Plasmonic Nanoparticle-in-nanoslit Antenna as Independently Tunable Dual-Resonant Systems for Efficient Frequency Upconversion

TL;DR

This paper investigates dual-resonant plasmonic nanoantennas, specifically nanoparticle-in-nanoslit structures, to enhance frequency upconversion efficiency through mode analysis and independent resonance tuning.

Contribution

It provides a detailed understanding of nanocavity-like modes in NPoS structures and introduces methods for independently tuning resonances to optimize nonlinear optical processes.

Findings

Identified methods for independently tuning visible and mid-infrared resonances.

Unveiled a fundamental resonance with higher field enhancement and better mode overlap.

Predicted a potential 5-fold increase in mid-infrared upconversion efficiency.

Abstract

Dual-band plasmonic nanoantennas, exhibiting two widely separated user-defined resonances, are fundamental building blocks for the investigation and optimization of plasmon-enhanced optical phenomena, including photoluminescence, Raman scattering, and various nonlinear effects such as harmonic generation or sum-frequency generation, parametric down-conversion, etc. The nanoparticle-on-slit (NPoS) or nanoparticle-in-groove (NPiG) is a recently proposed dual-band antenna with independently tunable resonances at mid-infrared and visible wavelengths. It was used to enhance the corresponding sum- and difference-frequency generation processes from optimally located molecules by an estimated $10^{13}$-fold. However, the theoretical understanding of such structures and their eigenmodes remains poor, hindering further optimization and limiting broader applications. Here, we explore a diverse range of nanocavity-like quasi-normal modes (QNMs) supported by NPoS structures, examining the contributions of both their near-field (i.e., giant photonic density of states) and far-field (i.e., spatial radiation patterns) characteristics to frequency upconversion. We identify methods for independently tuning the visible and mid-infrared resonances while conserving a good mode overlap in the near field, which is essential for efficient nonlinear processes. Moreover, through mode analysis, we unveil an experimentally unexplored fundamental resonance with greater field enhancement and much-improved mode overlap with the mid-infrared field, which could, in principle, further boost the mid-infrared upconversion efficiency by 5-fold compared to existing results. This work helps to rationalize and optimize the enhancement of nonlinear effects across a wide spectral range using a flexible and experimentally attractive nanoplasmonic platform.