John P. Loehr

Theoretical studies of the effect of strain on the performance of strained quantum well lasers based on GaAs and InP technology

A discussion is presented of the use of strain to improve the performance of quantum well laser structures. The deformation potential theory is used to study the effect of strain produced by the addition of excess indium on the conduction band and valence band properties. Full-band mixing effects are retained in the calculations. Using a numerical technique developed to study laser parameters in arbitrary quantum well structures, the authors study the effect of strain on the threshold current density and polarization dependence. Dramatic improvements are found due to the strain-induced band-structure changes. Optimization results are presented which show that single quantum well structures have the best performance. >

Analysis of microcavity VCSEL lasing modes using a full-vector weighted index method

Presents a semi-analytic full-vector method for calculating the spatial profile, optical confinement factor resonant frequency, absorption loss, and mirror loss of lasing modes in cylindrically symmetric microcavity vertical-cavity surface-emitting lasers (VCSELs). It can be shown that this method gives the best separable approximation for the electric and magnetic vector potentials. Our technique can model the entire VCSEL structure and can treat complex media. We apply the method to etched-post and oxide-apertured VCSELs designed for 980-nm emission and find a blueshift in cavity resonance as the cavity radius shrinks. We also find a minimum optical cavity radius below which radially bound lasing modes cannot be supported. This radius depends on the device geometry and lies between 0.5 and 1 /spl mu/m for the devices studied. Once this model is augmented to include diffraction losses-the dominant loss mechanism for conventional small aperture lasers-it will provide a complete picture of lasing eigenmodes in microcavity VCSELs.

Comparison of steady-state and transient characteristics of lattice-matched and strained InGaAs-AlGaAs (on GaAs) and InGaAs-AlInAs (on InP) quantum-well lasers

Numerical techniques are developed to study the output spectra and to solve the multimode coupled rate equations including transverse electric (TE) and transverse magnetic (TM) propagations for In/sub x/Ga/sub 1-x/As-Al/sub 0.3/Ga/sub 0.7/As and In/sub 0.53+x/Ga/sub 0.47-x/As-Al/sub 0.48/In/sub 0.52/ quantum-well lasers. Optical properties are calculated from a 4*4 k*p band structure, and strain effects are included with the deformation potential theory. It is found that an introduction of 1.4% compressive strain to the quantum well results in roughly 3-4 times improvement in the intrinsic static characteristics in terms of lower threshold current, greater mode suppression and lower nonlasing photon population in the laser cavity. The authors identify the effect of strain on the large signal temporal response. They also include calculated CHSH Auger rates in their model. >

Quasi-exact optical analysis of oxide-apertured microcavity VCSELs using vector finite elements

We report a new full vector finite element model for analyzing the optical properties of azimuthally symmetric oxide-apertured vertical-cavity surface emitting lasers (VCSELs). Our model allows for quasi-exact calculation of the lasing mode blueshift, threshold gain, and field profile. Through a detailed analysis of a sample VCSEL, we ascertain the physical effects which determine diffractive or parasitic mode loss. They are: 1) the background density of parasitic modes and 2) the coupling strength between the lasing mode and the parasitic mode continuum. The coupling strength is in turn determined by the relative alignment between the lasing and parasitic mode propagation vectors and the lasing mode penetration into the oxide region. This analysis improves our understanding of the optical physics of apertured VCSELs and should enable the next leap down in lasing threshold.

Refractive index and electro‐optic effect in compressive and tensile strained quantum wells

The effects of biaxial compressive and tensile strain on the excitonic resonances and associated changes in refractive index and electro‐optic effect in quantum wells have been calculated and measured. Theoretical calculations include the important heavy‐hole–light–hole band mixing effects. It is seen that the excitonic contributions dominate near the band edge. With increasing compressive strain the linear electro‐optic effect is slightly increased, while the quadratic effect is greatly enhanced. The effects are reversed in quantum wells under tensile strain.

