Unveiling Ultrafast Carrier Dynamics of Tellurium Microcrystals by Two-Color Asynchronous Sampling Infrared Transient Absorption Spectroscopy

Tellurium (Te) microcrystal with a bandgap of approximately 0.37 eV is a potentially useful semiconducting material exhibiting ultrafast electronic relaxation processes. To measure the intervalley and intravalley relaxation rates, we carried out two-color near-IR (NIR) pump and mid-IR (MIR) probe studies of rod-type Te microcrystals, employing a repetition-frequency-stabilized NIR (800 nm) laser and an MIR (3300 nm) frequency comb. Using interferometrically detected two-color asynchronous sampling (AS) transient absorption (TA) spectroscopy, we measured time- and frequency-resolved TA signals of rod-type Te microcrystals. The frequency-resolved and excitation-intensity-dependent AS-TA signals show that the charge carriers undergo relaxation processes with different time constants after photoexcitation. In this work, we found that there are three distinguishable relaxation components that correspond to an ultrafast (a few picoseconds) component associated with the MIR absorption of NIR-excited electrons in the conduction band, two stimulated emission processes associated with the recombination of electrons at the band edge with holes of the valence band with time constants of approximately 75 and 350 ps. We anticipate that the present NIR pump MIR probe spectroscopy with two repetition-frequency-stabilized lasers, which does not require any mechanical pump-probe time delay scanning devices, is useful for studying electron-hole dynamics in the MIR spectral range with femtosecond time resolutions and a few nanoseconds dynamic range measurements in semiconductor microcrystals with MIR band gaps.