Time-domain, field-resolving spectrometer for mid-infrared molecular spectroscopy

Abstract

We present an apparatus for sensitive and high-dynamic-range, time-domain, electric-field-resolved measurements of long-wave mid-infrared waveforms. A state-of-the-art Yb-modelocked-oscillator based femtosecond frontend provides a 28-MHz-repetition-rate train of 16-fs, 1-µm pulses with an average power of 60 W. These generate waveform-stable few-cycle mid-infrared transients via intrapulse difference-frequency mixing in a nonlinear crystal, and are also used for sub-optical-cycle temporal gating in electro-optic sampling. We report thermal-background-free, linear field detection of 13 orders of magnitude in intensity around a central wavelength of 8.6 µm, capable of measuring pulses with the lowest signal detectable amounting to 0.0001 photons per pulse. The sensitivity and high dynamic range of detection, the confinement of the infrared light to sub-two-cycle pulses with an intensity full width at half maximum of 56 fs, and the robustness against thermal background, render this system ideal for infrared vibrational spectroscopy of strongly absorbing samples, such as biofluids, live cells and biological tissue.