The influence of quantum-well composition on the performance ofquantum dot lasers using InAs-InGaAs dots-in-a-well (DWELL) structures

The optical performance of quantum dot lasers with different dots-in-a-well (DWELL) structures is studied as a function of the well number and the indium composition in the InGaAs quantum well (QW) surrounding the dots. While keeping the InAs quantum dot density nearly constant, the internal quantum efficiency η_i, modal gain, and characteristic temperature of 1-DWELL and 3-DWELL lasers with QW indium compositions from 10 to 20% are analyzed. Comparisons between the DWELL lasers and a conventional In_0.15Ga_0.85As strained QW laser are also made. A threshold current density as low as 16 A/cm² is achieved in a 1-DWELL laser, whereas the QW device has a threshold 7.5 times larger. It is found that η_i and the modal gain of the DWELL structure are significantly influenced by the quantum-well depth and the number of DWELL layers. The characteristic temperature T₀ and the maximum modal gain of the ground-state of the DWELL structure are found to improve with increasing indium in the QW It is inferred from the results that the QW around the dots is necessary to improve the DWELL laser's η_i for the dot densities studied     

Electronic band structures and optical gain spectra of strainedwurtzite GaN-AlxGa1-xN quantum-well lasers

The electronic band structures, density-of-states, and optical gain spectra for wurtzite GaN-Al_xGa_1-xN quantum wells are studied theoretically based on the Hamiltonian derived using the k.p method. We investigate the dependence of the optical gain and transparent current density on the well width, barrier height, and strain using a numerical approach with high accuracy. The mole fraction of Al in the barrier material is progressively increased to study the effects of quantum confinement and compressive strain. A higher Al mole fraction in the barrier leads to improvement of the TE optical gain and suppression of the TM optical gain. Furthermore, we demonstrate that a reduction of the well width offers improved modal gain over all radiative current densities. We also predict a transparent current density of 250 A/cm² for the GaN-Al_xGa_1-x N single quantum-well (QW) structure. Our results suggest that a suitable combination of thin well width and large barrier height should be selected in improving the TE optical gain in wurtzite GaN-Al_x Ga_1-xN single QW     

Gain saturation in traveling-wave semiconductor optical amplifiers

The gain saturation behavior of semiconductor traveling-wave optical amplifiers has been analyzed using a model that includes the specific dependence of gain on carrier concentration. Under the condition of a specific gain at a particular current, it is found that the saturation power strongly depends on the choice between quantum well (QW) or bulk amplifying medium but weakly on the detailed design of the device such as the number of QW's or the thickness of the bulk layer. The higher saturation power of the QW-based amplifier is caused by its logarithmic gain-current relation rather than its low optical confinement factor. Also, when the unsaturated device gain is specified, the designed saturation power can be obtained with the lowest drive current by using the highest optical confinement     

有强内电场的GaN-基阶梯量子阱中的线性与非线性光吸收

采用密度矩阵方法,考察了带强内建电场GaN-基阶梯量子阱中的线性与非线性光吸收系数.基于能量依赖的有效质量方法,在考虑了带的非抛物性情况下,推导了结构中的精确解析的电子本征态,给出了系统中简单解析的线性与非线性光吸收系数表达式.以AlN/GaN/AlxGa1-xN/AlN阶梯量子阱为例进行了数值计算.结果发现阶梯量子阱的阱宽Lw、阶梯垒宽Lb、阶梯垒的掺杂浓度x的减小将提高体系的吸收系数.而且,随着Lw,Lb和x减小,吸收光子的能量有明显的蓝移,总吸收系数的半宽度及饱和吸收强度均减小.计算获得的部分结果与最近的实验观察完全一致.     

