Trapped radiation in injection lasers

The effect of trapped radiation on the stimulated emission threshold and power in injection lasers is investigated theoretically and experimentally. The density of trapped radiation is determined by the relation between the gain and the loss coefficients for this radiation in the active region. The last value depends upon treatment of the cavity sidewalls and laser diode geometry. The absorption of spontaneous emission in the active diode region provides for an additional optical excitation of the crystal and can result in a decrease of threshold current. The GaAs laser diodes with Fabry-Perot cavities and four-ended resonators were studied. The lasing threshold for the axial modes is increased with the diode width and that for the nonaxial radiating modes is decreased with it. The luminescence spectra from the cavity ends and sidewalls are investigated as a function of diode length and current density. It is shown that the gained luminescence can be an essential source of energy losses in injection lasers and leads to limitations of dimensions of laser diodes with planar p-n-junctions. Various methods for suppression of harmful radiation and for increase of the stimulated emission power in injection lasers are discussed.     

High-radiance room-temperature GaAs laser with controlled radiation in a single transverse mode

A GaAs laser for room-temperature operation is reported with high radiance and short delay times. For high radiance, the radiation of a 200-μ-wide GaAs laser diode is confined to the lowest order transverse mode. This is accomplished by operating the laser diode with an optical cavity in which the transverse mode is controlled by aperture-limiting slits. In the cavity, built of spherical lenses and plane mirrors, the mode width and height coincide with the width and height of the active region of the injection laser. The diffraction losses of the higher order transverse modes introduced by the apertures are sufficiently high to yield controlled radiation in the TEM00mode ; this control was not always assured in the earlier design of the GaAs laser with the external resonator [1]. For short delays of the onset of stimulated emission, GaAs lasers are used for which the transition from short to long delays occurs above room temperature. Also, the external cavity is coupled closer to the active region of the semiconductor diode than reported previously [1] to minimize losses that would increase the delay times. Test results on the radiance, room-temperature delay times, and mode structure of the GaAs laser with the optical cavity are presented and the observed spiking pattern is discussed.     

Radiation fields of GaAs-(AlGa)As injection lasers

Theoretical radiation patterns from a multilayer model of a dielectric waveguide are fitted to single mode experimental profiles of three symmetrical double-heterojunction cavities to test the adequacy of the model and study the effect of the dielectric parameters on the beam pattern. The radiation from the normal TE modes is approximated by plane waves, while for the TM modes it is given by boundary value solutions of the Maxwell equations. The adequacy of the theory is shown by the faithfulness of the fit out to large beam angles and low intensities, and the agreement of the adjusted cavity parameters to the experimental values. Small changes in either the cavity thickness or the dielectric constant of the internal n-type region have similar first order effects on the angular position of the minima in the profile as well as in the amplitude of the sidelobes. Depths of the minima decrease with departures of the structure from planarity. Pattern distortion from mode coupling at the interfaces and facets is not observed.     

Mode dependence on mirror contrast in Fabry-Perot microcavitylasers

Data are presented comparing the modal characteristics of short Fabry-Perot lasers having either AlAs/GaAs or higher contrast ZnSe/CaF ₂ mirrors. The far field radiation patterns, thresholds, and spectral characteristics of the lasing modes are studied with a variation of the transverse dimension of the active region. The loss of the lasing mode is studied in each cavity by comparing lasing threshold and transverse dimension of the mode for a range of pump diameters     

Mechanisms of radiative recombination in InGaAsSb/InAsSbP lasers operating in the 3.0 to 3.6-µm spectral range

It is shown that type-I or type-II heterojunctions can be formed at heterojunction boundaries, depending on the composition of the active region and/or bounding layers. This is governed by differences in the mechanisms of radiative recombination, the temperature dependence of the radiation wavelength, the polarization type of the radiation, and the current-voltage characteristics. Fiz. Tekh. Poluprovodn. 33, 233–238 (February 1999)     

The radiative recombination coefficient and the internal quantum yield of electroluminescence in silicon

