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Highly-mismatched alloys constitute a promising approach to extend the operational range of GaAs-based quantum well (QW) lasers to telecom wavelengths. This is challenging using type-I QWs due to the difficulty to incorporate sufficient N or Bi via epitaxial growth. To overcome this, we investigate a novel class of strain-compensated type-II QWs combining electron-confining, tensile strained GaN y As 1−y and hole-confining, compressively strained GaAs 1−x Bi x layers. We systematically analyse the optoelectronic properties of W-type GaAs 1−x Bi x /GaN y As 1−y QWs, and identify paths to optimise their threshold characteristics. Solving the multi-band k•p Schrodinger equation self-consistently with Poisson’s equation highlights the importance of electrostatic confinement in determining the optical and differential gain of these QWs. Our calculations demonstrate that GaAs 1−x Bi x /GaN y As 1−y QWs offer broad scope for band structure engineering, with W-type structures presenting the possibility to combine high long-wavelength gain with the intrinsically low non-radiative Auger recombination rates of type-II QWs.

Highly-mismatched III-V semiconductor alloys containing dilute concentrations of bismuth (Bi) have attracted significant attention in recent years since their unique electronic properties open up a range of possibilities for practical applications in semiconductor lasers, photovoltaics, spintronics, photodiodes, and thermoelectrics. Research on dilute bismide alloys has primarily focused to date on $\text{GaAs}_{1-x}\text{Bi}_{x}$ , where incorporation of Bi brings about a strong reduction of the direct $\Gamma$ -point band gap ( $E_{g}{}^{\Gamma}$ ) –by up to 90 meV per % Bi at low Bi compositions $x$ –characterised by strong, composition-dependent bowing. This unusual behaviour derives from the large differences in size (covalent radius) and chemical properties (electronegativity) between As and Bi.Bi, being significantly larger and more electropositive than As, acts as an isovalent impurity which primarily impacts and strongly perturbs the valence band (VB) structure. This is in contrast to dilute nitride alloys, in which small electronegative nitrogen (N) atoms strongly perturb the conduction band (CB) structure in $\text{GaN}_{x}\text{As}_{1-x}$ and related alloys. Additionally, Bi, being the largest stable group-V element, has strong relativistic (spin-orbit coupling) effects. As such, the reduction of $E_{g}{}^{\Gamma}$ in $(\text{In})\text{GaAs}_{1-x}\text{Bi}_{x}$ is accompanied by a strong increase in the VB spin-orbit splitting energy ( $\Delta_{\text{SO}}$ ).

Adding dilute concentrations of nitrogen (N) or bismuth (Bi) into conventional III-V semiconductor alloys causes a large bowing of the bandgap energy due to the modification of the electronic band structure. This behaviour has attracted significant interest due to the resulting optical and electronic properties. Firstly, the authors present theoretical band structure models for GaAs-based dilute nitride, dilute bismide and dilute bismide-nitride alloys and then use them within current continuity equations to show the photovoltaic behaviour. To describe the band structures of these highly mismatched III-V semiconductor alloys, the authors introduce a 10-, 12- and 14-band k · p Hamiltonian for dilute nitride, dilute bismide and dilute bismide-nitride semiconductors, respectively. The authors then use this approach to analyse GaBiAs multi-quantum well p-i-n structures for photovoltaic performance. Through theoretical analysis the authors can: (i) elucidate important trends in the properties and photovoltaic performance of GaBiAs QW structures and (ii) comment generally on the suitability of GaBiAs alloys and heterostructures for applications in multi-junction solar cells. In particular, the authors identify and quantify the limitations associated with current GaBiAs solar cells, and describe the improvements in performance that can be expected pending further development of this emerging class of devices.

We present a theoretical analysis and optimisation of the properties and performance of mid-infrared semiconductor lasers based on the dilute bismide alloy In$_{x}$Ga$_{1-x}$As$_{1-y}$Bi$_{y}$, grown on conventional (001) InP substrates. The ability to independently vary the epitaxial strain and emission wavelength in this quaternary alloy provides significant scope for band structure engineering. Our calculations demonstrate that structures based on compressively strained In$_{x}$Ga$_{1-x}$As$_{1-y}$Bi$_{y}$ quantum wells (QWs) can readily achieve emission wavelengths in the 3 -- 5 $\mu$m range, and that these QWs have large type-I band offsets. As such, these structures have the potential to overcome a number of limitations commonly associated with this application-rich but technologically challenging wavelength range. By considering structures having (i) fixed QW thickness and variable strain, and (ii) fixed strain and variable QW thickness, we quantify key trends in the properties and performance as functions of the alloy composition, structural properties, and emission wavelength, and on this basis identify routes towards the realisation of optimised devices for practical applications. Our analysis suggests that simple laser structures -- incorporating In$_{x}$Ga$_{1-x}$As$_{1-y}$Bi$_{y}$ QWs and unstrained ternary In$_{0.53}$Ga$_{0.47}$As barriers -- which are compatible with established epitaxial growth, provide a route to realising InP-based mid-infrared diode lasers., Comment: Submitted version

