Investigation of visible emission induced by infrared femtosecond pulses in erbium-doped YVO4 and LuVO4 single crystals

Illumination of single crystal samples of erbium-doped YVO4 and LuVO4 by infrared femtosecond pulses brings about an intense green luminescence assigned to the 4S3/2 - 4I15/2 transition of Er3+ ions. When lowering the sample temperature an additional broad band in the blue related to the VO43− emission appears and next grows steadily. It has been observed that the variation of wavelength of incident infrared femtosecond pulses in the region 800 nm-1600 nm affects weakly the intensity of both the blue and green luminescence bands. Analysis of luminescence dynamics made it possible to reveal that upon infrared excitation the rise time of the 4S3/2 luminescence is longer by a factor of four, roughly as compared to that recorded upon direct excitation into high energy levels of Er3+. Observed temperature dependence of up-converted blue and green luminescence implies that the energy transfer from vanadate groups cannot be considered as a main mechanism involved in the excitation erbium ions. It has been supposed that erbium ions are likely to be excited by energy transfer from free electrons created in the conduction band of the host by multiphoton excitation and /or by non-resonant multi-step ETU process involving absorption of infrared light in multiphonon side bands of electronic transitions. Creation of free electrons has been corroborated by changes of electrical conductivity of crystals induced by an illumination with infrared femtosecond pulses.