Cross-polarized and Stable Second Harmonic Generation from Monocrystalline Copper

TL;DR

This paper demonstrates stable, anisotropic second-harmonic generation from monocrystalline copper microflakes, overcoming surface degradation issues and showcasing copper's potential in nonlinear nanophotonics and surface spectroscopy.

Contribution

It introduces an on-substrate synthesis method for stable, oxidation-resistant copper microflakes enabling reliable SHG measurements and revealing their strong cross-polarized response.

Findings

Stable SHG observed over minutes of femtosecond excitation

Strong cross-polarized SHG with C3v surface symmetry

Oxidation-resistant, atomically flat copper microflakes

Abstract

Second-harmonic generation (SHG) is a powerful surface-specific probe for centrosymmetric materials, with broad relevance to energy and biological interfaces. Plasmonic nanomaterials have been extensively utilized to amplify this nonlinear response. Yet, material instability has constrained most studies to gold, despite the significance of plasmonic metals such as copper for catalysis. Here, we demonstrate stable and anisotropic SHG from monocrystalline copper, overcoming long-standing challenges associated with surface degradation. By leveraging an on-substrate synthesis approach that yields atomically flat and oxidation-resistant Cu microflakes, we enable reliable SHG measurements and reveal a strong cross-polarized response with C3v surface symmetry. The SHG signal remains stable over several minutes of continuous femtosecond excitation, highlighting the optical robustness of the Cu microflakes. These results reinforce the viability of monocrystalline Cu as a robust platform for nonlinear nanophotonics and surface-sensitive spectroscopy, expanding the range of copper-based optical applications.