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

Abstract: Moving the detection wavelength in vibrational sum-frequency generation (VSFG) spectroscopy to the near-infrared (> 700 nm) can potentially enable the study of molecular interfaces absorbing in the visible and give access to buried bio-interfaces at minimal absorption, reduced scattering, and negligible autofluorescence. Here, we employ an ultra-narrow bandpass thin-film optical interference filter on 180-fs, 1.03-$\mu$m laser pulses to generate an upconversion beam yielding a spectral resolution of 5 cm$^{-1}$ and VSFG wavelengths between 890 and 980 nm for molecular vibrations in the fingerprint region. We demonstrate that the beam rejected by the filter can be utilized for driving a supercontinuum-seeded near-infrared optical parametric amplifier serving as the front-end of a broadband LiGaS$_{2}$-based mid-infrared amplifier. Benchmark data on a phospholipid monolayer at the air-water interface acquired using the resulting VSFG spectrometer show the possibility of achieving high resolution and signal-to-noise ratio at short acquisition times. The scheme can also be utilized in other types of vibrational spectroscopy 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.