Quantum cascade lasers: unipolar intersubband infrared lasers

The quantum cascade laser relies on only one type of carrier (unipolar laser) and on quantum jumps of electrons between discrete conduction band energy levels of quantum wells. We designed a structure where electrons make a vertical radiative transition essentially in the same well. This reduces considerably the width of the gain spectrum (FWHM=10 meV) and therefore the laser threshold current density. To prevent electron escape in the continuum, which is greatly reduced in the case of the diagonal transition, the superlattice of the digitally graded injector is designed to act as a Bragg reflector for electrons in the higher excited state and to simultaneously ensure swift electron escape from the lower states via a miniband facing the latter