Membrane potential detection with second-harmonic generation and two-photon excited fluorescence: A theoretical comparison

We theoretically compare the performance of TPEF and SHG microscopy for membrane potential imaging. We argue that electrochromic TPEF and SHG membrane potential responses are reflections of the same phenomenon, and can be described in a unified manner as resulting from the linear Stark effect. We also show that TPEF and SHG exhibit similar sensitivities in the case of both electrochromic and orientational response mechanisms. Despite their similar sensitivities, SHG nevertheless presents advantages over TPEF for membrane potential imaging because of its remarkable spatial and spectral contrast, and because of its insensitivity to non-radiative excited-state damping mechanisms.