No abstract

An experimental study on high-order harmonic generation from the interaction of 45 fs Ti:sapphire laser pulses with preformed plasma plumes of metal nanoparticles was carried out. Highly efficient harmonic generation in the range of 9th order to 19th order was observed for Ag nanoparticles. The stability of harmonic generation was enhanced by utilizing special target fabrication techniques and through optimizing the conditions of plasma plume formation. Broadband harmonic generation was observed through the optimization of femtosecond laser intensity and through the use of spectrally broadened laser pulses. The harmonic generation was compared for various target materials (nano and bulk) and for Ag nanoparticle targets prepared from different fabrication techniques. Efficient generation of even- and odd-order harmonics was observed through the use of two-colour pulses. The observations can be explained qualitatively from symmetry breaking of high-order harmonic generation through the introduction of second harmonic pulses. The spectral broadening and shift of harmonic radiation can be understood from the self-modulation of the laser and harmonic radiation in the plasma.

In a metal−semiconductor hybrid nanostructure, exchange of carriers between the two constituents is a crucial process determining its utility for various applications. Although the transfer of carriers from the metal to semiconductor (or vice versa) shows up in several ways, it is not easy to get a quantitative estimate of the process. In this paper, we show that it is possible to obtain a fair estimate of the number of carriers hopping from or to a metal nanoparticle by performing transient absorption measurement near the localized surface plasmon resonance peak. For demonstrating this concept, a specific Ag−CdTe hybrid nanostructure was prepared such that plasmon resonance of the Ag nanoparticles could be excited well below the band gap of the CdTe quantum dots. Using a simple Drude model and the changes in the number density of free electrons, we show that the transient optical response of the hybrid is governed by both free-electron temperature and the number of carriers transported in and out of the metal nanoplates. It is also shown that using the linear absorption spectra as reference, it is possible to estimate the number of carriers hopping between the two constituents of the hybrid. The estimate of the number of electrons hopping at ultrafast time scales can provide a way to optimize the design of a metal−semiconductor hybrid system for specific applications.

We have measured and compared the absolute values of nonlinear susceptibility of colloidal solutions containing silver nanospheres and nanodiscs at their respective plasmon peaks using a femtosecond laser. The nonlinear process responsible for the laser-induced grating formation in the sample is determined to be of third order. The ratio between the third-order susceptibility (|χ(3)|) and the linear absorption coefficient (α) of the nanodiscs at 590 nm is three times than that of the similar ratio for nanospheres at 398 nm. Using a randomly oriented ellipsoidal model, we have shown that the increase in |χ(3)|/α for a nanodisc at 590 nm can be attributed to the change in the field enhancement factor with shape.