Quantum susceptibilities in time-domain sampling of electric field fluctuations

TL;DR

This paper develops a quantum theoretical framework for electro-optic sampling of electric field fluctuations, revealing quantum susceptibilities and conditions where quantum effects dominate the measured response.

Contribution

It introduces a microscopic quantum model for electro-optic sampling, highlighting quantum contributions and susceptibilities accessible via quantum light, which were not previously characterized.

Findings

Quantum contributions can dominate the electro-optic response.

Classical approximation is valid under specific conditions.

Electro-optic sampling can probe pure quantum susceptibilities.

Abstract

Electro-optic sampling has emerged as a new quantum technique enabling measurements of electric field fluctuations on subcycle time scales. Probing a second-order nonlinear material with an ultrashort coherent laser pulse imprints the fluctuations of a terahertz field onto the resulting near-infrared electrooptic signal. We describe how the statistics of this time-domain signal can be calculated theoretically, incorporating from the onset the quantum nature of the electric fields involved in the underlying interactions. To this end, a microscopic quantum theory of the electro-optic process is developed using an ensemble of non-interacting three-level systems as a model for the nonlinear material. We find that the response of the nonlinear medium can be separated into a classical part sampling the terahertz field and quantum contributions independent of the state of the probed terahertz field. The quantum response is caused by interactions between the three-level systems mediated by the terahertz vacuum fluctuations. It arises due to cascading processes and contributions described by quantum susceptibilities solely accessible via quantum light. We show that the quantum contributions can be substantial and might even dominate the total response. We also determine the conditions under which the classical response serves as a good approximation of the electro-optic process and demonstrate how the statistics of the sampled terahertz field can be reconstructed from the statistics of the electro-optic signal. In a complementary regime, electro-optic sampling can serve as a spectroscopic tool to study the pure quantum susceptibilities of materials.