Seven-Photon-Excited Upconversion Lasing at Room Temperature

In contrast to conventional harmonic generation, realized using nonlinear crystal, the multiphoton-excited (MPE) laser has critical applications in fluorescence imaging probe and biological photonics. Here, lasing emission from a crystal plate under 7-photon excitation at room temperature is reported for the first time, which is the highest order MPE lasing reported up to date. Moreover, MPE lasing spikes and high-order harmonic peaks are observed simultaneously. The saturation behavior of harmonic generation and abnormal fluorescence enhancement identify that the interaction of photon–electron in 7-photon-excited lasing transits from photon-excited to a more field-driven regime. The results may pave the way for short wavelength laser by taking advantage of the processability and larger scale of solid crystals.