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Comparison of upconversion efficiencies between STEP and SHG mediated upconversion process in an Er3+ doped LiNbO3 crystal

The science for near infrared (NIR) pumped blue/green lasers has been an intense research field for display and data storage applications. Second harmonic generation (SHG) in nonlinear optical (NLO) crystals1 and upconversion lasing in rare earth doped solids2 have been promising approaches. The infrared to visible upconversion efficiencies for any case are of paramount importance for practical applications. Auzel3 compared the efficiencies for cw NIR laser excitations in different media, demonstrating that the fluorescence due to sequential two-photon excitation process (STEP) in a doped crystal is orders of magnitude stronger than SHG in a pure NLO crystal because real (resonant) transitions are involved in the STEP, in contrast to the SHG. However, with pulse laser excitations, the opposite can be true because SHG efficiency depends on the peak intensity of the pump pulse.