Field-resolved Infrared Differential Molecular Fingerprinting of Liquid Samples

Abstract

Field-resolved infrared spectroscopy (FRS) based on electro-optic sampling (EOS) measures directly in time domain the electric response field emitted by molecules, vibrationally excited by few-cycle mid-infrared (MIR) laser pulses [1]. The response field provides a fingerprint of the molecular composition of the sample, with potential applications in biomedical diagnostics [2]. For sufficiently long decoherence times, the sample signal can be separated from the excitation-pulse via time filtering, enabling background-free detection [1] - however, at the expense of losing the information at early times. Many biologically relevant molecules like proteins have significant overlap of the molecular signal with the excitation window. In order to reveal the full response signal and reduce the influence of the excitation noise at early times, we introduced differential molecular fingerprinting (dMF) [3,4]. The approach uses broadband interferometry to optically suppress the infrared excitation-pulse after interaction with the sample. Here, we demonstrate, for the first time, the application of octave-spanning mid-infrared dMF for the measurement of liquid samples.