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InAs/GaAs quantum dots (QDs) embedded within InP/GaP nanowires (NWs) were grown on Si substrates by Au-assisted and self-assisted vapor-liquid-solid (VLS) growth using molecular beam epitaxy (MBE). The morphology and structure of the NWs was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The samples were analysed using photoluminescence (PL) and photocurrent measurements to study the properties of NW-based QDs. The composition of InAs x P 1-x QDs embedded within InP NWs was varied from x = 0.25 to x = 1, demonstrating the tuning of quantum confined energy levels. PL measurements demonstrated an emission peak that shifted towards lower energy levels as the As composition was increased. This result was also observed for QD absorption peaks through wavelength-dependent room temperature photocurrent measurements. InP NWs were successfully passivated with an AlInP shell, which was demonstrated through PL analysis. The growth mechanism of patterned self-assisted GaP NWs on Si was studied through SEM and TEM analysis. It was found that for large V/III flux ratios the Ga seed particle reduced in volume throughout growth, which led to a smaller NW diameter. Conversely, for small V/III flux ratios the Ga seed particle increased in volume throughout growth, resulting in larger NW diameters. The dependence of V/III flux ratio on NW growth was characterized, allowing the tuning of NW diameter. iv GaP NWs with p-i-n junctions were fabricated on a Si substrate with GaAs QDs embedded within the intrinsic region. To the author’s knowledge, this is the first time such a device was demonstrated. The device demonstrated diode characteristics as expected for a p-n junction. Wavelength-dependent photocurrent measurements demonstrated the absorption of light within GaAs QDs, which was collected through electric field dependent tunneling and thermionic emission. The absorption of light extended beyond the bandgap of GaP due to the GaAs QDs. / Thesis / Doctor of Philosophy (PhD)

Luminescent quantum dots (QDs) or rods are semiconductor nano-particles that may be used for a wide array of applications such as in electro-optical devices, spectral bar coding, tagging and light filtering. In the case under investigation, the nano-particles are cadmium-selenide (CdSe), though they can be made from cadmium-sulfide, cadmium-telluride or a number of other II-VI and III-V material combinations. The CdSe quantum dots emit visible light at a repeatable wavelength when excited by an ultraviolet source. The synthesis of colloidal quantum dot nanoparticles is usually an organo-metallic precursor, high temperature, solvent based, airless chemical procedure that begins with the raw materials CdO, a high boiling point ligand, and a Se-trioctylphosphine conjugate. This investigation explores the means to produce quantum dots by this method and to activate the surface or modify the reaction chemistry with such molecules as trioctylphosphine oxide, stearic acid, dodecylamine, phenyl sulfone, aminophenyl sulfone, 4,4'dichlorodiphenyl sulfone, 4,4'difluorodiphenyl sulfone, sulfanilamide and zinc sulfide during the production to allow for further applications of quantum dots involving new chemistries of the outer surface. Overall, the project has been an interesting and successful one, producing a piece of equipment, a lot of ideas, and many dots with varied capping structures that have been purified, characterized, and stored in such a way that they are ready for immediate use in future projects. / Master of Science

Additive Manufacturing (AM) is a unique method of fabrication that, in contrast to traditional manufacturing methods, builds objects layer by layer. The ability of AM (when partnered with 3D scanning) to clone physical objects has raised concerns in the area of intellectual property (IP). To address this issue, the goal of this dissertation is to characterize and model a method to incorporate unique security features within AM builds. By adding optically detectable nanoparticles into transparent AM media, Physical Unclonable Function (PUFs) can be embedded into AM builds and serve as an anti-counterfeiting measure. The nanoparticle selected for this work is a Quantum Dot (QD), which absorbs UV light and emits light in the visible spectrum. This unique interaction with light makes the QDs ideal for a security system since the challenge (UV light) is a different signal from the response (the visible light emitted by the QDs). PolyJet, the AM process selected for this work, utilizes inkjet to deposit a photopolymer into layers, which are then cured with a UV light. An investigation into the visibility of the QDs within the printed PolyJet media revealed that the QDs produce PUF patterns visible via fluorescent microscopy. Furthermore, rheological data shows that the ink-jetting properties of the printing media are not significantly affected by QDs in sufficient concentrations to produce PUFs. The final objective of this study is to characterize the effects of the QDs on photocuring. The mathematical model to predict the critical exposure of the QD-doped photopolymer utilizes light scattering theory, QD characterization results, and photopolymer-curing characterization results. This mathematical representation will contribute toward the body of knowledge in the area of Additive Manufacturing of nanomaterials in photopolymers. Overall, this work embodies the first investigations of the effects of QDs on rheological characteristics of ink-jetted media, the effects of QDs on curing of AM photopolymer media, visibility of nanoparticles within printed AM media, and the first attempt to incorporate security features within AM builds. Finally, the major scientific contribution of this work is the theoretical model developed to predict the effects of QDs on the curing properties of AM photopolymers. / Ph. D.