We demonstrate a new class of GaAs-based type II quantum wells based on the highly mis-matched III–V semiconductor alloys GaAs 1-x Bi x and GaNyAs 1-y . We theoretically quantify and analyse the available design space for the growth of GaAs 1-x Bi x /GaN y As 1-y type II structures on GaAs. Our calculations indicate that, for alloy compositions and layer thicknesses comparable with epitaxial growth, these heterostructures offer optical emission and absorption at wavelengths up to 3 µm as well as the ability to grow strain-balanced structures. We present the results of experimental measurements on a GaAs 1-x Bi x /GaNyAs 1-y (x = 3.3%, y = 6.25%) structure, grown via metal-organic vapor phase epitaxy. X-ray diffraction measurements indicate good structural quality, which is confirmed by the observation of photoluminescence and optical absorption at room temperature. The measured photoluminescence peak wavelength of 1.72 µm, which is in good agreement with theoretical calculations is, to our knowledge, the longest emission wavelength that has been observed to date using a pseudomorphically grown GaAs-based quantum confined heterostructure.

GaAsBi offers the possibility to develop near-IR semiconductor lasers such that the spin-orbit-split-off energy (ΔSO) is greater than the bandgap (Eg) in the active region with lasing wavelengths in the datacom/telecom range of 1.3-1.6 μm. This promises to suppress the dominant efficiency-limiting loss processes as Auger recombination, involving the generation of “hot” holes in the spin-orbit split-off band (the so-called “CHSH” process), and inter-valence band absorption (IVBA), where emitted photons are re-absorbed in the active region, thereby increasing the internal optical losses and negatively impacting upon the laser characteristics being responsible for the main energy consumption. In addition to growth and fabrication processes refinement, a key aspect of efforts to continue the advancement of the GaAsBi material system for laser applications is to develop a quantitative understanding of the impact of Bi on key device parameters. In this work, we present the first experimental measurements of the absorption, spontaneous emission, and optical gain spectra of GaAsBi/AlGaAs QW lasers using a segmented contact method and a theoretical analysis of these devices, which shows good quantitative agreement with the experiment. Internal optical losses of 10-15 cm-1 and peak modal gain of 24 cm-1 are measured at threshold and a peak material gain is estimated to be 1500 cm-1 at current density of 2 kA/cm-2, which agrees well with the calculated value of 1560 cm-1. The theoretical calculations also enabled us to identify and quantify Bi composition variations across the wafer and Bi-induced inhomogeneous broadening of the optical spectra.

A detailed balance model is used with a blackbody radiation function todetermine the efficiency of an intermediate band solar cell including carrier losses from the intermediate band. The effect of the energy gap of the host semiconductor is examined as a function of the intermediate band position in the energy gap and the host semiconductor energy gap. Generally the optimum intermediate band level is found to decrease within the energy gap to mitigate the carrier losses and it is found that carrier losses are less detrimental to small energy gap materials. We therefore focus the study on the role of carrier losses in wide bandgap semiconductor intermediate band solar cell systems such as the GaN semiconductor with an Mn impurity band. Experimentally the Mn acceptor level in the GaN energy gap is found to be 1.8 eV above the valence band which is 199 meV off the ideal IB neglecting losses which reduces the efficiency to 21.36%. We demonstrate how carrier losses can be introduced into the system to shift the optimum IB position. Introducing carrier losses of 70% from the intermediate band, shifts the optimum intermediate band position to 1.8 eV above the valence band and increases the efficiency to 23.41%. We compare this to the effect of alloying the GaN and introducing biaxial strain to shift the effective position of the Mn impurity band within the bandgap to increase theefficiency.

A detailed balanced approach is used to model a quantum-dot intermediate band solar cell (QD-IBSC). A model assuming identical QDs with no carrier losses has peak efficiency of 63.24% for a bandgap of 1.95 eV with the intermediate band (IB) positioned either 1.24 eV above the valence band or below the conduction band. The effect of inhomogeneities in the QDs is introduced by incorporating an inhomogeneous broadening of the IB, and the homogeneous broadening describing life-time broadening effects is also introduced. A Gaussian inhomogeneous broadening of the ground state is considered initially and then other inhomogeneous broadenings are considered, including a ground plus excited state distribution. The inhomogeneous broadening removes state density from the ideal IB energy position and thereby reduces efficiency. However, the homogeneous broadening of the QD-IB levels allows more off-resonance QD states to be able to contribute to absorption of the optimum IB photon. This broadening also allows photons near the optimum IB photon energy to be absorbed as well. Both these effects lead to an increase in efficiency. The inter-play between these inhomogeneous and homogeneous broadening allows the efficiency in realistic inhomogeneous arrays of QDs to remain close to the ideal values.

