L. Pottier

Nonlinear effects in acousto-optic imaging

Acousto-optic (AO) imaging is a promising technique that is able to reveal optical properties in the millimeter range inside scattering media by tagging the photon paths with an ultrasonic beam. To increase both the contrast and the resolution of the AO images, we have explored the possibility of using the nonlinear response of the speckle modulation. Variation of the second-harmonic signal as the square of the ultrasonic amplitude has been found, and strong reduction of the tagged zone size has been demonstrated.

In situ local tissue characterization and imaging by backscattering acousto-optic imaging

We use an imaging system taking advantage of a focused ultrasonic field to mark the photon trajectories in a biological sample by modulating the optical paths in the focal zone. We show the applicability of this setup to various geometrical configurations, such as forward and backward scattering. The modulated speckle detected on a CCD camera reflects, after parallel processing, the local optical properties of the tissue in the focal zone. We propose to use the backscattering geometry to estimate the local effective attenuation coefficient. Finally we compare images which have been obtained through several centimeters of biological tissue, both in forward and in backward geometry.

Parity violation in forbidden transitions: Detection of the electroweak alignment or polarization in the upper state by stimulated emission

Abstract We present a new method for detecting the parity violation induced by weak neutral currents in a forbidden atomic transition such as the 6 S-7 S Cs transition. The 7 S atoms are produced by pulsed excitation and detected through the subsequent transient gain which appears at the 7 S-6 P frequency. A right-left asymmetry in this gain is expected as a result of parity violation, with possible amplification of this asymmetry when the vapor becomes optically thick. With respect to previous observation conditions, substantial improvement in signal/noise ratio is expected.

Optical orientation, via spin-orbit coupling, in the final state of the stark induced 6S–7S cesium transition

Abstract In the excitation process of the forbidden 6S 1 2 –7S 1 2 Cs transition by a circularly polarized laser beam in a static electric field, spin-orbit coupling causes the creation of an electronic polarization in the upper state with a component not only along the beam, but also transverse to it. This effect is studied experimentally by observing the Hanle effect in the 7S 1 2 state on the 7S 1 2 →6P 1 2 fluorescence light, in various field configurations. This provides a unique way of determining the relative sign of the scalar part and spin-dependent vector part of the polarizability which connects the transition electric dipole with the electric field.

New measurement of the Cs 6S → 7S scalar to vector polarizability ratio

Abstract We have measured the ratio |α/β| of the scalar to vector polarizabilities of the forbidden 6S 1 2 → 7S 1 2 transition of cesium by a method free of background difficulties. Our result, | α / β | = 9.90 ± 0.10, agrees with that of Hoffnagle et al. The slight disagreement with our earlier result is examined.

Setup for simultaneous imaging of optical and acoustic contrasts in biological tissues

Acousto-optic imaging consists in tagging multi-scattered photon paths with a focused ultrasonic beam. This technique should give optical information on hidden structures in several centimeter thick scattering media, with a millimetric resolution. We have coupled our previous acousto-optic imaging setup with a suitably designed echograph. Thanks to a single 3 MHz multi-ring emitter, working either in pulsed or c.w. mode, we can get acoustic as well as acousto-optic responses of structures in biological tissues.

Stark interference effects in a weak magnetic field on the 6S–7S forbidden transition of cesium

Abstract Hyperfine mixing in a weak magnetic field gives rise to new interference terms between the Stark-induced amplitudes of the 6S–7S forbidden transition of cesium. Compared with typical allowed transitions, effects of unusual size (circular dichroism of ∼10% in a 10 G field) and/or symmetry (3% upper state spin polarization induced by linearly polarized light in a direction normal to both the beam and the magnetic field) are observed. The circular dichroism yields a new method to determine the scalar to vector polarizability ratio; our result (α/β=-10.0±0.15) agrees with those of other techniques. The analysis of systematic effects in our recent measurement of parity violation required an understanding of these hyperfine mixing effects. One of them will be utilized to calibrate a new version of the parity experiment.

Hanle effect detection of the 6S12−7S12 single-photon transition of cesium, without electric field

Abstract Although the oscillator strength of the 6S 1 2 −7S 1 2 transition is only 4.0×10-15, by the Hanle effect in a zero electric field this transition gives rise to a very specific signal well above noise and background. From the hfs profile, we can test the magnetic dipole nature of the transition and obtain direct evidence for its perturbation by hyperfine mixing between the two S states. Our new values of Mhf1/M1 and M1 and M1/β agree with previous results, and the accuracy of Mhf1/M1 is improved.

Measuring the frequency jitter and spectral width of nanosecond, single-mode laser pulses

A pulse-by-pulse analysis system provides a simple and direct method for measuring the pulse-to-pulse frequency jitter and the spectral width of nanosecond pulses from single-mode lasers, up to repetition rates of a few kHz. To test the method, the jitter (<or approximately=1 MHz) and bandwidth ( approximately 80 MHz) of a pulsed laser system at lambda =540 nm, was measured.

Doppler-free inhibited fluorescence spectroscopy of a forbidden three-level system

Abstract In a three-level system driven by two lasers, one of which connects a populated state to a state of same parity through a forbidden transition, stimulated emission is detected through its competition with fluorescence in a different transition sharing the same upper level. “Inhibited fluorescence” appears as a sensitive sub-Doppler spectroscopy well suited to this parity-forbidden three-level system. It is applied to the 6S 1 2 −7S 3 2 −6P 3 2 configuration of atomic cesium, in view of its interest for par ity violation measurements. The 6P 3 2 hfs is fully resolved and the collisional damping of the 7S−6P optical coherence is measured. A theoretical model reproduces the spectra satisfactorily.