# Symmetry-controlled second-harmonic generation in a plasmonic waveguide

## TL;DR

This paper introduces a novel approach for second-harmonic generation in plasmonic waveguides, demonstrating symmetry-based control of nonlinear optical processes in centro-symmetric structures.

## Contribution

The study shows that optical mode symmetry alone can enable SHG in symmetric structures, with experimental validation using a plasmonic two-wire transmission line.

## Key findings

- SHG occurs only in symmetric modes when fundamental modes are symmetric or anti-symmetric.
- Emission can be switched to anti-symmetric mode by mixing fundamental modes.
- The approach enables new design possibilities for nonlinear nanophotonic devices.

## Abstract

A new concept for second-harmonic generation (SHG) in an optical nanocircuit is proposed. We demonstrate both theoretically and experimentally that the symmetry of an optical mode alone is sufficient to allow SHG even in centro-symmetric structures made of centro-symmetric material. The concept is realized using a plasmonic two-wire transmission-line (TWTL), which simultaneously supports a symmetric and an anti-symmetric mode. We first confirm the generated second-harmonics belong only to the symmetric mode of the TWTL when fundamental excited modes are either purely symmetric or anti-symmetric. We further switch the emission into the anti-symmetric mode when a controlled mixture of the fundamental modes is excited simultaneously. Our results open up a new degree of freedom into the designs of nonlinear optical components, and should pave a new avenue towards multi-functional nanophotonic circuitry.