Analysis of the broadband gain of a GaInNAs single quantum well (QW) semiconductor optical amplifier (SOA) is developed considering the tuneability of the gain in detail. The SOA is analyzed as a single device multiwavelength channel amplifier in a wavelength-division-multiplexing (WDM) network. The gain model includes the QW material gain derived using a band anti-crossing model and includes quantum dot (QD) fluctuations in the conduction band arising from compositional fluctuations of N within the QW. The material gain is broadened by adding the gain of the QD-like fluctuations and the QW confined level. Simultaneous amplification of two optical signals is analyzed, one at the peak of the QW gain and one at the peak of the QD distribution gain, and the linear and nonlinear regions are established. In addition, multi-channel signal amplification, appropriate for WDM applications, has been modeled across the frequency range of the QW and the QD-like fluctuations and no wavelength degradation between the channels was observed demonstrating the potential of dilute nitride QW as multiwavelength SOAs at optical communications wavelengths.

It has been observed experimentally that the band edge photoluminescence (PL) of GaxIn1-xNyAs1-yquantum well (QW) materials is broadened due to compositional fluctuations with N distribution. These fluctuations can be modelled as quantum dot-like region at the conduction band minimum in as-grown GaInNAs/GaAs QWs. We have developed a rate-equation approach to describe the distribution of electrons in the QW and QD-like fluctuations which include the carrier recombination from both the conventional 2D QW layer and the inhomogeneous dot-like fluctuations. The electron dynamics in the QW and QDs states are examined and the resulting carrier population and broadband gain is derived for a single QW. This is then applied to the design of a broadband gain multi-QW semiconductor optical amplifier (SOA). (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present an analysis of dilute bismide quantum well (QW) lasers grown on GaAs and InP substrates. Our theoretical analysis is based upon a 12-band k·p Hamiltonian which directly incorporates the strong impact of Bi incorporation on the band structure using a band-anticrossing approach. For GaBiAs QWs grown on GaAs we analyse the device performance as a function of Bi composition, and quantify the potential to use GaBiAs alloys to realise highly efficient, temperature stable 1.55 µm lasers. We compare our calculations to measured spontaneous emission (SE) and gain spectra for first-generation GaBiAs lasers and demonstrate quantitative agreement between theory and experiment. We also present a theoretical analysis of InGaBiAs alloys grown on InP substrates. We show that this material system is well suited to the development of mid-infrared lasers, and offers the potential to realise highly efficient InP-based diode lasers incorporating type-I QWs and emitting at > 3 µm. We quantify the theoretical performance of this new class of mid-infrared lasers, and identify optimised structures for emission across the application-rich 3 – 5 µm wavelength range. Our results highlight and quantify the potential of dilute bismide alloys to overcome several limitations associated with existing GaAs- and InP-based near- and mid-infrared laser technologies.

There is a need to improve the performance of materials underlying optical communications devices. Performance parameters such as modulation speed, temperature insensitively and band width can be tailored at the material level. In this talk I will show modelling and comparison with experiment of some highly-mismatched alloys: dilute nitride and dilute bismide materials and quantum dot materials.

We present a theoretical analysis of the electronic and optical properties of near-infrared dilute bismide quantum well (QW) lasers grown on GaAs substrates. Our theoretical model is based upon a 12-band kp Hamiltonian which explicitly incorporates the strong Bi-induced modifications of the band structure in pseudomorphically strained GaBixAs1􀀀x alloys. We outline the impact of Bi on the gain characteristics of ideal GaBixAs1􀀀x/(Al)GaAs devices, compare the results of our theoretical calculations to experimental measurements of the spontaneous emission (SE) and optical gain – a first for this emerging material system – and demonstrate quantitative agreement between theory and experiment. Through our theoretical analysis we further demonstrate that this novel class of III-V semiconductor alloys has strong potential for the development of highly efficient GaAs-based semiconductor lasers which promise to deliver uncooled operation at 1.55 m.

