We demonstrate experimentally and theoretically that frequency-modulated femtosecond laser pulses can be spectrally narrowed by self-phase modulation in optical fibers. We obtain a reduction of the spectral linewidth from 10.6 down to 2.7 nm, limited only by the laser power in the fiber. Applications for extracavity conversion of femtosecond lasers to narrow-linewidth picosecond sources are discussed.

We have dramatically enhanced the photoluminescence intensity emitted from a single quantum well (typically by factors of 3–6) by covering the sample surface with a thin semitransparent metallic film. Using a photolithographically prepared gold grating, we show that this enhancement is due to the excitation of surface plasmons on the metal. By selectively turning off the surface plasmon excitation via sample or light polarization rotation, the enhancement can be suppressed.

We report the observation of negative absolute mobility of electrons (i.e., a drift toward the negative electrode) in /^modulation-doped GaAs quantum wells. This unusual effect results from ''carrier drag" on the electrons by the high-mobility hole plasma via electron-hole scattering.

We report the first direct measurement of the relaxation of plasma oscillations in a solid. Via the second-harmonic autocorrelation of femtosecond laser pulses, the lifetimes of localized plasma oscillations in silver island films were investigated. Decay times of 40 ± 7 fs were obtained, which indicate that single-particle scattering is mainly responsible for the energy relaxation of the plasma oscillations. PACS numbers: 72.15.Lh, 73.20.Mf, 78.47.+p Considerable interest has been directed to the fundamental understanding of the behavior of plasma oscillations in solids [1]. In the literature, lifetimes and damping mechanisms of surface plasmons (SP) have been examined theoretically [2-7] and experimentally [8-12]. Studies of nonlocalized long-range plasma oscillations have been carried out in the infrared region to determine the mean free path, with typical values for the propagation length of long-range, photonlike SP in the mm range [12]. Reflection measurements in the visible using attenuated total reflection (ATR) methods or grating couplers were often limited by the spectral resolution of the experimental detection system [13]. Recently van Exter and Lagendijk [8] were successful in measuring the short-range SP propagation in the prism configuration with a picosecond pump-and-probe technique via heat distribution, giving a value of 48 fs. Lifetimes of localized SP in small systems (metal islands) of fermions [9,10] have been derived from optical absorption spectroscopy. However, the nonuniformity of the particle sizes gives rise to an inhomogeneous broadening of the absorption. Indeed the full width at half maximum (FWHM) varies between 0.1 and 1 eV depending on the size of the particles [10,11]. Quantum-mechanical and semiclassical theoretical calculations predict lifetimes which differ strongly from each other [2-6]. To the authors' knowledge, until now, no direct observation of the decay of SPs has been reported.In this Letter we report the first time-resolved measurement of the relaxation (damping) of plasma oscilla- tions in Ag. The experiments were performed using Ag island films in order to prevent the influence of propagation effects. A reference second-harmonic (SH) correlation signal of femtosecond pulses was first generated with a potassium dihydrogen phosphate (KDP) crystal, in which the second-harmonic generation (SHG) occurs quasi-instantaneously. In the experiment the KDP crystal was replaced by an Ag island film sample. The broadening of the second-harmonic autocorrelation signal on the island film in comparison to the autocorrelation of KDP is caused by the excitation of electron plasma oscillations with finite lifetimes rsp. We obtain a value of r S p =40 ±7 fs for the exponential decay of the oscillation energy. The optical properties of metal island films differ essentially from that of the corresponding bulk metal as they show a strong and broad absorption band in the visible [2-6] caused by the resonances of the collective electron plasma oscillations. Assuming the shape of islands as...

tion that all broadening in a-Si is due to surface chemical shifts of atoms surrounding voids. It must rather be attributed to random charge fluctuations as the result of bond length variations in the amorphous network. The average rms charge deviation is estimated to be 0.11 electron in a-Si, only about half as much as obtained by Guttman, Ching, and Rath. In a-Si:H the incorporation of hydrogen leads apparently to an overall reduction in the bond length fluctuations as manifested in the reduced 0' ph corresponding to charge fluctuations of only 0.09 electron. This is in agreement with the 20/p attenuation of the TO bands in the ir spectrum of a-Si upon hydrogenation. 'We thank the staff of Hasylab for their kind hospitality and in particular C. Kunz for making his spectrometer available to us.

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