Daniel Wright

High-speed photorefractive polymer composites

Two photorefractive polymer composites are presented that exhibit the fastest response times reported to date by an order of magnitude (τg≈5 ms at 1 W/cm2), while maintaining large gain coefficients (Γ≈230 and 130 cm−1). These materials show promise for video-rate optical processing applications. The factors limiting the photorefractive speed in these materials are investigated.

Spectroscopic determination of trap density in C60-sensitized photorefractive polymers

Abstract A new and simple method for the determination of the trap density in C 60 -sensitized photorefractive polymers is presented. We show that the radical anion of C 60 acts as the primary photorefractive hole trap, using the strong near-infrared absorption of C 60 − to quantify the anion concentration in situ. The spectroscopically determined concentration correlates well with the photorefractive trap density obtained from analysis of the photorefractive performance. In our model, optical irradiation is needed to activate the trapping sites, and the nonlinear optical chromophore acts as a temporary reservoir for the photogenerated holes (compensator). This model is further clarified using cyclic voltammetry.

Image amplification and novelty filtering with a photorefractive polymer

Energy transfer between two laser beams writing a volume hologram in a photorefractive polymer composite is applied to video-rate optical processing applications. A net increase in image intensity as high as a factor of 37 can be observed within one video frame time (33 ms) using a total beam intensity of 1 W cm−2 and 77 V μm−1 applied electric field. Moving object detection (novelty filtering) is also demonstrated.

Homodyne detection of ultrasonic surface displacements using two-wave mixing in photorefractive polymers

We demonstrate an improved laser-based receiver for the detection of small ultrasonic surface displacements. The receiver is based on a homodyne interferometer using a photorefractive polymer as an adaptive beam combiner. The interferometer requires no path-length stabilization and can process speckled beams from rough surfaces. In experiments using a 10 mW diode laser at 676 nm, we have measured a shot noise-limited surface displacement sensitivity that is within a factor of three of the ideal limit for a lossless, plane wave interferometer.

High-performance photorefractive polymer composite with 2-dicyanomethylen-3-cyano-2,5-dihydrofuran chromophore

A nonlinear optical chromophore for photorefractive applications containing a 2-dicyanomethylen- 3-cyano-2,5-dihydrofuran acceptor group is presented. When doped into a plasticized composite of poly(n-vinylcarbazole), large gain coefficients (Γ) are observed with photorefractive speed similar to the best composites reported in the literature while maintaining low sample absorption (∼15 cm−1).

High efficiency, multiple layer ZnO acoustic transducers at millimeter‐wave frequencies

We have fabricated multiple layer ZnO acoustic transducers for highly efficient sound wave generation at millimeter‐wave frequencies. The transducers consist of half‐wave‐thick layers of ZnO with alternating crystal structure. The two‐way untuned conversion loss values were 27 dB near 29 GHz and 50 dB at 96 GHz measured at liquid nitrogen temperatures.

Photochromic polymers for the optical homodyne detection of ultrasonic surface displacements

We present a novel scheme with which to detect small ultrasonic surface displacements by use of a photochromic polymer instead of a photorefractive material as an adaptive beam combiner in a two-wave mixing geometry. Poly(methyl methacrylate) is doped with a derivative of zinc tetrabenzoporphyrin that possesses a long-lived triplet state that can be efficiently populated in a reversible manner. The resulting dynamic hologram consists of local absorption and refractive-index gratings, which can process speckled beams reflected from rough surfaces. We believe that this is the first use of a local nonlinear medium for adaptive homodyne detection of ultrasonic surface displacements.

Dicyanomethylenedihydrofuran photorefractive materials

Derivatives of 2-dicyanomethylen-3-cyano-2,5-dihydrofuran (DCDHF) have been synthesized by different methods to be used as photorefractive (PR) chromophores. Structure modifications were performed on the donor, acceptor and conjugated π-system for improving properties such as glass formation. Structure-property relationships important for PR applications are discussed from the results of studies including UV-Vis, electrochemistry and DSC.

Synthesis and properties of glassy organic multifunctional photorefractive materials

Abstract We report the synthesis and characterization of several related amorphous multifunctional photorefractive (PR) materials with the charge transport agent covalently linked to a chromophore. DCTA (4,4′-di(carbazol-9-yl) triphenylamine) is used as the charge transport agent in conjunction with C60 to provide photoconductivity and DCST (an aminodicyanostyrene) is used as the birefringent and nonlinear optical chromophore. Chains with different lengths (2, 6 and 12 carbons) are used to covalently connect these two moieties. The linking units influence the glass transition temperature of the system, which in turn influences the PR performance. The longer linker allows faster orientation of the chromophore and results in higher PR speed but the response still appears to be limited by the molecular reorientation rather than photoconductivity in these systems.

Photorefractive polymers for laser-based ultrasound detection

Photorefractive polymers have recently shown an attractive combination of high two-beam-coupling gain coefficient (approximately equals 200cm-1), low absorption (approximately equals 5-10 cm-1), and fast response (few ms) at 1W/cm2 writing intensity. Such materials show promise as adaptive beamsplitters for homodyne detection of transient phase shifts due to laser-based ultrasound. The performance of a photorefractive polymer composite is explored for this application.