Electrically pumped GaAsBi/GaAs quantum well lasers are a promising new class of near-infrared devices where, by use of the unusual band structure properties of GaAsBi alloys, it is possible to suppress the dominant energy-consuming Auger recombination and inter-valence band absorption loss mechanisms, which greatly impact upon the device performance. Suppression of these loss mechanisms promises to lead to highly efficient, uncooled operation of telecommunications lasers, making GaAsBi system a strong candidate for the development of next-generation semiconductor lasers. In this report we present the first experimentally measured optical gain, absorption and spontaneous emission spectra for GaAsBi-based quantum well laser structures. We determine internal optical losses of 10–15 cm−1 and a peak modal gain of 24 cm−1, corresponding to a material gain of approximately 1500 cm−1 at a current density of 2 kA cm−2. To complement the experimental studies, a theoretical analysis of the spontaneous emission and optical gain spectra is presented, using a model based upon a 12-band k.p Hamiltonian for GaAsBi alloys. The results of our theoretical calculations are in excellent quantitative agreement with the experimental data and together provide a powerful predictive capability for use in the design and optimisation of high efficiency lasers in the infrared.

We present a theoretical analysis of the properties and performance of mid-infrared dilute bismide quantum well (QW) lasers grown on InP substrates. We analyse the band structure of strained InGaBiAs alloys and quantify their potential for the development of mid-infrared semiconductor lasers. In addition to identifying the permissible growth combinations for this class of laser structures, we perform a comprehensive analysis of the performance of a series of ideal laser structures. We investigate the variation of key material and device parameters on the alloy composition, QW thickness and epitaxial strain, and on this basis identify optimised laser structures for emission across the 3 – 5 m wavelength range. Our theoretical analysis suggests that InP-based dilute bismide alloys are an extremely promising candidate material system for the development of highly efficient and temperature stable laser diodes operating in the mid-infrared, and also that this class of laser structures is highly compatible with existing InP-based device architectures.

Very little, if any, has been published on optically pumped 850 nm vertical-cavity surface-emitting lasers (VCSELs), particularly for doped structures. This paper investigates GaAs-based VCSELs which have not been optimized for optical pumping work. Characterisation was carried out for both pulsed and continuous wave (CW). Pulsed operation causes a lower rise in temperature, thus postponing the onset of thermal rollover, and allowing the device to be operated at higher powers. A threshold of ∼160 kW/cm 2 , and single mode output with incident power density of up to 225 kW/cm 2 were obtained. From the simulation work done, it has been observed that for optically pumped VCSELs, at higher pump density, there was faster turn on and higher output power, and that dilute nitride active material give better output performance compared to GaAs.

The dependence of spectral, power versus current and small-signal modulation characteristics versus the etch depth in two types of surface-etched vertical-cavity surface-emitting lasers (VCSELs) are experimentally and theoretically investigated. One type has a photonic crystal (PC) fabricated in the top distributed Bragg reflector (DBR), whereas the second type has a micropillar (MP) created by removing the DBR surrounding it. The aim of both fabrication designs is to improve the single-mode high-power output. Theoretical and experimental results are found to be in qualitative agreement. It is shown that mode-selective optical losses, introduced by the etched holes of the PC in the DBR, control the optical modes of the PC-VCSEL. Single-fundamental-mode radiation is observed for deeply etched PC- and MP-VCSELs. In contrast, improved modulation characteristics are found for shallowly etched devices. Higher-order single-mode generation with improved modulation characteristics is demonstrated for PC-VCSELs with an etch depth of 1.54 μm. PC-VCSELs demonstrate higher slope efficiency, lower threshold current, and series resistance compared with MP-VCSELs of the same etching depth.

A dynamical model of oxide-confined Vertical-Cavity Surface-Emitting Lasers (VCSELs) with two-dimensional photonic crystals (PCs) incorporated within them so called PC-VCSELs is presented and used to optimise designs for high-power single-mode operation. Three PC-VCSEL designs are considered: (I) with holes in the top DBRs, (II) with PC holes situated between their DBRs and (III) with PC holes etched through the entire VCSEL. A simulated design for a PC-VCSEL of type (I) with holes of d = 2 μm diameter, a = 4 μm lattice constant (d/a = 0.5) and 2.2 μm depth was found to improve the single mode behaviour but not enough to establish single mode behaviour for large apertures. The modulation behaviour was not degraded by the PC. Simulations of type (II) and (III) PC-VCSELs, with the same parameters, have shown multimode operation and degraded modulation properties. Simulations of PC-VCSELs of type (III) with holes of d = 0.2 μm diameter and a = 0.4 μm lattice constant (d/a = 0.5) have shown improved modulation properties and enhanced single mode power for small apertures. In simulation, PC-VCSELs incorporating multiple PC-defects have shown order of magnitude increases in the single mode output power. However, the modulation properties of these VCSELs show degradation due to gain saturation and hopping of the optical modes localized within the PC defects.

We have investigated experimentally the pulse train (mode beating) stability of a monolithic mode-locked multi-section quantum-dot laser with an added passive auxiliary optical fiber cavity. Addition of the weakly coupled (∼ - 24 dB) cavity reduces the current-induced shift dv / dl of the principal peak in the RF spectrum (the effective pulse repetition frequency) by more than an order of magnitude, from -39.5 to - 2.3 kHz/mA. The rms timing jitter of the pulse train is simultaneously reduced from 1.4 to 0.9 ps. © 2009 IEEE.

