Threshold Current Reduction Research Articles

Significantly enhanced performance of InAs/GaAs quantum dot lasers on Si(001) via spatially separated co-doping.

We report the significantly enhanced performance of InAs/GaAs quantum dot (QD) lasers on Si(001) by spatially separated co-doping, including n-doping in the QDs and p-doping in the barrier layers simultaneously. The QD lasers are a ridge waveguide of 6 × 1000 µm2 containing five InAs QD layers. Compared with p-doped alone laser, the co-doped laser exhibits a large reduction in threshold current of 30.3% and an increase in maximum output power of 25.5% at room temperature. In the range of 15°C-115°C (under 1% pulse mode), the co-doped laser shows better temperature stability with higher characteristic temperatures of threshold current (T0) and slope efficiency (T1). Furthermore, the co-doped laser can maintain stable continuous-wave ground-state lasing up to a high temperature of 115°C. These results prove the great potential of co-doping technique for enhancing silicon-based QD laser performances towards lower power consumption, higher temperature stability, and higher operating temperature, to boost the development of high-performance silicon photonic chips.

High-power diode lasers with in-situ-structured lateral current blocking for improved threshold, efficiency and brightness

We present high-power GaAs-based broad-area diode lasers with a novel variant of the enhanced self-aligned lateral structure ‘eSAS’, having a strongly reduced lasing threshold and improved peak conversion efficiency and beam quality in comparison to their standard gain-guided counterparts. To realize this new variant (eSAS-V2), a two-step epitaxial growth process involving in situ etching is used to integrate current-blocking layers, optimized for tunnel current suppression, within the p-Al0.8GaAs cladding layer of an extreme-triple-asymmetric epitaxial structure with a thin p-side waveguide. The blocking layers are thus in close proximity to the active zone, resulting in strong suppression of current spreading and lateral carrier accumulation. eSAS-V2 devices with 4 mm resonator length and varying stripe widths are characterized and compared to previous eSAS variant (eSAS-V1) as well as gain-guided reference devices, all having the same dimensions and epitaxial structure. Measurement results show that the new eSAS-V2 variant eliminates an estimated 89% of lateral current spreading, resulting in a strong threshold current reduction of 29% at 90 μm stripe width, while slope and series resistance are broadly unchanged. The novel eSAS-V2 devices also maintain high conversion efficiency up to high continuous-wave optical power, with an exemplary 90 μm device having 51.5% at 20 W. Near-field width is significantly narrowed in both eSAS variants, but eSAS-V2 exhibits a wider far-field angle, consistent with the presence of index guiding. Nonetheless, eSAS-V2 achieves higher beam quality and lateral brightness than gain-guided reference devices, but the index guiding in this realization prevents it from surpassing eSAS-V1. Overall, the different performance benefits of the eSAS approach are clearly demonstrated.

Mid-IR quantum dot cascade VCSEL: feasibility study and feature extraction.

Quantum cascade lasers with quantum dot (QD) active regions have been developed to overcome the drawbacks of quantum-well-based cascade lasers. Low threshold current and enhanced bandwidth characteristics of QD lasers can be combined with promising energy-efficient features of vertical cavity surface emitting lasers (VCSELs) through introducing the novel, to the best of our knowledge, concept of a QD cascade VCSEL (QDC-VCSEL). Design steps and predicted features of a QDC-VCSEL operating at 4.5 µm wavelength are discussed in this paper, where the cavity and active region are optimized for TE mode using a high contrast grating design. Laser characteristics are investigated by developing rate equations for the cascade scheme. Threshold and L-I characteristics are obtained at different temperatures. Also, the dependency of threshold current, slope efficiency, and modulation bandwidth on design parameters, mainly the carrier tunneling time, is studied. Results exhibit output peak powers of ∼2 and ∼0.5mW at 150 and 273 K, respectively. In addition, threshold current increment and modulation bandwidth reduction are observed with the increase in tunneling time.

Promotion of temperature-dependent characteristics of midinfrared quantum cascade lasers under optical injection locking

Temperature-dependent characteristics of a midinfrared quantum cascade laser (QCL) including the threshold current, the output power, the modulation bandwidth, and the relative intensity noise (RIN) exhibit a deterioration with temperature from 77 to 300 K. Based on a comprehensive temperature-dependent model we show that optical injection locking can be applied to surpass the intrinsically limited thermal characteristics of a QCL through the modification of the carrier density in the active region. A modulation bandwidth improvement of $\ensuremath{\approx}35%$ and an almost temperature-independent RIN behavior for an injection-locked laser have been achieved compared to a free-running slave laser at room temperature. A threshold current reduction of $\ensuremath{\approx}20%$ has also been obtained at 300 K. Results indicate that the locking boundaries of an injection-locked QCL are suppressed with temperature. The output power and the small-signal characteristics of the laser have also been analyzed in the entire stable locking range.

