Melting-Induced Enhancement of the Second-Harmonic Generation from Metal Nanoparticles

A. M. Malvezzi, M. Allione, M. Patrini, A. Stella, P. Cheyssac, R. Kofman
<span title="2002-08-02">2002</span> <i title="American Physical Society (APS)"> <a target="_blank" rel="noopener" href="https://fatcat.wiki/container/pkmpevvs3bgkhal5dvrmsu5mvq" style="color: black;">Physical Review Letters</a> </i> &nbsp;
We report on the generation of second-harmonic signals by irradiating monolayers of high purity Ga nanoparticles, embedded in a SiO x matrix, with femtosecond laser pulses at 800 nm. A remarkable melting-induced enhancement of the second-harmonic generation is observed in correspondence of the phase transition. In addition, the hysteresis cycle of nonlinear transmittance is shown to be amplified a factor of 80-100 with respect to the linear response and interpreted in the framework of a
more &raquo; ... r effective-medium model. From the first observations and comprehensive theory of nonlinear optical phenomena developed in the early 1960s the number of systems and methods for generating nonlinear effects and, in particular, second-harmonic signals from solid surfaces has considerably increased [1]. In the 1970s, one-order-of-magnitude enhancement of surface second-harmonic generation was obtained through excitation of the surface plasmon resonance in silver [2] . In the next decades, the ability to control and grow metal nanoparticles embedded in dielectric matrices has provided structures maximizing the surface-to-volume ratio, which further enhances the nonlinear response [3] . While considerable efforts have been mainly focused on selection of new materials and fabrication techniques, in this work we adopt a different approach by studying the effect of melting (and more generally of phase transitions) on nonlinear optical phenomena in metallic nanoparticles. We observe a remarkable melting-induced enhancement of the secondharmonic signal generated at the phase transition in gallium nanoparticles. We further find that hysteresis cycles of nonlinear transmittance versus temperature across the phase transition are amplified up to a factor of 80-100 with respect to the linear ones. The generation of second-harmonic radiation, which is generally forbidden in centrosymmetric systems due to symmetry constraints, is enhanced by the excitation of surface plasmons in relatively large metallic nanoparticles (of the order of 5-50 nm radius) involving the participation of higher order multipoles. In order to study the effect of the phase transition on the second-harmonic generation (SHG) we choose the following as starting points: (i) to use gallium, which is expected to yield a large nonlinear response as compared to other metals [4]; (ii) to perform measurements on nanoparticles in which the surface-tovolume ratio for a given amount of active material increases with decreasing particle size; and (iii) to work in resonance with the surface plasmon (SP) peak, as determined by linear optical measurements. In this work we irradiate monolayers of high purity Ga nanoparticles, embedded in a SiO x matrix, with femto-second laser pulses at 800 nm. We measure the generated SH signals as a function of the nanoparticle radius from 2 to 100 nm, and we find for 30 nm an enhancement of the SH generation in correspondence with the resonance with the SP wavelength. Measurements are then repeated as a function of sample temperature from liquid-nitrogen temperature (LNT) to 320 K and back. The observed hysteresis behavior of the SH signal is interpreted in terms of solidification and melting of the nanoparticles, through a model which accounts for the increased SH generation in the liquid phase. The samples are grown by evaporation condensation (Vollmer-Weber mode) of high purity gallium in ultrahigh vacuum [5] over a dielectric SiO x (x 1) layer deposited on a silica or sapphire substrate. Because of the partial wetting character of Ga with respect to SiO x (the contact angle being 130 [6]), the formation of Ga nanodroplets occurs on the substrate at a temperature around the melting point of bulk Ga (300 K). Liquid nanoparticles with the shape of truncated spheres are then solidified by cooling the substrate down to LNT, and they keep the same shape. The crystalline structure of the solid nanoparticles is a mixture of and metastable phases, as determined by high-resolution transmission electron microscopy measurements [7] . A SiO x layer evaporated over the surface stabilizes the nanoparticle distribution and avoids ambient contamination. The average size turns out to be a linear function of evaporation time at constant deposition rate. Both size and size dispersion (20% of the nominal value) are determined by means of electron microscopy [5, 8] . The second-harmonic radiation is measured in transmission and reflection with a femtosecond Ti:sapphire laser at 800 nm, repetition rate 1 kHz, and pulse duration 150 fs. The maximum pulse fluence on the sample is 5 10 ÿ3 J=cm 2 at the 0.9 mm diameter focal spot of the 4 m focusing lens used in the experiment. These fluence levels are sufficiently low as to prevent any damage on nanoparticles even after long exposure to laser radiation. The SH signal travels through a combination of color and interference filters and is detected by a photomultiplier.
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