The emergence of macroscopic currents in photoconductive sampling of optical fields

TL;DR

This paper presents a comprehensive experimental and theoretical study of macroscopic current formation in ultrafast photoconductive sampling of optical fields, introducing a new model that links microscopic electron dynamics to macroscopic signals, enabling enhanced device sensitivity.

Contribution

The authors develop a rigorous Ramo-Shockley-based model and extensive PIC simulations that clarify the origin of macroscopic currents, challenging previous heuristic approaches and improving device design.

Findings

Most heuristic models need revision based on new insights.

Over an order of magnitude increase in signal is achievable.

The approach enables petahertz field measurements with high sensitivity.

Abstract

Photoconductive field sampling is a key methodology for advancing our understanding of light-matter interaction and ultrafast optoelectronic applications. For visible light the bandwidth of photoconductive sampling of fields and field-induced dynamics can be extended to the petahertz domain. Despite the growing importance of ultrafast photoconductive measurements, a rigorous model for connecting the microscopic electron dynamics to the macroscopic external signal is lacking. This has caused conflicting interpretations about the origin of macroscopic currents. Here, we present systematic experimental studies on the macroscopic signal formation of ultrafast currents in gases. We developed a theoretical model based on the Ramo-Shockley-theorem that overcomes the previously introduced artificial separation into dipole and current contributions. Extensive numerical particle-in-cell (PIC)-type simulations based on this model permit a quantitative comparison with experimental results and help to identify the roles of electron scattering and Coulomb interactions. The results imply that most of the heuristic models utilized so far will need to be amended. Our approach can aid in the design of more sensitive and more efficient photoconductive devices. We demonstrate for the case of gases that over an order of magnitude increase in signal is achievable, paving the way towards petahertz field measurements with the highest sensitivity.