Femtosecond pump-probe investigation of ultrafast silver nanoparticle deformation in a glass matrix

Single-color femtosecond pump-probe experiments were performed to investigate the time dependence of laser-induced ultrafast desorption and deformation processes of silver nanoparticles in glass. After laser excitation at wavelengths close to the surface plasmon resonance, transient extinction changes were found to exhibit dynamics on quite different time scales ranging from sub-picoseconds to some hundred ps. The slowest observed decay component is identified as characteristic for the deformation/desorption processes. Possible mechanisms for these processes are discussed. Received: 3 April 2000 / Revised version: 3 July 2000 / Published online: 20 September 2000     

Femtosecond surface plasmon resonance dynamics and electron-electron interactions in silver nanoparticles

Femtosecond excitation and relaxation of nonequilibrium electrons are investigated in silver clusters using a two color pump-probe technique with resonant excitation of the surface plasmon resonance and off resonant probing. The excitation process is shown to be identical to that in metal films, and permits creation of a strongly athermal single electron excitation in a time scale shorter than the duration of the pulses (25-30 fs), in agreement with the free-electron absorption model. Following the time evolution of the nonequilibrium distribution yields information on the internal energy redistribution dynamics of the conduction electrons and of its modification by confinement in metal clusters. Received 1st December 2000     

Surface plasmon dynamics of simple metal clusters excited with femtosecond optical pulses

We report a new mechanism on the dynamics of correlated electrons in simple metal clusters which manifests by a strong electron temperature dependence of the surface plasmon resonance spectral profile. This effect is revealed thanks to a theoretical approach based on the time-dependent local-density approximation at finite electronic temperature, and it should be experimentally observable using pump-probe femtosecond spectroscopy techniques.     

基于表面等离子体激元共振的飞秒抽运探测技术研究

对基于表面等离子体激元共振（SPR）的飞秒抽运探测技术进行了系统地数字分析.计算结果表明,利用SPR技术可将飞秒抽运探测信号强度ΔR/R提高2—3个数量级,其提高的比例依赖于金属膜的厚度和探测光的入射角,其中R和ΔR分别为探测光反射率的总量和变化量.特别地,对于金属-吸收介质体系,金属膜的介电常数变化对ΔR的影响仅为吸收介质影响的2%,所以ΔR/R主要反应吸收介质中的动力学过程,因此该技术对研究表面和界面的超 关键词： 超快光学 表面等离子体共振 飞秒抽运探测技术 信号强度     

Ultrafast nonlinear optical response of a single gold nanorod near its surface plasmon resonance

The ultrafast optical nonlinearity of an optically characterized single gold nanorod is investigated around its surface plasmon resonance, by combining a far-field spatial modulation technique with a high sensitivity pump-probe setup. The spectrally and temporally dependent response is quantitatively interpreted in terms of the bulklike optical nonlinearity enhanced by the plasmonic effect. The plasmon resonance dynamics is shown to be mostly governed by nonequilibrium electron and phonon processes. Their contributions to the nonlinear optical response of a single metal nano-object are elucidated, and the latter is connected to the nonlinearities of ensembles.     

Propagation of THz plasmon pulse on corrugated and flat metal surface

We report here experiments on surface plasmon excitation and propagation along corrugated and smooth aluminum surface in the terahertz frequency range. Narrowband plasmon excitation by a subpicosecond terahertz pulse is shown to be a transient process and plasmon propagation sufficiently changes its measured time profile. Plasmon propagation during its excitation and detection changes measured signal. We suggest to use parameters T (plasmon duration) and τ (plasmon lifetime) to describe the narrowband THz plasmon pulse. Plasmon duration and lifetime were defined and plasmon propagation lengths on smooth and corrugated surface were measured. Plasmon propagation length on flat surface turned out to be much smaller than it is predicted by the Drude model.     

Resolving dynamics of acoustic phonons by surface plasmons

We show that the dynamics of acoustic phonons generated by femtosecond impulsive optical excitation can be clearly resolved by a surface plasmon technique, with enhanced sensitivity orders of magnitude higher than regular optical probe measurements. Our calculations confirm that the enhanced sensitivity is an intrinsic property of the surface plasmon probe. Therefore, the surface plasmon technique is a promising tool to detect small signal changes in optical and mechanical properties on a microscopic scale.     

