Nonlinear optical signal generation mediated by a plasmonic azimuthally chirped grating

TL;DR

This paper demonstrates how a plasmonic azimuthally chirped grating enhances nonlinear optical signals like TPPL and SHG by providing broadband momentum for photon-plasmon coupling, confirmed through experimental mapping and analytical modeling.

Contribution

It introduces a novel azimuthally chirped grating design that effectively couples far and near fields for nonlinear signal enhancement, highlighting the grating's antenna role.

Findings

Experimental nonlinear signal distributions match analytical predictions.

The grating's antenna effect significantly boosts nonlinear signal generation.

Broadband momentum provided by the grating improves phase matching for nonlinear processes.

Abstract

The deployment of plasmonic nanostructures to enhance nonlinear signal generation requires effective far-to-near field coupling and phase matching for frequency conversion. While the latter can be easily achieved at plasmonic hotspots, the former is an antenna problem that requires dedicated structural design and optimization. Plasmonic gratings are a simple but effective platform for nonlinear signal generation since they provide a well-defined momentum for photon-plasmon coupling and local hotspots for frequency conversion. In this work, a plasmonic azimuthally chirped grating (ACG), which provides spatially resolved broadband momentum for photon-plasmon coupling, was exploited to investigate the plasmonic enhancement effect in two nonlinear optical processes, namely two-photon photoluminescence (TPPL) and second-harmonic generation (SHG). The spatial distributions of the nonlinear signals were determined experimentally by hyperspectral mapping with ultrashort pulsed excitation. The experimental spatial distributions of nonlinear signals agree very well with the analytical prediction based solely on photon-plasmon coupling with the momentum of the ACG, revealing the antenna function of the grating in plasmonic nonlinear signal generation. This work highlights the importance of the antenna effect of the gratings for nonlinear signal generation and provides insight into the enhancement mechanism of plasmonic gratings in addition to local hotspot engineering.