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We report results from ultrafast two-color optical pump-probe spectroscopy onbulk $\beta$-Ga$_2$O$_3$. A two-photon absorption scheme is used to photoexcitecarriers with the pump pulse and free-carrier absorption of the probe pulse isused to record the subsequent dynamics of the photoexcited carriers. Ourresults are consistent with carrier recombination via defect-assistedprocesses. We also observe transient polarization-selective optical absorptionof the probe pulse by defect states under nonequilibrium conditions. A rateequation model for electron and hole capture by defects is proposed and used toexplain the data. Whereas the rate constants for electron capture by defectsare found to be temperature-independent, they are measured to be stronglytemperature-dependent for hole capture and point to a latticedeformation/relaxation process accompanying hole capture. Our results shedlight on the mechanisms and rates associated with carrier capture by defects in$\beta$-Ga$_2$O$_3$.

Measurement of Ultrafast Dynamics of Photoexcited Carriers in $\beta$-Ga$_2$O$_3$ by Two-Color Optical Pump-Probe Spectroscopy