Quantum Dot Laser Diodes Research Articles

High-temperature and continuous wave-operation of all-MOCVD grown InAs/GaAs quantum dot laser diodes with highly strained layer and low temperature p-AlGaAs cladding layer

InAs/GaAs quantum dot laser diodes (QDLDs) on a GaAs substrate grown by utilizing all-metalorganic chemical vapor deposition (MOCVD) technology with a p-AlGaAs cladding layer are systematically investigated to produce 75 °C continuous-wave (CW) operation for the first time. First, the InAs quantum dots (QDs) in a dot-in-a-well (DWELL) structure formed by engineering the strained bottom InGaAs layer are successfully grown without detectable clusters, making it possible to increase the number of DWELL stacks effectively. To avoid degradation of the DWELL active layer during the high-temperature growth of the upper p-AlGaAs cladding layer, a detailed analysis of the p-AlGaAs cladding layer grown at low temperature is then carried out. The results of electron-channeling contrast imaging reveal that dislocations in the p-cladding layer are generated due to the accumulative strain of the DWELL and low-temperature growth. The fabricated InAs/GaAs QDLDs show good electrical and optical characteristics with O-band emission wavelength without high-reflection coating, indicating that the suggested growth technologies and the fabricated devices are promising options for future Si-photonic light source components.

The impact of lutetium doping on the structure and physical characteristics of aluminum arsenide quantum dots

This report mentioned the synthesis of self-assembly Al1-xLuxAs quantum dot nanocrystals using the L-Glutamine-assisted wet chemical route. The effect of Lu-doping on the structural phase, lattice strain, and deformation lattice energy was studied. The morphology and elemental compositions were emphasized using transmission electron microscopy, selected area of electron diffraction, and energy dispersive X-ray spectroscopy. The effect of Lu-doping on the oxidation state and vibrational modes was studied using X-ray photoelectron spectroscopy and Raman scattering spectroscopy. The influence of Lu-dopant on the optical bandgap and Urbach tail energy of AlAs nanocrystals was investigated. The tendency of Lu doping to create localized levels inside the band gap of AlAs gives rise to an increase in plasma frequency and a decrease in the high-frequency dielectric constant. The Lu-dopant improved the conductivity and the charge carrier mobility of AlAs nanocrystals. The luminescence measurements revealed the ability of Lu-dopant to suppress the crystal defects of the AlAs nanocrystals. The Lu doping improved the quantum yield (84 %) and the luminescent emission spectra of the AlAs QDs by 7-fold. The prepared self-assembly Al1-xLuxAs quantum dots revealed great encouragement for developing Al1-xLuxAs quantum dot diode lasers and optical broadbands.

Colloidal Semiconductor Nanocrystal Lasers and Laser Diodes.

Lasers and optical amplifiers based on solution-processable materials have been long-desired devices for their compatibility with virtually any substrate, scalability, and ease of integration with on-chip photonics and electronics. These devices have been pursued across a wide range of materials including polymers, small molecules, perovskites, and chemically prepared colloidal semiconductor nanocrystals, also commonly referred to as colloidal quantum dots. The latter materials are especially attractive for implementing optical-gain media as in addition to being compatible with inexpensive and easily scalable chemical techniques, they offer multiple advantages derived from a zero-dimensional character of their electronic states. These include a size-tunable emission wavelength, low optical gain thresholds, and weak sensitivity of lasing characteristics to variations in temperature. Here we review the status of colloidal nanocrystal lasing devices, most recent advances in this field, outstanding challenges, and the ongoing progress toward technological viable devices including colloidal quantum dot laser diodes.

Toward Nonepitaxial Laser Diodes.

