Quantum Dot Lasers
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The concept of carrier confinement has been of primary importance in the development of semiconductor laser. The invention of the double heterostructure, in which carriers in a narrow-gap material are confined by wide bandgap barriers, made possible the first continuous wave (CW) operation of semiconductor lasers at room temperature and their practical implementation. The next breakthrough occurred when Dingle and Henry proposed the idea to “exploit quantum effects in heterostructure semiconductor lasers to produce wavelength tunability” and to achieve “lower lasing thresholds” via “the change in the density of states which results from reducing the number of translational degrees of freedom of the carriers”. It was also shown that if the number of translational degrees of freedom of charge carriers is decreased below two, a singularity occurs in the density of states. This singularity increases light absorption or light amplification (gain). An ultimate case of size quantization is realized in QDs. A QD is a coherent inclusion of a narrow-gap material in a wide-gap matrix in which electrons are quantized in all three spatial directions. Thus a single semiconductor QD exhibits a discrete δ-function-like energy spectrum similar to that in a real atom, keeping the advantage of direct current injection, impossible in other types of lasers based on atomic transitions.
The physical advantages of QD lasers resulting from δ-function-like density of states are:
In addition, QD medium has some advantages, which are not directly related to size quantization effects:
Some of the basic advantages of QD lasers such as improvement in the temperature stability of the threshold current, high material gain, and the possibility of remarkable reduction in the laser threshold were already described in the first theoretical works on QD lasers. These works, however, were generally based on simplified assumptions such as infinite barriers, no QD size fluctuations, one confined electron and hole level, and ultrafast energy relaxation of injected carriers. More recent theoretical models had to take into account such complications as finite barriers, many electron and hole levels (effect of excited states), QD size fluctuations, many body effects, radiative and nonradiative recombination in the optical confinement layer (OCL; wide-gap matrix), charge neutrality violation in QDs, etc. It was shown that the characteristics of QD lasers depend dramatically on the parameters characterizing QD array (QD lateral size, height and corresponding position of energy levels, QD density, and size dispersion) as well as on the structure design: thickness of the OCL, doping profiles in the cladding layers and in the OCL, the band offsets at the interface between the OCL and the cladding layers. Depending on the abovementioned parameters the performance of QD lasers can be very good or poor. To get the best performance, an optimization of QD array parameters should be done consistently with the optimization of the structure design. Optimization for certain applications (low threshold current, high output power, etc.) should be done according to different criteria, and in many applications a combination of these criteria needs to be taken into account.
High temperature stability of the threshold current as the main fingerprint of QD lasers was demonstrated already in the first injection laser based on self-organized QDs. Further realization of the advantages of QD lasers was associated with the improvement in QD growth and the development of more sophisticated QD growth techniques. At present, all the advantages of QD lasers are basically experimentally proved. At the same time we believe that there is still room for improvement in the characteristics of QD lasers. Experimental and theoretical studies of QD lasers are exploding in scientific areas and further progress in the field of QD lasers can be foreseen. Recently, the first commercial QD lasers have been announced.