The results of the analysis of variations in the radiative recombination coefficient with varying doping level and concentration of excess electron-hole pairs are reported. It is shown that, along with the effect of narrowing of the band gap calculated in the many-electron approximation, the effect of screening of the Coulomb interaction responsible for the decrease in the excition binding energy should be taken into account. Both effects produce similar trends and decrease the radiative recombination coefficient with increasing levels of doping or injection. The contributions of excitonic radiative recombination and band-to-band radiative recombination to the total radiative recombination coefficient are separated from each other. It is shown that, in the region of room temperature, both contributions are comparable, while at liquid-nitrogen temperature, the excitonic component dominates over the band-to-band component. The results obtained by refined calculations of the limiting value of the internal quantum yield of electroluminescence for the silicon diodes and p-i-n structures are presented. It is shown that the internal quantum yield of electroluminescence can be as high as 14%. However, this values sharply decreases with increasing surface recombination rate and decreasing lifetime of excess charge carriers in the bulk.     

Radiative recombination of hot carriers in narrow-gap semiconductors

The mechanism of the radiative recombination of hot carriers in narrow-gap semiconductors is analyzed using the example of indium antimonide. It is shown that the CHCC Auger recombination process may lead to pronounced carrier heating at high excitation levels. The distribution functions and concentrations of hot carriers are determined. The radiative recombination rate of hot carriers and the radiation gain coefficient are calculated in terms of the Kane model. It is demonstrated that the radiative recombination of hot carriers will make a substantial contribution to the total radiative recombination rate at high carrier concentrations.     

Radiative and nonradiative recombination law in lightly doped InGaAsP lasers

We analyse data relating to radiative and nonradiative recombination in a 1.3 ?m InGaAsP laser operating below threshold. The results show that with a lightly doped active region the radiative recombination coefficient decreases with increasing carrier concentration in accordance with previous theoretical results. Further, they indicate that the Auger nonradiative recombination is relatively small.     

Study of photoluminescence and electroluminescence mechanisms in quantum-confined InSb/InAs heterostructures

Photoluminescence and electroluminescence in InSb/InAs heterostructures with ultrathin InSb insertions grown by molecular-beam epitaxy have been systematically studied. Measurements were made in the temperature range from 2 to 300 K on a large set of samples of various designs, with both the InAs matrix and ultrathin InSb insertions grown by different methods. The primary goal of the study was to identify the main radiative recombination channels in these heterostructures. It is shown that optical transitions associated with acceptor impurity centers in the InAs matrix represent an important mechanism diminishing the efficiency of luminescence from InSb insertions at room temperature. The results obtained are important for development of optimal growth modes and design of the active region of light-emitting devices based on quantum-confined InSb/InAs structures emitting in the range 3–5 μm.     

Controlled exciton spontaneous emission inoptical-microcavity-embedded quantum wells

A theory for polaritons and intrinsic spontaneous emission by excitons in quantum wells embedded in planar optical microcavities is presented. Excitons in ideal quantum wells are extended states, and the resulting spatial coherence leads to spontaneous-emission characteristics different from those for point dipoles in a planar cavity. It is pointed out that the quality factor Q of a planar cavity is typically strongly dependent upon the in-plane wave vector k&oarr; _∥ of excitation; in particular, we demonstrate that both strongly enhanced emission (low Q) as well as strongly inhibited emission accompanied by vacuum-field Rabi oscillations (high Q) occur in the same cavity at different k&oarr;_∥. Free-and localized-exciton radiative decay and radiation dynamics associated with confined and radiation modes of the optical field are considered, as well as the temperature dependence of the emission as measured in time-resolved photoluminescence spectroscopy. Other results obtained are a two-dimensional form of the longitudinal-transverse splitting for and dispersion of exciton polaritons in the presence of a cavity     

Amplification of terahertz radiation in a plasmon <Emphasis Type="Italic">n–i–p–i</Emphasis> graphene structure with charge-carrier injection

The absorption/amplification spectrum of terahertz radiation in inhomogeneous graphene (n–i–p–i structure) with a periodic dual metal grating is theoretically investigated. It is shown that the amplification of terahertz radiation sharply increases at the plasmon-resonance frequency, when losses due to electron scattering and emission are balanced by the plasmon gain (related to the stimulated radiative interband recombination of electron–hole pairs in the inverted region of graphene).     

