Martin M. Liphardt

Photoconductivity and grating response time of a photorefractive polymer

We report the photoconductive properties and photorefractive grating response time of a polymer mixture composed of 40-wt. % dissolved diethylamino-benzaldehyde diphenyl hydrazone (DEH) and the non-cross-linking epoxy polymer Bisphenol A 4,4′-nitroaminostilbene. The films have improved photoconductive sensitivities as high as 2.1 × 10−10 cm/(W Ω) at a wavelength of 650 nm with a corresponding reduction of the grating response time constant to 0.11 ± 0.02 s at an intensity of 1 W/cm2. The nitro-aminostilbene chromophore is deduced to be the source of photogenerated charge carriers on the basis of a comparison of the wavelength dependence of the photoconductivity and absorption coefficient. Degradation of the photoconductivity and the dark conductivity as well as of the photorefractive speed with sample age is attributed to precipitation of the DEH; this explanation is supported by x-ray diffraction observations of crystal growth in the polymer.

Measurement of the photorefractive grating phase shift in a polymer by an ac phase-modulation technique

The complex coupling constant of a photorefractive polymer was measured as a function of an applied electric field by use of a modified ac phase modulation technique. We determined that both a photorefractive index grating and a nonphotorefractive absorption grating were present. The electric field dependencies of the amplitude and the phase of the photorefractive gain coefficient were accurately described by standard photorefractive theory. The accuracy of the photorefractive phase shift as measured by this phase-modulation technique was ±1° near phase shifts of 90° and ±3° near phase shifts of 45°.

Effect of beam attenuation on photorefractive grating erasure

We investigate the influence of attenuation on the speed of erasure of photorefractive gratings by solving the coupled-wave equations in the undepleted pump approximation and by taking into account the attenuation and Gaussian intensity profile of all the beams. The extrinsic grating decay rate is significantly lower than the intrinsic photorefractive decay rate in samples with overall attenuation as low as 10%. The Gaussian beam profiles of the readout and the erasing beams result in a further reduction of the extrinsic decay rate. The results of these calculations are used to determine the spectrum of intrinsic decay rates in a photorefractive polymer.

Role of photoconductivity in molecularly doped photorefractive polymer

The photorefractive effect is a reversible mechanism of holographic grating formation in electro-optic materials, that has potential applications in integrated optics, optical data storage, optical computing, and several other areas. This effect was recently observed in photorefractive polymers doped with charge transport agents. The grating formation is initialized by the photoconductive response which includes charge generation, mobility, and trapping, occurring in successive order. Finally, the grating is formed as a result of modulation of the refractive index by the resulting space charge field, via the electro-optic (Pockels) effect. We present the results of photoconductive measurements as a function of temperature, applied electric field, and illuminated intensity. The investigation is focused on the nonlinear optical polymer bisphenol A 4-4-nitroaminostilbene mixed with 30 weight % of the hole transport agent diethylamino-benzaldehyde diphenyl hydrazone. We observed that the photoconductivity of the photorefractive polymer has the form exp[ aE1/2 + bE1/2/T2 - c/T2] in agreement with the diorder theory of the well- known hopping model developed for charge-transport in molecularly doped polymers. We also observed that the apparent hopping distribution bandwidth increases with increased intensity, consistent with an increasing density of states.

Metrology standards with ellipsometers

At the October 1997 NIST “Workshop for Thin Dielectric Film Metrology”, uses of ellipsometers, and the role of standard materials for semiconductor metrology were two of several important topics discussed. Much of the workshop discussion centered on metrology for thin gate oxides. This paper is an extension of some of the topics discussed at that workshop.