Up-conversion luminescence in dye-doped crystals of potassium hydrogen phthalate

Optically nonlinear single crystals of potassium hydrogen phthalate were grown in the presence of a variety of laser dyes. Up-conversion fluorescence (as shown in the Figure) in the visible part of the spectrum was observed as a result of two excitation mechanisms involving trapped, oriented luminophores: two-photon absorption in general orientations, and absorption of the second harmonic of the fundamental beam at 800 nm, when aligned with the phase-matching direction.