Surface plasmon interference excited by tightly focused laser beams

We show that interfering surface plasmon polaritons can be excited with a focused laser beam at normal incidence to a plane metal film. No protrusions or holes are needed in this excitation scheme. Depending on the axial position of the focus, the intensity distribution on the metal surface is either dominated by interferences between counterpropagating plasmons or by a two-lobe pattern characteristic of localized surface plasmon excitation. Our experiments can be accurately explained by use of the angular spectrum representation and provide a simple means for locally exciting standing surface plasmon polaritons.     

Analysis of dynamics of excitation and dephasing of plasmon resonance modes in nanoparticles

A novel theoretical approach to the dynamics analysis of excitation and dephasing of plasmon modes in nanoparticles is presented. This approach is based on the biorthogonal plasmon mode expansion, and it leads to the predictions of time dynamics of excitation of specific plasmon modes as well as their steady state amplitude and their decay. Temporal characteristics of plasmon modes in nanoparticles are expressed in terms of their shapes, permittivity dispersion relations, and excitation conditions. In the case of the Drude model, analytical expressions for time-dynamics of plasmon modes are obtained.     

Resonant charging of Xe clusters in helium nanodroplets under intense laser fields

We theoretically investigate the impact of multiple plasmon resonances on the charging of Xe clusters embedded in He nanodroplets under intense pump-probe laser excitation (τ = 25 fs, I ₀ = 2.5 × 10¹⁴   W/cm², λ = 800 nm). Our molecular dynamics simulations on Xe₃₀₉He_10 000 and comparison to results for free Xe₃₀₉ give clear evidence for selective resonance heating in the He shell and the Xe cluster, but no corresponding double hump feature in the final Xe charge spectra is found. Though the presence of the He shell substantially increases the maximum charge states, the pump-probe dynamics of the Xe spectra from the embedded system is similar to that of the free species. In strong contrast to that, the predicted electron spectra do show well-separated and pronounced features from highly efficient plasmon assisted electron acceleration for both resonances in the embedded clusters. A detailed analysis of the underlying ionization and recombination dynamics is presented and explains the apparent disaccord between the resonance features in the ion and electron spectra.     

分子激发中的表面等离激元增强效应

金属纳米粒子的表面等离激元增强效应是纳米科学领域的一个研究热点.针对染料分子与金属纳米粒子的耦合系统,应用偶极-偶极近似计算分子与金属纳米粒子的库仑相互作用,并应用密度矩阵理论描述在不同极化方向的电场作用下的电荷输运过程,分析了分子与金属纳米粒子在不同相对位置下分子激发态的动力学过程,发现表面等离激元的增强效应与分子和金属钠米粒子的相对位置以及等离激元的耗散系数有密切关系,详细讨论了分子与金属纳米粒子间的耦合强度、外场的极化方向、等离激元的寿命及共振激发条件对分子激发态及表面等离激元增强的影响,分析了分子-金属纳米粒子耦合系统中表面等离激元增强效应的物理本质.     

Experimental observation of different-order components of a vibrational wave packet in a bulk dielectric using high-order Raman scattering

We use high-order Raman scattering in a bulk dielectric to characterize coherent dynamics with precision typical for gas phase experiments. The experimental pump-probe approach allows for the simultaneous observation and separation in space and time of the individual contributions of different-order Raman processes to a coherent wave packet without relying on phase-matching conditions and within the same experimental geometry. We propose a novel technique to discriminate between stimulated excitation of vibronic levels in the impulsive and intermediate excitation regimes, futhermore allowing us to distinguish between different pathways contributing to the same fifth-order Raman processes.     

Research progress of femtosecond surface plasmon polariton

As the combination of surface plasmon polariton and femtosecond laser pulse,femtosecond surface plasmon polariton has both nanoscale spatial resolution and femtosecond temporal resolution,and thus provides promising methods for light field manipulation and light-matter interaction in extreme small spatiotemporal scales.Nowadays,the research on femtosecond surface plasmon polariton is mainly concentrated on two aspects:one is investigation and characterization of excitation,propagation,and dispersion properties of femtosecond surface plasmon polariton in different structures or materials;the other one is developing new applications based on its unique properties in the fields of nonlinear enhancement,pulse shaping,spatiotemporal super-resolved imaging,and others.Here,we introduce the research progress of properties and applications of femtosecond surface plasmon polariton,and prospect its future research trends.With the further development of femtosecond surface plasmon polariton research,it will have a profound impact on nano-optoelectronics,molecular dynamics,biomedicine and other fields.     

