# Spaser quenching by off-resonant plasmon modes

## TL;DR

This paper investigates how off-resonant plasmon modes influence the spaser threshold, revealing that gain coupling to higher-order modes causes frequency shifts and quenching effects, which can be mitigated at high gain levels.

## Contribution

It introduces an analytical model for spaser threshold considering off-resonant mode coupling and provides criteria to predict quenching onset.

## Key findings

- Off-resonant modes cause an upward shift in spaser frequency.
- Gain coupling to higher-order plasmons leads to quenching effects.
- High gain concentrations suppress spaser quenching.

## Abstract

We study the effect of off-resonant plasmon modes on spaser threshold in nanoparticle-based spasers. We develop an analytical semiclassical model and derive spaser threshold condition accounting for gain coupling to higher-order plasmons. We show that such a coupling originates from inhomogeneity of gain distribution near the metal surface and leads to an upward shift of spaser frequency and population inversion threshold. This effect is similar, albeit significantly weaker, to quenching of plasmon-enhanced fluorescence near metal nanostructures due to excitation of off-resonant modes with wide spectral band. We also show that spaser quenching is suppressed for high gain concentrations and establish a simple criterion for quenching onset, which we support by numerical calculations for spherical geometry.