Abstract
The thermal lens technique was carried out to experimentally determine the influence of the energy transfer upconversion (ETU) processes on fluorescence quantum efficiency (eta) in Nd3+-doped materials. The samples with high Nd3+ concentration present a considerable reduction in eta?with the increasing excitation power due to the efficient ETU processes. Besides, the energy migration was identified as the mechanism responsible for the upconversion losses. In addition, it was verified that the critical inversion density is not concentration independent, as previously stated, but it decreases with the Nd concentration. Our results point out the approach based on TL technique as a valuable alternative because of its sensitivity allowing the measurements to be performed in a pump power regime that avoids damages in the investigated material.
Highlights
Intense diode pumping of active materials doped with Nd3+ is a common approach to produce efficient, reliable, and compact high-power laser systems
In the high power regime, energy-transfer upconversion (ETU) causes a nonlinear increase of the thermal lens (TL) signal (θ) with excitation power [3,8]
We have demonstrated that the nonlinear dependence of the TL signal with excitation power can be used to study the energy-transfer upconversion in Nd-doped materials
Summary
Intense diode pumping of active materials doped with Nd3+ is a common approach to produce efficient, reliable, and compact high-power laser systems. It is well know that ETU is normally the dominant mechanism [10,11] This process involves the interaction of two excited Nd3+ ions in the 4F3/2 metastable laser level, such that one ion returns to the 4I11/2 and/or 4I13/2 while the other is promoted to the higher-lying excited state (4G7/2, 2K13/2, 4G9/2, 2D3/2, 4G11/2 and 2K15/2). These states decay by fast multiphonon relaxation back to the 4F3/2 level, generating heat, and can, be treated as a single state for our purpose. An accurate knowledge of the ETU rate is important for appropriate design of intensely pumped systems
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