Diode-Laser-Pumped 551-nm Upconversion Laser in YLiF4;Er3+

A single-mode diode laser at 802 nm was used to pump the 4I15/2➝4I9/2 transition of YLiF4Er3+, producing up to 2.3 mW of laser output at 551 nm (4S3/2 - 4I15/2) at 50 K; 0.5 mW of green output were obtained at 77 K. The upconversion mechanisms were found to depend on the pump wavelength. Energy transfer crossrelaxation is generally the dominant mechanism, but at certain pump wavelengths two-step absorption becomes important. Lasing at 560.6 nm from the 2H9/2 level is also reported. Although the upconversion excitation of 2H9/2 requires more than two pump photons, the efficiency of this laser is comparable to that at 551 nm which uses a two-step process.