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Photorefractive Polymers for Updatable 3D Displays

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

Topics:
Composite materials Displays Glass Imaging and visualization Optics Photonics Polymers Research and development
This Brief includes a Technical Support Package (TSP).
Document cover
Photorefractive Polymers for Updatable 3D Displays

(reference AFRL-0161) is currently available for download from the TSP library.

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    Defense Tech Briefs Magazine

    This article first appeared in the October, 2010 issue of Defense Tech Briefs Magazine (Vol. 4 No. 5).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document presents a final performance report on a project focused on the development of photorefractive polymers for updateable 3D displays, conducted by researchers Nasser Peyghambarian and Robert A. Norwood at the University of Arizona from January 2007 to November 2009. The project aimed to create a large-area, updateable 3D color display using a novel co-polymer based photorefractive composite.

    Key accomplishments of the project include the development of photorefractive copolymers that exhibit image persistence ranging from several minutes to hours, and the establishment of a holographic image recording setup capable of writing and displaying 3D images. Notably, the team demonstrated the first updateable 3D display utilizing these photorefractive polymers. They optimized the temporal response of the long-persistence polymer composites to accommodate single nanosecond pulse exposure, which is crucial for high-speed recording.

    The report details the technical aspects of the holographic 3D display system, including the writing and erasing dynamics of the holograms. The writing process involves using a 532 nm laser beam with specific irradiance and voltage settings to achieve high diffraction efficiency. The researchers found that the maximum diffraction efficiency increases with longer writing times, and they developed a system that allows for new images to be recorded as needed.

    Additionally, the project explored faster recording systems by increasing continuous wave (CW) laser power, which significantly reduced the write/erase cycle time from 230 seconds to 150 seconds while maintaining diffraction efficiency. The report emphasizes the potential for large thin-film devices to be fabricated and tiled together, indicating no technological limits to the achievable display size.

    The document concludes by highlighting the successful demonstration of refreshable color holograms and the feasibility of creating large area 3D display devices, with a specific example of a 6-inch by 6-inch sample. This work represents a significant advancement in the field of photorefractive materials and their application in dynamic 3D displays, paving the way for future innovations in visual technology.

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