Optical patterns and cavity solitons in quantum-dot microresonators

We study optical transverse instabilities in quantum-dot (QD) microresonators. We develop a model for the QD susceptibility taking into account the inhomogeneous broadening of the dot emission. A linear stability analysis and numerical integration of the resulting equations are performed. Special attention is paid to the formation of such structures as optical patterns and cavity solitons, which could play an important role in the field of optical information processing. Implications for actual QD materials are discussed in view of applications.     

Enhancement and spectral shift of optical gain in semiconductorsfrom non-Markovian intraband relaxation

We will investigate some aspects of the phenomenological treatment of transition (line shape) and level broadening arising from phase and energy relaxation of Bloch states, respectively. Calculating the absorption/gain via the spontaneous emission formula and performing the broadening within the latter circumvents certain artifacts for both level and transition broadening. When using k-independent relaxation times, Gaussian (non-Markovian) broadening functions are superior to Lorentzian (Markovian) ones. In contrast to transition broadening, level broadening may even enhance the gain over its whole spectral width. In contrast to Lorentzian transition broadening, Gaussian transition broadening yields a blue shift of the gain maximum. The direction and magnitude of the spectral shift arising from Gaussian level broadening depends on the degree of degeneracy of the electron and hole bands involved. The level broadening can have a significant influence on the carrier statistics, which, consequently, has to be included into a consistent treatment. Thus, the phenomenological model functions depend distinctly on which kind of relaxation process is faster, energy or phase relaxation. For GaAs-like semiconductors, the application of transition broadening-even when using the spontaneous emission formula-to cases of dominant intraband relaxation yields significant numerical deviations from the correct treatment of level broadening. Broadening the energy levels requires an additional convolution integration. We will present an approximation, which yields excellent results for the gain in GaAs-like semiconductors. This enables one to include the significant effects of level broadening without increasing the numerical effort and leads to favorable formulae for experimental data fitting and device modeling     

Circularly Polarized Microwave Cavity Filters

A new group of circularly polarized microwave cavity filters is described. With a single circularly polarized cavity, a reflectionless filter is achieved that couples nearly 100 per cent of the energy from the main waveguide at the cavity resonant frequency. Two degenerate cavity modes may be excited, to produce a circularly polarized field, by coupling to the transverse and longitudinal waveguide magnetic fields or to the transverse electric and magnetic waveguide fields. A theoretical analysis is presented as well as experimental results. The loss between the band-pass terminals of the reflectionless circularly polarized filter is identical with the loss in a conventional reflectiontype band-pass filter with the same bandwidth and cavity-wall losses. The null at resonance between the band-elimination terminals of the reflectionless circularly polarized filter is limited only by the asymmetries of the cavity and not by the cavity-wall losses. Design equations and curves are given for eight of the lower order, circularly cylindrical, degenerate cavity modes that are coupled to a rectangular waveguide at the point of circularly polarized waveguide magnetic fields.     

Polarization state of the emission of external grating diode lasers

The laser emission of transverse electric (TE) and transverse magnetic (TM) polarization, in alternating successive spectral ranges, that can occur when external grating diode lasers are wavelength tuned, is theoretically and experimentally investigated. The modulated threshold gain curves of the external cavity for the two polarizations can be made to intersect with each other if a grating is used in such a way as to practically eliminate the difference between the laser diode losses and confinement factors for TE and TM polarization. The result is a sequence of wavelength ranges where the threshold gain is alternately lower for the TE resp. TM polarization. Within each range, a wavelength tunable single mode laser emission is obtained. The phenomenon is described, taking into account the characteristics of the key components of the external cavity, i.e., laser diode structure, antireflective (AR) coating, and grating efficiency. Design tools are proposed for the characteristics of the components necessary for the development of such an external cavity. The theoretical and design concepts are experimentally confirmed in a number of external cavities, using diode lasers emitting around 1.5 μm, with different types of antireflective coatings, as well as different grating configurations     

Cavity Model for the Slot Radiation of an Enclosure Excited by Printed Circuit Board Traces With Different Loads

Traces on a printed circuit board (PCB) couple to the cavity consisting of the PCB ground plane and a metallic enclosure cover. We introduce this common-mode coupling of a PCB trace to the cavity field by an analytical model that is verified with 3-D simulations using HFSS. The cavity field causes radiated emission from the slots of the cavity. For an accurate calculation of the cavity field inside the enclosure and the radiated emission, we consider the radiation loss by a multiport approach. Comparisons of the analytical results for the radiated field to measurements show good agreement. Radiated emission can be calculated for arbitrary geometric enclosure shapes as a function of frequency, position of the trace on the PCB, and trace load/driver impedances.      

