Angle-resolved Measurements Reveal the Origin of Signal Anisotropy in Pump-probe Microscopy.

Nonlinear optical microscopy modalities have become widely used in applications ranging from material characterization to tissue diagnosis. In complex systems, multiple mechanisms often contribute to the signal; for example, two-color pump-probe microscopy provides rich molecular signatures of the samples based on several nonlinear processes. Here we uncover a surprising complication, that in many cases the detection direction drastically alters the signal components and the overall amplitude. To understand the origin of this signal anisotropy, we first calculate and measure a simple case, the pump-probe signal of gold nanoshells at various detection angles, attributing the effect to scattering changes due to the sample refractive index change; we then extend this analysis, showing a striking directional dependence arising from distinct scattering profiles of two dominant nonlinear processes. These results demonstrate that comparing epi (backward) and transmission (forward) signals provides additional information, enabling cleaner separation of nonlinear contributions in pump-probe measurements.