Harmonic generation from gold nanolayers: bound and hot electron contributions to nonlinear dispersion

Abstract

Understanding how light interacts with matter at the nanoscale is pivotal if one is to properly engineer nano-antennas, filters and other devices whose geometrical features approach atomic size. We report experimental results on second and third harmonic generation from 20 nm- and 70 nm-thick gold layers, for TE- and TM-polarized incident light pulses. We discuss the relative roles that bound electrons and an intensity dependent free electron density (hot electrons) play in third harmonic generation. While planar structures are generally the simplest to fabricate, metal layers that are only a few nanometers thick and partially transparent are almost never studied. Yet, transmission offers an additional reference point to compare experimental measurements with theoretical models. Our experimental results are explained well within the context of the microscopic hydrodynamic model that we employ to simulate second and third harmonic conversion efficiencies. Using our experimental observations we estimate ∣ χ 1064 n m ( 3 ) ∣≈ 10 − 18 ( m/V ) 2 , triggered mostly by hot electrons.

Near-infrared to ultra-violet frequency conversion in chalcogenide metasurfaces

“…Typical theoretical descriptions of NL optical interactions in nanostructures most commonly rely on assigning a phenomenological, effective volume nonlinear susceptibility χ (3) to describe THG, however, without specifying its origin and with the neglect of NL dispersion. We use a hydrodynamic model that preserves linear and NL material dispersions, allowing us to account for surface, magnetic, and bulk nonlinearities (described in Supplementary Note 1 ) 25 , 26 . Figure 2A shows the scanning electron microscopy (SEM) image of the As 2 S 3 pattern.…”

Retrieving Linear and Nonlinear Optical Dispersions of Matter: Combined Experiment-Numerical Ellipsometry in Silicon, Gold and Indium Tin Oxide

“…Using the same approach outlined above, we are now able to compare analytical results for gold with our simulations, which in turn are based on previously reported experimental results of THG in nanometer-thick gold layers, probed with both picosecond and femtosecond pulses (Rodríguez-Suné et al, 2021). The situation is different for gold compared to undoped silicon, because now a free electron gas and nonlocal effects may contribute to the dielectric constant.…”

“…The situation is different for gold compared to undoped silicon, because now a free electron gas and nonlocal effects may contribute to the dielectric constant. This difference is fully delineated in reference Rodríguez-Suné et al (2021), where the model is extended to include one free and two bound electron species to the dielectric constant. Accordingly, Eq.…”

“…7. The coefficient Λ relates to the nonlinear third order contributions of free (hot) electrons, whose Fermi surface is modified as a result of increased free electron density as a function of applied intensity (Rodríguez-Suné et al, 2021).…”

“…When third order effects are introduced, the χ (3) ω usually lacks information about dispersion (Suresh et al, 2021), is not always obtained under conditions that are directly pertinent to the problem at hand, and χ (3) 3ω remains undetermined. For instance, nanosecond pulses (Yang et al, 2004) are not representative of a fast electronic response (Rodríguez-Suné et al, 2021). Similar considerations apply to insulators, semiconductors, and conductive oxides.…”