双势阱中原子质心运动对自发辐射的抑制

研究了一个囚禁于对称双势阱中的二能级原子与单模腔场的相互作用.通过求解薛定谔方程,给出了整个系统波函数解析解和原子能级粒子数反转的解析表达式.分析了当腔场初始态分别为粒子数态、相干态以及热态时原子粒子数反转随时间的演化情况,并考虑了原子质心运动对粒子数反转的影响.结果表明通过选择合适的腔场初始态、势阱位置及相关因素,可以有效地控制原子的自发辐射率.     

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     

Modeling and Measurement of Radiated Field Emission From a Power/Ground Plane Cavity Edge Excited by a Through-Hole Signal Via Based on a Balanced TLM and Via Coupling Model

A balanced transmission line model (TLM) and via coupling model is proposed for efficient simulation of radiated field emission from a power/ground plane cavity edge, where the radiated field emission is excited by a through-hole signal via in a multilayer package and printed circuit board (PCB). The radiated field emission is simulated and measured with a series of test boards. The simulation agrees fairly well with the measurement confirming the preciseness and usefulness of the proposed model. It is shown that the through-hole signal via is a considerable source of the radiated field emission as well as the signal loss. When the signal trace is switching vertically stacked reference planes, the signal return current path is disconnected at the via and the impedance becomes extremely high. A significant amount of insertion loss and radiated field emission is generated at resonance frequencies of the plane cavity. The effect of a decoupling capacitor fence (De-Cap Fence) at the edge of the board to mitigate the radiated field emission is examined. The proposed model confirms that the De-Cap Fence changes the resonance mode and frequency of the plane cavity, and reduces the radiated field emission     

Emission-Line Width and α-Factor of 850-nm Single-Mode Vertical-Cavity Surface-Emitting Lasers Based on InGaAs/AlGaAs Quantum Wells

The emission-line width for 850-nm single-mode vertical-cavity surface-emitting lasers based on InGaAs/AlGaAs quantum wells is studied. The width of the emission line for a laser with a 2-μm oxide current aperture attains it minimum (~110 MHz) at an output power of 0.8 mW. As the optical output power is further increased, anomalous broadening of the emission line is observed; this is apparently caused by an increase in the α-factor as a result of a decrease in the differential gain in the active region under conditions of increased concentration of charge carriers and of high internal optical losses in the microcavity. The α-factor is estimated using two independent methods.     

Properties of the PbSe diode laser

Diode laser action has been obtained at 8.5μ with PbSe. This laser is of potential interest for terrestrial communications since its emission is in the 8-to-14-μ atmospheric window, a spectral region of high atmospheric transparency where attenuation due to scattering by haze is low. Fabrication techniques are described which are based on controlling carrier type and concentration by adjusting the Pb:Se ratio. Below threshold for laser action, the emission exhibits two spectral peaks, one near 8.5μ which increases superlinearly with current and another near 10.1μ which increases slowly with current. Laser action associated with the 8.5μ peak is observed above a threshold Current density of 2000 A cm^-2. From measurements which did not resolve the cavity mode structure, the emission peak was found to shift to higher energies in a [100] oriented magnetic field at the rate of7.1 times 10^{-8}eV per gauss, or 17 Mc/s per gauss. This is the expected shift if the emission is associated with band-to-band transitions. The threshold current decreased to a fraction of its zero field value in a magnetic field of approximately 10 kilogauss, then increased slowly with higher fields.     

Lifetime broadening in GaAs-AlGaAs quantum well lasers

Experimental observations of spontaneous emission spectra from GaAs-AlGaAs quantum well lasers shown that spectral broadening should be included in any realistic model of laser performance. A model of the lifetime broadening due to intraband Auger processes of the Landsberg type is described and developed for the case of electron-electron scattering in a 2-D system. The model is applied to the calculation of gain and spontaneous emission spectra and gain-current relationships in short-wavelength GaAs-AlGaAs quantum well lasers, and the results are compared with those obtained using both a fixed intraband scattering time and one that varies as n^-1/2, where n is the volume injected carrier density     

Emission Directionality of Semiconductor Ring Lasers: A Traveling-Wave Description

We use a traveling-wave model for explaining the experimentally observed changes in the directionality of the emission of semiconductor ring lasers and its different behavior when current is increased or decreased. The modulation of the cavity losses imposed by the light extraction sections together with the thermal shift of the gain spectrum and spatial hole burning in the carrier density play a crucial role in the directionality of the emission and its changes with operation current. The differences as the current is increased or decreased correspond to the different role played by spatial hole burning.     

