Preserving energy resp. dissipation in numerical PDEs using the Average Vector Field method

Highly mismatched semiconductor alloys such as GaNAs and GaBiAs have several novel electronic properties, including a rapid reduction in energy gap with increasing x and also, for GaBiAs, a strong increase in spin orbit- splitting energy with increasing Bi composition. We review here the electronic structure of such alloys and their consequences for ideal lasers. We then describe the substantial progress made in the demonstration of actual GaInNAs telecomm lasers. These have characteristics comparable to conventional InP-based devices. This includes a strong Auger contribution to the threshold current. We show, however, that the large spin-orbit-splitting energy in GaBiAs and GaBiNAs could lead to the suppression of the dominant Auger recombination loss mechanism, finally opening the route to efficient temperature-stable telecomm and longer wavelength lasers with significantly reduced power consumption.

/pdf/band-engineering-in-dilute-nitride-and-bismide-semiconductor-4zgdqfbmvz.pdf

Band engineering in dilute nitride and bismide semiconductor lasers

A new explicit fourth-order finite-difference time-domain (FDTD) scheme for three-dimensional electromagnetic field simulation is proposed in this paper. A symplectic integrator propagator, which is also known as a decomposition of the exponential operator or a general propagation technique, is directly applied to Maxwell's equations in the scheme. The scheme is nondissipative and saves memory. The Courant stability limit of the scheme is 30% larger than that of the standard FDTD method. The perfectly matched layer absorbing boundary condition is applicable to the scheme. A specific eigenmode of a waveguide is successfully excited in the scheme. Stable and accurate performance is demonstrated by numerical examples.

A three-dimensional fourth-order finite-difference time-domain scheme using a symplectic integrator propagator

An analytical expression for the low-temperature optical susceptibility of quantum-well semiconductor lasers is presented based on a simple parabolic band model. The optical susceptibility obtained keeps the nonlinear dependence on the carrier density, providing both a broad gain spectrum and a dispersion curve, so it can be used to analyze the dynamics of multimode devices or devices with large carrier density variations. The resulting peak gain, differential peak gain, and linewidth enhancement factor are discussed. cw operation of a single-mode laser is studied as a function of the frequency of the cavity resonance. An analytical approximation to the finite-temperature gain spectrum is also presented, although the refractive index spectrum must be determined numerically. @S1050-2947~98!07501-5# PACS number~s!: 42.55.Px, 78.66.2w The analysis of the static and dynamical properties of semiconductor lasers requires a knowledge of the coupling between the active semiconductor material and the optical field within the active region. In a semiclassical approach @1#, which constitutes the foundation for simpler descriptions as the rate equation ~RE! approximation @2#, the optical field is described by means of Maxwell’s equations, and its coupling to the material is described by the electrical susceptibility of the active medium. The imaginary part of the electrical susceptibility describes the energy exchange ~absortion or stimulated emission! between the field and the medium, while its real part describes the dispersive effect ~refractive

/pdf/simple-analytical-approximations-for-the-gain-and-refractive-86yxj2x7wy.pdf

Simple analytical approximations for the gain and refractive index spectra in quantum-well lasers

Rapid decrease of differential gain has been determined to dominate the temperature dependence of threshold current in 1.3-/spl mu/m multiquantum well and bulk active lasers giving rise to low values of T/sub 0/. Extensive experimental characterization of each type of device is described. Results are presented for the dependence of gain on chemical potential and carrier density as a function of temperature. The data indicate the important role of the temperature-insensitive, carrier density dependent chemical potential in determining differential gain. Modeling of the temperature dependence of threshold carrier density in MQW and bulk active lasers based on a detailed band theory calculation is described. The calculated value of T/sub 0/ depends on the structure of the active layer, e.g., multiquantum well versus bulk. However, the calculated values are substantially higher than measured. >

/pdf/analysis-of-gain-in-determining-t-sub-0-in-1-3-spl-mu-m-295z7tnfeo.pdf

Analysis of gain in determining T/sub 0/ in 1.3 /spl mu/m semiconductor lasers

The reflection coefficient at the dielectric interface orthogonal to the Yee-lattice axis in the finite-difference time-domain (FDTD) scheme is explicitly obtained. In the expression, the effective permittivities assigned to the nodes in the vicinity of the interface are included as parameters. The suitable effective permittivities for the accurate modeling of the interface are investigated theoretically based on the reflection coefficient. Regardless of the angular frequency, the incident angle, and the interface position relative to the lattice, second-order accuracy is achieved by the use of effective permittivities based on the weighted harmonic mean and arithmetic mean of the material permittivities. The second-order accuracy is demonstrated by numerical examples.

The second-order condition for the dielectric interface orthogonal to the Yee-lattice axis in the FDTD scheme

Low-chirp lasing operation in semiconductor lasers is addressed in a theoretical investigation of the possibility of reducing the linewidth enhancement factor ( alpha factor) in quantum-well (QW) lasers to zero. It is shown that in reducing the alpha factor it is essential that lasing oscillation be around the peak of the differential gain spectrum, not in the vicinity of the gain peak. The condition for such lasing oscillation is analytically derived. The wavelength dependence of the material gain, the differential gain, and the alpha factor are calculated in detail taking into account the effects of compressive strain and band mixing on the valence subband structure. The effect of p-type modulation doping in compressively strained QWs is discussed. It is shown that the alpha factor, the anomalous dispersion part in the spectrum, crosses zero in the region of positive material gain, which makes is possible to attain virtual chirpless operation by detuning. >

Theoretical study on enhanced differential gain and extremely reduced linewidth enhancement factor in quantum-well lasers

The effect of pure strain on the differential gain of strained InGaAsP/InP quantum-well lasers (QWLs) is analyzed on the basis of the valence band structures calculated by k*p theory. By using an InGaAsP quaternary compound as an active layer, it becomes possible to study the relationship between the differential gain and strain (both tensile and compressive) when both the quantum-well thickness and the emission wavelength are kept constant. It is shown that the tensile strain not only reduces the density of states in the valence band but also increases the energy spacings between the first two valence subbands. It is concluded that tensile strain has a more pronounced impact on the improvement of differential gain in InP-based, strained QWLs as compared with compressive strain. >

Pure strain effect on differential gain of strained InGaAsP/InP quantum-well lasers

Based on detailed valence band structure, a Monte Carlo analysis of the Auger recombination effects in compressively strained quantum-well diode lasers has been carried out. The recombination current is found to increase with carrier density, but at a rate far less rapidly than the conventional n/sup 3/-rule has suggested. At moderate carrier densities, the recombination current is also found to decrease exponentially with increasing strain. >

Suppression of Auger recombination effects in compressively strained quantum-well lasers

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