The femtosecond optical response of noble metal nanoparticles and its connection to the ultrafast electron dynamics are discussed in light of the results of high-sensitivity femtosecond pump-probe experiments. The physical origins of the nonlinear responses in the vicinity of the surface plasmon resonance and interband transition threshold are analyzed using extension of the theoretical models used in the bulk materials. These responses contain information on the electron interaction processes (electron-electron and electron-phonon scattering) that can thus be directly investigated in the time domain. Their size and environment dependences are discussed, and the results are compared to the ones in the bulk materials. Time-resolved techniques also permit direct study of the vibrational modes of metal nanoparticles and, in particular, the determination of their damping, which is a sensitive probe of the nature of the surrounding matrix and of the interface quality.

Electron-electron and electron-lattice interactions in noble metals are discussed in the light of two-color femtosecond pump-probe measurements in silver films. The internal thermalization of a nonequilibrium electron distribution created by intraband absorption of a pump pulse is followed by probing the induced optical property changes in the vicinity of the frequency threshold for the d band to Fermi surface transitions. This is shown to take place with a characteristic time constant of 350 fs, significantly shorter than previously reported in gold. This difference is ascribed to a weaker screening of the electron-electron interaction by the d-band electrons in silver than in gold. These results are in quantitative agreement with numerical simulations of the electron relaxation dynamics using a reduced static screening of the electron-electron Coulomb interaction, and including bound electron screening. Electron-lattice thermalization has been studied using a probe frequency out of resonance with the interband transitions. In both materials, the transient nonthermal nature of the electron distribution leads to the observation of a short-time delay reduction of the energy-loss rate of the electron gas to the lattice, in very good agreement with our theoretical model.

International audienceElectron-lattice energy exchanges are investigated in gold and silver nanoparticles with sizes ranging from 30 to 2.2 nm embedded in different environments. Femtosecond pump-probe experiments performed in the low-perturbation regime demonstrate a strong increase of the intrinsic electron-phonon interaction for nanoparticles smaller than 10 nm due to a confinement effect

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.

Time-resolved carrier dynamics in single-wall carbon nanotubes is investigated by means of two-color pump-probe experiments. The recombination dynamics is monitored by probing the transient photobleaching observed on the interband transitions of the semiconducting tubes. This dynamics takes place on a 1 ps time scale which is 1 order of magnitude slower than in graphite. Transient photoinduced absorption is observed for nonresonant probing and is interpreted as a global redshift of the pi-plasmon resonance. We show that the opening of the band gap in semiconducting carbon nanotubes determines the nonlinear response dynamics over the whole visible and near-infrared spectrum.

Progress in quantum computing and quantum cryptography requires efficient, electrically triggered, single-photon sources at room temperature in the telecom wavelengths. It has been long known that semiconducting single-wall carbon nanotubes (SWCNTs) display strong excitonic binding and emit light over a broad range of wavelengths, but their use has been hampered by a low quantum yield and a high sensitivity to spectral diffusion and blinking. In this Perspective, we discuss recent advances in the mastering of SWCNT optical properties by chemistry, electrical contacting and resonator coupling towards advancing their use as quantum light sources. We describe the latest results in terms of single-photon purity, generation efficiency and indistinguishability. Finally, we consider the main fundamental challenges stemming from the unique properties of SWCNTs and the most promising roads for SWCNT-based chip integrated quantum photonic sources.

Motional narrowing refers to the striking phenomenon where the resonance line of a system coupled to a reservoir becomes narrower when increasing the reservoir fluctuation. A textbook example is found in nuclear magnetic resonance, where the fluctuating local magnetic fields created by randomly oriented nuclear spins are averaged when the motion of the nuclei is thermally activated. The existence of a motional narrowing effect in the optical response of semiconductor quantum dots remains so far unexplored. This effect may be important in this instance since the decoherence dynamics is a central issue for the implementation of quantum information processing based on quantum dots. Here we report on the experimental evidence of motional narrowing in the optical spectrum of a semiconductor quantum dot broadened by the spectral diffusion phenomenon. Surprisingly, motional narrowing is achieved when decreasing incident power or temperature, in contrast with the standard phenomenology observed for nuclear magnetic resonance.PACS numbers: 78.67. Hc, 78.55.Cr, In the seminal work on motional narrowing by Bloembergen et al., relaxation effects in nuclear magnetic resonance were beautifully explained by taking into account the influence of the thermal motion of the magnetic nuclei upon the spin-spin interaction [1]. The general treatment of relaxation processes for a system interacting with a reservoir was later formulated by Kubo in a stochastic theory that assumes random perturbations of the system by a fluctuating environment [2]. Depending on the relative magnitude of the spectral modulation amplitude and the inverse of the modulation correlation time, the relaxation dynamics is either in the slow modulation limit, where the optical line-shape reflects directly the statistical distribution of the different system energies, or in the fast modulation limit where the fluctuation is smoothed out and the line-shape is motionally narrowed into a Lorentzian profile. The relevance of motional narrowing for the description of relaxation phenomena has spread throughout many different fields, such as spin relaxation in semiconductors [3], vibrational dephasing in molecular physics [4], or phase noise in optical pumping [5].The optical spectrum of a material system with localized, zero-dimensional electronic states provides a generic example of the influence of a fluctuating environment on the coherence relaxation dynamics. In that case, the perturbing interactions induce a stochastic shift over time of the optical spectrum, resulting in the so-called spectral diffusion effect, which was observed for rare-earth ions [6], molecules [7], or semiconductor quantum dots [8,9]. In this latter system, impurities, defects or localized charges in the vicinity of a quantum dot induce micro-electric fields that shift the quantum dot emission line through the quantum confined Stark effect. The fluctuation of the quantum dot environment thus randomize the emission energy over a spectral range Σ on a characteristic time scale τ c . Spectral dif...

The internal thermalization dynamics of the conduction electrons is investigated in silver nanoparticles with radius ranging from 13 to 1.6 nm using a femtosecond IR pump-UV probe absorption saturation technique. A sharp increase of the electron energy exchange rate is demonstrated for nanoparticles smaller than 5 nm. The results are consistent with electron-electron scattering acceleration due to surface induced reduction of the Coulomb interaction screening by the conduction and core electrons.