The Effect of Self-Heating on the Modulation Characteristics of a Microdisk Laser
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.