Infrared-Sensitive Photorefractive Polymer Composite Devices

August 1, 2008

Air Force Research Laboratory

Polymer composites that are photorefractive at visible and near- infrared wavelengths, and devices that exploit their photorefractivity, have been demonstrated. Potential applications for such devices could include real-time holography, medical imaging, imaging through light-scattering media, and beam cleanup in free-space optical communications. Especially notable products of this development effort include devices that exhibit one-photon photorefractivity with high diffraction efficiency at a wavelength of about 1 μm and devices that exhibit two-photon photorefractivity at a wavelength of about 1.5 μm. The polymer composites used in these devices are the first-demonstrated all-organic photorefractive materials suitable for wavelengths >0.83 μm, and are among the best infrared-sensitive photorefractive materials yet demonstrated under similar experimental conditions.

Dynamic Atmospheric-Like Wavefront Aberrations were introduced by use of an aberrator phase plate moved along a perpendicular to the line of sight and were corrected by use of a photorefractive polymer composite. These images were recorded at three different positions of the aberrator phase plate.

The development of these polymer composites started with exploration of (1) the high electron mobility of tetraphenyl diphenylamine molecules and (2) the chemical flexibility of the acrylic chain. Through choice of chemical components so as to effect careful manipulation of the electron-orbital energy levels of the components, the polymer composites thus formulated were made to exhibit efficient generation, transport, and trapping of electric charges:

•Poly(acrylic tetraphenyldiaminobi - phenyl) [PATPD] was formulated as the charge-transport matrix component;

•Two new chromophores C 3-(N,N-di-n-butylaniline-4-yl)-l- dicyanomethylidene-2- cyclohexene (DBDC) and 3-(4-(azepan-I-yl)-phenyl)-l- dicyanomethylidene-2- cyclohexene (APDC) C were formulated to increase conjugation lengths, and hence, dipole moments over those of previously known dicyanostyrene-based chromophores. These increases are expected to lead, in turn, to increases in figures of merit.

•Polymer composite samples containing a single previously known chromophore C 4-homopiperidino benzylidine-malonitrile C were prepared. Usually, loading of a single chromophore in a photorefractive composite must be limited to less than 40 weight percent to prevent crystallization, which is undesired. However, when chromophores are mixed, a greater loading can be achieved before the onset of phase separation. Hence, in addition, samples containing two chromophores were prepared.

•Adopting a poly(vinyl carbazole) composite recipe previously demonstrated to be successful, ethyl carbazole was used as a plasticizer and fullerene (C₆₀) was used as a photosensitizer at a wavelength of 633 nm.

•A new molecule C 2-[2-{5-[4-(di-n-butylamino) phenyl]-2,4-pentadienylidene}-1,1-dioxido-1-benzothien-3(2H)-ylidene] malononitrile (DBM) C was developed as the sensitizer for one-photon photorefractivity at a wavelength of 1 μm and two-photon photorefractivity at a wavelength of 1.5 μm.

Other accomplishments include the following:

The response times of the devices developed in this effort are of the order of 300°C, short enough for writing and erasing at video frame rates.
A photorefractive polymer composite for use at the important telecommunication wavelength of 1.55 μm was formulated. The components of this composite were optimized to yield a diffraction efficiency >40 percent.
It was shown that holograms can be fixed by reducing their temperatures and that, while thus fixed, they can be read nondestructively.
Devices that exhibit >90 percent internal diffraction efficiency at working voltages as low as 1 kV (lower than those of prior photorefractive polymer devices) were demonstrated.
In an experiment, a photorefractive polymer composite was shown to be capable of high-quality correction of dynamic atmospheric-like wavefront aberrations (see figure) with video-rate response.
It was shown that photorefractive devices can be used to perform a high-spatial-frequency-pass filter function for edge detection.
Photorefractive polymer composites were formulated for applications in which insensitivity to vibrations is required. An optimized composite of this type exhibited a diffraction efficiency >50 percent when written by a single 0.532-μm-wavelength laser pulse having an energy density of 4 mJ/cm² and read by use of a continuous-wave laser beam at a wavelength of 0.633 μm. Insensitivity to vibrations can be achieved because the duration of the writing pulse is of the order of a nanosecond. Moreover, these composites can be written and read at either or both wavelengths and, therefore, can be used for performing two-color holography. Further development may enable the use of these or related composites as dynamic full-color holographic recording media.

This work was done by Nasser N. Peyghambarian of Purdue University for the Air Force Research Laboratory.

AFRL-0033