Low-Threshold Epitaxially Grown 1.3-μm InAs Quantum Dot Lasers on Patterned (001) Si

A three-fold reduction of threshold current, with a minimum threshold current density of 286 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , a maximum operating temperature of 80 °C, and a maximum 3-dB bandwidth of 5.8 GHz was achieved for 1.3- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m InAs quantum dot lasers on patterned, on-axis (001) Si. This was enabled by the reduced threading dislocation density (from 7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> to 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> ), and optimized probe design. The patterned Si produced antiphase domain free material in the coalesced GaAs buffer layer with reduced misfit/threading dislocation nucleation, without the use of Ge/GaP buffers or substrate miscut. Utilizing aspect ratio trapping, cyclic thermal annealing, and dislocation filter layers, high-quality III–V on Si devices were grown, demonstrating the compelling advantages of this patterned Si template for a monolithic Si photonics integration platform.

Monolithic Integration of Buried-Heterostructures in a Generic Integrated Photonic Foundry Process

This paper demonstrates the integration of buried-heterostructure (BH) lasers and semiconductor optical amplifiers (SOAs) in a generic photonic foundry platform. The process-flow adaptations necessary for their integration with existing building blocks are presented. BH Fabry–Perot lasers fabricated within the platform are fully characterized and compared with shallow-ridge (SR) lasers already available as a standard building block. SR lasers were fabricated on the same wafer sharing the same layer stack. BH lasers showed a reduction in threshold current of 60% and a reduction in thermal resistance of 40%. The full integration is then validated by the characterization of a compact arrayed waveguide grating laser-based transmitter consisting of eight BH-SOAs and a deep-ridge waveguide. A side-mode suppression ratio of up to 57 dB and a linewidth of 1.5 MHz are achieved.

In situ annealing enhancement of the optical properties and laser device performance of InAs quantum dots grown on Si substrates.

The addition of elevated temperature steps (annealing) during the growth of InAs/GaAs quantum dot (QD) structures on Si substrates results in significant improvements in their structural and optical properties and laser device performance. This is shown to result from an increased efficacy of the dislocation filter layers (DFLs); reducing the density of dislocations that arise at the Si/III-V interface which reach the active region. The addition of two annealing steps gives a greater than three reduction in the room temperature threshold current of a 1.3 μm emitting QD laser on Si. The active region of structures grown on Si have a room temperature residual tensile strain of 0.17%, consistent with cool down from the growth temperature and the different Si and GaAs thermal expansion coefficients. This strain limits the amount of III-V material that can be grown before relaxation occurs.

Threshold current reduction and directional emission of deformed microdisk lasers via spatially selective electrical pumping

We report on laser threshold current reduction and directional emission from quadrupole-shaped AlGaInAs microdisk diode lasers by selective electrical pumping. The directional emission results from breaking the 2-fold rotation symmetry of the system by the introduction of a triangle-shaped contact geometry, and the laser threshold reduction results from a small current injection area. Room temperature laser operation is achieved in both pulsed and continuous-wave operation for a microdisk radius of 50 μm and deformation constant of ε = 0.09, with optical output power of more than 8 mW and 3 mW, respectively. Under pulsed operation, the minimum measured threshold current for selectively pumped microlasers is 42 mA, significantly lower than the minimum measured threshold current for uniformly pumped microlasers (58 mA) and standard ridge lasers (80 mA) of the same device size and material.

Threshold current reduction for the metal–insulator transition in NbO2−x-selector devices: the effect of ReRAM integration

The threshold current for inducing the metal–insulator transition in a NbO2−x selector element is shown to be affected by the properties of an adjacent memory element when integrated into a hybrid selector-memory device structure. Experimental results are reported for homogeneous NbO2−x/Nb2O5−y and heterogeneous NbO2−x/HfO2 device structures, and show that the threshold current is lower in both hybrid structures than in the selector element alone, and is lower in the heterogeneous structure than in the homogeneous structure. Finite element modeling of the selector-memory structure shows that this results primarily from current confinement produced by the filamentary conduction path in the resistive-switching memory layer (i.e. Nb2O5−y or HfO2), an observation that further implies a smaller diameter filament in HfO2 than in Nb2O5−y. The thermal and electrical conductivities of the memory layer are also shown to influence the threshold current, but to a lesser extent.

