Shamsul Arafin

Review of recent progress of III-nitride nanowire lasers

Abstract. One-dimensional compound semiconductor nanolasers, especially nanowire (NW)-based nanolasers utilizing III-nitride (AlGaInN) materials system, are an emerging and promising area of research. Significant achievements have been made in developing III-nitride NW lasers with emission wavelengths from the deep ultraviolet (UV) to the near-infrared spectral range. The types of lasers under investigation include Fabry-Pérot, photonic crystal, plasmonic, ring resonator, microstadium, random, polariton, and two-dimensional distributed feedback lasers. The lasing thresholds vary by several orders of magnitude, which are a direct consequence of differing NW dimensions, quality of the NWs, characteristics of NW cavities, and coupling with the substrate. For electrically injected, such as ultralow-threshold and continuous-wave III-nitride NW lasers that can operate at room temperature, the following obstacles remain: carrier loss mechanisms including defect-related nonradiative surface recombination, electron overflow, and poor hole transport; low radiative recombination efficiency and high surface recombination; poor thermal management; and highly resistive ohmic contacts on the p-layer. These obstacles must be overcome to fully realize the potential of these lasers.

Electric-field control of ferromagnetism in Mn-doped ZnO nanowires.

In this Letter, the electric-field control of ferromagnetism was demonstrated in a back-gated Mn-doped ZnO (Mn-ZnO) nanowire (NW) field-effect transistor (FET). The ZnO NWs were synthesized by a thermal evaporation method, and the Mn doping of 1 atom % was subsequently carried out in a MBE system using a gas-phase surface diffusion process. Detailed structural analysis confirmed the single crystallinity of Mn-ZnO NWs and excluded the presence of any precipitates or secondary phases. For the transistor, the field-effect mobility and n-type carrier concentration were estimated to be 0.65 cm(2)/V·s and 6.82 × 10(18) cm(-3), respectively. The magnetic hysteresis curves measured under different temperatures (T = 10-350 K) clearly demonstrate the presence of ferromagnetism above room temperature. It suggests that the effect of quantum confinements in NWs improves Tc, and meanwhile minimizes crystalline defects. The magnetoresistace (MR) of a single Mn-ZnO NW was observed up to 50 K. Most importantly, the gate modulation of the MR ratio was up to 2.5 % at 1.9 K, which implies the electric-field control of ferromagnetism in a single Mn-ZnO NW.

GaSb-Based VCSEL With Buried Tunnel Junction for Emission Around 2.3

In this paper, we present a device concept and results of an electrically pumped vertical-cavity surface-emitting laser in the (AlGaIn)(AsSb) material system grown on GaSb substrate. The structure consists of an n-doped GaSb/AlAsSb distributed Bragg reflector and a type-I GaInAsSb/AlGaAsSb active region, and incorporates a type-III p+-GaSb/n+-InAsSb buried tunnel junction for current as well as optical confinement. Continuous-wave operation up to 75degC has been achieved at a wavelength of 2.3 mum. The mid-IR emission, the large tunability over a wavelength range of more than 10 nm combined with its single-mode operation makes this device ideally suited for gas-sensing applications.

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In this paper, electrically pumped GaSb-based vertical-cavity surface-emitting lasers operating continuous wave at a record long emission wavelength of ∼2.6 μm are presented. Owing to the excellent thermal heat management, the devices exhibit single-mode operation up to a heat-sink temperature of 55 °C. Lateral current confinement and index guiding in the device are accomplished by utilizing the buried tunnel junction concept. Devices with aperture diameters of 6 μm show maximum output powers of 0.3 mW at room temperature with quantum efficiencies around 10%.

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Vertical-cavity surface-emitting lasers (VCSELs) are perfect light sources for spectroscopic applications, where properties such as continuous- wave (cw) operation, single-mode emission, high lifetime and often low power consumption are crucial. For applications such as tunable diode laser absorption spectroscopy (TDLAS), there is a growing interest in laser devices emitting in the near- to mid-infrared wavelength range, where many environmentally and technologically important gases show strong absorption lines. The (AlGaIn)(AsSb) material system based on GaSb is the material of choice for covering the 2.0-3.3µm range. In this paper, we report on electrically pumped single-mode VCSELs with emission wavelengths of 2.4 and 2.6µm, operating cw at room temperature and beyond. By (electro-) thermal tuning, the emission wavelength can be tuned mode-hop free over a range of several nanometers. In addition, low threshold currents of several milliamperes promise mobile application. In the devices, a structured buried tunnel junction with subsequent overgrowth has been used in order to achieve efficient current confinement, reduced optical losses and increased electrical conductivity. Furthermore, strong optical confinement is introduced in the lasers due to laterally differing cavity lengths.

