Quantum Dot Lasers Research Articles

Perspectives on Advances in Quantum Dot Lasers and Integration with Si Photonic Integrated Circuits

Epitaxially grown quantum dot (QD) lasers are emerging as an economical approach to obtain on-chip light sources. Thanks to the three-dimensional confinement of carriers, QDs show greatly improved tolerance to defects and promise other advantages such as low transparency current density, high temperature operation, isolator-free operation, and enhanced four-wave-mixing. These material properties distinguish them from traditional III–V/Si quantum wells (QWs) and have spawned intense interest to explore a full set of photonic integration using epitaxial growth technology. We present here a summary of the most recent developments of QD lasers grown on a CMOS-compatible (001) Si substrate, with a focus on breakthroughs in long lifetime at elevated temperatures. Threading dislocations are significantly reduced to the level of 1 × 106 cm–2 via a novel asymmetric step-graded filter. Misfit dislocations are efficiently blocked from the QD region through well-engineered trapping layers. A record-breaking extrapolated lifetime of more than 200000 hours has been achieved at 80 °C, forecasting that device reliability is now entering the realm of commercial relevance and a monolithically integrated light source is finally on the horizon.

Quantum Dot Lasers: High Speed Evanescent Quantum‐Dot Lasers on Si (Laser Photonics Rev. 15(8)/2021)

In article number 2100057, John E. Bowers, Yating Wan, Chao Xiang, and co-workers report record-setting quantum dot (QD) distributed feedback lasers on Si with a 3-dB modulation bandwidth of 13 GHz, a threshold current of 4 mA, a side-mode-suppression-ratio of 60 dB, and a fundamental linewidth of 26 kHz. The broadband QD lasers and the Si-on-insulator waveguiding platform can be expanded to build fully functional photonic integrated circuits throughout the O band.

Dynamic performance and reflection sensitivity of quantum dot distributed feedback lasers with large optical mismatch

This work reports on a high-efficiency InAs/GaAs distributed feedback quantum dot laser. The large optical wavelength detuning at room temperature between the lasing peak and the gain peak causes the static, dynamic, and nonlinear intrinsic properties to all improve with temperature, including the lasing efficiency, the modulation dynamics, the linewidth enhancement factor, and consequently the reflection insensitivity. Results reported show an optimum operating temperature at 75°C, highlighting the potential of the large optical mismatch assisted single-frequency laser for the development of uncooled and isolator-free high-speed photonic integrated circuits.

Nonlinear Dynamics of Two-State Quantum Dot Lasers under Optical Feedback

A modified rate equation model was presented to theoretically investigate the nonlinear dynamics of solitary two-state quantum dot lasers (TSQDLs) under optical feedback. The simulated results showed that, for a TSQDL biased at a relatively high current, the ground-state (GS) and excited-state (ES) lasing of the TSQDL can be stimulated simultaneously. After introducing optical feedback, both GS lasing and ES lasing can exhibit rich nonlinear dynamic states including steady state (S), period one (P1), period two (P2), multi-period (MP), and chaotic (C) state under different feedback strength and phase offset, respectively, and the dynamic states for the two lasing types are always identical. Furthermore, the influences of the linewidth enhancement factor (LEF) on the nonlinear dynamical state distribution of TSQDLs in the parameter space of feedback strength and phase offset were also analyzed. For a TSQDL with a larger LEF, much more dynamical states can be observed, and the parameter regions for two lasing types operating at chaotic state are widened after introducing optical feedback.

Effect of Shockley-Read-Hall recombination on the static and dynamical characteristics of epitaxial quantum-dot lasers on silicon

We semianalytically and numerically investigate the static and dynamical characteristics of quantum-dot (QD) lasers directly grown on silicon by considering the Shockley-Read-Hall (SRH) recombination. The static characteristics are studied through small-signal analysis, including the ${\ensuremath{\alpha}}_{H}$ factor, damping factor, and modulation dynamics. In addition, the feedback dynamics are analyzed through improved corpuscular rate equations based on the classical Lang-Kobayashi (LK) model with time series, bifurcation diagrams, and phase portraits. We find that a smaller ${\ensuremath{\alpha}}_{H}$ factor but larger damping factor are obtained by decreasing the nonradiative recombination lifetime. On top of that, in both the short- and long-external-cavity regimes, any decrease of the SRH recombination lifetime obliterates significantly chaotic regions and shifts the first Hopf bifurcation point to higher feedback values. Overall, this work provides insights into the understanding of QD laser physics, hence highlighting the influence of the SRH lifetime on the reflection sensitivity of epitaxial QD lasers on silicon. These results are qualitatively consistent with recent experiments and are therefore helpful for designing feedback-resistant lasers for future photonic integrated circuits operating without optical isolation.