Barium strontium titanate (Ba0.05Sr0.95TiO3) ferroelectric thin films have been prepared on single crystal [001] MgO substrates using the pulsed laser deposition method. The X-ray diffraction (XRD) analysis show the films were oriented with the [001] direction perpendicular to the plane of the substrate. The refractive index of Ba0.05Sr0.95TiO3 is determined from model fitting with the calculated data of the reflectivity of Ba0.05Sr0.95TiO3 in the wavelength 1450–1580 nm at the room temperature. The dispersion curve decreases gradually with increasing wavelength. The average value of the refractive index is found to be 1.985 in the wavelength 1450–1580 nm which is important for optoelectronic device applications.

Intermediate Band Solar Cells (IBSCs) were proposed [1] as a variation of a compound III–V semiconductor pn junction solar cell [2] with a proposed efficiency of 63% with concentration of light [3]-making them the highest efficiency pn junction solar cell possible. A schematic energy band profile of a p-i-n junction IBSC is given in Fig. 1. There are several possible approaches to create the Intermediate Band (IB): high density quantum dots, of a smaller band gap material can be introduced into a larger bandgap semiconductor or high density impurity states can also be introduced into the semiconductor host material to create an IB. A detailed balance approach is used to calculate the efficiency for a specific energy band gap with an IB positioned at the ideal position within the bandgap as shown in Fig. 2. This approach produces the high efficiency of 63% for a bandgap of 1.95 eV with the IB positioned 0.71 eV above the Valence Band (VB) or below the Conduction Band (CB). Experimentally efficiencies of < 20% have been observed for such devices employing QD-based IBs.

This paper presents results for focused ion beam (FIB) processing of two photonic devices: 1) a GaN laser with a fourth-order grating for vertical emission, and 2) a two-dimensional (2-D) photonic crystal (PhC) structure. For the GaN laser, both L-I and I-V results are shown before and after etching, and vertical emitted power as a function of the distance along grating is shown. A finite element (FE)-based electromagnetic model is developed to support the measured results and is used to predict the optimum grating depth. For the 2-D photonic crystal, direct FIB etching is used to create a PhC in a standard InP waveguide structure. Measured and modeled transmission results are compared, and there is good agreement for band edge position. A detailed study of hole shape is presented, and this leads to the development of a multistage etching procedure involving both reactive ion etching and inductively coupled plasma etching. This results in a much improved hole shape.

The potential of 1.3-/spl mu/m AlGaInAs multiple quantum-well (MQW) laser diodes for uncooled operation in high-speed optical communication systems is experimentally evaluated by characterizing the temperature dependence of key parameters such as the threshold current, transparency current density, optical gain and carrier lifetime. Detailed measurements performed in the 20/spl deg/C-100/spl deg/C temperature range indicate a localized T/sub 0/ value of 68 K at 98/spl deg/C for a device with a 2.8 /spl mu/m ridge width and 700-/spl mu/m cavity length. The transparency current density is measured for temperatures from 20/spl deg/C to 60/spl deg/C and found to increase at a rate of 7.7 A/spl middot/cm/sup -2//spl middot/ /spl deg/C/sup -1/. Optical gain characterizations show that the peak modal gain at threshold is independent of temperature, whereas the differential gain decreases linearly with temperature at a rate of 3/spl times/10/sup -4/ A/sup -1//spl middot//spl deg/C/sup -1/. The differential carrier lifetime is determined from electrical impedance measurements and found to decrease with temperature. From the measured carrier lifetime we derive the monomolecular ( A), radiative (B), and nonradiative Auger (C) recombination coefficients and determine their temperature dependence in the 20/spl deg/C-80/spl deg/C range. Our study shows that A is temperature independent, B decreases with temperature, and C exhibits a less pronounced increase with temperature. The experimental observations are discussed and compared with theoretical predictions and measurements performed on other material systems.

The 2-D lattice laser is a promising low-cost device for high-speed optical data communications applications.

Due to the keen interest in improving the high-speed and high-temperature performance of 1.3-/spl mu/m wavelength lasers, we compare, for the first time, the material gain of three different competing active layer materials, namely InGaAsP-InGaAsP, AlGaInAs-AlGaInAs, and InGaAsN-GaAs. We present a theoretical study of the gain of each quantum-well material system and present the factors that influence the material gain performance of each system. We find that AlGaInAs and InGaAsN active layer materials have substantially better material gain performance than the commonly used InGaAsP, both at room temperature and at high temperature.

