Few-femtosecond phase-sensitive detection of infrared electric fields with a third-order nonlinearity

TL;DR

This paper introduces a third-order nonlinear technique for phase-sensitive detection of infrared electric fields, enabling simpler and more direct measurement of electric-field transients and dielectric functions in the mid- and near-infrared range.

Contribution

It presents a novel third-order nonlinear method for measuring infrared electric fields, overcoming challenges of synthesizing short gate pulses required by traditional second-order methods.

Findings

Accurately reproduces experimental results with a theoretical model.

Demonstrates measurement of the real part of a sample's dielectric function.

Enables MIR-to-NIR time-resolved spectroscopy with spectral amplitude and phase retrieval.

Abstract

Measuring an electric field waveform beyond radio frequencies is often accomplished via a second-order nonlinear interaction with a laser pulse shorter than half of the field's oscillation period. However, synthesizing such a gate pulse is extremely challenging when sampling mid- (MIR) and near- (NIR) infrared transients. Here, we demonstrate an alternative approach: a third-order nonlinear interaction with a relatively long multi-cycle pulse directly retrieves an electric-field transient whose central frequency is 156 THz. A theoretical model, exploring the different nonlinear frequency mixing processes, accurately reproduces our results. Furthermore, we demonstrate a measurement of the real part of a sample's dielectric function, information that is challenging to retrieve in time-resolved spectroscopy and is therefore often overlooked. The new method paves the way towards experimentally simple MIR-to-NIR time-resolved spectroscopy that simultaneously extracts the spectral amplitude and phase information, an important extension of optical pump-probe spectroscopy of, e.g., molecular vibrations and fundamental excitations in condensed-matter physics.