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

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. Our 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. The traditional scheme to measure an electric-field waveform beyond radio frequencies requires the challenging synthesis of gate pulses shorter than half of the field's oscillation period. The authors circumvent this problem by measuring the electric field transient via a third-order nonlinear interaction without resorting to high-energy laser pulses.

Automated summary

Measuring electric field waveforms beyond radio frequencies typically requires the challenging synthesis of gate pulses shorter than half of the field's oscillation period. In this study, the authors demonstrate an alternative approach by directly measuring the electric-field transient via a third-order nonlinear interaction without the need for high-energy laser pulses.