On the limitations of the semi-classical picture in high harmonic generation

TL;DR

This paper explores the limitations of the semi-classical model in high harmonic generation, especially when quantum light sources like squeezed vacuum are used, revealing new questions about the process's validity.

Contribution

It introduces the consideration of non-classical light sources in high harmonic generation, challenging the traditional semi-classical framework and expanding the scope of quantum optical analysis.

Findings

Semi-classical model shows limitations with quantum light sources.

Bright squeezed vacuum can generate high harmonics despite zero mean electric field.

Raises questions about the validity range of classical approximations.

Abstract

The recent progress in the quantum optical formulation of the process of high harmonic generation has reached a point where the successful semi-classical model shows its limitations. So far the light source which drives the process was considered to be provided by a laser, in agreement with the classical picture. However, quantum optics allows to consider light fields beyond the classical realm, such as bright squeezed vacuum of photon number states. Both field states have vanishing mean electric field amplitudes, but can still lead to the generation of high harmonic radiation for sufficiently high intensities. This poses new questions about the range of validity of the semi-classical picture, and allows to extend the domain of questions which could possibly be asked.