Numerical Modeling of the Emission Characteristics of Semiconductor Quantum Dash Materials for Lasers and Optical Amplifiers

This paper deals with the simulation of the emission characteristics of self-assembled semiconductor quantum dash (QDash) active materials, characterized by high length-to-width and width-to-height ratios of the dash size and by a wide spreading of the dash dimensions. This significant size fluctuation requires to compute numerically the corresponding energy distribution of the electron and hole confined states. Furthermore, due to the long dash length, it is necessary to take into account the many longitudinal confined states that contribute to the emission spectrum. To implement a model that does not require excessive computation time, some simplifying assumptions have been introduced and validated numerically. Starting from good knowledge of the dash size, material composition, and optical waveguide dimensions, we have been able to simulate the amplified spontaneous emission and gain spectra of a quantum dash semiconductor optical amplifier with a good quantitative agreement with the measured data. As an application example, the model is used to predict the gain properties of different QDash ensembles having various size distributions

Analysis of the optical gain characteristics of semiconductor quantum dash materials including the band structure modifications due to the wetting layer

We present a numerical model for the calculation of the opto-electronic properties of a semiconductor InAs-InAlGaAs quantum dash active material including the presence of the wetting layer (WL), formed at the bottom of the dashes, and the quantum mechanical coupling among dashes caused by the high density of the InAs islands. The model calculates self-consistently the conduction and valence band energy diagram of the confined and unconfined states, the corresponding density of states, the electron and hole wavefunctions and the gain spectra. The results obtained are also compared with a more simple model that consider dashes as isolated and without the WL. The comparison evidences the role of the WL in limiting the gain performance such as the maximum gain, the differential gain and the optical gain bandwidth. The numerical tool is then used to design an improved quantum dash material, which allows to overcome these gain limitations even in presence of the WL and the high dash density

Numerical analysis of the frequency chirp in quantum dot semiconductor lasers

We present a numerical model for the analysis of the chirp dynamics of quantum-dot (QD) semiconductor laser under large signal current modulation. The model is based on the multi-population rate equation formalism, and it includes all the peculiar characteristics of the active QD material such as the inhomogeneous broadening of the gain spectrum, the presence of an excited state confined in the QDs and the presence of nonconfined states due to the wetting layer and the barrier. In this paper the model is applied to the analysis of the chirp of two,QD single-mode lasers emitting from the ground state and from the excited state, respectively. In order to make comparisons of the chirp in various operating conditions, we define some equivalent parameters for quantifying the adiabatic and transient contributions to the chirp. These parameters,are then used to analyze the chirp as function of the bias current, of the modulation depth and of the modulation frequency. All the various simulation results show that the carrier accumulation in the QD states, poorly involved in the stimulated emission process and the carrier dynamics in these states, can cause a nonzero chirp under current modulation even for the ideal condition of zero linewidth enhancement factor (or alpha-parameter) at the laser threshol

Coherent multi-mode dynamics in a Quantum Cascade Laser: Amplitude and Frequency-modulated Optical Frequency Combs

We cast a theoretical model based on Effective Semiconductor Maxwell-Bloch Equations and study the dynamics of a multi-mode mid-Infrared Quantum Cascade Laser in Fabry Perot with the aim to investigate the spontaneous generation of optical frequency combs. This model encompasses the key features of a semiconductor active medium such as asymmetric,frequency-dependent gain and refractive index as well as the phase-amplitude coupling of the field dynamics provided by the linewidth enhancement factor. Our numerical simulations are in excellent agreement with recent experimental results, showing broad ranges of comb formationin locked regimes, separated by chaotic dynamics when the field modes unlock. In the former case, we identify self-confined structures travelling along the cavity, while the instantaneous frequency is characterized by a linear chirp behaviour. In such regimes we show that OFC are characterized by concomitant and relevant amplitude and frequency modulation