Influence of electrostatic confinement on optical gain in GaInNAs quantum-well lasers

There remains controversy surrounding the cause of the magnitude and temperature sensitivity of the threshold current density of 1.3-/spl mu/m GaInNAs quantum-well (QW) lasers, with several authors attributing the strong temperature sensitivity to hole leakage, due to the relatively low valence band offset in GaInNAs/ GaAs QW structures. We use a Poisson solver along with a ten-band k.p Hamiltonian to calculate self-consistently the influence of electrostatic confinement on the optical gain in such lasers. We find that the inclusion of such effects significantly reduces the hole leakage effect, with the electrostatic attraction of the electrons significantly increasing the binding of heavy holes in the QW region. We conclude by comparison with previous theoretical and experimental studies that the room temperature threshold current is generally dominated by monomolecular recombination, while the temperature sensitivity can be explained as predominantly due to Auger recombination.     

The influences of refractive index dispersion on the modal gain ofa quantum-well laser

A new self-consistent method (SCM) for single-quantum-well (SQW) AlGaAs-GaAs diode lasers is introduced to study systematically the influences of refractive-index dispersion on TE modal gain. The refractive-index dispersion of QW layers is calculated by the density matrix method. It is affected by the effective propagation constant of guided mode. Likewise, the transverse guided mode of QW lasers, as obtained by the transfer matrix method, is also influenced by the refractive-index dispersion. SCM, using the density matrix and transfer matrix methods self-consistently, provides the TE modal gain spectra. SCM's calculated results are compared with those of Dumke's approximation and show a decrease in energy of modal gain peak and a decline of modal gain values at high emission energies. The differences between these two methods are seen to increase with an increase of well width and to be unrelated to barrier height. Although not treated formally in this paper, we suggest that SCM results show a significantly superior match to real phenomena     

Well width dependence of gain and threshold current in GaAlAs single quantum well lasers

The optical gain of single quantum well GaAs/GaAlAs laser diodes is studied theoretically. The model uses a nok-selection rule and Fermi statistics to obtain the gain coefficient expression. Gain-current characteristics are then reported and allow comparison of structures with well widths between 50 and 400 Å. Comparison is also made to previous models which use a strictk-selection rule. Then theoretical threshold current densities are calculated for typical single quantum well lasers where the optical confinement is performed using a five-layer slab waveguide. They are shown to be relatively insensitive to the well width as long as Lzis larger than 80 Å. Comparison between two different structures shows that optical confinement plays a critical role for optimizing the threshold Current and should be carefully studied, especially if thek-selection rule is relaxed.     

Threshold current analysis of compressive strain (0-1.8%) inlow-threshold, long-wavelength quantum well lasers

A comprehensive study of the effect of compressive strain on the threshold current performance of long-wavelength (1.5 μm) quantum-well (QW) lasers is presented. Model predictions of threshold currents in such devices identify QW thickness as a parameter that must be considered in optimizing laser performance when Auger currents are present. Experimental comparisons between strained and unstrained devices reveal strain-induced reductions in internal transparency current density per QW from 66 to 40 A/cm², an increase in peak differential modal gain from 0.12 to 0.23 cm/A, and evidence for the elimination of intervalence band absorption as compressive strain increases from 0 to 1.8%. However, most of these improvements arise in the first ~1% of compressive strain. To fabricate low-threshold 1.5-μm buried heterostructure (BH) devices in InP using the strained QW active regions an optimized design which shows that threshold current is at its lowest when the stripe width is approximately 0.6-0.7 μm is derived. Results for uncoated BH lasers are reported     

Electron nonelastic scattering by confined and interface polar optical phonons in a modulation-doped AlGaAs/GaAs/AlGaAs quantum well

The calculations of electron scattering rates by polar optical (PO) phonons in an AlGaAs/GaAs/AlGaAs quantum well (QW) with a different width and doping level are performed. The electron-and PO-phonon scattering mechanisms which are responsible for the alternate dependence of electron mobility on a QW width, as well as for the decrease of conductivity in the QW with increasing sheet electron concentration, are determined. It is shown that the enhancement of the scattering rate by PO-phonon absorption when the lower subband electron gas is degenerated is responsible for the decrease of QW conductivity with increasing sheet electron concentration. The competition between the decrease of the intrasubband scattering and the increase of the intersubband scattering by PO-phonon absorption is responsible for the alternate changes of the mobility with a QW width.     

