Two-photon excited fluorescence microscopy is now an indispensable imaging tool for studying biological samples because of its intrinsic optical sectioning. However, both of its contrast and penetration depth are still limited when imaging deep inside of scattering samples. Herein, we propose a general spectroscopy concept to enhance the image contrast and the fundamental depth limit of two-photon imaging. We show that the population transfer kinetics of the photoinduced molecular switches could generate additional high-order nonlinearity between the signal and the laser intensity. Due to the long-lived nature of these switchable states, the incident photons can operate in a sequential manner, and the nonlinearity effect could accumulate (up to sixth order) as the population is being cycled through these states. Conceptually different from conventional multiphoton processes mediated by transient virtual states, our strategy constitutes a new class of fluorescence microscopy with high-order nonlinearity that is mediated by population transfer.