Ph. Delaye

Transmission of time modulated optical signals through an absorbing photorefractive crystal

Abstract We solve the problem of the transmission of time modulated signals in a two-beam coupling photorefractive set-up using an absorbing material. We obtain an analytical expression that describes this phenomenon in absence of pump beam depletion due to beam coupling. We use this expression to analyse the influence of absorption on the photorefractive build-up. We also describe, using this model, the photorefractive beamsplitter response. It is also showed that the π 2 phase shift between illumination and index gratings is detrimental to a linear detection of the phase variation of the signal beam.

Correlations between microscopic properties and the photorefractive response for vanadium-doped CdTe

We present a summary of the experimental results, the attempts of interpretation at the present stage, and the conclusions of coupled investigations on semi-insulating, n-type and p-type vanadium-doped CdTe crystals. Combining electron paramagnetic resonance and magnetic circular dichroism permits the monitoring and the quantitative assessment of [V2+] and [V3+] defect concentrations. Photoabsorption results are presented and are discussed in terms of photoionization processes at various wavelengths. The temperature and the spectral dependence of the steady-state photoconductivity are carefully studied in the near infrared. The results are discussed in terms of electronic transitions for the two vanadium charge states. Finally, the photorefractive performances of the semi-insulating sample are analyzed in a two-beam-coupling experiment and are correlated to the properties of the V2+/V3+ donor states to derive photoionization cross sections.

Wavelength dependent effective trap density in CdTe : Evidence for the presence of two photorefractive species

An accurate determination of the effective trap density in CdTe:V is performed using both counterpropagating (small grating spacing) and copropagating (large grating spacing) two wave mixing. This enhanced precision allows to show a variation of the effective trap density with wavelength. We show that such a change is explained when two traps both coupled to the conduction and the valence bands are present. New theoretical expressions for the electron hole competition coefficient and Debye screening length are derived and used

Comparative study of CdTe and GaAs photorefractive performances from 1 μm to 1.55 μm

Abstract We present a photorefractive investigation of the vanadium doped CdTe at different wavelengths from 1.06 μm to 1.55 μm. The sensitivity and performances of different samples grown with different conditions are deduced. We compare these performances with the ones of the undoped GaAs in the same wavelength range.

Photorefractive vibrometer for the detection of high-amplitude vibrations on rough surfaces

We present and characterize theoretically and experimentally a photorefractive velocimeter. This device, based on two-wave mixing in a rapid photorefractive crystal, measures the instantaneous velocity of a vibrating target. It is particularly adapted to the measurement of high-amplitude (up to several mm) low-frequency (up to several kHz) vibrations. Instantaneous velocities as high as 25 mm s-1 are expected to be measured with common photorefractive semiconductors and CW lasers.

Bulk Photorefractive Semiconductors

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Bulk Photorefractive Semiconductors Philippe Delaye, H.J. Von Bardeleben, Gérald Roosen

Time resolved nonlinear spectroscopy at the band edge of 1D photonic crystals

Large refractive index changes have been measured at the band edge frequency of 1D photonic crystals (PCs). Results concerning both thin and thick samples of high and low refractive index contrasts, respectively, are presented. The very large value of the refractive index changes obtained at moderate pump powers thanks to the strong enhancement of the local intensity inside the PC opens the way to very small volume devices for optical signal processing. However, time resolved experiments demonstrate the photo-generation of high free carrier densities through two- or even three-photon absorptions, which are shown to be also strongly enhanced at the band edge of the PC. This drawback may most probably be circumvented by using lower pump intensities in PCs showing narrower resonances.

Diffraction and two-beam-coupling in GaAs along the [111]-direction

Abstract Diffraction and two-beam-coupling from mixed, i.e. phase and absorption gratings, with a grating vector in the (111)-plane are analyzed theoretically for cubic crystals of point symmetry 43 m . The diffraction efficiency does not depend on the grating orientation in the plane. Polarization of the diffracted beams varies with the grating orientation.

Implementation and comparative evaluation of various architectures of ultrasonic photorefractive sensors

A novel optical sensor for the detection of ultrasonic motion has been recently developed. It allows to reach 25 μm displacement sensitivity with a large frequency range bandwidth 100 MHz. We will present a comparative study between various architectures of this ultrasonic detection system we have implemented. It uses InP:Fe or CdZnTe:V holographic crystals and operates with CW laser at 1.06 μm. Two configurations called the direct detection and the anisotropic diffraction configurations work with either plane or speckled waves. A third discussed configuration works with depolarized speckled waves. We measured their relative detection limits as a function of the applied electric field that governs photorefractive efficiency of the materials. Experimental results are well described by theory, using photorefractive models relevant to the used crystals

EXPERIMENTAL INVESTIGATION OF A PHOTOREFRACTIVE RING PHASE MODULATION AMPLIFIER

Abstract We present the experimental investigation of a photorefractive ring phase modulation amplifier, that is not resonant and that can work with distorted beams. We show that with this geometry an amplification by a factor 5 of a phase modulation signal is possible. We also show that this strong amplification is alleviated by the presence of losses in the ring. These losses lead to a real gain in signal to noise ratio of a factor 3, that nevertheless expresses that with such a system the photon noise limit of laser ultrasonics systems can be overpassed. The losses are also responsible for the strong sensitivity of the system to the vibrations, which puts a strong limitation on the ring amplifier.