Thin-film organic, colloidal quantum dot, and metal halide perovskite semiconductors are all being pursued in the quest for a wavelength-tunable diode laser technology that does not require epitaxial growth on a traditional semiconductor substrate. Despite promising demonstrations of efficient light-emitting diodes and low-threshold optically pumped lasing in each case, there are still fundamental and practical barriers that must be overcome to reliably achieve injection lasing. This review outlines the historical development and recent advances of each material system on the path to a diode laser. Common challenges in resonator design, electrical injection, and heat dissipation are highlighted, as well as the different optical gain physics that make each system unique. The evidence to date suggests that continued progress for organic and colloidal quantum dot laser diodes will likely hinge on the development of new materials or indirect pumping schemes, while improvements in device architecture and film processing are most critical for perovskite lasers. In all cases, systematic progress will require methods that can quantify how close new devices get with respect to their electrical lasing thresholds. We conclude by discussing the current status of nonepitaxial laser diodes in the historical context of their epitaxial counterparts, which suggests that there is reason to be optimistic for the future.

Surfactant effect of Bi on InAs quantum dot laser diode

In this study, InAs multiple-stacked quantum dot lasers were investigated in the 1.55 μm band using the Bi surfactant effect. The Bi surfactant effect increased the size of the quantum dot and changed the emission wavelength from 1522 to 1554 nm. Moreover, the surfactant effect enhanced the internal quantum efficiency of the fabricated broad-area laser from 33% to 54%.

Impact of a defect trapping layer on the reliability of 1.3 μm quantum dot laser diodes grown on silicon

In this paper, we studied the impact of a defect trapping layer on the degradation kinetics of 1.3 μm quantum dot laser diodes (QD LDs) for silicon photonics (SiPh). The purpose of the analysis is to prove how the inclusion of trapping layers (TLs) for misfit dislocations can improve the performance and reliability of QD LDs. To this aim we stressed two kinds of devices, that only differ from the inclusion of TLs. By submitting the samples to current step-stress, we tested the devices from mild to highly accelerated conditions. Our results demonstrate that a) the addition of TLs shifts the roll-off of the optical characteristics to higher current densities; b) in devices with TLs, excited state (ES) emission is reached at higher current densities, compared to standard devices; c) in terms of reliability, the addition of TLs results in a slower degradation of the threshold current, to a lower concentration of misfit dislocations in the active region; d) degradation is ascribed to the lowering of the injection efficiency, driven by the escape of carriers out of the active region, which causes an increase in the concentration of non-radiative recombination centers.

Origin of the Diffusion-Related Optical Degradation of 1.3 μm Inas QD-LDs Epitaxially Grown on Silicon Substrate

This paper investigates the origin of the diffusion process responsible for the optical degradation of InAs quantum dot (QD) laser diodes epitaxially grown on silicon. By means of a series of constant-current stress experiments carried out at different temperatures, we were able to quantitatively evaluate the temperature acceleration of the degradation process. In addition, the presence of temperature thresholds above which the degradation rate drastically increases was ascribed to the onset of a recombination-enhanced degradation process, which is favored at high temperatures. Finally, the comparison of the experimentally determined diffusion coefficients with prior scientific reports suggests that degradation is related to the recombination-enhanced diffusion of Be, used here as p-type dopant, or of the lattice defects limiting Be diffusion. The original results of this work provide new insight on the microscopic origin of the gradual optical degradation of quantum-dot lasers, which will find wide application in silicon photonics.

Influence of potential well thickness on the carrier transport characteristics of InGaAs quantum dot laser diodes.

The characteristics of InGaAs quantum dot laser diodes have been investigated by inserting InGaAs wells of different thicknesses in the active zone. The simulation results indicated that the maximum output power of the InGaAs quantum dot laser diode with a 0 nm-thick InGaAs well is 0.72 W at an injection current of 1.1 A. However, the quantum dot laser diode with a 12 nm-thick InGaAs well has a maximum electro-optical conversion efficiency of 40.6%. The properties of carrier transport in the active zone were analyzed via the potential difference of the band offset among potential barrier, potential well, and waveguide layer. It is observed that electron leakage increases owing to the inserted InGaAs well. Auger recombination not only increases the carrier injection efficiency but also is acts as the key factor of carrier loss in the active zone. Moreover, the inserted InGaAs well can improve carrier transport in the active zone and increase the carrier utilization efficiency. The power-loss mechanism of the quantum dot laser diode was elucidated from the optical and carrier losses. It is found that optical loss is the key factor that affects the output power.

