Nonlinear vibrational spectrometer for bioapplications featuring narrowband 1-μm pulses and a recycled OPA pump beam

Moving the detection wavelength in vibrational sum-frequency generation (VSFG) spectroscopy to the near-infrared (> 700 nm) is crucial to avoid autofluorescence and electronic resonance effects distorting the molecular phase response or triggering unwanted chemical and photophysical reactions. Here, we employ an ultra-narrow bandpass interference filter on Yb laser pulses to generate an upconversion beam near 1.03 μm at a spectral resolution of 5 cm−1. We demonstrate that the beam rejected by the filter can be utilized for driving a supercontinuum-seeded near-infrared optical parametric amplifier, boosting the seed energy for a broadband LiGaS2-based mid-infrared amplifier. Through the example of a chromoprotein, we show that utilizing the scheme can extend the range of interfacial molecular systems that can be studied using VSFG spectroscopy, even in the case of high repetition rate Yb pump lasers with limited pulse energy. The scheme can also be utilized in other types of vibrational spectroscopie that derive their spectral resolution from bandpass-filtering of femtosecond near-infrared laser pulses, such as stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) spectroscopy.