Single-shot detection of mid-infrared spectra by chirped-pulse upconversion with four-wave difference frequency generation in gases

Summary form only given. Single-shot detection of an entire MIR spectrum (500-5000cm
^-1
) has been required for advanced molecular spec-troscopy such as pump-probe spectroscopy to trace ultrafast structural dynamics of molecules, real-time molecular imaging of biological tissues, etc. However, due to low pixel numbers, low sensitivity, and high cost of multi-channnel MIR detectors, the bandwidth has been limited to ~500 cm
^-1
at direct measurement of MIR spectra by using dispersive infrared spectrometers.Chirped-pulse upconversion is an alternative approach to detect an MIR spectrum with single-shot. By converting the wavelength of coherent MIR pulse to visible range, it becomes possible to detect MIR spectra with a visible spectrometer, which has much higher performance than MIR spectrometers. However, the bandwidth of the chirped-pulse upconversion has still been limited to ~1000 cm-1 because of the limited transmission range of the nonlinear crystals [1]. In this contribution, we have demonstrated ultrabroadband detection of MIR spectra on a single-shot basis using chirped-pulse upconversion with four-wave difference frequency generation (FWDFG) in gases. The schematic of the method is shown in Fig. 1(a). By using a gas as a nonlinear medium, the detection bandwidth becomes dramatically broad due to wide transmission range of gas media. Experimental demonstration of the scheme was realized with the system described as follows. We generated sub-single-cycle MIR pulses by using four-wave mixing of the fundamental and the second harmonic of Ti:sapphire amplifier (Femtopower compactPro, FEMTOLASERS) output through filamentation in air, which is basically the same generation scheme as that reported in Ref. 2 and 3. A small portion of the fundamental pulse (0.1 mJ) before the compressor of the Ti:sapphire amplifier was used as a chirped pulse, whose pulse duration was 10.3 ps. The chirped pulse Eref(t - τ) and the MIR pulse (EIR(t), 0.5 μJ) were focused into xenon with a parabolic mirror (f=50 mm) and generated a FWDFG signal, E2 ref(t - τ)EIR(t), which spread from 400 nm to 550 nm. The spectrum of the FWDFG signal was measured with a conventional spectrometer with a camera EMCCD (ProEM+1600, Princeton Instruments). The camera was synchronized with the repetition rate (1 kHz) of the laser and the spectrum was measured with a single shot, namely within 1 ms. A typical spectrum is shown as the upper curve in Fig. 1(b). By using retrieval algorithm from the upconverted spectrum to the original MIR spectrum [4] including the nonlinear chirp of the reference pulse, it was possible to retrieve the MIR spectrum shown as the lower curve in Fig. 1(b). Fine structure due to absorption of carbon dioxide (~2300 cm-1) and water vapor (~1600 cm-1 and ~3700 cm-1) in air was clearly observed. At the conference, we plan to show the application of the system to MIR absorption spectroscopy.