Impact of doping on InAs/GaAs quantum-dot solar cells: A numerical study on photovoltaic and photoluminescence behavior

We investigate the effect of doping on quantum dot (QD) solar cells by analysing their behavior in terms of photovoltaic characteristic, external quantum efficiency, and photoluminescence (PL) at room temperature. The analysis addresses the two most widespread methods for QD selective doping, namely modulation and direct doping, to gain a comprehensive device-level assessment of the impact of doping profile and density on the solar cell behavior. Devices are simulated using a physics-based model that accurately describes QD carrier dynamics within a semi-classical drift-diffusion-Poisson model. Different scenarios in terms of crystal quality are considered: in the high-quality material, close to radiative limit, large open circuit voltage recovery is predicted, due to the suppression of radiative recombination through QD ground state. In the defective material, significant photovoltage recovery is also attained owing to the suppression of both nonradiative and QD ground state radiative recombination. In both cases, PL emission from extended wetting layer states becomes dominant at high doping density. The interplay between nonradiative and QD radiative recombination channels, and how their interaction is modified by doping, are analyzed in detail. Strong influence on the cell behavior of unintentional background doping of interdot layers and markedly nonlinear behavior of open circuit PL with respect to excitation intensity are demonstrated. The resulting picture provides new insight on the experimental results in literature

Optical frequency combs in SiN hybrid lasers

We model the dynamical behavior of a III-V SiN hybrid laser accounting for the narrowband mirror effective reflectivity. We characterize the laser as a function of bias current and detuning of the laser emission frequency with respect to the effective reflectivity peak. Numerical simulations allow to address the potential physical triggers for longitudinal multimode dynamics. Among these, we assess the mechanism of generation of frequency combs and we preliminarily characterize this regime in terms of bandwidth and line separation

Time Domain Traveling Wave analysis of the multimode dynamics of Quantum Dot Fabry-Perot Lasers

In this paper we investigate with numerical simulations the rich multi-mode dynamics of Quantum Dot Fabry-Perot Lasers. We have used a Time Domain Traveling Wave approach including the electron and hole carrier dynamics in the various Quantum Dot confined states, the inhomogeneous broadening of the complex gain spectrum, the polarization dynamics and the effect of the carrier-photon interaction in the cavity. The role of the various non-linear interaction mechanism on the broadening of optical spectrum of the Quantum Dot laser has been investigated and the main parameters responsible for the phase locking between the longitudinal modes have been identified. We show that in some cases it is possible obtaining pulses after simulating the propagation of the laser output field in a dispersive medium. Many of the obtained simulation results are in good agreement with the experiments reported in the iterature

Open Circuit Voltage Recovery in Quantum Dot Solar Cells: a Numerical Study on the Impact of Wetting Layer and Doping

The authors present a numerical study on the influence of wetting layer states and doping on the photovoltage loss of InAs/GaAs quantum dot solar cells. Quantum-mechanical simulations are used to analyse how the reduction of wetting layer by Al(Ga)As overgrowth changes the quantum dot electronic states. Device-level simulations allow to correlate such changes with the achievable open circuit voltage. Almost full open circuit voltage recovery is predicted by combining wetting layer reduction, to realise thermal decoupling of barrier and quantum dot confined states, and doping to suppress radiative recombination through the quantum dot confined states

Self-pulsing in single section ring lasers based on Quantum Dot materials: theory and simulations

We studied theoretically coherent phenomena in the multimode dynamics of single section semiconductor ring lasers with Quantum Dots (QDs) active region. In the unidirectional ring configuration our simulations show the occurrence of self-mode-locking in the system leading to ultra-short pulses (sub-picoseconds) with a THz repetition rate. As confirmed by the Linear Stability Analysis (LSA) of the Traveling Wave (TW) Solutions this phenomenon is triggered by the analogous of the Risken-Nummedal-Graham-Haken (RNGH) instability affecting the multimode dynamics of two-level lasers