A method for correcting the excitation power density dependence of upconversion emission due to laser-induced heating

Upconversion (UC) materials enable the emission of a higher energy photon following the absorption of two or more lower-energy photons. The efficiency of UC is non-linear as it depends on the power density of the optical excitation. Hence, the measurement of the UC intensity as a function of excitation power density is a standard material characterization. Such a measurement reveals at which excitation power densities the UC material can be effectively used and hints at the underlying physical mechanisms of UC. One issue in the interpretation of these measurements is the decrease in the UC intensity due to the excitation-laser-induced heating of the sample. Here, we study the excitation-laser-induced heating of beta-NaYF4:Yb3+/Er3+ (18/2%) micro-powders, ascertaining the temperature as a function of time (and the steady-state temperature reached) for a series of excitation power densities by monitoring the thermally coupled peaks in the emission of the Er3+ ion. Based on this, we demonstrate a technique to correct the excitation power density dependence of the UC in order to remove the effect of sample heating from the measured data. Correcting the reduction in UC intensity at higher power densities (> 50 W/cm(2)), we recover the UC excitation power density dependencies that agree with theoretical predictions (that disregard thermal effects) in the high excitation power density regime. The reduction in UC intensity due to sample heating can be significant; for example, a 60 K temperature rise lead to a 24% decrease in UC intensity at an excitation power density of 250 W/cm2. However, the heating does depend on the nature of the sample. In contrast to the powder samples, beta-NaYF4:Yb3+/Er3+ (18/2%) nanoparticles in toluene solution showed no heating at excitation power densities up to 250 W/cm(2). Therefore, we conclude that the proposed correction can remove the effects of sample heating to allow accurate comparison to physical models, and is of especial relevance to powder samples due to their higher absorption of the excitation and poorer thermal transport.