Correlation of theory with experimental SOAs

Predictions for the near-traveling wave amplifier (NTWA) with multiple-quantum-well structures have been developed. The continuity equation for quantum wells (QWs) with high carrier densities is combined with the amplifier TW gain equation expressed in terms of stimulated lifetime. This formulation allows the signal gain to be related to the bias current and the optical input signal through Fermi energies. The charge neutrality condition also plays an important role for high carrier density QW amplifiers. Auger recombination and heating effects are incorporated as essential components of the model. Experimental measurements of gain versus bias current and output power for both /spl lambda/= 850- and 1550-nm devices are found to be very well matched by the calculated results.     

The effect of increased valence band offset on the operation of 2μm GaInAsSb-AlGaAsSb lasers

Two and four quantum-well (QW) GaInAsSb-AlGaAsSb lasers emitting at 2 μm are reported. In comparison to previously published data, it is found that higher Al content in the QW barrier improves the internal efficiency, saturated modal gain, and characteristic temperature of the lasers. These results are attributed to an increased valence band offset that provides superior hole confinement in the GaInAsSb QW. A differential efficiency of 74% is observed at 25°C under pulsed conditions for a 900-μm cavity length, 2-QW device, and a record characteristic temperature of 140 K is measured for a 4-QW laser     

Spontaneous Emission and Characteristics of Staggered InGaN Quantum-Well Light-Emitting Diodes

A novel gain media based on staggered InGaN quantum wells (QWs) grown by metal-organic chemical vapor deposition was demonstrated as improved active region for visible light emitters. Fermi's golden rule indicates that InGaN QW with step-function like In content in the well leads to significantly improved radiative recombination rate and optical gain due to increased electron-hole wavefunction overlap, in comparison to that of conventional InGaN QW. Spontaneous emission spectra of both conventional and staggered InGaN QW were calculated based on energy dispersion and transition matrix element obtained by 6-band k middotp formalism for wurtzite semiconductor, taking into account valence-band-states mixing, strain effects, and polarization-induced electric fields. The calculated spectra for the staggered InGaN QW showed enhancement of radiative recombination rate, which is in good agreement with photoluminescence and cathodoluminescence measurements at emission wavelength regime of 425 and 500 nm. Experimental results of light-emitting diode (LED) structures utilizing staggered InGaN QW also show significant improvement in output power. Staggered InGaN QW allows polarization engineering leading to improved luminescence intensity and LED output power as a result of enhanced radiative recombination rate.     

Tailoring light and heavy holes of GaAsP-AlGaAs quantum wells byusing interdiffusion for polarization-independent amplifier applications

A theoretical study of the polarization-independent (PI) optical gain using group III and group V interdiffusion for under- and over-strained GaAsP-AlGaAs quantum wells (QW's), respectively, is presented here. The group III interdiffusion generates a large enough Al concentration into the well of the under-strained QW for providing PI optical gain while this can be achieved in the over-strained QW through the reduction of the P concentration in the well by group V interdiffusion. When the well width increases, the required extent of interdiffusion to obtain PI optical gain increases for the case of group V diffused QW's, but, for the case of group III diffused QW's, the required extent of interdiffusion is not sensitive to the well width. In addition, the introduction of Al in the well layer of QW's can shorten and lengthen the group III and group V interdiffusion, respectively, for providing the PI optical gain. Similar results can also be obtained by increasing the P concentration in the well layer. Consequently, group III and group V interdiffusion can be used to achieve PI optical gain in the under- and over-strained QW's, respectively, for use in PI optical amplifiers. A range of the extent of interdiffusion can be used to obtain the PI gain although the value of the PI gain may reduce. Besides, the operating wavelength can be blue-shifted in group III interdiffusion and red-shifted in group V interdiffusion. Moreover, TE and TM optical gain peaks can be adjusted using interdiffusion for laser applications     

Reduction of noise figure in semiconductor laser amplifiers with Ga1-xInxAs/GaInAsP/InP strained quantum wellstructures

