Spectral gain narrowing in THz quantum cascade lasers

Terahertz time-domain spectroscopy is used to determine gain related parameters of terahertz quantum-cascade lasers. Due to the techniquespsila insensitivity to the QCL emission, spectral gain narrowing is observed above laser threshold. By correlating these measurements with bandstructure simulations, insight into the subband alignment as a function of the applied field is acquired.