Terahertz lasers based on germanium and silicon

Recent experimental and theoretical results of impurity doped germanium and silicon terahertz lasers are reviewed. Three different laser mechanisms exist in p-type germanium. Depending on the operating conditions and the properties of the crystal, laser transitions can occur between light- and heavy-hole subbands, between particular light-hole Landau levels or between impurity states. Electric and magnetic fields are required for laser operation. In n-type silicon lasing originates solely from impurity transitions of group-V donors, which are optically excited. The properties of these lasers depend upon the chemical nature of the impurity centre and the properties of the host material. The principles of operation are discussed in terms of their basic physical concepts. The state-of-the-art performance of these lasers is summarized.