# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1902.03539