High Speed Evanescent Quantum‐Dot Lasers on Si

AbstractSignificant improvements in III–V/Si epitaxy have pushed quantum dots (QDs) to the forefront of Si photonics. For efficient, scalable, and multifunctional integrated systems to be developed, a commercially viable solution must be found to allow efficient coupling of the QD laser output to Si waveguides. In this work, the design, fabrication, and characterization of such a platform are detailed. Record‐setting evanescent QD distributed feedback lasers on Si with a 3 dB modulation bandwidth of 13 GHz, a threshold current of 4 mA, a side‐mode‐suppression‐ratio of 60 dB, and a fundamental linewidth of 26 kHz, are reported. The maximum temperature during the backend III/V process is only 200 °C, which is fully compatible with CMOS process thermal budgets. The whole process is substrate agnostic and hence can leverage previous development in QD lasers grown on Si and benefit from the economy of scale. The broadband and versatile nature of the QD lasers and the Si‐on‐insulator low‐loss waveguiding platform can be expanded to build fully functional photonic integrated circuits throughout the O band.

Sole Excited-State InAs Quantum Dot Laser on Silicon With Strong Feedback Resistance

A feedback insensitive laser is a prerequisite for a desirable laser source for silicon photonic integration, as it is not possible to include an on-chip optical isolator. This work investigates the feedback insensitivity of an InAs/GaAs quantum dot laser epitaxially grown on an Si (001) substrate by operating in a sole excited state. The experimental results show that the sole excited-state lasing InAs quantum dot lasers on Si are less sensitive to external optical feedback than both Fabry-Perot and distributed-feedback quantum-well lasers. By comparing the laser behavior under different feedback levels, sole excited-state InAs quantum dot lasers on Si exhibit at least a 28 dB stronger feedback tolerance than quantum-well lasers. This result proposes a possible route for a high feedback insensitive laser as an on-chip light source towards Si waveguide integration with the absence of an optical isolator.

Fast dynamics of low-frequency fluctuations in a quantum-dot laser with optical feedback.

We experimentally investigate the complex dynamics of a multi-mode quantum-dot semiconductor laser with time-delayed optical feedback. We examine a two-dimensional bifurcation diagram of the quantum-dot laser as a comprehensive dynamical map by changing the injection current and feedback strength. We found that the bifurcation diagram contains two different parameter regions of low-frequency fluctuations. The power-dropout dynamics of the low-frequency fluctuations are observed in the sub-GHz region, which is considerably faster than the conventional low-frequency fluctuations in the MHz region. Comparing the dynamics of quantum-dot laser with those of single- and multi-mode quantum-well semiconductor lasers reveals that the fast low-frequency fluctuation dynamics are unique characteristics of quantum-dot lasers with time-delayed optical feedback.

Impact of Built‐In Electric Field on the Emission Characteristics of InAs/GaAs Quantum Dot Laser Structure

The impact of the built‐in electric field on the emission characteristics of an InAs/GaAs quantum dot (QD) laser structure is investigated by performing systematic spectroscopic measurements. Photoluminescence (PL) features related to the ground state (GS) and excited state (ES) transitions of a QD ensemble are clearly observed in the temperature range of 8–300 K. The cross‐sectional transmission electron microscopy image and a systematic analysis of temperature‐dependent PL data confirm the excellent crystalline and optical quality of the QD laser structure. It is found that the values of the activation energy associated with GS and ES transitions of the QD sample reasonably match with the energy splitting of PL features. This successfully explains the trends observed in temperature‐dependent PL spectra, where thermal transfer of carriers to the wetting/barrier layer via QD excited states is proposed to be a major decay channel. An interesting observation is made where the integrated intensity of PL features increases up to 75 K and falls thereafter during the heating cycle. Such a peak‐like behavior of the integrated intensity is explained by invoking the temperature dependence of the built‐in electric field, which is estimated from the analysis of Franz–Keldysh oscillations observed in photoreflectance spectra and is important in the design of QD lasers.

