Charles Hirlimann

Femtosecond white-light continuum pulses

We obtain gigawatt white-light continuum pulses that permit spectroscopic measurements with a time resolution of 80 fsec. These pulses extend continuously from 0.19 to 1.6 microm and have time sweeps as small as 10 fsec/1000 A. We find temporal, spatial, and spectral properties that are consistent with self-phase modulation having a prominent role in generation of the continuum.

Interferometric measurements of femtosecond group delay in optical components

We report direct measurements of the frequency dependence of the optical group delay for a number of optical components commonly used in femtosecond optics. We have investigated the group-delay errors that occur on reflection from metal and dielectric mirrors under various conditions and passage through devices that introduce angular dispersion. We obtain measurement accuracy of about ±1 fsec over the spectral range of 400–750 nm.

Photonic jet driven non-linear optics: example of two-photon fluorescence enhancement by dielectric microspheres.

The two-photon excited fluorescence from a dye solution is enhanced when a small amount of micro-meter sized silica beads are added. This observation is made in the simple scattering regime (inter-sphere distance four times larger than their radius) and is shown to depend on the concentration of the silica spheres. For a solution of rhodamine B, the enhancement can reach more than 30 %. As complementary experiments show that the fluorescence efficiency is unchanged, we argue that the non-linear absorption is enhanced due to focussing of the incident beam in the near-field of the spheres, a situation previously referred to as photonic (nano-)jets [3]. Our calculations indeed show that for the parameters of the spheres studied near-field focussing leads to an intensity concentration close to the sphere surface. We suggest that these photonic jets could be used to enhance other non-linear optical effects.

Two-dimensional colloid-based photonic crystals for distributed feedback polymer lasers

We report on a process to design highly ordered monolayers of two-dimensional photonic crystals, made of silica nanoparticules, that can be used for the development of organic optical devices. We have used a photopolymerization process to incorporate a dye gain medium into the nanoparticle layers in order to achieve a laser cavity. The high spatial coherence of the deposits allows for single-mode laser emission in the plane of the layer when the light excitation is perpendicular to the plane. Such periodic films should help in reducing the number of layers needed for future electrically pumped distributed feedback lasers.

Resonant light scattering

Abstract Resonant light scattering and hot luminescence are discussed in the unified framework of resonant secondary emission. Two examples are given: one concerning the resonant light scattering in GaSe, a lamellar semiconductor, which shows clearly a phonon cascade resonance process which is mediated by free excitons; the other concerns semiconducting alloys where the resonance process shows replica of the first order Raman spectrum with an intermediate state constituted by an exciton LO phonon complex.

A tunable femtosecond pulses amplifier

Abstract We have designed a multipass amplifier combined with a folded prisms optical compressor that generates 80 fs light pulses tunable between 580 and 630 nm when using sulforhodamine B in water. The external energy source is a 20 Hz repetition rate, doubled Nd: YAG laser, delivering 8 ns pulses. A numerical model has been developed that describes the dynamics of gain saturation and amplified spontaneous emission growth in a multipass pulses amplifier. This model predicts a gain Q-switched behavior of the multipass amplifier for high pumping levels.

Self-controlled growth and two-dimensional ordering of metallic nanoparticles

The control of synthesis, stabilization, and organization of metallic nanoparticles is one of the most active subjects in condensed matter science because of possible applications in high performance technology. A route to prepare a collection of ordered nanoparticles in large quantities consists of using the original combination of soft chemistry and surface science. A metal thin film deposited onto a self-assembled two-dimensional silica bead array can restructure through annealing. Pure metallic nanoparticles do form in an ordered array offering the opportunity of studying properties of an individual nanoparticle, thanks to a natural amplifier effect.

3 D Chemical Distribution of Titania–Alumina Catalyst Supports Prepared by the Swing-pH Method

Analytical electron tomography combined with more classical techniques such as X‐ray fluorescence and X‐ray photoelectron spectroscopy were performed with a series of titania–alumina catalyst supports prepared by using the swing‐pH method. The bulk proportion of titania and alumina was varied along the sample series. It was observed that, independently of this bulk proportion, only 30 % of the surface of the grains of the resulting catalysts was titania covered. It was also shown that the porosity of the material was driven by alumina, and for low titania contents, alumina formed a layer close the surface of the grains. At concentrations higher than 30 % titania, the layer was broken and aggregates covered by alumina were formed, which may have an impact on catalysis if these materials are used as supports for an active phase.

Resonant Raman scattering on the NN3 level of GaP:N

By tuning the excitation energy through the exciton bound to a NN3 pair ground-state energy, in GaP:N at 5K, the resonant Raman behaviour for the bulk LO mode of the crystal is obtained. Because the electronic levels associated with nitrogen in GaP are highly localised, a simple expression for the Raman cross section can be derived from the general Loudon expression. A calculation of the cross section is made, which involves three sublevels of the NN3 electronic states and then compared with the experimental results. This experiment shows interference between the three Raman contributions.

Interferometric detection of cross‐phase modulation in CdS

Using short 130 fs light pulses in a pump‐probe experiment we measured the transmission change under a strong irradiation (≊5 GW/cm2) of a Fabry–Perot‐like CdS sample, at room temperature, Using a simple model we deduced a nonlinear index n2=5×10−13 cm2/W. This index is only due to the electric field effect of the pulse for it is measured in a time shorter than the electronic relaxation time. We show that this experimental method could be used for measuring the duration of ultrashort light pulses at their fundamental frequency with intensities less than a few GW/cm2.