are two common types of 3D display based on the principles of stereopsis (perception of depth). One exploits binocular parallax (the displacement in the apparent position of an object viewed along two lines of site), and the other makes use of light-field reconstruction. However, the ultimate goal is holographic display, which provides the most realistic 3D images of objects or scenes. This is because it can reconstruct both intensity and phase information, enabling the perception of light as it would actually be scattered by a real object, without the observer needing special eyewear.
To achieve such displays, one approach is optical holography based on dynamic recording materials, which have a wide viewing angle and scalable display size. However, to show real-time, dynamic 3D images, there is a limited choice of suitable photorefractive materials with the necessary fast response and high modulation index to achieve a reasonable diffraction efficiency. This presents challenges in the choice of materials, devices, and system structures.
Here we present a real-time holographic display featuring a liquid crystal (LC) doped with an azo (synthetic) dye. This material enables a video-rate display, since we can refresh each hologram on the order of several milliseconds. We have successfully shown a real-time holographic video at a refresh rate of 25Hz, sourced from a spatial light modulator (SLM) and reconstructed with an azo-dye (DR1)-doped nematic LC cell. This approach does not require application of the electric field that conventionally enables LC displays. Figure 1 shows the setup of our system. The computer-controlled SLM modulates a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser beam, which illuminates the LC device, generating a hologram. The sapphire laser shines through that hologram to make the displayed 3D image.1
We measured the performance of the proposed device by studying the response time, including both the writing and erasing time. We found that, by adjusting parameters such as the recording intensity, polarization direction, and polarization state, we could measure response time on the order of several milliseconds, sufficiently fast for video-rate display applications.
Figure 2(a) shows snapshots of reconstructed images at different times, showing the dynamic display properties of our approach. The sample can also function with various colors of probe lights, offering potential for color displays using multiplexing. The materials we used here are widely available and the method is scalable, making this approach promising for future use in large, dynamic, colorful holographic displays.
However, due to the low solubility and high mobility of dye molecules in LC, the diffraction efficiency and thermal stability of this dye-doped system are insufficient. As an alternative, we considered semiconductor nanoparticle-doped LC materials. These are photorefractive, meaning that their refractive index can be altered by the electric field (the space charge field) generated when the crystal is exposed to coherent illumination, and by the polarization and propagation direction of the light (birefringence). By doping indium phosphide zinc sulfide (InP/ZnS) quantum dots into the LC,2 we can greatly enhance the photorefractive effect. The size of the quantum dots is not significant here, since our experiment exploits a photo-generated space charge effect that does not rely on photoluminescence. With an external voltage applied to the material, our device exhibited a fast response, and a significantly improved diffraction efficiency of 20%, up 33-fold from 0.6% in the case of DR1-doped LC. Figure 2(b) shows pictures taken from three diffracted holographic videos, when illuminated by light at wavelengths of 632.8, 532, and 488nm, respectively. We also demonstrated red/green/blue real-time videos and verified the system's feasibility as a color holographic display.
In the future we aim to realize the next-generation of true 3D display for practical applications. To do this, we will work on achieving high diffraction efficiency, large panel size, high resolution, wide viewing angle, and color multiplexing.
This work is sponsored by Program 973 (2013CB328804), National Natural Science Foundation of China (61307028), and the Science & Technology Commission of Shanghai Municipality (13ZR1420000).
Yikai Su is a professor in the electronic engineering department. He received a PhD in electronic engineering from Northwestern University, IL, in 2001. His research covers micro- and nanophotonic devices and real-time holographic 3D displays.