High-temperature reliable quantum-dot lasers on Si with misfit and threading dislocation filters

Direct epitaxial growth of III-V light sources on Si photonic chips is promising to realize low-cost and high-functionality photonic integrated circuits. Historically, high temperature reliability of such devices has been the major roadblock due to crystalline defects from heteroepitaxy. Here, by reducing the threading dislocation densities to ∼ 1 × 1 0 6 c m − 2 and efficiently removing misfit dislocations above and below the active region, 1.3 µm InAs quantum-dot lasers directly grown on industry standard on-axis Si (001) show record-breaking reliability at 80°C. The hero device shows minimum degradation after more than 1200 h of constant current stress. Statistical analysis shows an extrapolated lifetime of over 22 years for the median devices, bringing these devices one big step closer to real world applications.

Reduced dislocation growth leads to long lifetime InAs quantum dot lasers on silicon at high temperatures

We describe the effectiveness of filter layers, which displace misfit dislocation (MD) formation away from the active region, in improving high temperature reliability of epitaxially integrated InAs quantum dot lasers on on-axis silicon substrates. We find that inserting these “trapping layer (TL)” filters at either 80 nm or 180 nm from the active region substantially reduces device degradation at 60 °C. After 3000 h of continuous operation, the best trapping-layer-free device shows a 55% increase in threshold current while the best trapping layer (TL) devices each show less than a 9% increase. We explain these findings by correlating changes in individual device performance to changes in misfit dislocation (MD) structure. All MDs in devices without TLs show evidence of recombination enhanced dislocation climb (REDC); in contrast, adding trapping layers at 180 nm or 80 nm reduces the fraction of electrically active MDs to 9% and 1%, respectively. Reliability data after 3000 hours suggest that incorporating trapping layers a shorter distance from the active region (80 nm) is more effective than incorporating these layers further away. We conclude by identifying the mutually and self-reinforcing failure processes associated with REDC that TLs significantly remediate: increasing dislocation line length, increasing point defect densities, and increasing junction temperature. Overall, understanding and controlling crystal defects continues to be the most impactful avenue toward integrating light sources on photonic integrated circuits and closing the gap with native-substrate lasers.

Improved linewidth enhancement factor of 1.3-µm InAs/GaAs quantum dot lasers by direct Si doping

We report on the significantly improved linewidth enhancement factor (αH-factor) of 1.3-µm InAs/GaAs quantum dot (QD) lasers by direct Si doping, compared with ones having identical structures but without the Si doping. It is found that the αH-factors for the ground-state and first excited-state at their gain peak positions of the Si-doped QD lasers are 1.48 and 0.63 while those of the undoped QD lasers are 2.07 and 1.07, greatly decreasing by about 28.5% and 41.1%, respectively. Furthermore, theoretical calculation and analysis suggest that direct Si doping would increase the electron quasi-Fermi level in conduction, leading to the increase in population inversion. Meanwhile, the appearance of a built-in electric field caused by the Si doping would accelerate the capture of electrons into QDs and strengthen the confinement effect of electrons, resulting in an increased differential gain.

E‐band InAs quantum dot laser on InGaAs metamorphic buffer layer with filter layer

InAs quantum dots (QDs) are receiving attention as next-generation E-band light source that offers high-temperature operation and temperature insensitive operation. However, high-density crystal defects occur at the interface between the InGaAs buffer layer and GaAs, resulting in reduced device performance and shortened lifetime. Here, E-band QD lasers are demonstrated on InGaAs buffer layer, which suppressed the spread of dislocation by introducing a high-temperature annealing and a strained layer superlattice filter. In the device, the peak wavelength at room temperature is measured to be 1427 nm and the threshold current density was 440 A/cm2. This result indicates that E-band QD laser structures on low threading dislocation density are promising for the realisation of high-performance E-band lasers.

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.

Quantum Dot Lasers and Nanoporous Materials for Solar Cells [The Editors' Desk

Nanotechnology in photonics and renewable energy has been one of the most dynamic research topics in recent decades. This issue of <i>IEEE Nanotechnology Magazine</i> includes three articles that report on new developments in quantum dot lasers and amplifiers, and nanoporous-materials-related solar cells.

