To date, group-III-nitride has undergone continuous improvements to provide a broader range of industrial applications, such as solid-state lighting (SSL), visible light communications (VLC), and light projection. Recently, VLC has attained substantial attention in the field of wireless communication because it offers ~ 370 THz of bandwidth of unregulated visible spectrum, which makes it a critical factor in the evolution of the 5G networks and beyond. GaN-based light-emitting diode (LED) and laser diode (LD) have become increasingly appealing in energy-sufficient SSL replacing conventional light sources. However, III- nitride LEDs suffer from efficiency-droop in their external quantum efficiency associated with high current densities, and their modulation bandwidth is limited to 10 ~ 100 MHz. Although LDs have shown gigabit-modulation bandwidth, unfavorable artifacts, such as speckles are observed, which may raise a concern about eye safety. This dissertation is devoted to the fabrication and electrical and optical characterization of a new class of III-nitride light-emitter known as superluminescent diode (SLD). SLD works in an amplified spontaneous emission (ASE) regime, and it combines several advantages from both LD and LED, such as droop-free, speckle-free, low-spatial coherence, broader emission, high-optical power, and directional beam. Here, SLDs were fabricated by a focused ion beam by tilting the front facet of the waveguide to suppress the lasing mode. They showed a high-power of 474 mW on c-plane GaN-substrate with a large spectral bandwidth of 6.5 nm at an optical power of 105 mW. To generate SLD- based white light, a YAG-phosphor-plate was integrated, and a CRI of 85.1 and CCT of 3392 K were measured. For the VLC link, SLD showed record high-data rates of 1.45 Gbps and 3.4 Gbps by OOK and DMT modulation schemes, respectively. Additionally, a widely single- and dual-wavelength tunability were designed using SLD-based external cavity (SLD-EC) configuration for a tunable blue laser source. These results underscore the practicality of c-plane SLDs in realizing high-power, high data rate, speckle-free, and droop-free SSL-VLC apparatus. Additionally, the SLD-EC configuration allows a wide range of applications, including biomedical applications, optical communication, and high-resolution spectroscopy.

<p>Superluminescent diodes have become important for various applications, such as for biomedical imagining, due to their broad spectral width and high power.</p><p>This thesis demonstrates two-section superluminescent diodes fabricated using quantum well intermixing with strained Ga_0.75sln_0.25As quantum wells, grown on a GaAs substrate. A 100 nm capping layer of Ga_0.515In_0.485P grown at low temperature and having an excess of phosphorus, was removed from one section of the device to produce a relative bandgap shift between sections after rapid thermal annealing. The devices emitted at a wavelength of ~1μm with 60 nm of spectral width, and up to 38 mW of power at 20°C, depending on the currents applied to each section.</p><p>The combination of the spectral output from the two quantum well intermixed sections resulted in the broad spectral width. Angled facets at 7 ° were used to prevent the device from lasing. Additional power improvements were seen following the thermal anneal when a SiO2 capping layer was used on both sections. Depending on the applied currents, each section required 1.5 V to 3.0 V; and failed at 5.3 V ± 0.5 V.</p> / Thesis / Master of Applied Science (MASc)

