Abstract

Femtosecond time-resolved mid-infrared (MIR) spectroscopy based on chirped-pulse upconversion is a promising method for observing molecular vibrational dynamics. A quantitative study on nonlinear media for upconversion is still essential for wide applications, particularly at the frequencies below $2000{\mathrm{cm}}^{-1}$. We evaluate wide-bandgap nonlinear crystals of Li-containing ternary chalcogenides based on their performance as the upconversion medium for femtosecond MIR spectroscopy. The upconversion efficiency is measured as a function of the MIR pulse frequency and the chirped pulse energy. ${\mathrm{LiGaS}}_2$ is found to be an efficient crystal for the upconversion of MIR pulses in a wide frequency range of $1100–2700{\mathrm{cm}}^{-1}$, especially below $2000{\mathrm{cm}}^{-1}$. By using ${\mathrm{LiGaS}}_2$ as an efficient upconversion crystal, we develop a MIR pump–probe spectroscopy system with a spectral resolution of $2.5{\mathrm{cm}}^{-1}$, a time resolution of 0.2 ps, and a probe window of $120{\mathrm{cm}}^{-1}$. Vibrational relaxation dynamics of CO stretching modes of ${\mathrm{Mn}}_2{(\mathrm{CO})}_{10}$ in cyclohexane and bovine serum albumin in ${\mathrm{D}}_2\mathrm{O}$ are demonstrated with a high signal-to-noise ratio.

© 2016 Optical Society of America

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