Analytical Investigation of Electrons Capture Time Effect on the Threshold Current Density Reduction in QD Spin-Lasers

In this paper, we solve numerically the rate equations governing the GaAs InAs / semiconductor spin with un-polarized and polarized laser field based on quantum dot active region in which As Ga Al MnAs 9 . 0 1 . 0 / Schottky tunnel barrier treats as the spin injector. We demonstrate simultaneously the effects of electron capture time, and injected current polarization on threshold current density reduction and normalized spin-filtering interval. The threshold current density reduction and normalized spin-filtering interval increases simultaneously with electrons capture time reduction and increase of injected current polarization. The maximum obtained threshold current density reduction and normalized spin-filtering interval values are0.353 and 0.90, respectively. We also calculate the spinup optical gain and obtain the conditions for achieving optimum optical gain. The maximum obtained spin-up optical gain value is 17.70.

Scrutiny of Injected Electron Current Polarization Effect on the Threshold Current Density Reduction in QW Spin-Lasers

One of the mechanisms for threshold current density reduction is using spin polarized carriers generated by electrical spin injection. Electrical spin injection is spin-polarized carrier injection by using a magnetic contact. In this paper, we have solved numerically rate equations governing on semiconductor spin un-polarized and polarized laser with - based quantum well active region in which Schottky tunnel barrier treat as the spin injector. For the first time, we demonstrate simultaneously effect of normalized spin relaxation rate and injected current polarization on threshold current density reduction related to two form of spontaneous recombination. According to our result threshold current density reduction increases by simultaneously normalized spin relaxation rate reduction and increasing of injected current polarization. Maximum obtained threshold current density reduction values for linear and quadratic spontaneous recombination is 0.07 and 0.31. Moreover, we compute and compare the effect of value of injected electron current polarization on normalized spin-filtering interval for two types of recombination. Maximum obtained normalized spin-filtering interval values for linear and quadratic spontaneous recombination is 1.2 and 1.36. Finally we calculate spin-up optical gain and from this we obtained the conditions for achieving optimum optical gain. Maximum obtained spin-up optical gain valueis17.36.

Performance Optimization of Multiple Quantum Well Transistor Laser

A comprehensive physical model that emphasizes carrier tunneling between quantum wells (QWs) in the base of transistor lasers (TLs) is developed. This model relies on a set of multilevel coupled rate equations solved by computationally efficient numerical methods for simulating both steady state and transient TL operations. Our approach also features the explicit dependence of the structure design on device parameters such as optical confinement factor and carrier density-dependent gain. It also accounts for operation behaviors such as bandwidth roll-off and critical base width not yet addressed in the literature. Simulation results show significant enhancement in optical bandwidth as well as threshold current reduction when multiple QWs are incorporated within the base region. It predicts the dominance of tunneling transport of carriers for barriers thinner than 13.5 nm in a system with 7-nm QWs. However, the optimum QW number depends on the structure design as well as TL biasing conditions. For this purpose, we define a performance parameter as a TL figure of merit that can be maximized by optimizing both the base and the cavity designs.

Specific Jacket SMA-Connected TO-Can Package FPLD Transmitter With Direct Modulation Bandwidth Beyond 6 GHz for 256-QAM Single or Multisubcarrier OOFDM up to 15 Gb/s

A specifically designed SMA connector with jacket signal pin configuration is used to connect the transistor outline can (TO-can) packaged Fabry-Pérot laser diode (FPLD) for enhancing its direct modulation bandwidth beyond 6 GHz. The direct modulation of such an TO-can/jacket-SMA packaged FPLD with a 256 quadrature amplitude modulation (QAM) and single subcarrier optical orthogonal frequency-division multiplexing (OOFDM) format data stream at carrier frequency up to 6 GHz is approached with 10 MSa/s symbol rate. The optimized 256-QAM data with error vector magnitude (EVM) of 1.5% and signal-to-noise ratio (SNR) of 32.5 dB are observed, corresponding to a bit error rate as low as 1.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> and a highest carrier-to-noise ratio of 43.5 dB. Under injection locking, the tradeoff between the lengthened photon lifetime and the enlarged threshold current reduction of such an M-QAM-N-OOFDM modulated FPLD is theoretically and experimentally investigated to explain the saturation of SNR at 31.5 dB and EVM at 1.7%. Both 16-QAM and 64-QAM 200-subcarrier OOFDM formats with total transmission bit rates up to 15.6 and 23.4 Gb/s, respectively, are performed at carrier frequency of 2 GHz. The corresponding EVM and SNR for the 16-QAM OOFDM are determined as 11.67% and 20 dB, respectively. Under 64-QAM and 200-subcarrier OOFDM modulation, the received data stream shows a slightly degraded EVM and SNR of 12.85% and 19 dB, respectively.

Lateral carrier confinement and threshold current reduction in InGaN QW lasers with deeply etched mesa

Shallow etch depths may contribute to a reduction in the optical gain of multiple quantum well (MQW) lasers through the lateral diffusion of carriers away from the region of greatest optical intensity. Deeply etched mesas can prevent this lateral diffusion, but this practice may itself contribute to a degradation of optical gain if the sidewalls are not effectively passivated. Simulation results considering the effects of surface recombination velocity (SRV) at the edge of etched active layers indicate that SRV must be reduced below approximately 105cm/s in order for deep etch designs to provide benefit. Very few experimental studies quantify the efficiency of GaN surface passivation in terms of SRV. Further experimental studies are required to better assess the viability of deep etch MQW laser designs.

