Inversionless amplification of coherent terahertz radiation

We experimentally demonstrate inversionless amplification of the emission from electronic transitions in a solid state system inside a cavity. A semiconductor-based terahertz (THz) emitter is exposed to a coherent, phasematched THz background field during the emission process leading to an increase in the overall emitted THz power of more than one order of magnitude. The experimental concept and results are discussed within a three-level model and an extended density matrix approach.