Theoretical studies of the applications of resonant tunneling diodes as intersubband laser and interband excitonic modulators

We present a theoretical analysis of the optical applications of resonant tunneling diodes. The electronic properties are calculated with a self‐consistent traveling‐wave model that includes effective‐mass mismatches. The interband optical properties are calculated from a 4×4 k⋅p band structure in the dipole approximation. We find that it is possible to operate a conventional device as an intersubband laser if the transition energy is large (∼0.5 eV) and the linewidth in minimal (∼5 meV). A bound‐state device can produce a modulation ratio of 5:1 at the excitonic peak with an absorption length of ∼ 40 μm in a waveguide geometry.

Effect of strain on CHSH Auger recombination in strained In/sub 0.53+x/Ga/sub 0.47-x/As on InP

We calculate the mod conduction, heavy hole)- mod split-off hole, heavy hole) (CHSH) Auger rates in strained In/sub 0.53+x/Ga/sub 0.47-x/As on InP, a widely used material system for quantum well lasers. The bandstructure is obtained from an eight-band tight binding model with spin-orbit coupling, strain effects being included via the deformation potential theory. Adding excess In decreases the hole density of states: this effect acts to decrease the Auger rates. The excess In also decreases the bandgap, however, and increases the split-off gap: these effects act to increase the Auger rates. When we include both of these effects we find that the reduction in the net bandgap dominates; hence, the Auger rates increase with excess In for a fixed carrier concentration. We include these Anger rates in the threshold current calculation for a strained layer multi quantum well laser. We find that for x 0.15, however, the threshold carrier concentration changes little and the Anger rates continue to increase; thus, the threshold current begins to increase rapidly with x. >

Theoretical and experimental studies of the effects of compressive and tensile strain on the performance of InP-InGaAs multiquantum-well lasers

The authors have studied, both theoretically and experimentally, the effects of biaxial strain upon the performance characteristics of broad-area InP-InGaAsP-In/sub x/Ga/sub 1-x/As (0.33 >

Effects of strain on the high speed modulation of GaAs- and InP-based quantum-well lasers

A small-signal numerical analysis of pseudomorphic GaAs- and InP-based Fabry-Perot quantum-well lasers using calculated optical gain spectra with strain effects included is reported. Examination of the effect of lifetime broadening shows that the resonance frequency increases at a rate of approximately 250-MHz/meV reduction in the lifetime broadening for a GaAs-based strained layer laser. The modulation speed is limited by either device heating or facet damage. If the limitation is imposed by the optical power then the modulation speed increases as the laser cavity becomes shorter and the number of quantum wells increases. If the limitation is imposed by the injection current density, however, then the modulation speed decreases for the laser with shorter cavity length. The highest modulation speed is given by an optimum well number. A resonance frequency of approximately 16 GHz is predicted for a pseudomorphic GaAs-based laser with 30% excess In and average output power of approximately 5 mW. >

Calculation of microcavity VCSEL field modes using a doubly iterative weighted index method

We generalize the weighted index method for analysis of modal structure in various devices, including vertical cavity surface emitting lasers. Our model uses a doubly iterative process to calculate the bound modes for any dielectric device with an azimuthally symmetric geometry. In order to calculate the modes we assume a separable form for the electric and magnetic vector potentials. The scalar wave equation is then solved for the axial components of electric and magnetic vector potentials. Assuming a functional form of Az equals F((rho) )G(z) and Fz equals P((rho) )Q(z) we form coupled differential equations between F((rho) ) and G(z). These equations are then iteratively solved using the coupled boundary conditions for Az and Fz. Convergence by tracking the change in the eigenfrequency for the radial and axial eigenvalue equations. Our method allows rapid calculation, compared to an analogous finite element approach, and will handle any azimuthally-symmetric geometry with piecewise-constant indices of refraction. This model is particularly well suited to the calculation of bound modes in microcavity and oxidized structures where field confinement effects can be very important. The model can also, in principle, be adapted to obtain radiative modes, and should provide a valuable tool to analyze field behavior and quantum optics effects in microcavity devices.