F. Vallée

Coherent acoustic mode oscillation and damping in silver nanoparticles

Using a femtosecond pump-probe technique, the fundamental mechanical radial mode of silver nanoparticles is coherently excited and probed via its interaction with the electron gas. The mechanical oscillations are launched by an indirect displacive process and are detected via the induced modulation of the surface plasmon resonance frequency. The measured fundamental radial mode period and damping time are found to be proportional to the nanoparticle radius in the range of 3–15 nm, in agreement with theoretical predictions.

Electron dynamics and surface plasmon resonance nonlinearities in metal nanoparticles

Abstract The ultrafast surface plasmon resonance nonlinearities and their connection with the conduction band electron dynamics are discussed in metal nanoparticles in the light of the results of high sensitivity femtosecond pump-probe experiments in silver nanoparticles embedded in a glass matrix. The optical response is interpreted in terms of frequency shift and broadening of the surface plasmon resonance and is related to the changes of the metal nanoparticle dielectric function induced by ultrafast perturbation of the electron distribution. Alteration of the interband absorption is found to be responsible for the observed red shift and very short time delay broadening of the surface plasmon resonance, in agreement with numerical simulations and with measurements in silver films. On a longer time scale, a new nonlinear mechanism due to increase of the electron scattering off the surfaces is demonstrated. This mechanism, specific to confined system, plays an important role in the ultrafast nonlinear optical response of small nanoparticles.

Ultrafast response of nonlinear refractive index of silver nanocrystals embedded in glass

Ultrafast Kerr-type nonlinearities and relaxation dynamics of photoexcited electrons in silver nanocrystals embedded in glass have been investigated by means of femtosecond pump and probe spectroscopy. The transient absorption spectrum induced by the surface plasmon excitation shows a redshift and broadening of the surface plasmon band. The additional broadening is ascribed to the increase of surface plasmon damping and the redshift originates from a change in the real part of the dielectric function of the silver nanocrystals due to nonequilibrium electron heating. The observed redshift yields the nonlinear refractive index n2 of +2.4×10−10 esu and its time response is 1.9 ps.

Time-resolved investigation of the vibrational dynamics of metal nanoparticles

Abstract Time resolved excitation and detection of the coherent vibrational motion of metal nanoparticles are discussed in the light of femtosecond pump-probe experiments performed in silver nanoparticles with radius ranging from 2.1 to 15.3 nm. Analysis of the phase of the observed sample absorption oscillations shows that coherent excitation is dominated by an indirect displacive mechanism due to subpicosecond heating of the lattice by fast electron-lattice energy transfer for large nanoparticles (R>10 nm). For smaller particles, the results suggest an additional contribution from direct coupling with the non-equilibrium electron gas. Both mechanisms, being related to an isotropic particle expansion, the fundamental radial mode is preferentially excited because of its better spatial matching with the excitation process. Optical control of the acoustic nanoparticles vibration is also demonstrated.

Environment effect on the acoustic vibration of metal nanoparticles

Abstract The impact of the environment on the frequency and damping of the breathing acoustic mode of noble metal nanoparticle is discussed using the model of isotropic homogeneous elastic spheres embedded in an elastic medium. The results are compared to the experimental investigations performed in glass embedded silver nanoparticles and gold colloids using a time-resolved pump–probe technique.

Femtosecond hole thermalization in bulk GaAs

Ultrafast hole relaxation dynamics is selectively investigated in intrinsic bulk GaAs using a high sensitivity two‐wavelength pump‐probe technique. Nonequilibrium carriers are photoexcited close to the bottom of their respective bands and hole heating is followed by monitoring the transient bleaching of optical transitions involving higher momentum states. Hole heating is found to be dominated by hole‐phonon interactions with a thermalization time of the order of 150 fs for carriers densities in the 1017 cm−3 range.

Biphonon dynamics in crystals: Nitrous oxide

Abstract Coherent picosecond excitation and probe techniques are employed to measure the dephasing times of the very weak 2v2 two-phonon resonance in solid N2O as a function of temperature. T2 times are also obtained for the v1 one-phonon transition which is in feeble Fermi interaction with 2v2.

Tunable coherent vacuum ultraviolet generation in carbon monoxide in the 1150 Å range

Abstract Coherent and tunable over 500 cm-1 radiation in the vacuum ultraviolet (VUV) in the 1150 A region has been generated by resonantly enhanced 4-wave sum frequency mixing in carbon monoxide. Theoretical estimates of the third-order nonlinear susceptibility are shown to be in reasonable agreement with the observed conversion efficiency in CO.

Time resolved investigation of coherent acoustic mode oscillations in silver nanoparticles

Abstract Using a femtosecond pump-probe technique, we show that the fundamental mechanical radial mode of metal nanoparticles can be coherently excited and probed via its coupling with the electrons. The results permit determination of the frequency and damping of this mode as a function of the nanoparticle radius.

Vacuum ultraviolet generation in phase matched carbon monoxide

Abstract Resonantly enhanced four-wave sum frequency mixing in phase matched carbon monoxide leads to the generation of continuously tunable over 1200 cm -1 coherent VUV radiation in the 1150 A range. Experiments allow to determine the VUV absorption cross sections and indices of refraction of CO. Multiphoton photolysis of CO has been observed and confirmed by the VUV absorption in atomic carbon.