Recombination dynamics in GaAs/AlxGa1- xAs quantum well structures

Time-resolved and excitation-dependent photoluminescence of GaAs/AlxGa1-xAs quantum well structures reveal that recombination takes place between free carriers, not excitons, at room temperature for carrier densities at and above the mid -10¹⁶cm^-3level. Other samples show trapping and release of carriers from traps, evidence of dynamic Shockley, Hall, and Read recombination for optically active traps. The traps can be saturated to a large extent. Further studies show that they are associated with interfaces between different materials and that they become active at a temperature around 150 K. Results from all samples indicate that the bimolecular radiative recombination coefficientBfor quantum wells is no larger than the value ofBfor bulk GaAs, and may in fact be smaller. This is one of the first studies of time-resolved luminescence of impurities in quantum well structures. Impurity decays at low temperatures are found to be quite slow. A spectral line which appeared to be longitudinal optical phonon-shifted emission is shown to be due to an acceptor impurity.     

Minority carrier lifetimes and lasing thresholds of PbSnTe heterostructure lasers

A new calculation of radiative minority carrier lifetime in PbSnTe is presented which yields lifetimes an order of magnitude greater than previously accepted values. These results are used in combination with recent measurements of minority carrier lifetime and interface recombination velocity in PbSnTe double heterojunction laser structures to calculate DH laser thresholds. It is demonstrated that radiative recombination is a relatively inefficient process in present PbSnTe lasers, and that when this inefficiency is taken into account the magnitude of the experimentally observed threshold and its variation with active region width and temperature can be accurately predicted, even at low temperatures where all previous models have failed.     

1.3-μm CW lasing characteristics of self-assembled InGaAs-GaAsquantum dots

This paper presents the lasing properties and their temperature dependence for 1.3-μm semiconductor lasers involving self-assembled InGaAs-GaAs quantum dots as the active region. High-density 1.3-μm emission dots were successfully grown by the combination of low-rate growth and InGaAs-layer overgrowth using molecular beam epitaxy. 1.3-μm ground-level CW lasing occurring at a low threshold current of 5.4 mA at 25°C with a realistic cavity length of 300 μm and high-reflectivity coatings on both facets. The internal loss of the lasers was evaluated to be about 1.2 cm^-1 from the inclination of the plots between the external quantum efficiency and the cavity length. The ground-level modal gain per dot layer was evaluated to be 1.0 cm^-1, which closely agreed with the calculation taking into account the dot density, inhomogeneous broadening, and homogeneous broadening. The characteristic temperature of threshold currents T₀ was found to depend on cavity length and the number of dot layers in the active region of the lasers. A T₀ of 82 K was obtained near room temperature, and spontaneous emission intensity as a function of injection current indicated that the nonradiative channel degraded the temperature characteristics. A low-temperature study suggested that an infinite T₀ with a low threshold current (~1 mA) is available if the nonradiative recombination process is eliminated. The investigation in this paper asserted that the improvement in surface density and radiative efficiency of quantum dots is a key to the evolution of 1.3-μm quantum-dot lasers     

Intrinsic modes of radiation in ferrite patch antennas

We have found two types of radiation modes for patch antennas loaded with ferrite materials. Each mode of radiation is a linear combination of normal modes of propagation in parallel plate waveguide separated by a slab of ferrite material. We have introduced new boundary conditions in which only TE modes of oscillation in the patch antenna cavity result. According to different propagation directions relative to the applied DC field these TE modes are distinguished as transverse modes and longitudinal modes, and they possess mutually perpendicular radiation polarizations. While the longitudinal TE modes are found to form discrete modes in the frequency domain, the radiation frequency of a transverse TE mode can be continuously tuned over a wide frequency range by varying the biasing magnetic field. Circularly polarized radiations may result from simultaneous excitations of these two modes. Ferrite patch antennas of square geometry have been fabricated and tested. The measured resonant frequencies compared very well with our theory     

