Broadband interferometric subtraction of ultrashort pulses

Abstract

We present a simple, cost-effective method to optically subtract ultrashort pulses spanning a super-octave spectrum (950 – 2100 nm). Achromatic extinction is achieved in a Mach-Zehnder-like interferometer with an intensity extinction of 6.2×10^-4 by unbalancing the number of Fresnel reflections off optically denser media in the two interferometer arms. By introducing a methane gas sample in one interferometer arm, we isolate the coherent molecular vibrational emission from the broadband, impulsive excitation. We predict a potential improvement in detectable concentration compared to direct transmission geometry by more than one order of magnitude. The presented concept will benefit sensing applications requiring high detection sensitivity and dynamic range, including time-domain and frequency-domain spectroscopy and affords the potential of separating the nonlinear polarization response of a sample from the linear one, upon excitation with intense laser pulses.