High-contrast fluorescence polarization microscopy through stimulated emission

In this work, we demonstrate the contrast enhancement through polarization-resolved pump-probe microscopy, implemented by measuring the spontaneous fluorescence loss induced by stimulated emission. The pump-probe measurement is compared with the conventional fluorescence polarization microscopy. The anisotropy values thus obtained are 0.11 and 0.01, respectively. The contrast improvement is attributed to the multiphoton mechanism in sharpening the point-spread function and the polarization resolving. In addition, the pump-probe technique promises higher temporal resolution in lifetime measurements than time-correlated single-photon counting, enabling more precise determination of the fluorescent molecules' rotational diffusion time constant, which is often in the sub-nanosecond regime.