Direct-gap optical gain of Ge on Si at room temperature

Lasers on Si are crucial components of monolithic electronic–photonic integration. Recently our theoretical analysis has shown that Ge, a pseudodirect bandgap material compatible with Si complementary metal oxide semiconductor technology, can be band engineered by tensile strain and n-type doping to achieve efficient light emission and optical gain from its direct gap transition. We report on what is to our knowledge the first experimental observation of optical gain in the wavelength range of 1600–1608 nm from the direct-gap transition of $n^{+}$ tensile-strained Ge on Si at room temperature under steady-state optical pumping. This experimental result confirms that the band-engineered Ge on Si is a promising gain medium for monolithic lasers on Si.