Small-signal dynamic response of semiconductor quantum dot (QD) lasers with asymmetric barrier layers was studied. Semiconductor lasers are used in many communication systems. Fiber optic communication systems use semiconductor lasers in order to transmit information. DVD and Blu-ray disk players feature semiconductor lasers as their readout source. Barcode readers and laser pointers also use semiconductor lasers. A medical application of semiconductor lasers is for minor soft tissue procedures. Semiconductor lasers are also used to pump solid-state and fiber lasers. Semiconductor lasers are able to transmit telephone, internet, and television signals through fiber optic cables over long distances. The amount of information able to be transferred is directly related to the bandwidth of the laser. By introducing asymmetric barrier layers, the modulation bandwidth of the laser will improve, allowing for more information to be transferred. Also, by introducing asymmetric barrier layers, the output power will be unrestricted, meaning as more current is applied to the system, the laser will get more powerful. An optimum pumping current was found which maximized modulation bandwidth at -3dB, and is lower in QD lasers with asymmetric barrier layers (ABL) as opposed to conventional QD lasers. Modulation bandwidth was found to increase with cross section of carrier capture before reaching an asymptote. Both surface density of QDs and cavity length had optimum values which maximized modulation bandwidth. Relative QD size fluctuation was considered in order to see how variation in QD sizes effects the modulation bandwidth of the semiconductor QD laser with ABLs. These calculations give a good starting point for fabricating semiconductor QD lasers with ABLs featuring the largest modulation bandwidth possible for fiber optic communication systems. In semiconductor QD lasers, the electrons and holes may be captured into excited states within the QDs, rather than the ground state. The particles may also jump from the ground state up to an excited state, or drop from the excited state to the ground state. Recombination of electron-hole pairs can occur from the ground state to the ground state or from an excited state to an excited state. In the situation if the capture of charge carriers into the ground state in QDs takes place via the excited-state, then this two-step capture process makes the output power from ground-state lasing to saturate in conventional QD lasers. By using ABLs in the QD laser, it is predicted that the output power of ground-state lasing will continue to rise with applied current, as the ABLs will stop the electrons and holes from recombining in the optical confinement layer. Thus, ABL QD lasers will be able to be used in applications that require large energy outputs. / Master of Science / Semiconductor lasers (also known as diode lasers) have been used in numerous applications ranging from communication to medical applications. Among all applications of diode lasers, of particular importance is their use for high speed transmission of information and data in fiber optic communication systems. This is accomplished by direct conversion of the diode laser input (electrical current) to its output (optical power). Direct modulation of the laser optical output through varying electrical current helps cut costs by not requiring other expensive equipment in order to perform modulation. The performance of conventional semiconductor lasers suffers from parasitic recombination outside of the active region – an unwanted process that consumes a considerable fraction of the laser input (injection current) while not contributing to the useful output and thus damaging its performance. Asymmetric barrier layers were proposed as a way to suppress parasitic recombination in semiconductor lasers. In this study, the optimal conditions for semiconductor quantum dot lasers with asymmetric barrier layers were calculated in order to maximize their modulation bandwidth – the parameter that determines the highest speed of efficient information transmission. This includes finding the optimal values of the dc component of the pump current, quantum dot surface density and size fluctuations, and cavity length. As compared to conventional quantum dot lasers, the optimal dc current maximizing the modulation bandwidth is shown to be considerably lower in quantum dot lasers with asymmetric barrier layers thus proving their outperforming efficiency. In the presence of extra states in quantum dots in conventional lasers, the optical output of needed ground-state lasing may be heavily impacted – it may remain almost unchanged with increasing the laser input current. As opposed to conventional lasers, the output power of ground-state lasing in devices with asymmetric barrier layers will continue growing as more input current is applied to the system.

