Describing High-Order Harmonic Generation Using Quantum Optical Models

Péter Földi,István Magashegyi,Ákos Gombköto,Sándor Varró

doi:10.3390/photonics8070263

Abstract

Optical generation of high-order harmonics is a prototypical example of nonlinear light–matter interactions in the high-field regime. Quantum optical effects have recently been demonstrated to have a significant influence on this phenomenon. These findings underline the importance of understanding the dynamics of the quantized electromagnetic field during high-order harmonic generation. In the following, we discuss the challenges that are related to the theoretical description of this process and summarize the results that were obtained using the high-field, multimode generalization of well-known quantum optical models that are based on the concept of the two-level atom.

Highlights

High-intensity electromagnetic radiation is effectively described via time-dependent classical fields, i.e., using a model without the concept of photons
The reason for the validity of this approximation is related to the number of photons: While, e.g., for fundamental cavity QED experiments, the average photon numbers can be close to unity [1], a strong laser pulse can correspond to 1015 photons
We summarize the results that were obtained using a two-level atom—a concept that is widely used in quantum optics

Summary

Introduction

High-intensity electromagnetic radiation is effectively described via time-dependent classical fields, i.e., using a model without the concept of photons. Realistic theoretical models should reproduce the well-established experimental findings, such as the qualitative properties of the HHG spectra, which means the appearance of well-defined peaks at integer multiples of the central frequency of the excitation The heights of these peaks decrease suddenly with increasing frequency (HH order), a plateau region with comparable peak heights follows, and an intensity-dependent cutoff (when the HH peaks vanish) is observed. We summarize the results that were obtained using a two-level atom—a concept that is widely used in quantum optics This model cannot account for material-specific effects, but it is useful to see the general, qualitative properties of the HHG process.

Models

Summary and Outlook