Threshold current reduction in spin-polarized lasers: Role of strain and valence-band mixing

We show that valence-band mixing (VBM) plays a dominant role in determining the current threshold ${I}_{\mathrm{th}}$ of spin-pumped lasers. The degree of VBM is controlled by tuning the strain through the selection of alloy composition and layer structure. In a well-designed In${}_{x}$Ga${}_{1\ensuremath{-}x}$As structure, VBM can lead to a nearly a fourfold decrease in ${I}_{\mathrm{th}}$. However, it can also lead to a 25-fold increase for a different composition, in marked contrast to common expectations. In certain cases, larger threshold reductions are achieved for smaller injected spin polarization, a counterintuitive result.

Optimization of Current Injection Area for Low-Threshold Operation of 3-D Lasers

We analyze the reduction of threshold current by optimizing the area of the active layer into which the carriers are injected. The optimal area is shown to depend on the size of the optical mode, cavity lifetime, and the optical properties of the active layer. We introduce a critical value for the cavity lifetime that depends only on the properties of the active layer and show how the optimal area can be estimated depending on the cavity lifetime with respect to its critical value. For optimization, two cases are considered, with and without optical absorption in the active layer outside of the injection area. The active layer is modeled as a double heterostructure with parameters relevant to nitride semiconductors and the optical mode is taken as for a micrometer-size cavity.

Quantum dot polarized light sources

The design, operation and performance of quantum dot spin-polarized vertical cavity surface emitting lasers (VCSELs) and single-photon sources are described and discussed. The effects of spin-induced gain anisotropy on output polarization and threshold current reduction have been studied along with the high-frequency response in a spin-polarized VCSEL. While the output circular polarization in a VCSEL follows the out-of-plane magnetization characteristics of the ferromagnetic spin injector, the output polarization of the spin-polarized single-photon source shows a switching behavior which is explained by invoking the exciton fine structure in the quantum dots and the effects of electron–hole exchange splitting due to in-plane quantum dot rotational asymmetry.

Systematic study on the confinement structure design of 1.5-μm InGaAlAs/InP multiple-quantum-well lasers

An in-dept analysis on the separate confinement heterostructure (SCH) design parameters of 1.5-μm InGaAlAs/InP multiple quantum-well (MQW) lasers is reported. Theoretical calculations show a drastic enhancement on threshold current and slope efficiency from step-index SCH (STEP-SCH) to graded-index SCH (GRIN-SCH) design, but the effect ceases beyond a critical number of grading steps. This finding implies ease of the growth process and reduction in cost. The overall GRIN-SCH’s thickness is found to exert great influence over the achievable laser’s threshold current and slope efficiency. An average of 27 mA threshold current reduction and more than 32% of slope efficiency increment were achieved by optimizing the GRIN-SCH thickness. Increasing the grading energy range of the GRIN-SCH decreases the slope efficiency, but is found to effectively reduce carrier leakage at elevated temperature leading to a less temperature-sensitive threshold current MQW ridge lasers were fabricated and characterized out of two laser materials, one with a reference STEP-SCH and another with a proposed optimized GRIN-SCH profile. The laser with optimised SCH design has shown a 36% reduction in room temperature threshold current as compared to that with the STEP-SCH design, which is in good agreement to the theoretical simulation. In addition, a record high characteristic temperature (T0) of 105 K was obtained on the GRIN-SCH laser structure, which is more than three fold increment as compared to the STEP-SCH design.

Mid infrared interband cascade lasers for sensing applications

The growth of Interband Cascade Laser material to cover the wavelength range from 3–4 μm is presented along with the fabrication and characterization of Broad Area (BA) and Ridge Waveguide (RWG) devices based on this material. Pulsed operation slightly below room temperature is observed for both device types, and a strong reduction of threshold currents can be observed in the RWG lasers. Variation of the active Quantum Well width in the epitaxial structures enables laser emission in the 3–4 μm wavelength region.

Optical Polarization Modulation and Gain Anisotropy in an Electrically Injected Spin Laser

The effects of spin-induced gain anisotropy on output polarization and threshold current reduction of electrically pumped spin-polarized lasers have been studied. Analytical forms of these parameters are derived by considering diffusive transport from the spin injector to the active region. The calculated values of the parameter are in excellent agreement with values obtained from measurements made at 200 K on an InAs/GaAs quantum dot spin-polarized vertical cavity surface-emitting laser. Electrical modulation of the output polarization of the laser is demonstrated with a peak modulation index of 0.6.