Mid-infrared asynchronous and interferometric transient absorption: Coherent averaging with sustained ergodicity for observing ultrafast dynamics in heterogeneous samples

Asynchronous and interferometric transient absorption (AI-TA) spectroscopy is a powerful technique for probing ultrafast dynamics, offering both time-resolved insights and frequency-domain information with a single-point detector. In this study, we successfully measured the transient absorption signal of rod-shaped tellurium microcrystals under low pump fluence using a mid-infrared (MIR) AI-TA setup. By leveraging exceptionally stable frequency combs for both the pump and probe beams, coherent averaging in the MIR AI-TA enabled the clear observation of transient absorption signals, even in heterogeneous samples under low pump fluence. The standard deviation of the TA signal decreases with averaging time (τavg) following a 1/τavg dependence, confirming the ergodicity of the TA measurement over extended averaging periods. These results highlight the significant potential of MIR AI-TA spectroscopy for advancing our understanding of ultrafast carrier dynamics in MIR-active materials.