Ultrafast carrier dynamics in GeSn/GeSi quantum wells with inter-subband resonances

Group IV material systems represented by GeSn alloys have come forth as a promising candidate for next-generation silicon-based photonic integrated circuits, thanks to their favorable properties, including adjustable band structures and strong light–matter interaction. However, experimental investigation of carrier relaxation dynamics in these materials remains scarce, hindering deeper insights into the optical physics of the systems as well as the development of high-speed infrared optoelectronics. Here, we report a comprehensive study of both steady-state and time-resolved optical absorption in GeSn multiple-quantum-well (MQW) structures with GeSi barrier layers. Steady-state absorption measurements identify a series of step-like features, revealing cutoff wavelengths that are in excellent agreement with theoretical band-structure calculations. Furthermore, a broadband degenerate ultrafast pump–probe spectroscopy covering both the fundamental and high-order inter-subband transitions of the MQW near 2 μm wavelength is carried out. Interestingly, while typical transient reflection curves are characterized by a relatively strong plateau signal, at wavelengths in resonance with inter-subband transitions the long-lived plateaus are significantly suppressed, which is attributed to the highly efficient intervalley scattering processes. This suggests efficient relaxation pathways for the on-resonance cases on one hand and also offers a spectroscopic method for uncovering quantum transitions using time-resolved signals. Our findings provide critical perspective on band-structure engineering in GeSn-based MQW structures and guide the design of near- to mid-infrared photonic and optoelectronic devices using GeSn.