Effect Of Carrier Diffusion Research Articles

Fourier-optical transverse mode selection in external-cavity broad-area lasers: experimental and numerical results

Broad-area lasers (BALs) with external Fourier-optical cavities with spatial filter for transverse mode selection have been studied experimentally and theoretically. The transverse mode structure of BALs is modeled using a three mirror cavity approach. The model accounts for gain saturation, carrier diffusion, and anti-guiding effects. Near- and far-field distributions are calculated and compared to experimental results. Transverse mode selection is achieved for BALs with a residual front facet reflectivity of 10% at low pump currents. For BALs having a very low front facet reflectivity of 0.001%, transverse modes can be selectively excited up to pump currents more than 200% above laser threshold. Calculations show that BALs having a 0.001%-antireflection coating with an external Fourier-optical cavity high above threshold can operate in a self-Q-switched-like mode with pulse durations of 2-4 ns and repetition rates of 100-200 MHz. Experimental results obtained with a hybrid integrated-optical external cavity for transverse mode selection are also presented.

A general method for estimating the duration of avalanche multiplication

The time lapse between the injection of the primary carrier and the termination of the resulting avalanche current in an APD is a random variable. Its distribution is calculated allowing for the effects of carrier diffusion and also for an arbitrary distribution of ionization events in both space and time, following primary carrier injection. While diffusion plays a secondary role, it is found that the enhancement in the average speed of those carriers which ionize early in their trajectories speeds up the avalanche process significantly in short devices.

Acousto-optic modulation in magnetised diffusive semiconductors

The modulation of an intense electromagnetic beam induced by the acousto-optic (AO) effect has been analysed in a transversely magnetised semiconductor-plasma medium. The effect of carrier diffusion on the threshold field and gain profile of the modulated wave has been extremely investigated using coupled mode theory. The origin of the AO interaction is assumed to lie in the induced nonlinear diffusion current density of the medium. By considering the modulation process as a four wave parametric interaction an expression for effective third-order AO susceptibility describing the phenomena has been deduced. The modulation is greatly modified by propagation characteristics such as dispersion and diffraction due to dielectric relaxation of the acoustic mode. The threshold pump field and the steady state growth rates are estimated from the effective third-order polarisation in the plasma medium. Analytical estimation reveals that in the presence of enhanced diffusion due to excess charge carriers the modulated beam can be effectively amplified in a dispersionless acoustic wave regime. The presence of an external dc magnetic field is found to be favourable for the onset of diffusion induced modulational amplification of the modulated wave in heavily doped regime.

Density-functional theory approach to ultrafast laser excitation of semiconductors: Application to theA1phonon in tellurium

Calculations of the ${A}_{1}$ phonon frequency in photoexcited tellurium are presented. The phonon frequency as a function of photoexcited carrier density and phonon amplitude is determined, including anharmonic effects. The sensitivity of the ${A}_{1}$ mode to photoexcitation is related to the Peierls mechanism for stabilizing the \ensuremath{\alpha}-Te structure. The assumptions of slow and fast carrier recombination are investigated and it is found that the two regimes give qualitatively different predictions for the excitation dependence of the phonon frequency. Recent pump-probe experiments are compared with the calculations. The predictions based on fast carrier recombination are not in agreement with experiment. The reflectivity oscillations expected to occur in pump-probe experiments are simulated, including the coupled effects of optical absorption, carrier diffusion, and phonon dynamics. Using the calculated dependence of phonon frequency on carrier density (assuming slow carrier recombination) and experimental values for the optical dielectric constants, the derivative of the frequency peak for reflectivity oscillations with respect to pump fluence is found to be $\ensuremath{-}0.085 \mathrm{}\mathrm{THz}$ per ${\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^{2},$ compared to an experimental value of $\ensuremath{-}0.07 \mathrm{THz}$ per ${\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}$ in the low-fluence regime. The ambipolar diffusion constant for the optically excited carriers is estimated to be 10 ${\mathrm{cm}}^{2}/\mathrm{s},$ substantially smaller than its equilibrium value. The effects of carrier diffusion are found to be more important than phonon anharmonicity in the observed changes of phonon frequency within the first few cycles of motion after laser excitation. Greatly increased damping of the reflectivity oscillations at high pump fluences, which was reported in recent experiments, is not found in the simulations.

