High-harmonic generation from an epsilon-near-zero material

TL;DR

This paper demonstrates enhanced high-harmonic generation from an epsilon-near-zero material, indium-doped cadmium oxide, achieving up to the 9th harmonic with spectral shifts, opening new avenues for compact attosecond light sources.

Contribution

It introduces the use of ENZ materials to significantly boost HHG efficiency and control, a novel approach in solid-state nanophotonics.

Findings

High-harmonic emission up to the 9th order observed

Spectral red-shift and linewidth broadening due to electron heating

ENZ medium enables efficient, compact HHG sources

Abstract

High-harmonic generation (HHG) from a compact, solid-state medium is highly desirable for applications such as coherent attosecond pulse generation and extreme ultra-violet (EUV) spectroscopy, yet the typically weak conversion of pump light to HHG can largely hinder its applications. Here, we use a material operating in its epsilon-near-zero (ENZ) region, where the real part of its permittivity vanishes, to greatly boost the efficiency of the HHG process at the microscopic level. In experiments, we report high-harmonic emission up to the 9th order directly from a low-loss, solid-state ENZ medium: indium-doped cadmium oxide, with an excitation intensity at the GW cm-2 level. Furthermore, the observed HHG signal exhibits a pronounced spectral red-shift as well as linewidth broadening, resulting from the photo-induced electron heating and the consequent time-dependent resonant frequency of the ENZ film. Our results provide a novel nanophotonic platform for strong field physics, reveal new degrees of freedom for spectral and temporal control of HHG, and open up possibilities of compact solid-state attosecond light sources.