The central research goal of Callsen's research group is to understand how heat forms and moves through nanoscale structures – structures widely used in electronics and optoelectronics. Unlike their large-scale counterparts, heat and charge transport in micro and nanoscale structures remain, in many cases, poorly understood. To help close that gap, Callsen's research group is developing experimental methods in solid-state spectroscopy, with the goal of testing their "nanothermometry" results against the latest theoretical models. Together, this work addresses one of the central open questions in solid-state physics, with strong potential for high-impact, internationally visible research.
 

Callsen earned his diploma in physics from Technische Universität Berlin (TU Berlin) and an honors degree from the University of Technology Sydney (UTS). He completed his PhD in quantum dot spectroscopy through a cotutelle agreement between the two universities. After a role as a postdoc at TU Berlin, he joined the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, where he also held an EU Marie Skłodowska-Curie Fellowship for two years. Callsen came to the University of Bremen in October 2020 as a junior professor, a position from which he has now transitioned into his current professorship.
 

Callsen won the Physikalische Gesellschaft zu Berlin's Physics Student Prize in 2010, and in 2015, the same society awarded him the Carl Ramsauer Prize for his doctoral thesis.