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Recent efforts have demonstrated efficient Cr2+ :II-VI chalcogenide (e.g. ZnSe, ZnS) broadly tunable (1.9-3.3μm) lasers under direct intra-shell Cr2+ optical excitation. We report on the spectroscopic study of Cr2++:ZnSe/ZnS under visible excitation into the charge transfer band of Cr2+ ions. Polycrystalline samples prepared by thermal diffusion method were studied. Middle-infrared (mid-IR) photo-luminescence (PL) of Cr2+ ions was compared under continuous wave (CW) direct 1532nm (5T2→5E) excitation and under 532nm excitation into charge transfer band. The quantum yield of Cr:ZnSe mid-IR luminescence under CW green excitation was estimated as close to 100% at room temperature. To estimate Cr excitation rate via charge transfer band under short pulse excitation, mid-IR PL kinetic measurements were performed with the use of 532nm picosecond and nanosecond pumping. Mid-IR PL kinetics of Cr:ZnSe under pulsed green excitation exhibit a relatively slow growth reaching a peak at ~5-10μs for nanosecond and picosecond excitations, respectively, while PL kinetics in Cr:ZnS reveal shorter measured rise time (<1μs) limited by the response time of the detector. This rise of the PL intensity under 532nm pulsed excitation implies that 5E population continues to grow after the excitation pulse due to slow relaxation processes from higher-lying excited levels of Cr2+ to the upper laser level 5E. At the same time for nanosecond excitation the excited level is pumped at a rate faster than it is depleted and, hence, it is reasonable to expect that the population of the 5E level could be inverted. For laser experiments we used 5ns radiation from BBO based optical parametric oscillator tunable over 450-700nm. Cr:ZnSe lasing at 2.5μm induced by 2+→1+→2+ ionization transitions of chromium under visible excitation was achieved.
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