Using time-resolved pump-and-probe experiments we show that the acoustic vibrations of silver nanoparticles excited by ultrashort laser pulses are triggered by two mechanisms: the lattice anharmonicity and the thermal pressure of the hot electrons.

We investigate the vibration dynamics of ellipsoidal silver nanoparticles, using time-resolved optical pump-probe spectroscopy. When excited with femtosecond laser pulses, the particles execute anisotropic shape oscillations. We show that these vibrations are triggered by the thermal expansion of the optically heated particles. The time dependence of the vibrations indicates that this expansion is caused by two mechanisms: The lattice anharmonicity and the extremely large pressure of the hot conduction electrons.

We investigate non-equilibrium relaxation processes in optically excited large gold and silver clusters. Time-resolved pump-probe experiments and model calculations show that optical excitation of the clusters by femtosecond laser pulses results in a heating of the electron system, which is followed by electron cooling via phonon emission. The electron heating leads to an enhanced damping of the surface-plasmon resonance in the clusters. This enhanced damping is caused by an enhancement of the Landau damping and electron scattering rates at high electron temperatures. Furthermore, we find that the rate of electron cooling in the clusters changes with electron temperature; this is a consequence of the temperature-dependent specific heat of the conduction electrons. Finally, pump-probe experiments on ellipsoidal silver clusters show that the thermal expansion of the heated clusters triggers mechanical vibrations at the acoustic eigenfrequencies of the clusters.

A Comment on the Letter by M. Perner, et al., Phys. Rev. Lett. 78, 2192 (1997). The authors of the Letter offer a Reply.Received 19 October 1998DOI:https://doi.org/10.1103/PhysRevLett.82.3188©1999 American Physical Society

A Reply to the Comment by Gregory V. Hartland, Jose H. Hodek, and Ignacio Martini.Received 4 January 1999DOI:https://doi.org/10.1103/PhysRevLett.82.3189©1999 American Physical Society

Noble-metal nanoparticles show large optical resonances associated with surface plasmons (SP's), i.e. collective oscillations of conduction electrons [1], Ellipsoidal nanoparticles with axis dimensions a=b

Summary form only given. The FOLANT (focusing of laser radiation in the near-field of a tip) technique developed by Dickmann et al. (1997) for nanostructuring of surfaces has attracted considerable attention. In the FOLANT configuration, intense laser pulses illuminate the gap region between the tip of a scanning probe microscope (SXM) and a sample surface. Above a certain threshold, each laser pulse causes modifications of the surface enabling the writing of complex patterns as the tip is moved across the sample surface. The physical mechanism for generating these nanostructures is still under debate. While Dickmann et al. attribute the surface modifications to material ablation resulting from a tip-induced focusing of laser radiation onto the surface, it was often argued that the observed structures are caused by a thermal expansion of the tip leading to a crash of the tip into the sample surface. Here we present time-resolved SXM measurements after short laser pulse excitation that clearly demonstrate that thermal tip expansion is the dominating mechanism. Our results show that an STM combined with intense ultrafast laser pulses should be considered a microscopic light-driven air hammer rather than a nanometric laser drill.

We report a detailed investigation of optical gain narrowing processes in thin films of a conjugated ladder-type poly(p-phenylene) polymer. The intensity-dependent optical emission spectra of samples of varying thickness are compared. For thin high-quality spin-coated samples spectral narrowing of the emission is observed for pump pulse energies as low as 10 nJ. Picosecond time-resolved measurements show that the spectral narrowing is accompanied by an accelerated emission for high pump energies. Our experimental findings are in full agreement with the assumption that amplified spontaneous emission in a lateral direction is the underlying physical mechanism.

Using a near-field optical antenna effect, we measure the homogeneous line shape of the surface-plasmon resonance in single gold nanoparticles. The surface-plasmon dephasing times extracted from the near-field spectra of the individual particles vary around 8 fs. This mean value agrees with calculations based on Mie theory, which neglect surface effects. Deviations of individual particles from this value are interpreted as being due to variations in the local nanoenvironment. We also observe double-peaked line shapes caused by electromagnetic coupling between close-lying particles.

We study the damping of the surface-plasmon resonance in gold nanoparticles that is caused by high-intensity optical excitation of the electron gas. Femtosecond pump-probe experiments are performed on gold nanoparticles embedded in dielectric matrices. Optical excitation of single-electron interband transitions leads to a pronounced broadening of the surface-plasmon line, which reflects the excitation-induced damping of the collective electron oscillation. The time evolution of the damping rate follows that of the electron temperature, showing that the damping rate is strongly influenced by transient variations in the electronic scattering rate. The dependence of the damping rate on the excitation energy shows evidence for fast relaxation of the optically generated d-band holes. In addition, cw transmission experiments performed with a scanning near-field optical microscope (SNOM) on single gold nanoparticles give access to the homogeneous line width of the surface plasmon.

Summary form only given.The near-field transmission spectra for a series of individual Au particles were taken. We find that the homogeneous linewidths and the spectral positions of the surface plasmon (SP) resonance vary slightly from particle to particle and thus are not always reproducible by application of Mie theory using bulk values for the dielectric function of Au. Surface scattering events and possibly also quantum size effects might contribute to the SP lineshape opening up the exciting possibility of sensoring the nano-environment of a single Au particle.

Using femtosecond pump-probe experiments we measure the interplay between the two, perpendicularly polarized and spectrally separate, surface plasmons of silver nano-ellipsoids. Pulsed optical excitation leads to two normal-mode vibrations of the ellipsoidal particles.

Collective stimulated emission processes in conjugated polymers makes these materials potential candidates for laser applications. The fabrication of a low-cost flexible distributed feedback laser (see Figure and also the cover) by spin-coating a conjugated polymer onto a specially structured, flexible plastic substrate is reported. Single-mode laser emission in the blue-green spectral region has been achieved.

The surface plasmon damping induced by high excitation of the electron gas is studied in femtosecond pump-and-probe experiments on gold colloids embedded in a sol-gel matrix. Optical excitation of single-particle interband transitions leads to a pronounced broadening of the surface plasmon line. A similar behavior is observed for resonant excitation of the surface plasmon. This broadening is the dominant optical nonlinearity of the system, and reflects the excitation-induced damping of the surface plasmon resonance. The time evolution of the damping rate follows that of the electronic scattering rate. [S0031-9007(97)02694-X]

Summary form only given. We have performed photoluminescence (PL) and PL excitation (PLE) experiments on strain-induced InGaAs QDs showing remarkably high optical quality. As depicted schematically InP islands have been grown on an InGaAs-GaAs single quantum well structure leading to a strain-induced lateral potential profile of considerable energetic depth in the region below the InP stressors.

Femtosecond white-light experiments on Au-colloids embedded in a sol-gel matrix show that excitation of single-particle interband transitions leads to an initial plasmon-line-broadening caused by electron scattering.

The dynamics of optical excitations in a PPP-type ladder polymer have been investigated by means of femtosecond pump-probe experiments. Stimulated emission (SE) is observed around the S1 → S0 transition while photoinduced absorption (PA) dominates the differential transmission at lower energies. The stimulated emission is due to singlet exciton recombination. Comparative studies of the temporal behaviour of the SE and the PA demonstrate that the SE and the PA signals originate from different species. Our results provide evidence that the PA is due to spatially separated electron-hole pairs (indirect excitons).

Summary form only given, as follows. Near-field optical and ultrafast nonlinear optical experiments on various metal nanostructures provide important information on the dynamical properties of surface plasmons and their use in nano-optics.