Lineshape of harmonic generation on metal nanoparticles and metallic Photonic Crystal slabs

TL;DR

This paper investigates the optical lineshapes of metal nanoparticles and photonic crystal slabs, revealing limitations in determining linewidths from harmonic generation experiments and introducing a classical model to explain observed spectral features.

Contribution

It provides a corrected analysis of linewidth determination in harmonic generation experiments and introduces a simple coupled oscillator model for metallic photonic crystal slabs.

Findings

Femtosecond harmonic-generation experiments cannot reliably determine homogeneous linewidths.

A classical coupled oscillator model accurately describes Fano-like lineshapes in photonic crystal slabs.

Experimental THG spectra show strong dependence on time delay and agree with theoretical predictions.

Abstract

We study the linear- and nonlinear-optical lineshapes of metal nanoparticles (theory) and metallic photonic crystal slabs (experiment and theory). For metal nanoparticle ensembles, we show analytically and numerically that femtosecond second- or third-harmonic-generation (THG) experiments together with linear extinction measurements generally do not allow to determine the homogeneous linewidth. This is in contrast to claims of previous work in which we identify a technical mistake. For metallic photonic crystal slabs, we introduce a simple classical model of two coupled Lorentz oscillators, corresponding to the plasmon and waveguide modes. This model describes very well the key experimental features of linear optics, particularly the Fano-like lineshapes. The derived nonlinear-optical THG spectra are shown to depend on the underlying source of the optical nonlinearity. We present corresponding THG experiments with metallic photonic crystal slabs. In contrast to previous work, we spectrally resolve the interferometric THG signal, and we additionally obtain a higher temporal resolution by using 5 fs laser pulses. In the THG spectra, the distinct spectral components exhibit strongly different behaviors versus time delay. The measured spectra agree well with the model calculations.