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Semiconductor quantum dots (QDs) are prime candidates for single and entangled photon sources in quantum information technologies due to their unique optical properties. This thesis investigates the development of QD sources operating at the telecom C-band ---around 1550 nm---, a critical aspect for long-distance applications in optical fibers. The research focuses on the design and optimization of InAs/GaAs QDs for efficient single photon emission within the telecom C-band. This thesis delves into the optimization of the quantum dot environment, by etching its matrix as a microlens (ML). The design process utilizes both simulations and lab fabrication techniques to achieve a source with high single photon throughput, a key requirement for quantum key distribution (QKD). To achieve this, the design optimizes factors such as material growth techniques, device structures, and microlens array configuration to enhance light collection efficiency by a microscope objective and Purcell effect for higher single-photon emission rate. The optimized microlens geometries, particularly the Gaussian and hemispherical shapes, significantly enhanced light extraction efficiency by the objective, achieving up to 40\% and 35\% respectively. The combined fabrication techniques of FIB milling, photolithography, and dry etching resulted in upgraded optical properties and minimal scattering in the microlenses. Furthermore, this work builds upon previous research conducted at the Royal Institute of Technology (KTH). The Quantum Nano Photonics (QNP) group successfully employed QDs to generate entangled photon states. This thesis extends this research by focusing on the design and optimization of a telecom C-band QD source suitable for long-distance transmission through existing fiber optic infrastructure over the Greater Stockholm Metropolitan Area, i.e. between the QNP-group lab at KTH AlbaNova campus and Ericsson HQ, in Kista, Stockholm. The feasibility of such transmission is explored by demonstrating the transmission of single photons from a QD source in the QNP lab at KTH to Ericsson's lab. This thesis contributes to the advancement of QD-based telecom C-band photon sources for future quantum communication networks, with a specific focus on microlens design and fabrication for enhanced single-photon emission efficiency. / Halvledarkvantprickar (QD) är utmärkta kandidater för enstaka och sammanflätade fotonkällor i kvantinformationsteknik på grund av deras unika optiska egenskaper. Denna avhandling undersöker utvecklingen av QD-källor som strålar på telekom C-band ---cirka 1550 nm---, en kritisk aspekt för långdistansapplikationer i optiska fiber. Forskningen fokuserar på design och optimering av InAs/GaAs QDs för effektiv emission av enstaka fotoner inom telekom C-bandet. Denna avhandling fördjupar sig i utformningen av kvantprickarkällan, med hjälp av en mikrolins (ML) array. Designprocessen använder både simuleringar och tillverkningstekniker för att uppnå en källa med hög enfotonrenhet, ett viktigt krav för kvantnyckeldistribution (QKD). För att uppnå detta optimerar designen faktorer som materialtillväxttekniker, enhetsstrukturer och mikrolinskonfiguration för att förbättra ljusinsamlingseffektiviteten och Purcell-effekten för ljusare och snabbare emission av enstaka fotoner. De optimerade mikrolinsgeometrierna, särskilt de gaussiska och halvsfäriska formerna, förbättrade avsevärt ljusextraktionseffektiviteten och nådde upp till 40\% respektive 35\%. De kombinerade tillverkningsteknikerna FIB-fräsning, fotolitografi och torretsning resulterade i uppgraderade optiska egenskaper och minimal spridning i de mikrolinserna. Vidare bygger detta arbete på tidigare forskning som bedrivits vid Kungliga Tekniska Högskolan (KTH). Quantum Nano Photonics-gruppen (QNP) använde framgångsrikt QD för att generera sammanflätade fotontillstånd. Denna avhandling utvidgar denna forskning genom att fokusera på design och optimering av en telekom C-band QD-källa lämplig för långdistansöverföring genom befintlig fiberoptisk infrastruktur över Storstockholmsområdet, dvs. mellan QNP-gruppens labb på KTH AlbaNova campus och Ericssons huvudkontor i Kista, Stockholm. Genomförbarheten av sådan överföring undersöks genom att demonstrera överföringen av enstaka fotoner från en QD-källa i QNP-labbet på KTH till Ericssons labb. Denna avhandling bidrar till utvecklingen av QD-baserade C-bandsfotonkällor för framtida kvantkommunikationsnätverk, med ett specifikt fokus på mikrolinsarraydesign för förbättrad renhet och emissionseffektivitet för enskilda fotoner. Arbetet bygger på tidigare forskning om generering av kvantsammanflätning och banar väg för säkra kvantkommunikationsnätverk över långa avstånd.

A simplified model of a double quantum dot is presented and analyzed, with applications to spin-qubit quantum computation. The ability to trap single electrons in semiconductor nanostructures has led to the proposal of quantum computers with spin-based qubits coupled by the exchange interaction. Current theory predicts an exchange interaction with a -1 power-law dependence on the detuning ϵ, the energy offset between the two dots. However, experiment has shown a -3/2 power-law dependence on ϵ. Using WKB analysis, this thesis explores one possible source of the modified dependence, namely an ϵ-dependent tunnel coupling between the two wells. WKB quantization is used to find expressions for the tunnel coupling of a one-dimensional double-well, and these results are compared to the exact, numerical solutions, as determined by the finite difference method and the transfer matrix method. Small ϵ-dependent corrections to the tunnel coupling are observed. In typical cases, WKB correctly predicts a constant tunnel coupling at leading-order. WKB also predicts small ϵ-dependent corrections for typical cases and strongly ϵ-dependent tunnel couplings for certain exceptional cases. However, numerical simulations suggest that WKB is not accurate enough to analyze the small corrections, and is not valid in the exceptional cases. Deviations from the conventional form of the low-energy Hamiltonian for a double-well are also observed and discussed.

A simplified model of a double quantum dot is presented and analyzed, with applications to spin-qubit quantum computation. The ability to trap single electrons in semiconductor nanostructures has led to the proposal of quantum computers with spin-based qubits coupled by the exchange interaction. Current theory predicts an exchange interaction with a -1 power-law dependence on the detuning ϵ, the energy offset between the two dots. However, experiment has shown a -3/2 power-law dependence on ϵ. Using WKB analysis, this thesis explores one possible source of the modified dependence, namely an ϵ-dependent tunnel coupling between the two wells. WKB quantization is used to find expressions for the tunnel coupling of a one-dimensional double-well, and these results are compared to the exact, numerical solutions, as determined by the finite difference method and the transfer matrix method. Small ϵ-dependent corrections to the tunnel coupling are observed. In typical cases, WKB correctly predicts a constant tunnel coupling at leading-order. WKB also predicts small ϵ-dependent corrections for typical cases and strongly ϵ-dependent tunnel couplings for certain exceptional cases. However, numerical simulations suggest that WKB is not accurate enough to analyze the small corrections, and is not valid in the exceptional cases. Deviations from the conventional form of the low-energy Hamiltonian for a double-well are also observed and discussed.