Research On Low Dimensional Near/Mid-infrared Quantum Lasers And Their Characteristics

Aiming for potential applications in next-generation ultrafast large-scale optical network with ultralow power consumption,biomedical imaging,gas phase spectroscopy,public security monitoring,internet of things,and ultrafast wireless communication,two kinds of low dimensional quantum lasers--near-infrared?NIR?quantum dot lasers?QDLs?and mid-infrared?MIR?quantum cascade lasers?QCLs?are investigated in this dissertation,mainly in the aspects of device modeling,theoretical mechanisms and characteristics investigations which are given as followings:?1?The developments of laser and its low-dimensional trend are introduced firstly,while the critical importance of the above two kinds of low dimensional quantum lasers--NIR QDLs and MIR QCLs are described subsequently.Also,the recent developments of those two devices are then given.?2?The device models of the NIR QDLs and a recently proposed MIR transistor-injected QCLs?TI-QCLs?as well as the growth processes of corresponding active regions,quantum dots?QDs?and superlattice?SL?,are introduced.In order to modeling the QDLs and the TI-QCLs,the fourth-order Runge-Kutta method for numerical solution of rate equations,matrix and iteration algorithms of Poisson equations,highly efficient Numerov Schrodinger equation solver,self-consistent Schrodinger-Poisson algorithm,and Numerov Schrodinger equation solver with complex potential boundaries?NCPB?algorithm are given in detail to provide solid foundations for further researches in following chapters.?3?To investigate the dynamic process of the ground state spectral splitting?GSSS?as well as the tunable GSSS phenomena occurred in uncoupled quantum dot lasers?UQDLs?,a multi-population rate equations model is established and numerically solved by the fourth order Runge-Kutta method.Then,the dynamic mechanism of the GSSS in time-domain and the current density dependent tunability of the GSSS are systematically investigated and specifically interpreted based on the gain saturation effect.Calculated results show that when combined with frequency difference technology,QDLs with GSSS are promising candidates for tunable THz generation with high stability.Meanwhile,our calculated results agree well with reported experimental results,and our gain saturation effect and carrier competition explanation are consistent with the reported state-filling effect,both indicating the accuracy of our model and simulating results.?4?To explore the spectral and carrier transfer characteristics of the coupled quantum dot lasers?CQDLs?,a probabilistically coupled multi-population rate equations model is developed and solved with fourth Runge-Kutta method.Analyses of the calculated results of the CQDLs lasing spectrum under different current injection show that the coupling strength between different subbands in different QDs different sizes changes with the increasing current injections,which is further explained by both the carrier competition theory and coupled theory,giving a systematic understanding to the operation of CQDLs systems.Based on the equivalent homogeneous broadening effect,temperature characteristics of the CQDLs are theoretically investigated.Calculated results show that the output spectra at low temperatures are much flatter than the ones at high temperatures and explanations are given by mode competition theory.With a lower threshold,CQDLs have a much broader output range with higher inner-quantum efficiency compared to the uncoupled QDLs,indicating that CQDLs can be the excellent light source for not only long-haul ultrahigh capacity optical communications but also on-chip photonics integrated circuits with low power consumption.?5?Aiming for more compact,designing-and fabricating-friendly tunable TI-QCL for wide MIR and THz generation,detailed theoretical analyses of the TI-QCL design,modeling and operation principle are performed,and two critical requirements,the quantum impedance matching issue for the p-doped base and n-doped SL regions and the conditions for achieving quantum cascading lasing characteristics in the SL region,are proposed and investigated systematically for the normal lasing of the TI-QCL.Calculated results show that by inserting an i-n structure between the SL in the collector and the base region,improvements of the electron injection efficiency into the SL collector region can be achieved by fulfilling the quantum impedance matching condition.By optimizing the inserted quantum impedance matching area?QIMA?structure,the total current density injected into the first active region is enhanced to be a current density of4.71 kA/cm² above threshold.By investigating the effect of space charge induced by the injection carriers in the SL region from the Poisson equation's point of view,we confirm the requirement of selective n-doping to maintain the quasi-neutrality and constant electric field in the SL region for cascading lasing,where the quasi-neutrality of the SL region can be easily fulfilled by varying the base-emitter bias V_be of the TI-QCL.?6?To investigate the electrical tunability of the TI-QCL,the SL miniband spectra are calculated by the NCPB algorithm,the roles of each level in the minibands are then ascertained by analyzing the transition dipole moments?TDMs?,and the variations of energy separation and TDMs between different levels as functions of the electric field at the SL region and corresponding spectrum gain cross sections are calculated and analyzed respectively.Calculated results of the miniband spectra in each period of SL by NCPB algorithm are found to be in a good uniformity among all SL cells.By changing the emitter-base bias V_cb,the lasing wavelength of a given SL can be tuned over a wide range.Under a particular bias,the TI-QCL is predicted to be able for dual-or multi-color lasing with wide wavelength separation.The calculated results of the spectrum gain cross section method are consistent with the ones by the TDMs analysis,promising the TI-QCL's potential as compact,designing-and fabricating-friendly tunable structure for wide MIR and THz generations.?7?Based on the TI-QCL structure,a novel design-friendly device called the transistor-injected dual doping quantum cascade laser?TI-D²QCL?with two different doping in each stack of a homogeneous superlattice is proposed for broad flat gain spectrum generation,and corresponding operation and design principles of above dual doping SL stack?D²SLS?are then given in detail.After solving the D²SLS with the NCPB algorithm for the minibands and ascertaining the roles of all levels by the TDMs analysis,the individual gain spectrum cross section of each given SL cell?SLC?,as well as the total gain spectrum cross section of corresponding designed D²SLS are calculated,calculations showed that a broad flat gain spectrum operation in the MIR window can be generated by simply designing the dual doping profile all SLCs.In addition,wideband THz generation can also be achieved by applying the dual doping approach to a demonstrated functional THz lasing SLC structure with the D²SLS through the same roadmap.One can expect that.By optimizing the doping levels,and increasing the numbers of SLC and D²SLS,respectively,our novel proposal of TI-D²QCL could be a powerful approach with high common-fit property for ultra-broad flat gain spectrum generations as well as in advanced MIR and THz frequency comb technologies.