François Ramaz

Magnetic circular dichroism and the optical detection of magnetic resonance for the Bi antisite defect in Bi12GeO20

We report on the optical detection of electron paramagnetic resonance at 36 GHz, 67 GHz and 70 GHz for all the absorption bands of Bi12GeO20 (BGO) in the visible spectral range. This completes our previous studies and demonstrates clearly that the three major magnetic circular dichroism features do belong to the same isotropic centre, i.e., a Bi3+Ge+h defect with the hole being delocalized onto the four surrounding oxygen atoms. We provide a simple explanation for the unusual sign of ODMR and the transient behaviour of the magnetic circular dichroism at resonance.

Charge transfer in photorefractive CdTe:Ge at different wavelengths

Abstract The charge transfer processes in photorefractive CdTe:Ge were modeled using the data of optical absorption, magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectroscopies. Within the developed model the variations in the photorefractive properties of different CdTe:Ge samples are explained by differences in the relative concentrations of donor and trap centers. The existence of two different centers of comparable concentrations, each in two charge states, allows charge redistribution between them and gives rise to optical sensitization of some CdTe:Ge samples for photorefractive recording under an auxiliary illumination. In the present article we follow the proposal of pseudo-3D presentation of light-induced absorption to distinguish the main charge transfer processes at different excitation energies and explain the sensitization of CdTe:Ge for photorefractive recording at 1.06, 1.32 and 1.55 μm by light with appropriate wavelength.

Magnetooptical characterization of ligand field bands and charge transfer processes in sillenite oxides

This paper contains two main sections. In the first one, we illustrate the technological interest of crystals of the sillenite family and we present our powerful methods of investigation in some details. These are essentially optical absorption and magnetic circular dichroism at liquid helium temperatures, down to 4000 cm-1. The latter is also used to detect electron paramagnetic resonance optically and to tag a given absorption component to a specific paramagnetic, intrinsic or extrinsic, defect. In the second part, we present a variety of experimental results and interpretations dealing with undoped crystals and with samples doped with transition ions of the iron (Mn, Cr, Fe, Co, Cu) group. Emphasis is put on the assignment of ligand field bands in the near infrared, since these are used to monitor the site, charge state, and amount of the dopants in a thermally bleached state or after various illuminations.

Monitoring of V2+ and V3+ in vanadium-doped CdTe : a combined magnetic circular dichroism and electron paramagnetic resonance approach

Magnetic circular dichroism (MCD) is demonstrated to allow the simultaneous characterization of the V2+ and V3+ ions in vanadium-doped Bridgman grown CdTe bulk samples. The identification of the MCD bands relies on the analysis of n- and p-type co-doped samples as well as on spectroscopic arguments. A direct correlation of the strength of certain MCD bands with the V3+ concentrations determined by electron paramagnetic resonance spectroscopy allowed the calibration of our spectra for V3+. Based on photo-MCD experiments, we also provide a calibration of MCD bands related to V2+. Finally, the [V2+]/[V3+] ratio can be derived simply from the shape of the MCD spectrum in the low-energy region (E<1.1 eV).

Spectroscopic characterization of photorefractive CdTe : Ge

Abstract We report on the photoabsorption, magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) of Ge-doped CdTe. Strong photoinduced absorption changes have been detected for two samples at ambient and low temperatures (down to 4.3xa0K). They reveal the existence of four absorption bands around 0.94, 1.10, 1.22 and 1.36xa0eV at 4.3xa0K. MCD experiments at 1.4xa0K allowed the tagging of different paramagnetic defects to the two bands at lower energies. EPR demonstrated that one of them is Ge+Cd. We propose that the second one is an isolated or Ge-related A-centre. A diagram showing the various transport processes is given.

Absorption and MCD study of photochromism in Bi12SiO20 doped with 4D or 5D transition metal ions

We present the first spectroscopic investigation concerning the role of four ions (Ru, Rh, Os, Re) of the platinum group on the photochromic behavior of the photorefractive crystal Bi12SiO20. Absorption and magnetic circular dichroism (MCD) spectra were taken in the thermally bleached state and after various illuminations so as to determine the coloration thresholds. Ruthenium and Osmium are demonstrated to enter the crystal, whereas the answer is not yet certain in the case of rhodium and rhenium. We believe that the doping occurs at the pseudo-octahedral Bi site.

Optical and magnetooptical study of photorefractive germanium-doped cadmium telluride

Abstract A photorefractive CdTe:Ge single crystal was investigated via photo-absorption at room temperature, 98 K and 4.3 K, and magnetic circular dichroism at 1.4 K. At all wavelengths, except near 1064 nm, absorption spectra suffer relatively large changes under various illuminations. Magnetic circular dichroism demonstrates unambiguously the presence of two paramagnetic defects, one of them being the ionised donor Ge + at the Cd site. Our results further reveal the existence of four absorption components around 0.94 eV, 1.10 eV, 1.22 eV and 1.36 eV. The two extreme ones are assigned to the photoneutralisation of Ge Cd + and to the photoionisation of Ge Cd 0 respectively.

Fourier transform acousto-optic imaging with a custom-designed CMOS smart-pixels array

We report acousto-optic imaging (AOI) into a scattering medium using a Fourier Transform (FT) analysis to achieve axial resolution. The measurement system was implemented using a CMOS smart-pixels sensor dedicated to the real-time analysis of speckle patterns. This first proof-of-principle of FT-AOI demonstrates some of its potential advantages, with a signal-to-noise ratio comparable to the one obtained without axial resolution, and with an acquisition rate compatible with a use on living biological tissue.

Multi‐modal acousto‐optic/ultrasound imaging of ex vivo liver tumors at 790 nm using a Sn2P2S6 wavefront adaptive holographic setup

Biological tissues are very strong light-scattering media. As a consequence, current medical imaging devices do not allow deep optical imaging unless invasive techniques are used. Acousto-optic imaging is a light-ultrasound coupling technique that takes advantage of the ballistic propagation of ultrasound in biological tissues to access optical contrast with a millimeter resolution. We have developed a photorefractive-crystal-based system that performs self-adaptive wavefront holography and works within the optical therapeutic window. As it works at an appropriate wavelength range for biological tissues imaging, it was tested on ex vivo liver samples containing tumors as a pre-clinical study. Optical contrast was obtained even if acoustical one was not significant. Ultrasound image (left) and acousto-optic image (right) of a liver biopsy with tumors. Acousto-optic imaging exhibits tumors that are not detected through ultrasound.

Fast wavefront adaptive holography in Nd:YVO4 for ultrasound optical tomography imaging.

Several approaches exist to perform acousto-optic imaging of multiple-scattering media such as biological samples. Up to now, most of the coherent detection methods use holographic setup based on photorefractive crystals such as BSO or SPS. One of the issue of these techniques is the moderate response time compared to the speckle decorrelation time in biological sample. We introduce a new approach for the holographic detection based on two-wave mixing in a Nd:YVO4 gain medium enabling us to perform a fast wavefront adaption (50 μs) of the speckle field from a multiple-scattering sample.