At the Niels Bohr Institute, he carried out research in elementary particle physics, particularly analytic S-matrix theory and electromagnetic corrections to hadron scattering.
Several co-workers including Henning Olesen, Gunnar Jacobsen, Jens Henrik Osmundsen, Finn Mogensen, Kristian Stubkjær, Jesper Mørk, Xing Pan, Hans Erik Lassen and Björn Jónsson contributed to this work over a period of almost 15 years until 1995.
At TeleDanmark Research in the late 1980s and early 1990s Tromborg and colleagues worked to study the dynamics of active semiconductor materials in order to understand the physical relaxation processes at play, their strength and characteristic time scales.
Tromborg led the effort to identify this as a topic that would remain important for many years and argued that Denmark should work to lead in the field.
He also proposed theoretical methods that could be used to estimate the size of these ultrafast dynamical effects and their role in understanding the origin of nonlinear gain suppression in semiconductor lasers.
He also applied general techniques within stochastic theory and signal analysis to develop improved descriptions of noise spectra in nonlinear semiconductor optical amplifiers.