Laser Characteristic and Strain Distribution Dependence on Embedding Layer Thickness of Quantum Dots Laser Diodes Grown on InP(311)B Substrate

Herein, 14 layer‐stacked quantum dots laser diodes (QD‐LDs) are fabricated with different thicknesses of embedding layers grown on InP(311)B substrate, and the lowest threshold current (Ith) of 21.6 mA is demonstrated in an as‐cleaved ridge‐structured QD‐LD with 13 nm‐thick embedding layers. Ith has the minimum value when the thickness is 13 nm. The factors unique to QDs grown on an InP(311)B substrate are assumed as the overlap integral of the wavefunction of electrons and holes and the decrease in gain owing to the formation of a miniband. According to the simulation results of the local strain profile in the embedding and QD layers, the strain in the QD and embedding layer is higher in the case of 7 nm thickness than in the case of 20 nm thickness. As a result of the large strain, because the thickness of the embedding layer decreases, a large internal electric field tends to be generated, causing the wavefunctions to become largely separated and localized. Therefore, the overlap integral of the wavefunction becomes smaller, and because of the decrease in the radiation recombination probability, the internal quantum efficiency also decreases, causing the characteristic of Ith on the thickness of the embedding layer.

Degradation mechanisms of 1.3 μm C-doped quantum dot lasers grown on native substrate

The aim of this paper is to analyse the degradation modes of 1.3 μm InAs quantum dot laser diodes (QD LDs) grown by using carbon as a p-type dopant, as an alternative to beryllium. The devices were stressed at high current densities, to favor the onset of excited-state (ES) emission, and to study the related degradation phenomena. The study of QD LDs is of fundamental importance for the photonic integrated circuits (PICs). The investigation is based on two types of stress tests: 1) a current step stress and 2) a constant current stress. With these experiments we demonstrate that a) the current is a determining factor for the onset of the ES during the operation of such devices. b) the onset of ES is responsible for a rapid quenching of the ground state (GS) emission. c) stress induces a significant increase in the threshold current of the devices, that is ascribed to the lowering of the injection efficiency, which in turn can be explained by the easier escape of carriers from quantum dots (QDs) from ES energy levels. d) the contribution of defects on the optical and electrical degradation is also discussed.

Prospects and challenges of colloidal quantum dot laser diodes

Colloidal quantum dots (QDs) combine the superior light-emission characteristics of quantum-confined semiconductors with the chemical flexibility of molecular systems. These properties could, in principle, enable solution-processable laser diodes with an ultrawide range of accessible colours. However, the realization of such devices has been hampered by fast optical gain decay due to non-radiative Auger recombination and poor stability of QD solids at the high current densities required for the lasing regime. Recently, these problems have been resolved, which resulted in the development of electrically pumped optical gain devices operating at ultrahigh current densities of around 1,000 A cm−2. The next step is the realization of a QD laser diode (QLD). Here we assess the status of the QD lasing field, examine the remaining challenges on the path to a QLD and discuss practical strategies for attaining electrically pumped QD lasing. Colloidal quantum dots may offer solution-processable lasers, with a wide range of colours available. Technical hurdles and progress towards realization of useful quantum dot laser diodes is reviewed.

Feedback-induced locking in semiconductor lasers with strong amplitude-phase coupling

The influence of optical feedback on semiconductor lasers has been a widely studied field of research due to fundamental interests as well as the optimization of optical data transmission and computing. Recent publications have shown that it is possible to induce a periodic pulsed like output in quantum-dot and quantum-well laser diodes utilizing the locking of the external cavity modes and the relaxation oscillation frequency. We present an in-depth analysis of this effect. We choose submonolayer quantum dots as a gain system, as these provide a relatively strong amplitude-phase coupling, which has proven to be very beneficial for these locking effects to occur. Introducing a new theoretical model we can correctly reproduce the essential features of the gain system and validate them by comparison to our experimental results. From this starting point we can further explore how the staircase behavior of the oscillation frequency with increasing pump current can be influenced by changing various laser parameters. The staircase behavior is induced by a reordering of the Hopf bifurcations giving birth to the regular pulsed-like oscillations.

