Applicability limits of the particle-in-сell (PIC) method for the calculation of jet gasdynamic flows under conditions of pressure variations by four or five orders of magnitude are studied. Three approaches permitting one to determine real limits of the model adequacy from the side of low pressures are considered. Based on the analysis of the results, it is shown that the PIC method adequately operates in the pressure range of 5–105 Pa in spite of the fact that, formally, the PIC method can operate also at lower pressures.
A new variant of the method of probability density distribution recovery for solving topical modeling problems is described. Disadvantages of the Gibbs sampling algorithm are considered, and a modified variant, called the “granulated sampling method,” is proposed. Based on the results of statistical modeling, it is shown that the proposed algorithm is characterized by higher stability as compared to other variants of Gibbs sampling
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.
Results of an experimental study of the noise temperature (Tn) and noise bandwidth (NBW) of the superconductor NbN hot-electron bolometer (HEB) mixer as a function of its temperature (Tb) are presented. It was determined that the NBW of the mixer is significantly wider at temperatures close to the critical ones (Tc) than are values measured at 4.2 K. The NBW of the mixer measured at the heterodyne frequency of 2.5 THz at temperature Tb close to Tc was ~13 GHz, as compared with 6 GHz at Tb = 4.2 K. This experiment clearly demonstrates the limitation of the thermal flow from the NbN bridge at Tb ≪ Tc for mixers manufactured by the in situ technique. This limitation is close in its nature to the Andreev reflection on the superconductor/ metal boundary. In this case, the noise temperature of the studied mixer increased from 1100 to 3800 K.
Organic photovoltaic cells with a bulk heterojunction have been manufactured in which the photoactive layer consists of a mixture of bithiophene copolymer or related rotaxane with a fullerene derivative (PC70BM). The mobility of charge carriers in photoactive layers has been determined, the current–voltage characteristics of photovoltaic cells have been measured, and the energy level diagram of cell components has been constructed. It is established that the polyrotaxane component (macrocycle) insulates a part of thiophene fragments of the macromolecule, thus hindering the transport of carriers and leading to large energy losses for exciton dissociation, which results in a decreasing photovoltaic effect.
The performance of quantum dot microdisk lasers operating at room temperature without thermal stabilization was experimentally investigated, and the highest modulation bandwidth of microdisks of various diameters was calculated. It is shown that taking into account the self-heating effect of the microlaser at high bias currents, which manifests itself in a decrease in the maximum modulation frequency and in an increase in the current at which the maximum speed is reached, allows us to describe the experimental data well. Self-heating effect has the greatest impact on microlasers of small diameter (less than 20 µm).
AlGaAs/GaAs microdisk lasers with InAs/InGaAs quantum dots region were transferred onto a silicon wafer using indium bonding. Microlasers have a joint electrical contact put over a residual n+ GaAs substrate, whereas their individual addressing is achieved by placing them p-contact down to separate contact pads. No effect of non-native substrate on electrical resistance, threshold current, thermal resistance, and spectral characteristics was revealed. Microdisks lase in continuous-wave mode without external cooling with the threshold current density of 0.7 kA/cm2. Lasing wavelength remains stable (<0.1 nm/mA) against injection current increment.
We report on the possibility to detect laser emission of microdisk laser having diameter of 24 μm with an active region based on InGaAs/GaAs quantum well-dots using a waveguide photodiode (100x4000 μm) with a similar active region. A photocurrent of ~ 10 μA was obtained under a continuous-wave laser operation, injection current of 20 mA and a distance between the facets of the microlaser and photodiode of about 100 μm. The photodiode’s sensitivity was estimated to be ~ 9 μA / 10 μW.
A model of the equation of state for classical gases consisting of nonpolar molecules is constructed under the assumption that the spinodal, critical isochore, and second virial coefficients of the gas have been set. The corresponding thermodynamic distributions are determined. It is shown that the isotherms constructed in the framework of the proposed model coincide with the isotherms of the van der Waals model obtained on a different basis.
The problem of interaction of strong and weak discontinuities is solved in the general case for a system of quasilinear hyperbolic equations with two independent variables. It is proved that the product of the left eigenvector of the system by a derivative of the vector function in a strong discontinuity direction remains constant during the interaction. Examples of using this fact in solving the problems of gasdynamics are presented.
Perovskite solar cells with photoactive layer of methylammonium lead iodide and hole transport layer based on a polyaniline complex with poly(2-acrylamido-2-methyl-1-propanesulfonic acid) have been developed for the first time. The power conversion efficiency of obtained cells is comparable with that of known analogs. Results of simulation of the optical parameters of cells in the framework of the Maxwell–Garnet model showed that the experimentally observed weak dependence of the power conversion efficiency on the perovskite layer thickness within 350–500 nm is related to the absence of significant variation of both the energy absorbed by the photoactive layer and the exciton generation rate.
Numerical simulations show that the introduction of aluminum nanoparticles into one layer of a bulk-heterojunction organic solar cell leads to an increase in the rate of exciton generation in the active layer of the cell. According to calculations of the optical absorption in the cell, which have been performed in the effective refractive index approximation using the Maxwell-Garnet model, a maximum relative increase in the rate of exciton generation due to plasmonic nanoparticles is about 4%.