Abstract
<title>Abstract</title> <p>Thermal management is a major challenge in densely integrated photonic systems, yet the mechanisms governing heat generation in operating nanoscale devices remain poorly understood. Here, the spatio-temporal evolution of thermal loads in an electrically driven GeSn/SiGeSn multiple-quantum-well microdisk laser is resolved using a multimodal approach combining time-resolved operando Dark-Field X-ray microscopy, electro-thermal finite-element simulations, and three-dimensional focused ion beam–scanning electron microscopy tomography. With nanosecond temporal and sub-200 nm spatial resolution, this methodology directly visualises heat-induced lattice strain and enables quantitative reconstruction of the corresponding temperature field under realistic electrical injection conditions. Heat generation is strongly localised within the electrical contact area in Ge layer rather than in the gain medium. Current crowding at the contacts creates nanoscale hot areas that induce asymmetric strain fields extending into the active region, resulting in spatially non-uniform optical gain. Contact architecture, current injection uniformity, and interfacial defects emerge as the primary factors governing thermal loading and device-to-device performance variability. These results establish contact-mediated Joule heating as the dominant thermal bottleneck in electrically pumped group-IV microlasers and identify contact engineering as a direct route to improved performance. More broadly, they show that thermal behaviour in nanoscale photonic devices is governed primarily by current injection pathways and structural design, thereby providing a framework for developing thermally robust, CMOS-compatible light sources.</p>