Broadband light emitters operation, which covers multiple wavelengths of the electromagnetic spectrum, has been established as an indispensable element to the human kind, continuously advancing the living standard by serving as sources in important multi-disciplinary field applications such as biomedical imaging and sensing, general lighting and internet and mobile phone connectivity. In general, most commercial broadband light sources relies on complex systems for broadband light generation which are bulky, and energy hungry. Recent demonstration of ultra-broadband emission from semiconductor light sources in the form of superluminescent light emitting diodes (SLDs) has paved way in realization of broadband emitters on a completely novel platform, which offered compactness, cost effectiveness, and comparatively energy efficient, and are already serving as a key component in medical imaging systems. The low power-bandwidth product is inherent in SLDs operating in the amplified spontaneous emission regime. A quantum leap in the advancement of broadband emitters, in which high power and large bandwidth (in tens of nm) are in demand. Recently, the birth of a new class of broadband semiconductor laser diode (LDs) producing multiple wavelength light in stimulated emission regime was demonstrated. This very recent manifestation of a high power-bandwidth-product semiconductor broadband LDs relies on interband optical transitions via quantum confined dot/dash nanostructures and exploiting the natural inhomogeneity of the self-assembled growth technology. This concept is highly interesting and extending the broad spectrum of stimulated emission by novel device design forms the central focus of this dissertation. In this work, a simple rate equation numerical technique for modeling InAs/InP quantum dash laser incorporating the properties of inhomogeneous broadening effect on lasing spectra was developed and discussed, followed by a comprehensive experimental analysis of a novel epitaxial structure design. The layered structure is based on chirping the barrier layer thickness of the over grown quantum dash layer, in a multi-stack quantum dash/barrier active region, with the aim of inducing additional inhomogeneity. Based on material-structure and device characterization, enhanced lasing-emission bandwidth is achieved from the narrow (2 u m)ridge-waveguide LDs as a result of the formation of multiple ensembles of quantum dashes that are electronically different, in addition to improved device performance. Moreover, realization of SLDs from this device structure demonstrated extra-ordinary emission bandwidth covering the entire international telecommunication union (O- to U-) bands. This accomplishment is a collective emission from quantum wells and quantum dashes of the device active region. All these results lead to a step forward in the eventual realization of more than 150 nm lasing bandwidth from a single semiconductor laser diode.

The main objectives of this thesis are to develop viable process and/or device technologies for bandgap tuning of 1300-nm InGaAs/GaAs quantum-dot (QD) laser structures, and broad linewidth 1300-nm InGaAsP/InP quantum well (QW) superluminescent diode structures. The high performance bandgap-engineered QD laser structures were achieved by employing quantum-dot intermixing (QDI) based on impurity free vacancy diffusion (IFVD) technique for eventual seamless active-passive integration, and bandgap-tuned lasers. QDI using various dielectric-capping materials, such as HfO2, SrTiO3, TiO2, Al2O3 and ZnO, etc, were experimented in which the resultant emission wavelength can be blueshifted to ∼ 1100 nm ─ 1200 nm range depending on process conditions. The significant results extracted from the PL characterization were used to perform an extensive laser characterization. The InAs/GaAs quantum-dot lasers with QDs transition energies were blueshifted by ~185 nm, and lasing around ~1070 – 1190 nm was achieved. Furthermore, from the spectral analysis, a simultaneous five-state lasing in the InAs/InGaAs intermixed QD laser was experimentally demonstrated for the first time in the very important wavelength range from 1030 to 1125 nm. The QDI methodology enabled the facile formation of a plethora of devices with various emission wavelengths suitable for a wide range of applications in the infrared. In addition, the wavelength range achieved is also applicable for coherent light generation in the green – yellow – orange visible wavelength band via frequency doubling, which is a cost-effective way of producing compact devices for pico-projectors, semiconductor laser based solid state lighting, etc. [1, 2] In QW-based superluminescent diode, the problem statement lies on achieving a flat-top and ultra-wide emission bandwidth. The approach was to design an inhomogeneous active region with a comparable simultaneous emission from different transition states in the QW stacks, in conjunction with anti-reflection coating and tilted ridge-waveguide device configuration. In this regard, we achieved 125 nm linewidth from InGaAsP/InP multiple quantum well (MQW) superluminescent diode with a total output power in excess of 70 mW with an average power spectral density of 0.56 mW/nm, and a spectral ripple of ≤1.2 ± 0.5 dB. The high power and broadband SLD with flat-top emission spectrum is a desirable as optical source for noninvasive biomedical imaging techniques employing low coherence interferometry, for instance, optical coherence tomography (OCT).