The introduction of a deep-etched hexagonal lattice of round holes around a short section of the ridge waveguide of a Fabry-Perot laser can be used to create single-mode lasers suitable for high-speed data communications. A detailed characterization of the operating characteristics of the two-dimensional-lattice laser is presented. In addition, modeling of gain, reflectivity, and waveguide properties is used to explain experimentally observed results.

The three-dimensional finite-difference time-domain method is used to obtain loss per unit length in a two-dimensional photonic crystal membrane waveguide by simulating three different length guides. Results are shown for propagation both above and below the light line. The results are compared with a Fourier expansion method and good agreement is obtained above and below the light line.

The influence of compositional fluctuations of N in GaInNAs Quantum Well (QW) lasers has been studied using a rate equation model. These fluctuations can be treated as Quantum-Dot (QD)-like fluctuations at the Conduction Band Edge (CBE). The gain model includes the QW material gain derived using a Band Anti-Crossing (BAC) model and includes QD fluctuations in the conduction band. For low N, (N ~ 1%), the QD-like fluctuations act as defect-related non-radiative centres. However as N is increased (N > 2%), the density of QD-like fluctuations increase and can support lasing. The dynamics of the electrons and photons in both the QW and the QD-like fluctuations is explored. Lasing can occur at either or both of the QW and QD states with the carrier densities being strongly coupled. In addition, short pulse generation from the QW is observed due to interaction with the carriers within the QDs demonstrating the potential of dilute nitride QW for short pulse generation at optical communications wavelengths.

Summary form only given. Dilute nitride GaInNAs/GaAs quantum well (QW) lasers have been subject to intensive study since being first proposed by Kondow et al. [1]. Dilute nitride GaInNAs materials have a wide range of applications such as long wavelength infrared laser diodes, high efficient multi-junction solar cells, broad band semiconductor optical amplifiers (SOA) and tuneable lasers. The GaInNAs/GaAs material system has a large band-gap bowing which results in a large conduction band offset [2] and this system has the potential to cover a range of optical communication wavelengths by controlling the N composition. Also, the reduced temperature sensitivity and observed broad-band gain have made GaInNAs a promising candidate for un-cooled and tuneable communication lasers at 1.3 μm.Incorporation of N into GaInAs results in low photon luminescence (PL) intensities with wide line-widths [3], and thus for lasers, tuneability over a broad gain higher albeit with increased threshold current densities [3]. We have modelled the gain in GaInNAs/GaAs QW lasers this using a Band Anti-crossing (BAC) model [2] including the spatial compositional fluctuations of the N that lead to quantum dot (QD)-like fluctuations at the conduction band minimum. Therefore we use an array of inhomogeneous broadened QDs to represent the CBE fluctuations. We model this system using a rate equation approach. This gives us the population of the electrons in the QW energy level and within the energy levels of the inhomogeneous array of QD-like fluctuations which can be used to calculate the gain from the QW and the QD resulting photon output. Positive gain only occurs for levels with electron densities above transparency while absorption (negative gain) occurs below this electron density. At low nitrogen composition (N=1%), due to small density of states (DOS) of the QD-like fluctuations, the electron density is insufficient to reach the lasing threshold of the QD system. These fluctuations act like defected-related non-radiative centres. We compare this rate equation analysis to one considering the monomolecular (defect-related) recombination process and find good agreement with the experimental increase in threshold current density. As the N composition increases we observe an increase in the lasing threshold as shown in Fig. 1 (a). For N=2% the density of the QD-like fluctuations is enough to allow lasing from electrons in these QD states. In this case we see simultaneous lasing occurs at both QW and QD energy states. We also observe carrier dynamics between the two systems which can result in short pulse lasing generation shown as Fig. 1 (b).The electron-photon dynamics can be used to calculate the material gain arising from both the QW confined level and from the QD-like fluctuations. It is observed to be broadened relative to the gain from the QW level only. We evaluate the gain for a single QW system as a function of input current. This model can be extended to a multi-quantum well system to further broaden the gain spectrum for use in comb generators.

A novel duobinary RZ modulation format having a compact spectrum, complete removal of the carrier, and significant suppression of both the side-lobes and discrete components is proposed, with the best immunity to fiber nonlinearities and dispersion.

