N Vogiatzis

Investigations of Repetition Rate Stability of a Mode-Locked Quantum Dot Semiconductor Laser in an Auxiliary Optical Fiber Cavity

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 d¿/dI 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.

Density of states for dilute nitride systems: calculation of lifetime broadening.

We present calculations for the band structure of bulk and confined quantum well and quantum wire GaInNAs structures. To treat this non-randomly alloyed material system we follow previous approaches in using an Anderson impurity model where the nitrogen localized states interact with the GaInAs conduction band states. We solve this model using Matsubara Greens functions and the associated self-energies which produce a complex band structure where both the real and imaginary components depend on the concentration of nitrogen. In particular this approach gives a definite nitrogen dependent lifetime broadening and is different from previous work in that no artificial input parameters are used. The density of states of the conduction band, derived from these functions, is strongly altered by interaction with the nitrogen states. The density of states is required for further optical and transport investigations involving this system.

Negative differential velocity in ultradilute GaAs1−xNx alloys

We present theoretical results on steady state characteristics in bulk GaAs1−xNx alloys (x ≤ 0.2) using the single electron Monte-Carlo method. Two approaches have been used; the first assumes a GaAs band with a strong nitrogen scattering resonance and the second uses the band anti-crossing model, in which the localized N level interacts with the GaAs band strongly perturbing the conduction band. In the first model we observe two negative differential velocity peaks, the lower one associated with nitrogen scattering while the higher one with polar optical phonon emission accounting for the nonparabolicity effect. In the second model one negative differential velocity peak is observed associated with polar optical phonon emission. Good agreement with experimental low field mobility is obtained from the first model. We also comment on the results from both Models when the intervalley Г → L transfer is accounted for.

Modeling Dilute Nitride 1.3 μm Quantum Well Lasers: Incorporation of N Compositional Fluctuations

Compositional fluctuations of N in Ga0.68In0.32NxAs1-x result in quantum dot (QD)-like fluctuations in the conduction band edge (CBE). The influence of these compositional fluctuations on the performance of Ga 0.68In0.32 NxAs1-x /GaAs quantum well (QW) lasers has been studied using a rate equation approach. Adding N into InGaAs has been observed to reduce the photon luminescence (PL) intensity, broaden the line width, and increase the laser threshold. For low N composition (N ≈ 1%), due to the small density of QD-like fluctuations, the electron density within the fluctuations is below the lasing threshold and they act as defect-related nonradiative centers. However, as N increases (N ≥ 2%), the density of the QD-like fluctuations increases allowing lasing to occur from the QD-like fluctuations. The dynamics of the electrons and photons in both the 2-D QW and the QD-like fluctuations are evaluated. In addition, by adding the gain of the QD-like fluctuations and the QW confined level gain, a broad-band material gain results can be exploited in tuneable lasers.

Fabrication and characterization of GaInNAs/GaAs semiconductor optical amplifiers

The constraints on dilute-nitride Semiconductor Optical Amplifiers (SOAs) for multi-wavelength amplification have been evaluated. SOAs have been fabricated by angling the facets of a GaInNAs/GaAs edge emitting laser using gas enhanced focused ion beam etching. The original laser has been characterized in terms of its temperature dependence and net modal gain. A full width half maximum (FWHM) of 40nm has been found at 298K. Good temperature stability has also been found with a value of 0.35nm/K for the lasing wavelength. The good temperature stability of the device has been explained in terms of the role that the monomolecular recombination plays in the temperature dependence of the device. The monomolecular recombination has been reported temperature independent having two key effects; reduction of the temperature performance and reduction of the dynamic performance in terms of an increase in the threshold current and a decrease of the high speed potential. Iodine gas enhanced focused ion beam etching (GAE-FIB) has been used for the fabrication of the SOA, the iodine gas increasing the etching rate by a factor of 2.5. The fabrication has been completed in two steps; in the first one the facets have been angled and in the second step a cross-section procedure has been employed for smoothing of the facets. Once the SOA has been fabricated its potential for simultaneous multiple channel amplification has been studied. A flat gain spectrum over a range of 40nm has been obtained. This value and the wavelength range have good agreement with the net modal gain measured in the original laser device. In addition, minimum channel interspacing has been achieved with no wavelength degradation.

Theoretical study of dilute nitride 1.3 µm quantum well semiconductor lasers for short pulse generation: Effect of incorporation of N compositional fluctuations

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.

Broad-band gain incorporating quantum dot fluctuations for a GaInNAs semiconductor optical amplifier

It has been observed experimentally that the band edge photoluminescence (PL) of GaxIn1-xNyAs1-y Quantum Well (QW) materials is broadened. This has been attributed to compositional fluctuations of the N. These fluctuations can be modelled as quantum dot-like fluctuations in the conduction band edge of the GaInNAs/GaAs QW. We have developed a rate-equation approach to evaluate the distribution of electrons in the QW energy level and the QD-like fluctuations which includes 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 broad band gain is derived for a single QW. This is then applied to the design of a broad band gain semiconductor optical amplifier (SOA).

Carrier dynamics and gain characteristics of 1.3 µm GaInNAs Quantum Well lasers on GaAs substrate

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.

Modelling dilute nitride 1.3 μm quantum well lasers: Incorporation of N compositional fluctuations

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.

Investigating dilute nitride materials for broad band SOAs for optical communications

The dilute nitride GaInNAs/GaAs quantum well material has been subject to intensive study since it was first proposed by Kondow et al. It has wide applications such as in long wavelength infrared laser diodes, high efficient multi-junction solar cells and broad band semiconductor optical amplifiers (SOA). 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. 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. 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 and observed broad-band gain 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 and the resulting lasers have high threshold current densities, 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.