Excited-state absorption measurement of Tm³⁺-doped crystals

The excited-state absorption (ESA) spectra from the long-lived 3F4 energy-level of several crystals doped with trivalent thulium (Tm3+) ions have been measured using a pump-probe technique employing high-brightness narrowband (FWHM ~15 nm) light emitting diode (LED) as the probe. Our aim is to determine the strength of ESA channels at wavelengths addressable by commercially available laser diodes around 630-680 nm, pivotal for building waveguide upconversion lasers. The favorable lifetime of the 3F4 energy-level and negligible ground-state absorption for the red-wavelength second-step excitation, ensures a direct and efficient excitation route for a dual-wavelength pumping scheme of the thulium ion, which will enable blue-green laser emission from its 1G4 manifold.ESA measurements in the red spectral region were conducted at room temperature utilizing the pump-probe technique. The advantage of the LED probe over a broadband incandescent lamp is that stimulated-emission at the probe wavelength can be eliminated, simplifying the analysis of the measurement and the determination of the magnitude of the ESA cross-section. Four Tm-doped crystalline hosts Y3Al5O12 (YAG), YAlO3 (YAP), LiYF4 (YLF), and KYGdLu(WO4)2 (KYGdLuW) were investigated, with polarized ESA spectra measured for the non-isotropic samples. Excitation of the Tm3+ ions into the intermediate 3F4 manifold was achieved via non-radiative and cross-relaxation energy transfer from the 3H4 manifold, pumped by a current-modulated fibre-coupled GaAs laser diode operating at ~790 nm. To ensure that ESA at all wavelengths of interest were resolved, the LED's spectra were tuned via the modulated drive-current amplitude and junction temperature. As such, with direct pump and probe modulation, we eliminated the need for mechanical choppers, simplifying the optimization of frequency-resolved ESA measurements needed to isolate the other possible ESA transition (3H4 to 1D2). Vastly improved ESA spectra have been obtained with respect to those reported in the literature