Nanosecond kinetics of multiphoton upconversion in an optically trapped single microcrystal

Abstract

Recently, erbium-doped nanomaterials have been demonstrated to achieve multiband upconversion luminescence (UCL) via high excitation power and material alteration. In such a scenario, a large number of energy levels of rare-earth ions are populated, emitting light at characteristic wavelengths. However, understanding how the energy flows between these energy levels after intense excitation is rarely studied. Here, we built a setup that can optically trap single microcrystals (MCs) in solution, and record time-resolved luminescence at a nanosecond timescale. Under 976 nm nanosecond laser excitation, we observed UCL (white light) from a single β-NaYF₄:Yb/Er microcrystal (MC). Surprisingly, the Er³⁺ ions are populated through four-photon upconversion (UC) processes, except for the traditional two-photon UC processes. Two populating pathways of the four-photon UC processes were observed, i.e. pathway A (⁴I_15/2 → ⁴I_11/2 → ⁴F_7/2 → ²H_11/2 → ⁴S_3/2 → ²G_7/2 → ⁴G_11/2 → ²H_9/2 → ⁴F_5/2 → ²K_13/2) and pathway B (⁴I_15/2 → ⁴I_11/2 → ⁴I_13/2 → ⁴F_9/2 → ²H_9/2 → ²D_5/2), and A was more efficient than B. Our results suggest that pathway A (which occurs first) can promote the operation of pathway B by non-radiative relaxation processes (nRPs) and back energy transfer (BET). This can provide a method to study the kinetic process of UC systems, which may facilitate the application of MCs in color displays and waveguide-based optical devices in the future.