Latest advances in high-performance light sources and optical amplifiers on silicon

Efficient light generation and amplification has long been missing on the silicon platform due to its well-known indirect bandgap nature. Driven by the size, weight, power and cost (SWaP-C) requirements, the desire to fully realize integrated silicon electronic and photonic integrated circuits has greatly pushed the effort of realizing high performance on-chip lasers and amplifiers moving forward. Several approaches have been proposed and demonstrated to address this issue. In this paper, a brief overview of recent progress of the high-performance lasers and amplifiers on Si based on different technology is presented. Representative device demonstrations, including ultra-narrow linewidth III–V/Si lasers, fully integrated III–V/Si/Si3N4 lasers, high-channel count mode locked quantum dot (QD) lasers, and high gain QD amplifiers will be covered.

Influence of quantum-confined device fabrication on semiconductor-laser theory

Among Professor Arthur Gossard’s many contributions to crystal growth are those resulting in important improvements in the quality and performance of quantum-well and quantum-dot semiconductor lasers. In celebration of his 85th birthday, we review the development of a semiconductor laser theory that is motivated and guided, in part, by those advances. This theory combines condensed matter theory and laser physics to provide understanding at a microscopic level, i.e., in terms of electrons and holes, and their interaction with the radiation field while influenced by the lattice.

Quantum-dot semiconductor lasers with prominent relative intensity noise and spectral characteristics.

Relative intensity noise (RIN) behavior of a quantum dot laser (QDL) at free running and under optical injection locking (OIL) has been reported in detail. Considering the mutual effects of the inhomogeneous and homogeneous broadenings, different and also non-routine RIN characteristics of a QDL at low, intermediate, and high homogeneous broadening regimes have been discussed. Results demonstrate that an injection-locked quantum dot laser exhibit enhanced characteristic over free-running operation along with low-frequency noise behavior in a wide range of homogeneous broadening values. RIN reduction of about 25, 55, and 7 dB/Hz have been achieved for low, intermediate, and high values of homogeneous broadening which implies the spontaneous emission noise suppression even at low injection power. The promising impact of the OIL on side-mode suppression for the intermediate homogeneous broadening regime has also been discussed. Our results guarantee the single-mode and low-noise operation of the OIL-QDL at any homogeneous broadening value.

Experimental study of neuromorphic node based on a multiwaveband emitting two-section quantum dot laser

In this work, we present experimental results concerning excitability in a multiband emitting quantum-dot-based photonic neuron. The experimental investigation revealed that the same two-section quantum dot laser can be tuned through a simple bias adjustment to operate either as a leaky integrate and fire or as a resonate and fire neuron. Furthermore, by exploiting the inherent multiband emission of quantum-dot devices revealed by the existence of multiple lasing thresholds, a significant enhancement in the neurocomputational capabilities, such as spiking duration and firing rate, is observed. Spike firing rate increased by an order of magnitude that leads to an enhancement in processing speed and, more importantly, neural spike duration was suppressed to the picosecond scale, which corresponds to a significant temporal resolution enhancement. These new regimes of operation, when combined with thermal insensitivity, silicon cointegration capability, and the fact that these multiband mechanisms are also present in miniaturized quantum-dot devices, render these neuromorphic nodes a proliferating platform for large-scale photonic spiking neural networks.

Dynamic and nonlinear properties of epitaxial quantum-dot lasers on silicon operating under long- and short-cavity feedback conditions for photonic integrated circuits

This work reports on an investigation of the dynamics of $1.3\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\text{m}$ epitaxial quantum dot (QD) lasers on silicon subject to delayed optical feedback. Operating the device under different feedback conditions, we experimentally identify various dynamical states of periodic oscillations. In the long-cavity feedback regime, the device remains chaos-free up to $\ensuremath{\approx}70% (\ensuremath{-}1.55\phantom{\rule{0.16em}{0ex}}\mathrm{dB})$ feedback strength. This remarkable result is in agreement with prior studies and is attributed to the particular design and properties of the QD-based active region. Shortening the external cavity length to the short-cavity regime being on the scale of photonics integrated circuits (PICs), the onset of periodic oscillations only takes place under extremely high feedback strength, which is much higher than those in PICs. The devices studied exhibit strong resistance to chip-scale back reflections in absence of any unstable oscillation. Our results also demonstrate that p doping is an efficient technique to further improve the feedback tolerance. These results point out the potential of QD lasers as an on-chip light source not requiring an optical isolator and gives insights for developing ultrastable silicon transmitters for PIC applications.