Electrically pumped continuous-wave vertical-cavity surface-emitting lasers at ∼2.6 μm

The GaSb and InAs(Sb) material combination results in a type-III (broken gap) band alignment and is of particular interest for use as an ohmic, low-resistive intra-cavity contact in complex optoelectronic devices, such as buried-tunnel-junction vertical-cavity surface-emitting lasers. In this work, we report electrical characteristics of MBE-grown p+-GaSb/n+-InAs tunnel junctions. The investigated structures exhibit ultra-low resistive behavior, yielding specific resistivity values below 2.8 × 10−7Ω cm2. This value is nearly ten times better than previously reported best values.

Single-mode electrically pumped GaSb-based VCSELs emitting continuous-wave at 2.4 and 2.6 µm

GaInAsSb-GaSb strained quantum-well (QW) ridge waveguide diode lasers emitting in the wavelength range from 2.51 to 2.72 mum have been grown by molecular beam epitaxy. The devices show ultralow threshold current densities of 44 A/cm2 (L rarr infin) for a single QW device at 2.51 mum, which is the lowest reported value in continuous-wave operation near room temperature (15degC) at this wavelength. The devices have an internal loss of 3 cm-1 and a characteristic temperature of 42 K. By using broader QWs, wavelengths up to 2.72 mum could be achieved.

Ultra-low resistive GaSb/InAs tunnel junctions

An integrated heterodyne optical phase-locked loop was designed and demonstrated with an indium phosphide based photonic integrated circuit and commercial off-the-shelf electronic components. As an input reference, a stable microresonator-based optical frequency comb with a 50-dB span of 25 nm (~3 THz) around 1550 nm, having a spacing of ~26 GHz, was used. A widely-tunable on-chip sampled-grating distributed-Bragg-reflector laser is offset locked across multiple comb lines. An arbitrary frequency synthesis between the comb lines is demonstrated by tuning the RF offset source, and better than 100Hz tuning resolution with ± 5 Hz accuracy is obtained. Frequency switching of the on-chip laser to a point more than two dozen comb lines away (~5.6 nm) and simultaneous locking to the corresponding nearest comb line is also achieved in a time ~200 ns. A low residual phase noise of the optical phase-locking system is successfully achieved, as experimentally verified by the value of -80 dBc/Hz at an offset of as low as 200 Hz.

Low-Threshold Strained Quantum-Well GaSb-Based Lasers Emitting in the 2.5- to 2.7-

GaInAsSb/GaSb based quantum well vertical cavity surface emitting lasers (VCSELs) operating in mid-infrared spectral range between 2 and 3 micrometres are of great importance for low cost gas monitoring applications. This paper discusses the efficiency and temperature sensitivity of the VCSELs emitting at 2.6 μm and the processes that must be controlled to provide temperature stable operation. We show that non-radiative Auger recombination dominates the threshold current and limits the device performance at room temperature. Critically, we demonstrate that the combined influence of non-radiative recombination and gain peak – cavity mode de-tuning determines the overall temperature sensitivity of the VCSELs. The results show that improved temperature stable operation around room temperature can only be achieved with a larger gain peak – cavity mode de-tuning, offsetting the significant effect of increasing non-radiative recombination with increasing temperature, a physical effect which must be accounted for in mid-infrared VCSEL design.

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This paper discusses several performance-related aspects of electrically-pumped GaSb-based buried tunnel junction VCSELs with an emission wavelength of 2.6 μm based on theoretical and experimental results. These results allow a deeper insight into the internal device physics, such as radial diffusion of carriers, maximum continuous-wave operating temperature, diffraction loss, internal temperature, gain and loss parameters, internal quantum efficiency of the active region etc. These parameters can be taken into account while designing mid-infrared lasers which leads to an improved device performance. A simple thermal model of the devices based on the two-dimensional (2-D) finite element method using the material data from the literature is also presented. In addition, an application-based result utilizing these lasers for the measurement of absolute water vapor concentration by wavelength modulation spectroscopy (WMS) method are also described, hinting that devices are well-suited for the targeted sensing applications.