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Earlier we investigated the non-steady-state photo-EMF effect in PVK-based photorefractive polymer films in the conventional transmittance tilted configuration [1]. Here we apply this effect and dynamic TWM for the characterization of reflectance gratings in similar polymer films. Note that both experimental configurations utilize periodically phase-modulated recording beams and allow us to probe shifted and unshifted components of the space charge grating (by detecting fundamental and second harmonics (SH) in the output signals).

Green region of spectrum is one of most promising for interferometric applications since the sensitivity grows with reduction of wavelength, and powerful CW lasers are commercially available now. Photorefractive Bi12SiO20 (BSO) crystal is the most sensitive one in this spectral region, which makes it attractive for adaptive detection of laser ultrasonic via two-wave mixing (TWM). We report basic dependencies of TWM of phase modulated beams in this PRC in its conventional holographic (110) cut (in particular on external dc field, input light polarization etc.). Since the amplitude noise of laser is usually dominating, the sensitivity of the whole system can be essentially increased using anisotropic (i.e. with polarization plane rotation) light diffraction in BSO.

The relatively high (typically several MΩ) resistance of conventional GaAs photo-EMF detectors is one of the most important disadvantages of these devices, especially for laser ultrasonic detection. Indeed, to ensure the 10-30 MHz frequency band necessary for this application, one has to use feedback resistors in the input transimpedance amplifier of values less than 10 kΩ. This is clearly very far from optimal matching conditions, and the input noise of the bipolar-input operational amplifier (≈ 1 pA/Hz) proves to be dominating. Improving the current responsivity of the photodetector is of a vital importance in this situation. This can be done, in particular, in longitudinal configuration of the photodetector, or by using planar asymmetric interdigitated system of electrodes.