Оценка вклада поверхностной рекомбинации в микродисковых лазерах с помощью высокочастотной модуляции
The development of a fast semiconductor laser is required for the realization of next-generation telecommunication applications. Since lasers operating on quantum dot ground state transitions exhibit only limited gain due to the saturation effect, we investigate lasing from excited states and compare its corresponding static and dynamic behavior to the one from the ground state. InAs quantum dots (QDs) grown in dot-in-well (DWELL) structures allowed to obtain light emission from ground and three excited states in a spectral range of 1.0–1.3 μm. This emission was coupled to whispering gallery modes (WGMs) of a 6 μm microdisk resonator and studied at room temperature by steady-state and time-resolved micro-photoluminescence. We demonstrate a cascade development of lasing arising from the ladder of quantum dot states, and compare the lasing behavior of ground and excited state emission. While the lasing threshold is being increased from the ground state to the highest excited state, the dynamic behavior is improved: turn-on times and lifetimes of WGMs become shorter paving the way towards high frequency direct driven microlasers.
A model is developed that makes it possible to analytically determine the threshold current of a microdisk laser with consideration for its self-heating as a function of the ambient temperature and the microlaser diameter. It is shown that there exists a minimum microdisk diameter determined by self-heating, up to which continuous-wave lasing can be reached at a given temperature. Another manifestation of the self-heating effect is the existence of the ultimate working temperature, which is lower, the smaller the microlaser diameter. Reasonable agreement between the predictions of the model and the available experimental data is shown.
The operation speed of microdisk lasers with quantum dots working at room temperature without thermal stabilization has been experimentally examined, and the widest modulation bandwidth of microdisks with various diameters has been calculated. It was shown that taking into account the effect of self-heating of a microlaser at high bias currents, which is manifested in a decrease of the ultimate operation speed and in an increase in the current at which the widest modulation bandwidth is reached, enables a good description of the experimental data. The self-heating most strongly affects microlasers with a small diameter (less than 20 μm).
Considerable attention has been given in recent years to microlasers based on microdisk and microring cavities with an active region based on quantum dots (QDs), which is due to the possibility of achieving small device sizes (down to 1 μm under optical pumping and to less than 10 μm under injection pumping ) and low threshold currents (250 A/cm2 at room temperature ) combined with the ease of fabrication of microlasers of this kind. There is no need to use distributed Bragg reflectors, current apertures, and multiple-stage lithography for fabricating these lasers, nor for epitaxial heterostructures similar to those in fabrication of stripe-contact lasers. One of the main proposed applications of microdisk lasers is optical data transmission to ultrashort distances and, in the limiting case, within an optoelectronic integrated circuit, including those based on silicon. Therefore, one of the most important device characteristics of a microdisk laser is modulation bandwidth f3 dB, defined as the frequency at which the efficiency of direct modulation decreases by 3 dB relative to its low-frequency value.
The modulation frequency can be limited due to a multitude of factors , one of which is the increase in the temperature of a device through which a high-density electric current is passed. The self-heating phenomenon is characteristic to the greatest extent of lasers with small current flow area and, therefore, has been actively studied for vertical cavity surface emitting lasers, VCSELs [4, 5]. At the same time, the influence exerted by the self-heating on the high-frequency characteristics of microdisk lasers has not, to our knowledge, been studied [6, 7]. In the present study, we examine by comparing experimental data with results of a numerical simulation the relative contribution of the self-heating to the limitation of the maximum modulation frequency of injection-type microdisk lasers with QDs, which operate at room temperature without forced cooling.
The experimental values of modulation bandwidth f3 dB reported in this Letter were determined from small-signal amplitude–frequency characteristic A(f) measured in the frequency range of 0.1–20 GHz at various bias currents. We analyzed the results obtained in studying microlasers with high-density (In,Ga)As QDs . The microlasers were formed by deep etching of an epitaxial heterostructure, followed by fabrication of electrical contacts to the substrate and to the top of the cylindrical mesa. Microlasers of this kind currently demonstrate the widest modulation bandwidth exceeding 6 GHz , which made it possible to perform an optical data transmission at a rate of 10 Gb/s .
The microlaser parameters used in our calculations are listed in Table 1. The threshold current of the microdisk lasers under study is characterized by a two-component dependence on the microlaser diameter: the summand proportional to the device area can be associated with the recombination in the bulk of the active region, while the summand proportional to its perimeter may be connected with the surface recombination on the lateral walls. The K-factor shows no regular dependence on the microlaser diameter, in agreement with theoretical predictions . According to these predictions, the diameter-dependent radiation loss caused by the cavity curvature becomes noticeable only when the cavity size is comparable with the emission wavelength. The nonlinear gain saturation coefficient is negligible, which is due to the low optical power of microdisk lasers.
The dynamics of a two-component Davydov-Scott (DS) soliton with a small mismatch of the initial location or velocity of the high-frequency (HF) component was investigated within the framework of the Zakharov-type system of two coupled equations for the HF and low-frequency (LF) fields. In this system, the HF field is described by the linear Schrödinger equation with the potential generated by the LF component varying in time and space. The LF component in this system is described by the Korteweg-de Vries equation with a term of quadratic influence of the HF field on the LF field. The frequency of the DS soliton`s component oscillation was found analytically using the balance equation. The perturbed DS soliton was shown to be stable. The analytical results were confirmed by numerical simulations.
Radiation conditions are described for various space regions, radiation-induced effects in spacecraft materials and equipment components are considered and information on theoretical, computational, and experimental methods for studying radiation effects are presented. The peculiarities of radiation effects on nanostructures and some problems related to modeling and radiation testing of such structures are considered.
Let G be a semisimple algebraic group whose decomposition into the product of simple components does not contain simple groups of type A, and P⊆G be a parabolic subgroup. Extending the results of Popov , we enumerate all triples (G, P, n) such that (a) there exists an open G-orbit on the multiple flag variety G/P × G/P × . . . × G/P (n factors), (b) the number of G-orbits on the multiple flag variety is finite.
I give the explicit formula for the (set-theoretical) system of Resultants of m+1 homogeneous polynomials in n+1 variables