Influence of carrier diffusion on the response of a resonant-cavity enhanced detector

We have investigated the effects of carrier diffusion on the impulse response of a resonant-cavity enhanced photodetector using our analytic expression for a short-circuit current. Our theoretical results indicate that diffusion introduces a significant tail in the temporal response of photodetectors. We show that the influence of diffusion becomes significant as the absorption width decreases. A discrepancy in bandwidth as high as 35% between our results and previous theoretical predictions is observed.

The effect of carrier diffusion and recombination in semiconductors on the photoacoustic signal

A quantitative derivation is presented for the heat transport in bipolar semiconductors, taking into account generation and heating of carriers on the surface due to an incident modulated laser beam on the surface and finite carrier diffusion and recombination in the solid. The temperature distribution as function of the position and time in the semiconductor is calculated using appropriate boundary conditions according to the photoacoustic experimental conditions. In addition, special emphasis is made on the heat power density generated in the sample due to the recombination of the electron–hole pair and the effect of the inhomogeneous temperature distribution on the thermal generation rate of carriers in the photoacoustic signal.

Turn‐on transient dynamics of a semiconductor laser with optical feedback

AbstractWe study the effect of lateral carrier diffusion on the transient turn‐on of a semiconductor laser with optical feedback. The numerical simulations are based on the Lang–Kobayashi model, and on an extension of the model that includes lateral profiles for the carrier density and the optical field. For moderately strong feedback, intensity pulses characterize the dynamics of the turn‐on, with a repetition rate at the external cavity round‐trip time. We find that carrier diffusion has an important effect when the laser is biased close to the solitary threshold. For low carrier diffusion the intensity drops to a nearly zero value in between the pulses; for fast carrier diffusion, the spatial holes in the carrier profile burned by the intensity pulses are quickly replenished, and the intensity does not decay to a zero value in between the pulses. The results of our analysis show that the effect of diffusion is qualitatively well modelled by a non‐linear gain saturation coefficient different from zero in the space‐independent Lang–Kobayashi model. Copyright © 2001 John Wiley & Sons, Ltd.

Photoluminescence spectroscopy and decay time measurements of polycrystalline thin film CdTe/CdS solar cells

The results of room temperature photoluminescence spectroscopy and decay time measurements carried out on CdTe/CdS solar cells are reported. The as-grown structures were annealed in air at temperatures in the range 350–550 °C. For excitation via the CdTe/CdS interface, longer photoluminescence decay times were observed as the anneal temperature was increased, this is attributed to localized passivation of nonradiative states possibly due to the effect of S interdiffusion. When the photoluminescence is excited via the CdTe free surface, the decay curves consist of a fast and slow component. The fast component (<130 ps) of the photoluminescence is attributed to nonradiative recombination at grain boundaries or the CdTe free surface. The slow component is attributed to the effects of carrier drift and diffusion and subsequent recombination at the CdTe/CdS interface. Modeling of the transport process has led to the extraction of a value of 0.20±0.03 cm2 s−1 for the minority carrier diffusion coefficient of the CdTe for the sample annealed at 450 °C. These results are correlated with improvements in device efficiency determined from illuminated current density–voltage measurements.

Dynamics of GaAs/AlGaAs microdisk lasers

Dynamic response of a GaAs/AlGaAs microdisk laser has been experimentally investigated using femtosecond optical pumping. Below the lasing threshold, the delay time of the emission pulse from the microdisk hardly changes with the pump power. Above the lasing threshold, the delay time is shortened dramatically, and it decreases with increasing the pump power. The theoretical simulation based on the rate equations reproduces the experimental observation after the effect of carrier diffusion is taken into account. The simulation result illustrates that the speed of a microdisk laser is limited mainly by the carrier diffusion in the disk plane.

