Multifrequency room‐temperature continuous diode and Ar* laser‐pumped Er3+ laser emission between 2.66 and 2.85 μm

cw pumped laser emission on six different wavelengths is obtained for the first time in a LiYF4:15% Er3+ crystal at room temperature under diode laser excitation for 2.66, 2.716, 2.81 μm emissions and under argon ion laser excitation for 2.77, 2.81, 2.84, 2.85 μm emissions. Threshold of 5 mW absorbed power at 0.795 μm (diode) is obtained for the 2.81 μm wavelength. Time and power evolution of the laser emissions are presented. The type of up‐conversion taking place during 2.81 μm laser emission is shown to be of the addition of photons by energy transfer (APTE) type involving a two‐photon summation which depopulates the 4I11/2 rather than the 4I13/2 state as up to now believed.