Ultrabroadband mid-infrared spectroscopy with four-wave difference frequency generation

摘要

Four-wave difference frequency generation (FWDFG) is a third-order optical parametric process, which is generally explained as omega(1) + omega(2) - omega(3) -> omega(4) or omega(1) - omega(2) - omega(3) -> omega(4), where three input frequencies are omega(1), omega(2), and omega(3), and the output frequency is omega(4). Here we report the use of FWDFG for chirped-pulse upconversion (CPU) of an ultrabroadband mid-infrared (MIR) supercontinuum and the application of the technique for MIR spectroscopy. When the CPU technique is used for MIR spectroscopy, ultrashort MIR pulses are converted into visible ones. This way, the spectra can be recorded with a visible spectrometer, which has much higher performance than MIR spectrometers. In the previous experiments, the CPU has been performed by using sum-frequency generation (SFG) with a solid crystal, and the bandwidth has been limited to less than 1000 cm(-1) due to the phase matching condition of the SFG. This limitation can be removed by using FWDFG, which is a third-order nonlinear process that allows us to use centrosymmetric nonlinear media such as gases for the upconversion. Since gaseous media have much less dispersion than solid media, the bandwidth of the phase-matching condition for the upconversion process becomes very broad. In our experiments, the entire spectrum of theMIR supercontinuum spanning from 200 to 5500 cm(-1) was upconverted by using a 4.9 ps chirped pulse to visible wavelength radiation, which was detected with a conventional visible dispersive spectrometer. The technique has been applied to attenuated total reflectance MIR spectroscopy. Absorption spectra of liquids in the range from 200 to 5500 cm(-1) were measured with a visible spectrometer on a single-shot basis.