多次外光反馈下垂直腔面发射激光器非线性动态特性理论研究

在SIMULINK平台下建立了垂直腔面发射激光器(VCSEL)动态仿真模型，利用该模型对多次外光反馈下垂直腔面发射激光器的非线性行为进行了研究。结果表明，短外腔时，光子密度随外腔长呈周期为半激射波长的余弦关系；长外腔时，随外腔长增加，垂直腔面发射激光器依次经历混沌、多周期、倍周期和单周期区域，增大外腔反射率时同样存在这些非线性区域，但出现顺序相反。进一步得出：考虑单次反馈时由于忽略了占有较多能量的高次反馈导致上述非线性效应偏弱；增大自发辐射因子或减小线性展宽因子可抑制系统的非线性行为。     

Peculiarities of the total energy distribution of field emission electrons from graphene-like structures

It is demonstrated that the field emission from graphene-like structures (GLSs) has a very low excitation threshold (about two orders of magnitude lower than that of metals and semiconductors). It is shown that the total energy distribution (TED) of field emission electrons from GLSs differs considerably from the spectra obtained for metals. The existence of two maxima of this distribution (spaced 0.7–1.7 eV apart), significant broadening of the energy distribution, and a shift of these maxima depending on the electric field intensity are reported. The resonance nature of low-threshold field emission from GLSs is confirmed.     

Noise in the Presence of Strong Signals

The electrical noise at millimeter wavelengths generated by an absorbing gas in a cavity is computed as a function of the intensity of an applied coherent signal. The phase of the noise components is strongly correlated with the phase of the coherent signal and the absorption and emission spectrum are similar in the neighborhood of the coherent signal frequency. As the intensity of the coherent field is reduced, the noise emitted by the gas-filled cavity becomes random and can be described by Nyquist's equation.     

Theoretical interpretation of the enhanced single-mode stability in extended cavity VCSELs

Paraxial eigenmode analysis demonstrates that the extended cavity stability follows from the geometric scaling of the diffractive cavity losses; the same principle also explains the narrow aperture stability. Because diffraction losses scale as (L/b)/sup 2/ = (L/spl lambda///spl pi/w/sub o//sup 2/)/sup 2/, one can enhance diffraction to differentiate thresholds among modes either by shrinking the mode waist w/sub o/ (smaller aperture) or by extending the cavity length L. The second approach does not limit the vertical cavity semiconductor laser (VCSEL) output power. Our predictions agree with experimental trends, and suggest that diffraction limited modes apply to extended cavity VCSELs, since guided mode theory (effective index or thermal lensing) does not allow strong stability dependence on cavity length. The extended length stabilization holds when factors other than diffraction (such as mode wing clipping or aperture scattering) significantly contribute to cavity losses.     

Reduction of absorption loss in asymmetric twin waveguide laser tapers using argon plasma-enhanced quantum-well intermixing

Lasers based on asymmetric twin waveguide integration technology are limited by the necessity of pumping the tapers to avoid absorption losses within this section of the active region. Here, we demonstrate that the threshold current is reduced by argon plasma-enhanced quantum-well intermixing in the taper. Intermixing induces a (57/spl plusmn/5) nm wavelength blue shift in the emission peak, accompanied by a <12-nm linewidth broadening of the photoluminescence spectrum, indicating minimal material degradation. The threshold current of a 0.6-mm-long laser is reduced by (18/spl plusmn/1)% to (27/spl plusmn/1) mA using an intermixed taper as compared to a nonintermixed structure.     

Splitting and shift of hot magnetophonon resonance peaks in a two-dimensional electron gas

The splitting of the magnetophonon resonance peaks in a two-dimensional electron system is investigated as function of the electric field (or average electron velocity) for different values of the broadening of the Landau levels. We found that for small broadening the maxima in the magnetophonon oscillations are split into two peaks. A new physical interpretation is presented for this splitting which is based on the separate contributions of LO-phonon absorption and emission processes. A shift of the resonance maxima is found when the broadening of the Landau levels is large. A new explanation is given for the apparent temperature and electron density dependence of the optical phonon frequency in heterostructures as determined from magnetophonon resonance experiments. A mechanism is proposed which is able to produce the observed shift in the resonant position and which is consistent with an interaction with the optical phonon mode of the bulk material.     

Performance of a low-voltage high-current-density electron probe

A low-voltage high-current-density electron probe has been constructed. Electrons are extracted at 7 kV from a zirconiated tungsten thermal field emission source, pass through a two-lens collimated optical system, and are decelerated by a retarding field at the sample. A focused probe is obtained at 1-keV landing energy with 40- nm diameter and 320-nA current. The corresponding peak current density is 1.3 × 10⁴A/cm². This result shows, for the first time, that space-charge beam broadening is much better controlled in these retarding field probes than in conventional low-voltage column designs where the extraction and landing energies are equal.     

Excess noise factor in laser with extra cavity devices

The effect of the nonorthogonality of the longitudinal eigenmodes in laser with an extra cavity device described by the extra cavity distributed losses, is analyzed. An expression for the excess noise factor is derived. It is shown that the extra cavity devices changes remarkably the behaviour of the noise characteristics in comparison with conventional two-mirror laser