Tech Briefs

Photorefractive (PR) polymer composites developed for 3D display applications contain a copolymer as the hole-transporting host matrix. The copolymer approach is followed to reduce the phase separation typical in guest-host polymer systems with low glass transition temperature (Tg), thus allowing increased loading of functional components such as NLO chromophores. The copolymer consists of a polyacrylate backbone with pendant groups tetraphenyldiaminobiphenyltype (TPD) and carbaldehyde aniline (CAAN) attached through an alkoxy linker (PATPD-CAAN). A fluorinated dicyanostyrene (FDCST) NLO chromophore was added to provide sufficient refractive index change and charge generation at the wavelength of interest (532 nm). The plasticizer Nethyl carbazole (ECZ) was also used to reduce the Tg to room temperature. In some composites, a fullerene derivative [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) was used to provide improved sensitization.

The Holographic 3D Display System. The hologram for 3D display is generated using integral image holography. Dozens of 2D perspectives of an object are processed on a computer and then optically multiplexed onto the recording medium in a manner such that when reconstructed, the sensation of depth is created via parallax.
Samples were prepared by melt processing a composite of PATPD-CAAN/ FDCST/ECZ (50/30/20 wt%) (C1) between two indium-tin-oxide (ITO) coated glass slides. The thickness was set using glass spacer beads. Samples for nanosecond pulse writing were prepared with PCBM (C2= PATPD-CAAN/FDCST/ ECZ/PCBM (49.5/30/20/0.5 wt%)). Those samples used in the display showed no degradation or damage for several months over hundreds of write/erase cycles, or showed no phase separation in an accelerated aging test at 600 °C for seven days.

The PR thin-film devices (C1) showed almost 90% diffraction efficiency at an applied voltage of 4kV using 532-nm writing beams and a 633-nm reading beam in a typical four-wave mixing (FWM) measurement. Using the same irradiance, the two-beam coupling (TBC) gain coefficient for the samples at 5kV is around 200 cm-1. Large-area devices made of the composite showed no degradation or dielectric breakdown for extended periods of usage (several months) under very harsh conditions such as high applied fields and high-power, focused laser beams.

The holograms recorded in the thin-film devices can persist for several hours in the dark (without writing beams) at an applied voltage of 4 kV while continuously being probed with a red (633-nm) laser beam for which the samples are transparent. The total recording time of a 3D display that employs this material needs to be brought to around a few minutes to achieve a high FOM. A new technique was developed to improve the writing speed of organic PR materials based on manipulation of the applied voltage, called the “voltage kick-off.”

In conventional recording of PR polymers, a fixed external voltage is usually applied across the polymer to pole the NLO chromophores. In the kick-off approach, an increased voltage (i.e. 9kV) is applied across the polymer to increase the writing speed during the hologram recording, and then the voltage is reduced to its optimum value of 4kV after recording is complete. The temporarily increased voltage facilitates efficient separation of electron-hole pairs and improves the drift characteristics, forcing them to travel faster, and increasing the orientational order parameter and speed of the NLO chromophores. The reduction of the voltage to its optimum value after recording ensures hologram persistency. The overall benefit of the voltage kick-off is the reduction of the writing time per hologram to less than a second by fine-tuning of the applied voltage.

A high diffraction efficiency of 55% was achieved with a total writing time of 0.5 second and several hours of hologram persistency in this composite using voltage kick-off. 3D displays (4×4" in size) were recorded with complex and high-quality images within a few minutes using horizonatal parallax only (HPO) imaging. The 3D display exhibits a total horizontal viewing angle of 45 degrees. The images are viewable up to three hours directly on the photorefractive thin-film device without the need for intermediate projection tools or magnification between the recorded image and the viewer. The images can be completely erased within minutes by uniform illumination of the display using a 532-nm beam, and new images can be recorded when desired. There is no technological limit to the achievable display size, as large thin-film devices can be fabricated and even tiled together. For larger, full parallax displays a short pulsed recording can be employed.

This work was done by Nasser Peyghambarian and Robert A. Norwood of the University of Arizona for the Air Force Office of Scientific Research and the Office of Naval Research. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Photonics category. AFRL-0161

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