Sub-second hyper-spectral low-frequency vibrational imaging via impulsive Raman excitation

Real-time vibrational microscopy has been recently demonstrated by various techniques, most of them utilizing the well-known schemes of coherent anti-stokes Raman scattering and stimulated Raman scattering. These techniques readily provide valuable chemical information mostly in the higher vibrational frequency regime (>400 cm⁻¹). Addressing the low vibrational frequency regime (<200 cm⁻¹) is challenging due to the usage of spectral filters that are required to isolate the signal from the Rayleigh scattered excitation field. In this Letter, we report on rapid, high-resolution, low-frequency (<130 cm⁻¹) vibrational microscopy using impulsive coherent Raman excitation. By combining impulsive excitation with a fast acousto-optic delay line, we detect the Raman-induced optical Kerr lensing and spectral shift effects with a 25 μs pixel dwell time to produce shot-noise limited, low-frequency hyper-spectral images of various samples.