François Kerhervé

Applications des methodes d'optique non-lineaire a l'etude des films plastiques

Resume The dielectric strength of some plastic films is of the order of 3 × 10 6 Vcm −1 . This exceptional circumstance allows us to observe conveniently non-linear optical effects associated with the presence of a strong static field. Under certain conditions which we shall state precisely the effects are irreversible.

Bremsstrahlung and photoconduction in dense gases

Abstract We report bremsstrahlung experiments in dense gases. The first conduction electrons required for these observations are obtained by a two-steps ionization process acting on dopant molecules.

Emissions Optiques Et Electroniques De Solides Intensement Eclaires

Abstract In order to analyze the processes involved in the optical and electronic emissions induced on various targets by laser pulses, we have developed pyrometric methods having a time resolution of 10 −8 s. The initial temperature of the target is an important parameter which we control over a wide range. Our measurements suggest that many non-linear emission effects shown by various metals come from a non-metallic superficial layer and that they may not be accounted for by a simple heating effect. On tungsten and platinum, this superficial layer is easily destroyed in vacuum around 1200°C, which brings to the two photons photoelectric yield a decrease of at least three orders of magnitude.

A laser system for optical waves mixing experiments

Abstract We built a neodymium glass laser delivering two simultaneous pulses having a frequency separation tunable by steps from 10 to 90 cm-1. We used it to measure non-linear polarizabilities in glases and carbon disulfide by conventional coherent second Stokes and anti-Stokes techniques with unusual spectral filtering requirements. With the device described one can study in the 10-13–10-14 s range the kinetics of effects induced by the square of an electric field.

Elements optiques utilisant des gaz en vibration

Abstract We have studied the optical properties of cylindrical resonant cavities filled with gases in which radial acoustical standing waves are excited. When the oscillation mode is of the Bessel J 0 kind, the system is somewhat equivalent to a lens alternately convergent and divergent. With these optical components, it is easy to modulate light and to trigger lasers of any colour.

Laser-induced amplification and visualization of β-ray ionized tracks in dense gases

Abstract We describe some experiments on Bremsstrahlung energy transfer from an intense laser light towards electrons sorrounded by a dense gas. The electrons gain eventually enough energy to radiate light and to excite and ionize the surrounding atoms. We have measured the spectra of the light emitted in these conditions and the electron number multiplication factor resulting from successive ionization. In krypton we observe that a static field enhances significantly the rate of energy transfer from light to electrons. At the pressure of 200 bar we obtained controllable electron multiplication factors up to 106. At this level of amplification the ionized track of a 90Yβ-ray becomes visible. This latent electronic image development takes place in 10−8s.

Etude d'impulsions lumineuses breves par interferometrie a deux ondes

Abstract We have studied the evolution of the light transmitted through matter in which an optical breakdown is taking place. With a two-waves interferometer made a birefringent plate between polarizers we get informations about the time correlation function of the fields. With an interferometer made of two identical harmonic generating plates separated by a dispersion medium, we learn about the correlation function of the square of the fields. Both systems act as spectral filters with a notch cosine-shaped centered at the incident frequency. We have also observed a transient angular spreading of the beam not previously emphasized which brings a further complication in the picture.

Application of stimulated Raman scattering to brightness measurements of thermal sources of light

Abstract We have studied the efficiency of the Stokes to anti-Stokes conversion process in dispersionless Raman-active media. Experiments show under which conditions this efficiency is large enough to make possible fast measurements of brightness temperatures in the 105 K range.

Influence d'un fort champ electrique statique sur le spectre d'absorption du polyester mylar

Abstract Electrical fields induce a shift towards lower frequencies of the absorption spectrum of mylar, this shift, proportional to the square of the field, is three orders of magnitude smaller than a typical Franz-Keldysh effect and is sensitive to the relative orientation of the static field with respect to the polarization vector of the light. We suggest that the measured shift is mostly due to Stark effects in this amorphous material. This kind of measurement could be used to estimate the degree of crystalline order in partially amorphous solids.