The dilute nitride GaInNAs/GaAs quantum well material has been subject to intensive study since it was first proposed by Kondow et al. [1]. It has wide applications such as in long wavelength infrared laser diodes [2], [3], high efficient multi-junction solar cells [3], [4] and broad band semiconductor optical amplifiers (SOA) [5]. Conventional materials for these emission applications based on GaInAsP/InP have poor temperature stability due to a small conduction band discontinuity resulting in poor electron confinement [6]. The GaInNAs material system has a a bandgap that can be tuned, whilst remaining lattice matched to GaAs. In addition, it was found experimentally that a large band-gap bowing reduced the bandgap even further and resulted in a large conduction band offset [7]. Thus this dilute nitride system has the potential to cover a range of optical communication wavelengths by controlling the small nitrogen concentration. Also, the reduced temperature sensitivity [8] and observed broad-band gain [7] have made GaInNAs a promising candidate for broad-band laser and SOA design. Dilute nitride has been found to be one of a class of such materials known as highly mismatched alloys (HMA) in which the addition of one constituent strongly affects the alloy properties such as band-gap and effective mass. Since the emergence of dilute nitride other such HMA have been discovered and these follow similar trends. Recent applications for this broader class of HMAs are as intermediate band solar cells and as Gunn-type electronic diodes. Incorporation of N into GaInAs results in low PL intensities with wide line-widths [9], [10] and the resulting lasers have high threshold current densities [9], which have been attributed to the difference between the N and As atoms in the lattice structure of GaInNAs. This has been successfully analysed using a Band Anti-crossing (BAC) model [7] in which the N acts as a defect on the GaInAs conduction band mixing with it and pushing it downwards. The N defect level also alters the effective mass of the conduction band. Spatial variation in the N composition leads to quantum dot (QD)-like fluctuations at the conduction band edge(CBE) as shown schematically in Fig 1. Therefore it is crucial to understand the effect of these QD-like fluctuations in GaInNAs material systems.

Monte Carlo has become a powerful tool for describing complex systems with many degrees of freedom. It involves simulating a combination of deterministic and stochastic processes. Here, after a basic introduction to the technique, we focus on its application in the analysis of carrier transport in semiconductors. This method is applied to GaAs and to dilute nitride materials.

We report for the first time the transmission of a 2.5-Gb/s SCM channel over a 300-m length of installed-grade 62.5-/spl mu/m core-diameter multimode fiber. This result demonstrates that, in a "worst-case" scenario, subcarrier modulation can be successfully used to allow improved quality transmission over multimode fiber links compared with conventional baseband transmission techniques.

Summary form only given. GaInNAs/GaAs quantum wells emitting around 1300 nm have been the subject of intense interest for optical data- and tele-communications applications. Indeed the observation of 1550 nm emission with GaInNAsSb has increased their potential usefulness. Their ability to be made into VCSELs using AlGaAs-based DBRs, their high gain, high differential gain and their fast modulation speed offers much. The study of the material aspects of dilute nitride, where the nitrogen acts as a defect, not fully incorporating into the lattice, has been incorporated within the Band Anti-crossing (BAC) model. To derive gain from this model is fundamentally difficult requiring improved treatments which treat a complex band structure. In addition, spatial fluctuations of N cause a lowering of the conduction band and a possible trapping of electrons in these quantum-dot (QD)-like fluctuations. Also the position of the N within the QW affects the strength of its coupling to the conduction band.

Summary form only given. It has been observed experimentally that the band edge photoluminescence of GaInNAs Quantum Well (QW) materials is broadened resulting from band-tailing, localised states or conduction band edge fluctuations. We have developed a model for N compositional fluctuations causing conduction band edge fluctuations which localise electrons into the resulting Quantum Dots (QDs). The electron dynamics in both QW and QDs states are examined using a four-rate-equation considering gain processes from both QW and QDs, which is shown in Eq. (1). The mechanism was proved to lead to broad gain in GaInNAs QW structure which could be useful for broad-band Semiconductor Optical Amplifier (SOAs) for optical communications.

We present an analysis of the scattering of hot carriers by conduction electrons in heavily doped semiconductors within the random-phase approximation (RPA). Different approximations to the temperature-dependent RPA are considered: (i) the two-pole approximation developed by Rorison and Herbert, (ii) the plasmon-pole approximation, and (iii) the Lindhard dielectric function. We present a range of results for n-doped GaAs for different carrier energies, doping levels, and temperatures, and rue examine the ranges of validity of the different approximations. As an extension of our theory rue include, within the two-pole approximation, the coupling of optical phonons to the electron system

A calculation of the exciton binding energy (E x ) and oscillator strength for quantum dots in type-II semiconductor systems is presented. These structures consist of a spherical dot of one semiconductor embedded in a second semiconductor. As in the type-II exciton systems in quantum wells the electron is confined in one semiconductor and the hole is confined in the other due to band lineups in the two materials which make this arrangement energetically favorable. We have considered the systems (i) GaAs/AlAs where the electron is confined in the X state in the AlAs while the hole is confined in the GaAs dot (for a dot radius of less than 56 A) and (ii) InAs/GaSb where the electron is confined in the InAs dot while the hole is confined in the GaSb (for a dot of radius less than 87 A)

The key parameters in the fabrication of deep-etch high-order λ/4 Bragg gratings for short-wavelength nitride-based lasers are investigated. Calculations indicate that, for an air-gap thickness of 1.73 μm and single-spot Gaussian beam profile, the reduction in grating reflectivity due to light diffraction in the air gaps is only 17% with respect to a first-order structure with 0.1 μm air gaps. Scanning electron microscopy and microphotoluminescence characterizations confirm the validity of the numerical predictions and show that the 28%–38% reflectivity obtained from prototype focused-ion-beam-etched air/nitride gratings is mainly limited by imperfections and material disorder due to etching. Improving the etching technique would, therefore, allow standard lithographic fabrication of reduced-threshold GaN lasers.

