Lock-in amplitude-phase correlations in coherent Raman microscopy: application to lithium ores

In Stimulated Raman Scattering (SRS) Microscopy, which is typically based on modulation transfer techniques, the Lock-In amplitude and phase provide easily measured, complementary information. This information characterizes both the strength of a signal and the lifetime of the underlying process on a per-pixel basis. Despite these advantages, interpreting the phase information becomes challenging when several nonlinear optical processes occur simultaneously, as is particularly noticeable when pump-probe type signals are present. Here, we use spectral focusing SRS Microscopy to characterize a lithium ore sample. Spectral focusing SRS microscopy is based on linearly chirping the input pulses so as to achieve high Raman spectral resolution through the narrowed instantaneous bandwidth. By scanning the inter-pulse time delay, the instantaneous difference frequency and consequently, the Raman shift probed is tuned. Unfortunately, this also makes the process sensitive to delayed pump-probe signals, as shown here for lithium ore samples. We show that by using both phase and amplitude lock-in amplifier information, we can fully characterize the competing nonlinear processes which contribute during a pump-probe time-delay scan.