Nonlinear saturation behaviors of high-speed p-i-n photodetectors

We present numerical simulations of the ultrafast transport dynamics in an ultrahigh-speed double-heterostructure p-i-n photodetector. Nonlinear saturation behaviors under high field and high power illumination are investigated with the external circuit response considered. Damping constants and diffusion constants are both treated as electric-field- and carrier-concentration dependent in our model in order to take into account the effect of carrier scattering. We have also considered the carrier trapping at the heterostructure interfaces for the first time. Besides the drift-induced space charge screening effect, we find that saturation of external circuit and carrier-trapping-induced screening effect are also the dominant mechanisms contributed to the nonlinear bandwidth reduction under high power illumination. On the other hand, previously reported plasma oscillations are found to be greatly suppressed by including strong carrier diffusion effect in the model.

Nonlinear analysis of quantum-well lasers with the effects of carrier transport

An improved nonlinear quantum-well (QW) laser model, which takes into account the effects of quantum capture and escape processes, is presented, based on laser rate equations and Volterra theory. This model is further expanded by proper parameter transform to include the effect of carrier diffusion in the separate confinement heterostructure region. Various QW laser distortions have been evaluated using this model and compared with the results obtained from the previous model where the transport effects are absent. The results shows that the effect of transport processes on laser dynamic nonlinearity can be significant.

Differential carrier lifetime in oxide-confined vertical cavity lasers obtained from electrical impedance measurements

Differential carrier lifetime measurements were performed on index-guided oxide-confined vertical cavity surface emitting lasers operating at 980 nm. Lifetimes were extracted from laser impedance measurements at subthreshold currents, with device size as a parameter, using a simple small-signal model. The carrier lifetimes ranged from 21 ns at 9 μA, to about 1 ns at a bias close to threshold. For a 6×6 μm2 oxide aperture device the threshold carrier density was nth∼2×1018 cm−3. The effect of carrier diffusion was also considered. An ambipolar diffusion coefficient of D∼11 cm2 s−1 was obtained.

Lateral mode dynamics of semiconductor lasers

The interaction between two lateral modes in a waveguide semiconductor laser is studied. The effects of carrier diffusion are included into the governing rate equations. A linear stability analysis is used to identify regions of frequency locking between the lateral modes. Numerical integration shows the effects of the diffusion length and other system parameters on the lateral mode dynamics. It is shown that variations in the carrier diffusion length give rise to regimes of single-mode dominance, oscillation, chaos, and frequency locking. Direct experimental observations of higher order lateral modes in lasers exhibiting beam instabilities are presented. Observations of bistable beam steering with hysteresis are presented and explained.

Ultrasmall and ultralow threshold GaInAsP-InP microdisk injection lasers: design, fabrication, lasing characteristics, and spontaneous emission factor

We have calculated lasing characteristics of current injection microdisk lasers of several microns in diameter, taking account of the scattering loss at center posts and the carrier diffusion effect. We found that the optimum width of the disk wing exposed to the air is 0.6-0.7 /spl mu/m and the minimum threshold current is nearly 10 /spl mu/A for the disk diameter of 2 /spl mu/m. The internal differential quantum efficiency can be 95% if the transparent carrier density is reduced to 7.5/spl times/10/sup 17/ cm/sup -3/ and the diffusion constant is increased to 8 cm/sup 2//s. In the experiment, we have obtained the room temperature continuous-wave operation of a GaInAsP-InP device of 3 /spl mu/m in diameter, for the first time, with a record low threshold of 150 /spl mu/A. This achievement was mainly owing to the reduction of the scattering loss at the disk edge, and hence the reduction of the threshold current density. The spontaneous emission factor was estimated to be 6/spl times/10/sup -3/. This value was much reduced by the large detuning of the lasing wavelength against the spontaneous emission peak. A larger value over 0.1, which is expected for such a small device, will be obtained by the wavelength tuning and the narrowing of the spontaneous emission spectrum.