The noise characteristics of semiconductor laser amplifiers (SLAs) in the Ga_1-xIn_xAs/GaInAsP/InP strained quantum well (QW) system are theoretically calculated and analyzed using density-matrix theory and taking into account the effects of band mixing on both the valence subbands and the transition dipole moments. The numerical results show that a reduced noise figure can be obtained in both tensile and compressively strained QW structures due to the increase in differential gain and the decrease in transparent carrier density. From a comparison among compressively strained (x=0.70), unstrained (x=0.53), and tensile strained (x=0.40) QW SLAs at a fixed carrier density and optical confinement factor, it is found that the noise figure of the tensile strained QW reaches its lowest value of 3.4 dB at average input optical power of -20 dB     

Nonlinear gain effects on spectral dynamics in quantum well lasers

A theoretical analysis is presented to show how nonlinear gain affects the spectral dynamics of quantum well (QW) lasers. The results indicate that the nonlinear gain, which is enhanced by the quantum confinement of carriers, causes an increase in the linewidth enhancement factor alpha . This enhancement of alpha results in spectral rebroadening: under high power output conditions. These properties should be taken into account when quantum well lasers are designed for highly coherent lasers.<>     

Time-Resolved Linewidth Enhancement Factors in Quantum Dot and Higher-Dimensional Semiconductor Amplifiers Operating at 1.55 $mu{hbox{m}}$

We report time-resolved measurements of the linewidth enhancement factors (-factors) , and , associated with the adiabatic carrier recovery, carrier heating, and two-photon absorption dynamical processes, respectively, in semiconductor optical amplifiers (SOAs) with different degrees of dimensionality-one InAs/InGaAsP/InP quantum dot (0-D), one InAs/InAlGaAs/InP quantum dash (1-D), and a matching InGaAsP/InGaAsP/InP quantum well (2-D)-all operating near 1.55- wavelengths. We find the lowest values in the QD SOA, 2-10, compared to 8-16 in the QW, and values of and that are also lower than in the QW. In the QD SOA, the -factors exhibit little wavelength dependence over the gain bandwidth, promising for wide-bandwidth all-optical applications. We also find significant differences in the -factors of lasers with the same structure, due to the differences between gain changes that are induced optically or through the electrical bias. For the lasers we find the QW structure instead has the lower -factor, having implications for directly modulated laser applications.     

New mechanism of excitonic enhanced optical gain for wide-gap quantum wells

New mechanism of optical gain in quantum wells are proposed using excitonic effects. Exciton in wide-gap semiconductors plays an important role in optical phenomena since it has a large binding energy and could be stable at room temperature. However, its bound state is constructed by the electron-hole Coulomb interaction and should be related to the electron and hole distributions when the ground state has many electron and holes. We have evaluated the current-current correlation function, i.e. conductivity, treating the mechanism of optical gain and exciton on equal footing. It is shown that the recombination of the exciton does not yield optical gain directly but that excitonic effects enhance an oscillator strength of the coupled transition. Taking into account a localized level in the energy gap, the optical gain in terms of the population inversion between the localized level and one of the band edge subband states is produced with the very small carrier concentration. Simultaneously, the excitonic absorption occurs due to the band edge electron-hole interaction. It is found that the former optical gain is enhanced extremely by the latter excitonic effect through the coupling between the two transitions. This enhanced optical gain might show a possibility of very low threshold current density for wide-gap laser diodes.     

Differential optical gain in a GaInN/AlGaN quantum dot

Electronic and optical properties are obtained with the increase in indium alloy content (x) in a Ga_1-xIn_xN/Al_0.2Ga_0.8N quantum dot. The barrier height with the different In alloy contents is applied to acquire the confinement potentials. The results are obtained taking into consideration geometrical confinement effect. The optical absorption coefficient with the photon energy is observed in a Ga_1-xIn_xN/Al_0.2Ga_0.8N quantum dot. The optical output with the injection current density and the threshold optical pump intensity for various In alloy contents are studied. The differential gain as functions of indium alloy content, charge density and the dot radii in the Ga_1-xIn_xN/Al_0.2Ga_0.8N quantum dot are investigated. The exciton binding energy is calculated in order to obtain the exciton density, the optical gain and the threshold current density in the Ga_1-xIn_xN/Al_0.2In_0.8N quantum dot. The results show that the red shift energy with an increase in In alloy content is found and the differential gain increases with the charge carrier density.