Degradation of 1.3 μm InAs Quantum-Dot Laser Diodes: Impact of Dislocation Density and Number of Quantum Dot Layers

This paper investigates the impact of dislocation density and active layer structure on the degradation mechanisms of 1.3 μm InAs Quantum Dot (QD) lasers for silicon photonics. We analyzed the optical behavior of two sets of samples, having different dislocation densities and different number of quantum dot layers in the active region. The samples were subjected to a short-term step-stress experiment and to long-term constant current operation in order to investigate the dominant degradation processes. The results indicate that: (i) the temperature stability is much higher in the devices grown on native substrate, thanks to the lower defect density; (ii) the roll-off current is considerably higher for the devices with higher number of layers, due to the lower density of carriers in the QDs; (iii) in nominal ground-state operating regime, the degradation rate is limited by the density of dislocations, that may serve as preferential paths for the diffusion of non-radiative recombination centers; (iv) at extreme injection levels and operating temperatures, the devices exhibit a blue shift of the spectral emission; possible explanations for this process are discussed in the paper.

Optically pumped colloidal-quantum-dot lasing in LED-like devices with an integrated optical cavity

Realization of electrically pumped lasing with solution processable materials will have a revolutionary impact on many disciplines including photonics, chemical sensing, and medical diagnostics. Due to readily tunable, size-controlled emission wavelengths, colloidal semiconductor quantum dots (QDs) are attractive materials for attaining this goal. Here we use specially engineered QDs to demonstrate devices that operate as both a light emitting diode (LED) and an optically pumped laser. These structures feature a distributed feedback resonator integrated into a bottom LED electrode. By carefully engineering a refractive-index profile across the device, we are able to obtain good confinement of a waveguided mode within the QD medium, which allows for demonstrating low-threshold lasing even with an ultrathin (about three QD monolayers) active layer. These devices also exhibit strong electroluminescence (EL) under electrical pumping. The conducted studies suggest that the demonstrated dual-function (lasing/EL) structures represent a promising device platform for realizing colloidal QD laser diodes.

A comparative study of the operational characteristics of CdSe quantum dots and BeZnCdSe quantum well laser diodes

In this study, the operational durability of laser diodes (LDs) with CdSe quantum dot (QD) active region is examined through a detailed comparison with its BeZnCdSe quantum well (QW) counterpart. To focus on the impact on the difference caused by the nature of the active region, other layers such as the waveguide layer, cladding layer, and contact layer, which influence the mode profile of the optical confinement and electrical properties, are maintained under the same condition. Although the threshold current of the CdSe QD LD is higher than that of the BeZnCdSe QW LD by a factor of 1.6–1.7 under a fresh condition, the degradation of the former proceeds much more slowly than that of the latter. This is confirmed by a reliability test conducted under the current injection condition both below and above the lasing threshold. The result indicates that the stronger carrier localization in the CdSe QDs compared to that in the BeZnCdSe QWs prevents the interaction with remote defects in the active layer. Consequently, the CdSe QD LD is less sensitive to degradation due to recombination-enhanced defect reactions.

Automatic Cross Carrier-Envelope Phase Locking Within a Dual-Peak Mode-Locked Quantum-Dot Diode Laser

We demonstrated automatic cross carrier-envelope phase locking between two synchronized pulse trains at different wavelengths generated within a single InAs/InP quantum-dot diode laser cavity. The resulting waveforms, verified by optical auto-/cross-correlation and explained by a theoretical model, show multipeaks or quasi-periodic subpulses with a central-subpulse pulse duration of about 183 fs.

