Benjamin J. Stevens

p-doped 1.3 μm InAs/GaAs quantum-dot laser with a low threshold current density and high differential efficiency

A modification of the thickness of the low-growth-temperature component of the GaAs spacer layers in multilayer 1.3μm InAs∕GaAs quantum-dot (QD) lasers has been used to significantly improve device performance. For a p-doped seven-layer device, a reduction in the thickness of this component from 15to2nm results in a reduced reverse bias leakage current and an increase in the intensity of the spontaneous emission. In addition, a significant reduction of the threshold current density and an increase of the external differential efficiency at room temperature are obtained. These improvements indicate a reduced defect density, most probably a combination of the selective elimination of a very low density of dislocated dots and a smaller number of defects in the thinner low-growth-temperature component of the GaAs spacer layer.

Tuning Superluminescent Diode Characteristics for Optical Coherence Tomography Systems by Utilizing a Multicontact Device Incorporating Wavelength-Modulated Quantum Dots

This paper details the development of broadband sources at 1050 nm for optical coherence tomography applications. A method for obtaining a broad and smooth emission and gain spectrum for 1050 nm quantum dot (QD) layers is presented. The design, fabrication, and operating characteristics of multicontact superluminescent diodes are then set out, and the operating characteristics of this device, incorporating the broadband QD material, are described. It is shown that this device allows the tuning of the emission spectrum peak position, peak shape, emission bandwidth, and power, which are advantageous for imaging applications. A simplified device, utilizing a single current source, is achieved by incorporating a resistor network. The operation of the multicontact device under various drive topologies is discussed.

Persistent template effect in InAs/GaAs quantum dot bilayers

The dependence of the optical properties of InAs/GaAs quantum dot (QD) bilayers on seed layer growth temperature and second layer InAs coverage is investigated. As the seed layer growth temperature is increased, a low density of large QDs is obtained. This results in a concomitant increase in dot size in the second layer, which extends their emission wavelength, reaching a saturation value of around 1400 nm at room temperature for GaAs-capped bilayers. Capping the second dot layer with InGaAs results in a further extension of the emission wavelength, to 1515 nm at room temperature with a narrow linewidth of 22 meV. Addition of more InAs to high density bilayers does not result in a significant extension of emission wavelength as most additional material migrates to coalesced InAs islands but, in contrast to single layers, a substantial population of regular QDs remains.

High-Power and Broadband Quantum Dot Superluminescent Diodes Centered at 1250 nm for Optical Coherence Tomography

Quantum dot (QD) superluminescent diodes (SLDs) exhibiting 8 mW and 95 nm full-width at half-maximum centered at 1270 nm are demonstrated with a flat-topped spectral profile. This is achieved using 3 times 2 dots in compositionally modulated wells technique. Furthermore, techniques for realization of high-power SLDs are also demonstrated. A continuous-wave output power of 42 mW is achieved for narrowband devices centered at 1250 nm.

Strain Balancing of Metal-Organic Vapour Phase Epitaxy InAs/GaAs Quantum Dot Lasers

Incorporation of a GaAs_0.8P_0.2 layer allows strain balancing to be achieved in self-assembled InAs/GaAs quantum dots (QDs) grown by metal organic vapor phase epitaxy. Tuneable wavelength and high density are obtained through growth parameter optimization, with emission at 1.27xa0μm and QD layer density 3 × 10^10xa0cm^–2. Strain balancing allows close vertical stacking (30xa0nm) of the QD layers, giving the potential for increased optical gain. Modeling and device characterization indicates minimal degradation in the optical and electrical characteristics unless the phosphorus percentage is increased above 20%. Laser structures are fabricated with a layer separation of 30xa0nm, demonstrating low temperature lasing with a threshold current density of 100xa0A/cm² at 130 K without any facet coating.

Optimization of the Epitaxial Design of High Current Density Resonant Tunneling Diodes for Terahertz Emitters

We discuss the numerical simulation of high current density InGaAs/AlAs/InP resonant tunneling diodes with a view to their optimization for application as THz emitters. We introduce a figure of merit based upon the ratio of maximum extractable THz power and the electrical power developed in the chip. The aim being to develop high efficiency emitters as output power is presently limited by catastrophic failure. A description of the interplay of key parameters follows, with constraints on strained layer epitaxy introduced. We propose an optimized structure utilizing thin barriers paired with a comparatively wide quantum well that satisfies strained layer epitaxy constraints.

Operating Characteristics of GaAs/InGaP Self Aligned Stripe Lasers

We demonstrate a novel process for fabrication of GaAs based self-aligned lasers based upon a single overgrowth. A lattice matched n-doped InGaP layer is utilized for both electrical and optical confinement. We present operating characteristics such as external differential quantum efficiency, T-zero and far field as a function of stripe width.

A Quantum Dot Swept Laser Source Based upon a Multisection Laser Device

An all semiconductor swept laser source relying on state filling in quantum dot lasers is discussed. The use of a multi-contact laser operating at the saturated gain of the ground state allows a new type of swept laser source to be realised. Analysis of the prototype device is made and design rules for optimised operation are discussed.

Fabrication, Characterisation, and Epitaxial Optimisation of MOVPE-Grown Resonant Tunnelling Diode THz Emitters

Resonant tunnelling diodes (RTDs) are a strong candidate for future wireless communications in the THz region, offering compact, room-temperature operation with Gb/s transfer rates. We employ the InGaAs/AlAs/InP material system, offering advantages due to high electron mobility, suitable band-offsets, and low resistance contacts. We describe an RTD emitter operating at 353GHz, radiating in this atmospheric transmittance window through a slot antenna. The fabrication scheme uses a dual-pass technique to achieve reproducible, very low resistivity, ohmic contacts, followed by accurate control of the etched device area. The top contact connects the device via the means of an air bridge. We then proceed to model ways to increase the resonator efficiency, in turn improving the radiative efficiency, by changing the epitaxial design. The optimization takes into account the accumulated stress limitations and realities of reactor growth. Due to the absence of useful in-situ monitoring in commercially-scalable metal-organic vapour phase epitaxy (MOVPE), we have developed a robust non-destructive epitaxial characterisation scheme to verify the quality of these mechanically shallow and atomically thin devices. A dummy copy of the active region element is grown to assist with low temperature photoluminescence spectroscopy (LTPL) characterisation. The resulting linewidths limits the number of possible solutions of quantum well (QW) width and depth pairs. In addition, the doping levels can be estimated with a sufficient degree of accuracy by measuring the Moss-Burstein shift of the bulk material. This analysis can then be combined with high resolution X-ray diffractometry (HRXRD) to increase its accuracy.

Mode Control in Photonic Crystal Surface Emitting Lasers Through External Reflection

In this paper, we show the effect of lateral external optical feedback on an all semiconductor photonic crystal surface emitting laser (PCSEL). Initially, a PCSEL is grown and fabricated with a square lattice of triangles, the device is shown to operate electrically driven at room temperature under continuous wave condition. We investigate, theoretically and experimentally, the effect of lateral feedback on the performance of photonic crystal lasers. Demonstrating a reduction in mode competition and a modification to spatial mode distribution, opening routes to all electronic beam shaping and divergence control.