Terahertz Stimulated Emission from Silicon Doped by Hydrogenlike Acceptors

Stimulated emission in the terahertz frequency range has been realized from boron acceptor centers in silicon. Population inversion is achieved at resonant optical excitation on the 1Γ+8 → 1Γ−7, 1Γ−6, 4Γ−8 intracenter transitions with a midinfrared free-electron laser. Lasing occurs on two intracenter transitions around 1.75 THz. The upper laser levels are the 1Γ−7, 1Γ−6, and 4Γ−8 states, and the lower laser level for both emission lines is the 2Γ+8 state. In contrast to n-type intracenter silicon lasers, boron-doped silicon lasers do not involve the excited states with the longest lifetimes. Instead, the absorption cross section for the pump radiation is the dominating factor. The four-level lasing scheme implies that the deepest even-parity boron state is the 2Γ+8 state and not the 1Γ+7 split-off ground state, as indicated by other experiments. This is confirmed by infrared absorption spectroscopy of Si:B.