Multiphonon-assisted lasing beyond the fluorescence spectrum

As laser action emerges from fluorescence, its emission wavelength lies within the fluorescence spectrum. Exploiting multiphonon processes can take the laser emission far beyond the spectral limits defined by a material's intrinsic fluorescence. Lasing typically starts from fluorescence, and the laser wavelength is therefore limited to the fluorescence spectrum of the gain material. Accessing wavelengths beyond this intrinsic emission spectrum requires emission assisted by multiphonon processes. However, this is much weaker than the fluorescence originating from direct transitions between electronic energy levels, which only involve a small number of phonons. Here, we present the realization of lasers far beyond the fluorescence spectrum in Yb-doped YCa4O(BO3)(3) crystals. We selectively amplify three- to eight-phonon processes and suppress all the fewer-phonon ones, which we attribute to constructive interactions of vibrational modes from free oxygen sites. We obtain an overall spectral tuneability of 1,110-1,465 nm in five segments, each with its own configuration, corresponding to the three- to seven-phonon cases. The longest (eight-phonon) lasing wavelength can reach 1,518 nm, over 400 nm beyond the fluorescence spectrum. Our results shed light on strengthening the original weak electron-phonon-photon interaction and give rise to a search for on-demand lasers operating outside the fluorescence spectrum.