Single-photon fluorescence enhancement in IR144 by phase-modulated femtosecond pulses

► The single-photon fluorescence in IR144 solution can be enhanced by manipulating the laser spectral phase. ► We theoretically and experimentally validate that the single-photon fluorescence enhancement should be attributed to the non-resonant two-photon absorption from the state S1 → S2. ► Our proposed mechanism is further confirmed by repeating the SPF experiment in the low laser intensity, where the non-resonant two-photon absorption from the sate S1 → S2 is eliminated.