High power high efficiency electron-hole and unipolar quantum dot lasers
The goal of this research work is to develop and analyze Quantum Dot (QD) lasers aimed at improving high power performance which is crucial for numerous scientific, military and industrial applications. Fundamentally two dissimilar types of lasers are investigated: namely bipolar electron-hole laser and unipolar quantum cascade laser. Planar quantum well (QW) laser diodes are already well-established as commercially available high power semiconductor lasers. However these lasers are unable to deliver power greater few 10's of watts due to reduction in efficiency at longer cavity lengths. This limitation arises from inherent optical losses tied to the two-dimensional density of available states in QWs. A novel approach is proposed here to circumvent this limitation by introducing self-assembled QDs into the laser cavity which due to their delta-like discrete density of states promise to reduce the optical losses by at least an order of magnitude, hence allowing cavity length to increase proportionally. Detailed analysis based on harmonic oscillator model and solution at quasi-equilibrium condition reveal that total internal losses as low as 0.05 per cm⁻¹ can be achieved in a QD laser enabling it to deliver 50 watts of power from each bar while maintaining efficiency close to 90%. In order to take full advantage of the discrete atom-like behavior, it is also of utmost importance to reduce the inhomogeneous broadening of the dot distribution originating from size fluctuation. Experimental data of ultra narrow linewidth InAs quantum dots having linewidth of only 22 meV is presented. Research attempt has been taken to integrate these narrowly distributed dots into a workable structure. Preliminary data shows that these dots are extremely sensitive to the laser material which calls for careful optimization of the entire structure. As for the unipolar QCL, it is shown that internal absorption caused by phonon emission of electrons in a planar quantum cascade laser represents a possible limitation to the maximum operating efficiency. Possibility of reducing this absorption is explored and it is optimistically asserted that introducing QDs into the gain stage of a QCL can eliminate this internal loss mechanism, thus greatly improving high power operating characteristics.