P. Langot

Coherent acoustic vibration of metal nanoshells

Using time-resolved pump-probe spectroscopy, we have performed the first investigation of the vibrational modes of gold nanoshells. The fundamental isotropic mode launched by a femtosecond pump pulse manifests itself in a pronounced time-domain modulation of the differential transmission probed at the frequency of nanoshell surface plasmon resonance. The modulation amplitude is significantly stronger, and the period is longer than that in a gold nanoparticle of the same overall size, in agreement with theoretical calculations. This distinct acoustical signature of nanoshells provides a new and efficient method for identifying these versatile nanostructures and for studying their mechanical and structural properties.

Second-order non-linear optical response of metallo-organic compounds: towards switchable materials

A series of coordination compounds, [M(PM-L)2(NCS)2], with large aromatic ligands (PM-L) and a divalent metal ion (MII=Mn, Fe, Co, Ni and Zn) have been synthesized. The magnetic data are consistent with divalent metal ions in an octahedral ligand field environment. Hyper-Rayleigh scattering measurements show an enhancement of the molecular hyperpolarizabilities (β) from Ni compounds (with two unpaired electrons) to Mn compounds (five unpaired electrons). In the case of the Fe(II) metal ion, the influence of the ligand on the β values is reported. This work provides attractive information for the design of switchable materials based on the spin-crossover phenomenon.

Optical properties and relaxation processes at femtosecond scale of bimetallic clusters

The optical properties of Au-Ag and Ni-Ag clusters are measured by linear optical absorption spectroscopy and the time-resolved pump-probe femtosecond technique allowing a study of the influence of alloy or core-shell structure.

Acoustic Vibration Modes and Electron–Lattice Coupling in Self-Assembled Silver Nanocolumns

Using ultrafast spectroscopy, we investigated electron-lattice coupling and acoustic vibrations in self-assembled silver nanocolumns embedded in an amorphous Al2O3 matrix. The measured electron-lattice energy exchange time is smaller in the nanocolumns than in bulk silver, with a value very close to that of isolated nanospheres with comparable surface to volume ratio. Two vibration modes were detected and ascribed to the breathing and extensional mode of the nanocolumns, in agreement with numerical simulations.

Acoustic vibrations of Au nano-bipyramids and their modification under Ag deposition: a perspective for the development of nanobalances.

We investigated the acoustic vibrations of gold nanobipyramids and bimetallic gold-silver core-shell bipyramids, synthesized by wet chemistry techniques, using a high-sensitivity pump-probe femtosecond setup. Three modes were observed and characterized in the gold core particles for lengths varying from 49 to 170 nm and diameters varying from 20 to 40 nm. The two strongest modes have been associated with the fundamental extensional and its first harmonic, and a weak mode has been associated with the fundamental radial mode, in very good agreement with numerical simulations. We then derived linear laws linking the periods to the dimensions both experimentally and numerically. To go further, we investigated the evolution of these modes under silver deposition on gold core bipyramids. We studied the evolution of the periods of the extensional modes, which were found to be in good qualitative agreement with numerical simulations. Moreover, we observed a strong enhancement of the radial mode amplitude when silver is deposited: we are typically sensitive to the deposition of 40 attograms of silver per gold core particle. This opens up possible applications in the field of mass sensing, where metallic nanobalances have an important role to play, taking advantage of their robustness and versatility.

Femtosecond investigation of the non-instantaneous third-order nonlinear suceptibility in liquids and glasses

The symmetry of the noninstantaneous Kerr susceptibility is investigated in different liquids and glasses using a time-resolved two-beam energy transfer technique with pulses in the 100–15 fs range. The pulse duration dependence of the ratio of the isotropic χxxxx(3) to anisotropic χxyyx(3) tensor elements is demonstrated in liquid CS2 and in silica, showing that the elementary mechanisms mediating the observed optical beam energy transfers are modified for short pulses. This effect is ascribed to alteration of the nuclear contribution and increasing influence of the electronic one.

Femtosecond spectroscopy of defect modes in silica glasses

The D1 and D2 defect modes are investigated in the time domain in normal and densified silica using a high sensitivity femtosecond pump-probe technique with 15fs pulses. Their frequency and width are precisely determined even when they are partly masked by a broad background in spontaneous Raman spectroscopy. These results open up many possibility for investigation of local structures in glasses.

Measurement of non-instantaneous contribution to the χ(3) in different liquids using femtosecond chirped pulses

Abstract In the Born–Oppenheimer approximation and considering a Debye nuclear motion, a theoretical computation of pump–probe two-beam coupling in liquids using femtosecond chirped pulses is proposed. This technique makes it possible to specifically evidence the non-instantaneous contribution to the third-order susceptibility χ (3) . Our model, which is an extension at the femtosecond scale of the one proposed by Dogariu et al., describes the temporal evolution of the probe signal as a function of different parameters such as the linear laser chirp, the ratio between the pulse duration and the nuclear response time. Experimentally, this method is applied to characterize the non-instantaneous χ (3) contribution in transparent liquids such as CS 2 , benzene and toluene. Time resolved pump–probe coupling data using parallel and perpendicular linear polarizations fit well with the model developed. The experimental ratio R between both fast and slow non-instantaneous χ (3) XXXX and χ (3) XYYX elements of the tensor is equal to 1.33±0.01 in all the liquids studied, and is in good agreement with the expected liquid nuclear symmetry.

High repetition rate nonlinear generation of synchronized frequency tunable femtosecond pulses

Abstract Pulses delivered by a high power Ti:sapphire femtosecond oscillator are frequency broadened in an optical fiber and subsequently filtered to generate two synchronized frequency tunable femtosecond pulses at a repetition rate of 76 MHz. The created pulses are transform limited with a duration in the range 70–120 fs, and independently tunable over 220 nm. The possibilities offered by this source for high sensitivity two-color femtosecond spectroscopy are demonstrated in GaAs.

Cold-phonon effect on electron heating in GaAs

Abstract Heating of cold electrons is investigated in bulk GaAs at room temperature using a femtosecond high sensitivity absorption saturation technique. The electron thermalization time is found to increase with the photoexcited carrier density in the range 5 × 1015−3 × 1017cm−3. This effect is attributed to LO phonon underpopulation and is shown to be controlled by the LO phonon lifetime in agreement with a numerical simulation of the nonequilibrium carrier dynamics.