Resonance and composition effects on the Raman scattering from silver-gold alloy clusters

Low-frequency Raman scattering experiments have been performed on thin films consisting of pure gold or gold-silver alloy clusters embedded in alumina matrix. It is clearly shown that the quadrupolar vibrational modes are observed by Raman scattering because of the effect of resonance with the excitation of the electronic surface dipolar plasmon. This is due to the strong coupling between the collective electronic dipolar excitation and the quadrupolar vibrational modes. This effect of resonance does not exist with the core electron excitations. The mixing of the conduction electron dipolar excitation (surface plasmon) with the core electrons leads to the quenching of the resonant Raman scattering. Received 16 November 2000     

Ultrafast dynamics of intersubband excitations in a quasi-two-dimensional hole gas

We present the first study of ultrafast hole dynamics after resonant intersubband excitation in a quasi-two-dimensional semiconductor. p-type Si0.5Ge 0.5/Si multiple quantum wells are studied in pump-probe experiments with 150 fs midinfrared pulses. Intersubband scattering from the second heavy-hole back to the first heavy-hole subband occurs with a time constant of 250 fs, followed by intrasubband carrier heating within 1 ps. Such processes give rise to a strong reshaping of the intersubband absorption line, which is accounted for by calculations of the subband structure, optical spectra, and hole-phonon scattering rates.     

Photoemission from multiply excited surface plasmons in Ag nanoparticles

Two-photon photoemission spectroscopy using femtosecond laser pulses is used to investigate the excitation and decay mechanisms of the surface plasmon resonance in Ag nanoparticles grown on graphite. The resonant excitation of this collective excitation leads to a two-orders-of-magnitude-enhanced two-photon photoemission yield from a graphite surface with Ag nanoparticles compared to the yield from pure graphite. From the shape of the photoemission spectra, the polarization dependence of the photoemission yield and the excitation probabilities for different excitation pathways we conclude that excitation with 400-nm femtosecond laser pulses leads to the coherent multiple excitation of the surface plasmon in the Ag nanoparticles. This multiply excited plasmon mode can decay via the coupling to a single-particle excitation leading to the emission of an electron if its final state is located in the continuum. The surface plasmon in metallic nanoparticles is a model system to investigate collective excitations in multiphoton processes. Received: 26 June 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Hybrid waveguide-surface plasmon polariton modes in a guided-mode resonance grating

We analyze the coupling between surface plasmon polaritons in a metal grating and the guided modes of a dielectric waveguide. Our model structure is a gold wire grating on a slab waveguide made of silicon nitride on silica wafer. The excitation of guided-mode resonances, surface plasmon polariton modes and hybrid waveguide-plasmon modes are observed in numerical simulations. Our experiments verify the existence of the predicted modes. These hybrid modes add significant degrees of freedom in designing structures for plasmonic applications.     

Femtosecond photoemission microscopy

We developed a novel experiment for time-resolved photoemission microscopy by combining a commercial photoemission electron microscope (PEEM) with a pulsed Ti:sapphire laser oscillator. The laser system, the setup of the delay stage for pump-probe experiments, and the interface between the PEEM and the laser system are discussed. We use self-organization of Ag islands and nanowires on Si(1 1 1) and 4° vicinal Si(0 0 1) to generate structures with a plasmon resonance that matches the photon energy of our laser (?ω = 3.1 eV after frequency doubling). In two-photon photoemission (2PPE) the photoemission yield then directly visualizes the plasmons in the nanostructures. Accordingly, the photoemission yield depends on the size and shape of the nanostructures, and on the polarization of the laser pulses as well. In Ag nanowires, we observe surface plasmon polariton (SPP) waves by a beating that is formed by interference of the SPP wave and the incident laser light. In a pump-probe experiment, we can directly visualize the propagation of the SPP on a femtosecond time scale.     

Angular pattern of electron diffraction by reflection in the plasmon channel of energy losses

We have constructed a theory for the excitation of plasmons by a fast charged particle that undergoes diffraction in a single crystal and then is scattered elastically and incoherently through a large angle. The theory allows the 30-year-old experimental results that have seemed strange to be explained. An increase in the diffraction contrast in the channel of inelastic electron scattering related to the excitation of a bulk plasmon compared to the diffraction contrast of elastically and incoherently reflected electrons was observed in these experiments. Based on this theory, we show that the excitation of a surface plasmon affects only slightly the angular diffraction pattern, leaving it almost the same as that for elastically reflected electrons. These peculiarities of elastic and inelastic diffraction can be used to identify the type of energy plasma loss.