Magneto-spectroscopy and development of terahertz quantum cascade lasers

In this work we concentrate our efforts on the generation of laser emission at low THz frequencies (3-1 THz range) employing the quantum cascade technology. Quantum cascade (QC) lasers are unipolar semiconductor lasers based on intersubband transitions in quantum wells that cover a large portion of the Mid and Far Infrared electromagnetic spectrum. Two main research lines have been followed: (i) the development of quantum cascade lasers based on population inversion between parabolic subbands and (ii) the development of low frequency QC lasers based on a three-dimensional electron confinement induced by an external magnetic field. (i) : Gain and laser action have been demonstrated in different systems at frequencies of 3.4 3.6 THz exploiting bound-to-bound and bound-to-continuum optical transition. A QC laser emitting at 3.6 THz and based on a vertical transition and resonant tunneling in a single quantum well has been demonstrated. To overcome the limitations in performance of such a system, an heterostructure laser based on a bound-to-continuum transition has been developed. The structure was the first one to operate above the technologically important temperature of liquid nitrogen. With a further development of the bound-to-continuum design that includes lower state lifetime reduction by optical phonon emission, laser action was successfully achieved at 115 K. A study of different waveguide mechanisms suitable for different THz frequencies has also been carried out. (ii) : THz quantum cascade lasers based on the in-plane confinement introduced by a strong magnetic field applied perpendiculary to the plane of the layers have been developed. A model system based on large single quantum wells (50-60 nm wide) has been exploited to study this gain mechanism. Such an approach led to the extension of the frequency range of operation of QC lasers, with the demonstration of laser action at 1.39 THz (220 m) which is the lowest frequency observed to-date for this kind of technology. The size confinement induced by the magnetic field radically modifies the physics of the system allowing laser action with extremely reduced threshold current densities (0.65 A/cm2), a factor of 70 lower than any other quantum cascade laser and among the lowest ever observed for a semiconductor laser in general. Electron wavefunction localization is at the basis of the observed effects. Resonances in transport and in laser emission characteristics have been observed and attributed to many-body phenomena. The magnetic field has been used also as a spectroscopic tool to investigate the structures developed in the research line (i). Multi-frequency operation obtained by selectively injecting carriers in the excited states of a single quantum well structure has also been demonstrated.

Thèse de doctorat : Université de Neuchâtel, 2005 ; 1882