Analysis of nearly degenerate four-wave mixing in multisection laser diodes

The characteristics of nearly degenerate four-wave mixing (NDFWM) in above-threshold two-section single cavity Fabry-Perot (TSSC FP) laser diodes subject to weak or strong probe-injection power, are investigated numerically. The ND-PWM behaviors are significantly dependent on optical probe injection into either section of a TSSC FP laser diode, and the injected probe power. It is shown that both carrier diffusion and pump depletion effects play very important roles in determining the reflectivity efficiency of conjugate wave, and that a greater than /spl sim/3.5-dB enhancement of reflectivity efficiency can be obtained in an asymmetric laser diode, compared to a symmetric laser, for strong injection probe power. Calculations suggest that the TSSC FP laser diodes may be applied to control the influence of carrier diffusion on NDFWM reflectivity efficiency.

Parabolic-confocal unstable-resonator semiconductor lasers-Modeling and experiments

Surface-emitting all-grating-based unstable-resonator lasers, suitable for integration with diffractive beam-forming elements, have been experimentally and theoretically studied. The lasers exhibit single spatial mode operation for a device width of 160 /spl mu/m. The device performance is modeled using a beam propagation method, which accounts for spatial hole burning as well as thermal and carrier diffusion effects. Near- and far-field calculations are incorporated to facilitate the comparison with the experimental results. To achieve high-quality beam forming, it is essential that the wavefront of the guided optical mode is well defined and stable. Using the model, we study the dependence of the wavefront distortion on various parameters and show how these distortions affect the far-field characteristics. The results from the simulations agree well with the experimental work. We find that the laser performance is to a large extent controlled by thermal effects. At low-power operation, these effects can be compensated for, for a range of the injection current, by modifications of the resonator or outcoupler grating geometries.

Effect of carrier diffusion in oxidized vertical-cavity surface-emitting lasers determined from lateral spontaneous emission

The effect of carrier diffusion on threshold current is studied quantitatively for the oxide-aperturbed 780 nm vertical-cavity surface-emitting lasers (VCSEL) from the measured spontaneous emission. The oxide aperture size of 2–10 μm is prepared and characterized. The diffusion coefficient of 4.6 cm2/s and the nonradiative recombination coefficient of 6.8×107/s are obtained using the independence of the nonradiative recombination coefficient on the laser aperture size. The contribution of carrier diffusion loss is 42% of the threshold current for the 2 μm VCSEL. The relative contribution of the diffusion becomes smaller for the larger devices as is expected.

Mode-partition noise in vertical-cavity surface-emitting lasers

We present a numerical study of the effects of carrier diffusion and spatial hole-burning on the noise characteristics of vertical-cavity surface-emitting lasers (VCSEL's) under both single-mode and multimode operations. In the case of single-mode operation, VCSEL noise characteristics are similar to those of edge-emitting lasers except for a diffusion-induced damping of relaxation oscillations. Under multimode operation, VCSEL's exhibit low-frequency noise enhancement due to mode partition. However, depending on the spatial distributions of the transverse modes excited, the mode-partition noise characteristics can be quite different.

Optimum sensitivity and two-dimensional modeling of microwave detected photoconductance decay carrier lifetime measurement

Optimization of the measurement sensitivity using closed form analytical expressions derived from an electrical equivalent circuit is carried out for microwave detected photoconductance decay system used for nondestructive carrier lifetime measurement in silicon wafers. The effect of transverse inhomogeneity in the sample conductivity on the microwave power reflection is discussed using the equivalent circuit model. The effect of lateral inhomogeneity in the sample conductivity due to local illumination by a source with a laterally varying intensity is studied using an elaborate analytical two-dimensional model. Finally, the effect of transient lateral carrier diffusion in a sample subjected to a pulsed illumination on the extracted value of the lifetime is investigated.

Effects of spatial hole burning on gain switching in vertical-cavity surface-emitting lasers

We present a numerical study of the effects of carrier diffusion and spatial hole-burning in vertical-cavity surface-emitting lasers under gain switching. Our model includes spatial and temporal dependences of both the optical field and the carrier density. Results show that spatial hole burning places a limit on the minimum achievable pulse width. We demonstrate that spatial hole-burning tan be avoided and shorter pulses can be obtained by using an appropriate pumping geometry. We also consider the case in which the laser operates simultaneously in two transverse modes and show that transverse-mode competition induced by spatial hole burning leads to period doubling and other interesting nonlinear behavior.