High harmonic generation from surface states of solids

TL;DR

This paper shows that high-harmonic generation in solids mainly comes from surface states, with bulk contributions being significantly suppressed, supported by experiments and a new theoretical model, enabling advanced spectroscopy and metrology applications.

Contribution

The study provides the first comprehensive experimental and theoretical evidence that HHG predominantly originates from surface states in solids, highlighting their importance over bulk states.

Findings

HHG from surface states is dominant over bulk states by 1-4 orders of magnitude.

A new theory accurately describes harmonic generation from surface and interface states.

Bulk HHG is significantly weaker due to phase mismatch and non-perturbative effects.

Abstract

We demonstrate that high-harmonic generation (HHG) in solids dominantly originates from strongly localized surface states through non-perturbative processes. Measurements reveal that HHG from bulk states is suppressed by at least 1-2 orders of magnitude due to the lack of phase matching, when generated perturbatively, or by at least 3-4 orders of magnitude, when generated non-perturbatively. We derive a theory that fully supports this observation and quantitatively describes the generation of harmonics from the surface states as well as from interfaces between solids; it also predicts a much weaker generation of harmonics from the bulk states. Our results pave the way for the development of very high repetition rate high harmonic sources for vacuum ultraviolet spectroscopy and high precision frequency comb metrology using surface states from solids.