Lateral mode discrimination in surface emitting DBR lasers withcylindrical symmetry

The effective reflectivity and the threshold gain of all lateral modes in a surface emitting DBR laser with cylindrical symmetry are analyzed using the coupled-mode approach. Our results indicate that when a cavity is designed for operation in the odd modes, all even modes are effectively eliminated. A small perturbation is introduced into the complex dielectric constant of the active region to suppress the unwanted lateral modes of odd symmetry. The model predicts that lasing will occur in a single mode with a narrow far-field pattern     

EMI from cavity modes of shielding enclosures-FDTD modeling andmeasurements

Electromagnetic interference (EMI) from slots and apertures resulting from coupling of interior sources through enclosure cavity modes in a rectangular test enclosure is reported. EMI from a specially designed test enclosure with slots or apertures excited by interior sources was studied experimentally and with finite-difference time-domain (FDTD) modeling. The measurements and FDTD modeling agree well. The results indicate that radiation at cavity mode resonances through slots and apertures of nonresonant dimensions can be as significant as the radiation at aperture or slot resonances. The agreement between the FDTD modeling and measurements demonstrates the usefulness of FDTD for investigating aspects of shielding enclosure design such as coupling to slots and apertures and slot interactions     

Energy Transport by Radiation in Hyperbolic Material Comparable to Conduction

Radiation as a heat transfer mode inside a bulk material is usually negligible in comparison to conduction. Here, the contribution of radiation to energy transport inside a hyperbolic material, hexagonal boron nitride (hBN), is investigated. With hyperbolic dispersion, i.e., opposite signs of dielectric components along principal directions, phonon polaritons contribute significantly to energy transport due to a much greater number of propagating modes compared to that in a normal material. A many‐body model is developed to account for radiative heat transfer in a material with a nonuniform temperature distribution. The total radiative heat transfer through hBN is found to be largely contributed by the high‐κ modes within the Reststrahlen bands, and is comparable to phonon conduction. Experimental measurements of temperature‐dependent thermal transport also show that radiative contribution to thermal transport is of the same order as that from phonons. Therefore, this work shows, for the first time, radiative heat transfer inside a material can be comparable to phonon conductive heat transfer.     

First- and second-harmonic radiations from a near-millimeter-wave gyromonotron

First- and second-harmonic radiations in frequency regions centered around 120 and 240 GHz, respectively, have been observed from a gyromonotron employing magnetic fields between 34 and 54 kG and a magnetron injection gun preducing a 30-kV, 1- to 2-A electron beam. Extensive redesign of the cavity, electron beam tunnel holders, and vacuum envelope resulted in greatly improved tube performance over that obtained previously. Nine first-harmonic modes and three second-harmonic modes were identified with specific TE cavity modes. These identifications were based on agreement between the measured frequency of a radiation and the calculated resonant frequency of a TE mode, and one or both of the following: (a) the agreement between the magnetic field at which the radiation occurred and that predicted by linear theory calculations of threshold current versus magnetic field for that mode; and (b) the similarity between the far-field antenna pattern measured with calibrated, waveguide-mounted detectors and that predicted for that mode. Output power was measured with a Scientech 362 Calorimeter and also estimated by integrating the far-field antenna patterns measured with the calibrated detectors. The power ranged from a fraction of a watt to 200 W for second-harmonic modes, and from tens of watts to 2 kW for first-harmonic modes.     

降低光子晶体谐振腔Q值的方法分析

该文在分析计算金属光子晶体的正三角形晶格TE模式的色散特性、全局带隙分布图的基础上,针对金属光子晶体结构谐振腔Q值较高的问题,对降低光子晶体谐振腔Q值的方法进行了分析和设计。采用加载介质柱的混合结构和介质微扰两种方法分别对谐振腔的Q值进行有效的控制,并分析了两种方法对谐振腔模式选择性的影响。结果表明,两种方法都能在不改变谐振腔模式选择性和场分布的基础上有效降低Q值,而介质微扰的方法还同时清除了与TE04竞争的两种杂模,提高了模式选择性。