Steady state lasing in strained germanium microbridges as fundamental measure for the crossover to direct band gap

The realization of a laser on silicon (Si) from group IV materials is a long-cherished wish of the semiconductor industry; it would enable the mass production of photonic systems at low cost. However, the path towards an efficient light emitter requires material with a direct band gap, in line with all the typical group III-V lasers platforms [1]-[4]. Such configuration can be achieved by loading Ge with tensile strain [5] by alloying with Sn [6] or both [7], [8]. Here, we demonstrate steady state lasing at low temperature in strained germanium microbridges and establish this finding as a fundamental probe for the conduction band line-up between the Γ and L minima.