Investigation of Current-Driven Degradation of 1.3 μm Quantum-Dot Lasers Epitaxially Grown on Silicon

This work investigates the degradation processes affecting the long-term reliability of 1.3 μm InAs quantum-dot lasers epitaxially grown on silicon. By submitting laser samples to constant-current stress, we were able to identify the physical mechanisms responsible for the optical degradation. More specifically, the samples (i) exhibited a gradual increase in threshold current, well correlated with (ii) a decrease in sub-threshold emission, and (iii) a decrease in slope efficiency. These variations were found to be compatible with a diffusion process involving the propagation of defects toward the active region of the device and the subsequent decrease in injection efficiency. This hypothesis was also supported by the increase in the defect-related current conduction components exhibited by the electrical characteristics, and highlights the role of defects in the gradual degradation of InAs quantum dot laser diodes. Electroluminescence measurements were used to provide further insight in the degradation process.

System of ideal quantum dots with Coulomb interaction

The wave functions of single-electron states localised on a system of single-level ideal quantum dots in a semiconductor matrix are constructed. The standard Kane theory, which describes the renormalisation of the effective mass of electrons in bulk III − V semiconductors, is transformed, for the first time as far as we know, for small-size quantum dots. The renormalised electron mass in a quantum dot depends on its ground state energy, which, in turn, depends on this mass. Thus, a self-consistent problem is obtained for calculating the electron binding energy. The radius of the Debye screening of the Coulomb interaction in a system of quantum dots is calculated at room temperature. The Coulomb repulsion of electrons localised on dots limits from above the possible number of filled dots. The condition is formulated for the optimal concentration of quantum dots. A classical distribution function for the probability of quantum dot filling is constructed. It is found the state, where electrons fill exactly half the maximum possible number of filled dots, exhibits the minimal energy (and therefore the most stable), the condition being fairly universal. In particular, it can be used to estimate the limiting efficiency of quantum-dot diode lasers.

Optical Gain and Lasing Properties of InP/AlGaInP Quantum-Dot Laser Diode Emitting at 660 nm

We investigated the optical gain properties and lasing characteristics of a pulsed electrically pumped laser structure, which consists of a single layer of self-assembled InP quantum dots (QDs) assembled in (Al 0.10 Ga 0.90 ) 0.52 In 0.48 P barriers. The optical gain and absorption spectra were obtained by analyzing the amplified spontaneous emission as a function of the excitation length. At a current density of 1.2 kA/cm 2 , an internal optical loss value of 5 ± 2 cm -1 and a peak modal gain of 39.3 cm -1 corresponding to a material gain of approximately 2675 cm -1 per QD layer were determined at room temperature. For a 2.24-mm-long laser with uncoated facets emitting at 660 nm, a low threshold current density of 281 A/cm 2 and an external differential quantum efficiency of 34.2% were determined. The internal quantum efficiency of 66% and the transparent current density of 65.8 A/cm 2 for a single layer of QDs were also demonstrated.

Impact of InGaAs carrier collection quantum well on the performance of InAs QD active region lasers fabricated by diblock copolymer lithography and selective area epitaxy

Quantum dot (QD) laser diodes, where the active region consists of wetting-layer-free InAs QDs, were demonstrated by block copolymer (BCP) lithography and subsequent selective area metal-organic vapor phase epitaxy, which results in the QD density of ∼4 × 1010 cm2. In this work, we show that an In0.1Ga0.9As QW located in close proximity to the wetting-layer-free InAs QDs leads to an enhanced carrier injection into the QDs, allowing for lasing at room temperature (RT). Devices employing InAs QDs grown with and without In0.1Ga0.9As QW carrier collection layer exhibit lasing at 80 K, while only the lasers grown with the In0.1Ga0.9As QW exhibits lasing at RT. Driving current-dependent electroluminescence measurements reveals a low carrier injection into the QDs in the device grown without In0.1Ga0.9As QW carrier collection layer. In addition, the appearance of lasing emission, corresponding to high energy transitions, indicates a relatively flat gain profile for this InAs QD active region grown by SA-MOVPE. In addition, EL measurements at 80 K on devices employing varying thickness (dInAs) QDs indicate the peak emission wavelength varies from 920 nm (dInAs ∼ 1.5 nm) to 974 nm (dInAs = 3 nm), although the devices employing dInAs ∼ 3 nm did not exhibit lasing, possibly because of a significant degree of strain relaxation for this QD thickness.