F. Hache

Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots: assignment of the first electronic transitions

CdSe is used as a prototype to show the implications of valence-band degeneracy for the optical properties of strongly quantum-confined nanocrystals. Absorption spectra and photoluminescence spectra obtained under intermediate and strong pulsed excitation show the presence of new structures. The energy levels for the electron and the hole are calculated with the spherical confinement, the nonparabolicity of the conduction band, and the valence band degeneracy taken into account. The oscillator strengths of the dipole-allowed transitions are also calculated. This model is found to be in good agreement with the experimental observations, which originate mainly from the quantization of the energy spectrum of holes with due account given to valence-band degeneracy.

Optical nonlinearities of small metal particles: surface-mediated resonance and quantum size effects

Time- and frequency-dependent measurements of optical phase conjugation in gold colloids are reported. It is shown that the nonlinear response is fast on a 5-psec time scale and can be assigned to the electrons of the small metal spheres. The frequency-dependent measurements were performed in the neighborhood of the surface-plasma resonance and give experimental evidence of the validity of our resonant enhancement model. We also report the first model calculation of the third-order Kerr susceptibility of small metal particles. The dominant terms of this electric-dipole contribution are emphasized and lead to an expression for χ(3) that varies roughly as the inverse third power of the radius of the particles. This model accounts for the observed anisotropy of this susceptibility and provides an estimate of its magnitude, in agreement with the measured values. The limited size effect on the linear susceptibility is also discussed.

The optical kerr effect in small metal particles and metal colloids: The case of gold

We first consider theoretically the various mechanisms contributing to the Kerr nonlinearity in small gold particles. The major ones are the conduction electron intraband contribution, the saturation of direct interband transitions and the change in dielectric constant due to hot photoexcited electrons. We present experimental results obtained using optical phase conjugation in gold-doped glasses. By varying several parameters, we were able to ascertain the origin of the nonlinear response: the main contributions are the hotelectron and the interband contributions. All experimental results, including saturation behaviour, are fully understood.

Sub-20-fs tunable pulses in the visible from an 82-MHz optical parametric oscillator

We have produced pulses tunable in the 590-666-nm range, with durations down to 13 fs, using an 82-MHz Ti:sapphire second-harmonic-pumped, high-bandwidth, beta-barium borate optical parametric oscillator in a fused-silica prism group-delay-dispersion-compensated, six-mirror folded ring cavity.

Carbon nanotubes for optical limiting

This paper reviews the optical limiting properties of carbon nanotubes. The nonlinear optical properties of nanotubes were investigated in water and in chloroform suspensions. Nonlinear transmittance measurements were reported for various pulse durations and wavelengths and show that carbon nanotubes are good candidates for effective optical limiting over broad temporal and laser energy ranges. Z-Scans and pump-probe time-resolved experiments were achieved to identify the origin of optical limiting in nanotubes. The main phenomenon is a strong nonlinear scattering, originating from solvent vapour bubble growth and sublimation of nanotubes at high fluences. Heat transfer from particles to solvent is particularly effective as compared to carbon black suspensions because of the large surface area of the carbon nanotubes.

Ti:sapphire second-harmonic-pumped visible range femtosecond optical parametric oscillator

Abstract A β-barium borate (BBO) femtosecond optical parametric oscillator (OPO) pumped by the second-harmonic of a modelocked 82 MHz Ti:sapphire laser is described. With intracavity dispersion compensation, pulse durations down to 30 fs are obtained with a total average power up to 100 mW. The current wavelength range, with a 400 nm pump, is from 566 nm to 676 nm. Unusual tuning characteristics, including bichromatic emission, are presented in detail and explained theoretically.

SINGLE-WALL CARBON NANOTUBES FOR OPTICAL LIMITING

Abstract We report on the non-linear optical transmittance of single-wall carbon nanotubes (SWNT) in a water/surfactant suspension. Optical limiting is observed at low-energy thresholds both in the visible and near-infrared. We find small non-linear thresholds and large optical densities that match or overpass the performances of other good optical limiters (C60 and carbon black) both in the visible and in the near-infrared, which makes SWNT very promising systems for broadband optical limiting.

Optical limiting properties of singlewall carbon nanotubes

We report on the optical limiting performances of singlewall carbon nanotubes. The nonlinear transmission of the samples is investigated using nanosecond Nd:YAG laser pulses at 532 nm and 1064 nm. Z-scan experiments are carried out at different incident energies in order to identify the effects responsible for nonlinear transmission. We find that nonlinear scattering and nonlinear refraction are the dominant mechanisms. Optical limiting efficiencies are compared with those of multiwall nanotubes, carbon black suspensions and fullerenes.

Time‐resolved measurements of carrier recombination in experimental semiconductor‐doped glasses: Confirmation of the role of Auger recombination

Using three different time‐resolved techniques: degenerate four wave mixing, nonlinear absorption, and luminescence, we show that, for our CdSxSe1−x‐doped glasses, under high excitation conditions, Auger recombination is the dominant recombination process. This explains the broad range of luminescence lifetimes previously reported. After photodarkening, we also observe a reduction in the nonradiative lifetime.

Quantum‐confined Stark effect in very small semiconductor crystallites

Experimental study of the change δα in the absorption spectrum due to a static electric field is reported for very small CdSSe crystallites. We observe oscillations of δα as a function of the wavelength, which are well explained as a Stark effect for the quantized electronic levels. A perturbation calculation is performed, which gives good theoretical fits of the experimental curves.