Evidence of High-Order Nonlinearities in Supercontinuum White-Light Generation from a Gold Nanofilm

Abstract

White-light supercontinuum generation can be readily observed when gold nanostructures are irradiated with short pulses of light. It is believed that the nanostructures enhance the optical elds, which facilitates the supercontinuum white-light generation from the surrounding environment or the substrate. Here, we investigate the dierent nonlinear processes that contribute to the generation of the supercontinuum from plasmonic nanostructures themselves using a technique that isolates the dierent nonlinear contributions. By exciting a gold nanolm with a pair of frequency shifted optical frequency combs, we demonstrate multiple modulation frequencies in the supercontinuum. and supercontinuum (SC) generation 5,10 can be observed at higher intensities. SC generation is also routinely observed when intense pulsed lasers are focused onto any dielectric medium, 11 and it has been suggested that the SC from the gold nanostructures originates due to the emission from the surrounding dielectric medium. The spectral broadening leading to the SC generated from the dielectrics is mainly due to the third-order nonlinear processes such as self-phase modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM). In such a process, the spectral width of the broaden spectrum, ∆ω, is proportional to the product of the intensity and the interaction length, ∆ω ∝ Iz (here I is the intensity and z is the interaction length). 11 In a typical dielectric medium, an excitation intensity of 10 13 10 14 W/cm 2 and the interaction length of few millimeters are necessary to observe appreciable spectral broadening. Alternatively, one can substantially increase the interaction length by using guided propagation of pulses in nonlinear optical (NLO) bers in order to generate broad spectrum at lower excitation intensity. 1214 The SC from gold nanostructures, on the other hand, is observed at a relatively lower excitation intensity (∼ 10 10 − 10 11 W/cm 2 ) as well as very short interaction length (∼ few tens of nanometers). 10 Although gold 2 nanostructures can locally enhance the electric eld, the intensity that is required to generate the SC within the interaction length of few nanometers from the surrounding substrate by SPM, XPM and FWM is about 10 18 W/cm 2 . As metal nanostructures cannot withstand such high intensities, it is unlikely that SC from the gold nanostructures and the dielectric medium originate from the same nonlinear processes.The SC generation has not been observed from the bulk gold despite its high intrinsic nonlinearity. 15 Thus it is reasonable that the optical properties of gold that only manifest in the nanostructures contribute to the nonlinear process. The nonlocal response of free electrons to the incident light elds in metals is one of such properties. 16 Generally, lightmatter interactions in dielectric materials are described by the local-response approximation (LRA) in which the electric displacement eld D(r, ω) induced at a point r by an incident light eld of frequency ω is proportional to the...

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