High-power Few-cycle MID-IR Pulse Generation for Vibrational Spectroscopy

Abstract

High-power, high-energy, ultrashort, mid-infrared (MID-IR) laser systems operating at high repetition rates are of significant interest for coherent vibrational spectroscopy investigations within the "fingerprint region'' (6-16 um). First of all, the optical properties of Li-based nonlinear crystals (NLC) for MID-IR generation are evaluated under high power laser irradiation at 1030\,nm central wavelength using a thermal imaging method. It turns out that lithium gallium sulfide (LGS) crystal exhibits a relatively low linear absorption coefficient < 0.002 cm^{-1}, the lowest nonlinear absorption coefficient < 3.2 x 10^{-4} cm/W and a nonlinear refractive index < 6.4 x 10^{-15} cm^{2}/W, positioning it as a highly promising material candidate for MID-IR optical parametric chirped-pulse amplifier (OPCPA) applications.

Based on the LGS crystal, a versatile design of a MID-IR OPCPA laser system is developed, featuring two complementary operation modes differ that in the group-delay dispersion (GDD) of the signal pulse. One scheme provides a wavelength-tunable source (from 4.2 to 11 um) at ~ 1 ps pulse width, while the other scheme generates a broadband pulse (from 7 to 11 um) centered at 9 um with 114 fs pulse duration, which corresponds to about 3 optical cycles. Both MID-IR laser operation modes exhibit high average power exceeding 200 mW and high pulse energy of 1.2 uJ operating at 200 kHz, having significant potential for vibrational spectroscopy and microscopy making use of characteristic molecular fingerprints in the MID-IR spectral range.

Moreover, by utilizing the broadband ultrashort MID-IR pulse, the ultra-broadband vibrational sum-frequency generation (BB-VSFG) spectroscopy is demonstrated using glucose pellets as a proof-of-principle sample, revealing 8 characteristic vibrational modes spanning from 800 to 1400 cm^{-1}. Notably, the carbon-oxygen bond stretching mode at 1035 cm^{-1} shows high sensitivity to biologically relevant 10 mM of glucose solution in the VSFG spectra. Consequently, this methodology holds promise for blood sugar monitoring in diabetic individuals.