G. Grillon

Non-thermal melting in semiconductors measured at femtosecond resolution

Ultrafast time-resolved optical spectroscopy has revealed new classes of physical, chemical and biological reactions, in which directed, deterministic motions of atoms have a key role. This contrasts with the random, diffusive motion of atoms across activation barriers that typically determines kinetic rates on slower timescales. An example of these new processes is the ultrafast melting of semiconductors, which is believed to arise from a strong modification of the inter-atomic forces owing to laser-induced promotion of a large fraction (10% or more) of the valence electrons to the conduction band. The atoms immediately begin to move and rapidly gain sufficient kinetic energy to induce melting—much faster than the several picoseconds required to convert the electronic energy into thermal motions. Here we present measurements of the characteristic melting time of InSb with a recently developed technique of ultrafast time-resolved X-ray diffraction that, in contrast to optical spectroscopy, provides a direct probe of the changing atomic structure. The data establish unambiguously a loss of long-range order up to 900 Å inside the crystal, with time constants as short as 350 femtoseconds. This ability to obtain the quantitative structural characterization of non-thermal processes should find widespread application in the study of ultrafast dynamics in other physical, chemical and biological systems.

Polarization selectivity in time-resolved transient phase grating

Abstract A selective polarization approach to transient phase grating experiments is presented. The high potentiality of such a technique lies in the fact that nonlinear signals show not only a temporal behaviour but also symmetry properties associated to each physical process involved, through the polarization states of the four coming beams. Subpicosecond resolution measurements of the kinetics of liquid CS 2 transient anisotropy have been performed in this way. We demonstrate that even using ultra short pulses, polarization analysis alone allows full discrimination between electronic and molecular processes, both of which intervene in the induced nonlinearity. Futhermore, a temporal delay in the onset of the molecular processes is evidenced for the first time. It is accounted for by a non instantaneous orientation of the molecules along the actinic optical field.

Generation of high-peak-power tunable infrared femtosecond pulses in an organic crystal: application to time resolution of weak infrared signals

High-intensity femtosecond pulses tunable in the 0.8–1.6-μm range have been generated by parametric amplification of a continuum white light in a new organic crystal, N-(4-nitrophenyl)-L-prolinol (NPP). The traditional concept of noncritical phase matching was revised in view of requirements linked to the observation of ultrafast subpicosecond nonlinear phenomena. The notions of θ (noncritical) and λ (noncritical) phase matching are introduced together with their applications. An experimental determination of phase-matching curves for both second-harmonic generation and three-wave mixing has been carried out. A θ noncritical phase-matching configuration for second-harmonic generation at 1.15 μm and a quasi-λ noncritical phase-matching configuration in the near IR for three-wave mixing were evidenced. Frequency and pump-intensity dependences of the gain have also been studied. Parametric emission at degeneracy was observed, with the emitted bandwidth extending from 1.0 to 1.4 μm. Time resolution of the amplified signal has been carried out by cross correlating the pump with the incoming signal, evidencing a reduced time broadening of the interacting pulses; a new spectroscopic method with subpicosecond time resolution is derived from the previous nonlinear optical characterization experiments by replacing the IR continuum from the water cell by any sample emitting in the same frequency range. This method, termed parametric amplification and sampling spectroscopy, was used for temporal analysis of amplified and emitted infrared signals generated in an NPP crystal.

Generation of high peak power subpicosecond pulses in the 1.0–1.6 μm range by parametric amplification in an organic crystal

High peak power subpicosecond optical pulses have been generated by parametric amplification of a white‐light femtosecond continuum in a new organic crystal, N‐(4‐nitrophenyl)‐L‐prolinol or (NPP), with an emission spectrum extending in the 1.0–1.6 μm range.

Electronic nonlinear optical susceptibilities of silicate glasses.

Femtosecond techniques have been used to measure the electronic contribution to the nonlinear susceptibility of several standard glasses. The relation between their nonlinear efficiency and the content of network-modifying cations has been determined.

Cross-correlation measurements of femtosecond extreme-ultraviolet high-order harmonics

He atoms have been ionized by a combination of two pulses the fundamental (800 nm, 150 fs) from a Ti:sapphire laser, and its 21st harmonic (38 nm). The resulting above-threshold ionization spectra comprise peaks that scale as the cross-correlation function, which can be mapped out by variation of the time delay between the two pulses. Using this technique, we have carried out the first measurement, to our knowledge, of the duration of high-order harmonic pulses with subpicosecond resolution. The result indicates a duration of 40 fs. This duration is much shorter than the duration of the fundamental pulse, in agreement with existing model calculations.

Generation of 27 fs pulses of 70 kW peak power at 80 MHz repetition rate using a cw self‐pulsing Ti:sapphire laser

We have built a self‐mode‐locked Ti:sapphire cw oscillator delivering up to 2 W of average output power. This oscillator provides 100 fs pulses tunable in the 770–807 nm range. Using a fiber‐prism compressor at the cavity output, the pulses have been compressed down to 27 fs at 785 nm, while keeping 200 mW average power.

Femtosecond multiphoton generation of the self‐trapped exciton in α‐SiO2

Nonlinear optical excitation of crystalline quartz with intense femtosecond UV pulses yields the 2.8 eV recombination luminescence of the self‐trapped exciton. The relation between the excitation and emission intensities reveals two‐ and three‐photon kinetics for photon energies of 4.4 and 4.0 eV at excitation densities below 1018 cm−3. These power laws are not sizably influenced by transient linear absorption, self‐focusing, and filamentation.

Theoretical and experimental studies of laser-produced plasmas driven by high-intensity femtosecond laser pulses

A theoretical and experimental study of the dynamics of the electron density gradient in near-solid-density plasmas produced by the interaction of ultra-short laser pulses with solid targets at intensities between 1013 and 1016 W/cm2 and pulse duration between 0.12 and 2.5 ps is presented. X-ray spectroscopy of n=3 to n=1 resonance and dielectronic satellite lines is used to determine the range of electron densities in the plasma. Frequency-domain interferometry is employed to measure the expansion velocity and the electron density gradient scale length as a function of laser pulse duration and intensity. Quantitative agreement is noticed with one-dimensional hydrodynamic simulations which include the solution of the wave equation for the laser field.

Third-order electronic susceptibilities of liquids measured by femtosecond kinetics of optical Kerr effect

Intense femtosecond optical pulses are used to discriminate temporally among the different processes involved in the third-order-induced polarization. The tensor elements deduced from the instantaneous measured components are compared with previous frequency-domain experimental results.