The effect of the nature of nitrogen incorporation in the InGaAsN/GaAs quantum well system on gain is investigated. The nitrogen is considered to be either fully incorporated within the lattice or to be incorporated as a localized acceptor. In the latter case this results in conduction-band anticrossing, causing nonparabolicity. The resulting gains from the two extreme limits are compared and found to be similar. This shows that the nature of the nitrogen incorporation is not a key issue in the performance of InGaAsN/GaAs quantum well lasers for 1.3 μm applications.

The device is pumped with an electrical current injection. The magnitude of carrier density in QD is higher than in QW since it is the summation of carrier densities in all groups of dots. The corresponding output remains minimal until the carrier concentration in the QD states reaches threshold and then increases rapidly which reduces the carrier density and carrier density rises again through recovery of carriers from current injection. This cycle repeats several time until steady state is achieved. The probability of carrier concentration in the QD states is shown in Fig. 2. The carriers occupy the lower energy states first as their probability being highest then drops all the way with the increase of energy. Once the current reaches the threshold, we can observe a “hole” near central energy, which is known as the spectrum hole burning (SHB) and this SHB becomes more significant with more current injection.

Recently, it has been shown that the etching of deep trenches in close proximity to GaAs vertical-cavity surface-emitting laser (VCSEL) apertures causes the linearly polarized TE emission to be pinned in a direction parallel to the line etch. In this letter, we show that etching introduces compressive strain or relaxes tensile strain through the creation of free interfaces. An anisotropic variation of strain is the origin of the polarization pinning effect. We report on the enhancement of polarization pinning by postannealing after etching. Photoluminescence and Raman measurements of the VCSEL wafer were taken before and after etching and annealing. The observed shift in the Fabry–Perot mode was used to model the strain, giving 4×108 dyn/cm2, or 0.05%, compressive strain perpendicular to the etch.

Using a many impurity Anderson model, we describe the interaction of localized N states with GaInAs conduction states. N dependent DOS and material gain reflect features from strong mixing with N pairs/clusters,suggesting its broadband tunability.

The gain and refractive index dynamics of dilute nitride antimonide semiconductor optical amplifiers are studied using heterodyne pump probe spectroscopy, both in forward and reverse bias regimes. In the forward biased absorption regime, both gain and refractive index relax on the same timescale indicating that both quantities are linked to the same relaxation process, interband recombination. Above transparency, in the forward biased gain regime, the gain and phase exhibit differing timescales resulting in a dynamical alpha factor that varies strongly with time. Reversed bias measurements suggest a recombination dominated absorption recovery where the recovery timescale increases with increasing reversed bias, possibly due to charge separation effects. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3246781]

This work presents results of theoretical study of semiconductor vertical-cavity surface-emitting lasers (VCSELs) with two-dimensional photonic crystal (PC) waveguide incorporated into laser cavity. The model is designed for the understanding and optimization of VCSELs with incorporated PCs. Static and dynamic characteristics were investigated. It was found that the computed modulation response of the VCSEL with the PC has a dip at 1.5 GHz modulation frequency, which has been observed in our experimental results. Therefore, a qualitative agreement with experimental results is observed.

Focused ion beam etching (FIB) was used to fabricate a two dimensional photonic crystal (PC)into the top distributed Bragg reflector (DBR) of an oxide-confined vertical-cavity surface-emitting laser (VCSEL) to create a PC-VCSEL. The influence of the geometrical parameters of the PC such as its lattice constant, the diameter and the depth of holes on the static and dynamic characteristics of the laser was investigated. It was shown that PC-VCSELs reduce transverse mode competition with single-mode radiation being observed an etching depth of 2.34 mum.The small signal modulation bandwidth was found to increase with the incorporation of PCs but is optimised for a shallower etching depth. Experimental results are analysed with various dynamical models of VCSELs with PCs.

In this work, AlGaAs-GaAs semiconductor vertical-cavity surface-emitting laser (VCSELs) with incorporation of two-dimensional photonic crystal (PC) waveguides into their top distributed Bragg reflector are experimentally and theoretically investigated. This is done to control the transverse optical modes in the cavity and is also seen to improve the modulation characteristics. Results for VCSELs with PC are compared to VCSELs without the PC and with VCSELs with micropillar microcavities.