The construction of a high-resolution optical microscope system for micro-photoluminescence (µ-PL) spectroscopy is described, and a range of time-integrated and time-resolved experimental work on single InGaN quantum dots (QDs) is presented. Time-integrated measurements demonstrate the existence of InGaN QDs in three different samples via the presence of sharp exciton recombination lines in the µ-PL spectra. The narrowest peaks display a linewidth Γ of ~230 µeV, implying a decoherence time T2 ≥5.7 ps. Time-resolved measurements on exciton recombination lines from single self-assembled InGaN QDs reveal typical lifetimes of ~2.0 ns (which decrease with increasing temperature), while typical lifetimes for excitons in single selectively-grown micropyramidal InGaN QDs are found to be ~0.4 ns. The shorter exciton recombination lifetime in selectively-grown QDs is believed to be due to a stronger coupling of these QDs to the underlying quantum well. Temporal fluctuations (on a timescale of seconds) in the energy, intensity and FWHM of µ-PL peaks arising from the recombination of excitons in single self-assembled InGaN QDs are observed. These are attributed to transient Stark shifts induced by a fluctuating local charge distribution as carriers become trapped in defect states in the vicinity of the QDs. Time-integrated power-dependent measurements are used to demonstrate the presence of biexciton states in single self-assembled InGaN QDs. The exciton–biexciton energy splitting is found to be ~41 meV, in agreement with values predicted by theoretical calculations. Time-resolved studies of the biexciton and exciton decay curves reveal a coupling as the exciton population is refilled by biexciton decays. The biexciton lifetime is found to be ~1.4 ns, compared to an exciton lifetime of ~1.0 ns. Lateral electric fields are applied to a single self-assembled InGaN QD using aluminium electrodes lithographically defined on the sample surface. Application of fields of the order of ~0.17 MVcm-1 is found to cause both a red-shift and a reduction in the intensity of the exciton recombination peak in the µ-PL spectrum.

In this thesis, we demonstrate a new type of resonant, nonlinear, light-matter interaction facilitated by the unique electromagnetic vacuum density-of-state (DOS) structure of Photonic Band Gap (PBG) materials. Strong light localization inside PBG waveguides allows extremely strong coupling between laser fields and embedded two-level quantum dots (QD). The resulting Mollow splitting is large enough to traverse the precipitous DOS jump created by a waveguide mode cutoff. This allows the QD Bloch vector to sense the non-smoothness of the vacuum structure and evolve in novel ways that are forbidden in free space. These unusual strong-coupling effects are described using a "vacuum structure term" of the Bloch equation, combined with field-dependent relaxation rates experienced by the QD Bloch vector. This leads to alternation between coherent evolution and enhanced relaxation. As a result, dynamic high-contrast switching of QD populations can be realized with a single beam of picosecond pulses. During enhanced relaxation to a slightly inverted steady state at the pulse peak, the Bloch vector rapidly switches from anti-parallel to parallel alignment with the pulse torque vector. This then leads to a highly inverted state through subsequent coherent "adiabatic following" near the pulse tail, providing a robust mechanism for picosecond, femto-Joule all-optical switching. The simultaneous input of a second, weaker (signal) driving beam at a different frequency on top of the stronger (holding) beam enables rich modulation effects and unprecedented coherent control over the QD population. This occurs through resonant coupling of the signal pulse with the Mollow sideband transitions created by the holding pulse, leading to either augmentation or negation of the final QD population achieved by the holding pulse alone. This effect is applied to ultrafast all-optical logic AND, OR and NOT gates in the presence of significant (0.1 THz) nonradiative dephasing and (about 1%) inhomogeneous broadening. Further numerical studies of pulse evolutions inside the proposed devices demonstrate satisfactory population contrast within a PBG waveguide length of about 10 micro meter. These results provide the building blocks for low-power, ultrafast, multi-wavelength channel, on-chip, all-optical computing.

Quantum Dot (QD) nanobeacons distinguish themselves from molecular beacons with the promise of non-linear activation, tunability, and multi-functionality. These unique features make them highly attractive for cancer detection imaging with opportunities for increased signal-to-background ratio and tunable sensitivity. In this thesis, a nanobeacon was designed to target matrix metalloproteinase-7 (MMP-7), known to be over-expressed by a wide array of tumours. The nanobeacon is normally dark until specifically activated by MMP-7. The overall design strategy links single QDs to multiple energy acceptors by GPLGLARK peptides that can be cleaved specifically by MMP-7. However, design details such as the choice of energy acceptor and conjugation method was found to drastically alter the function of the nanobeacon. Studies of nanobeacons synthesized with Black Hole Quencher-1 or Rhodamine Red by either covalent conjugation or electrostatic self-assembly revealed that peptide conformation and bonding flexibility are both important considerations in nanobeacon design due to QD sterics.

In this thesis, we demonstrate a new type of resonant, nonlinear, light-matter interaction facilitated by the unique electromagnetic vacuum density-of-state (DOS) structure of Photonic Band Gap (PBG) materials. Strong light localization inside PBG waveguides allows extremely strong coupling between laser fields and embedded two-level quantum dots (QD). The resulting Mollow splitting is large enough to traverse the precipitous DOS jump created by a waveguide mode cutoff. This allows the QD Bloch vector to sense the non-smoothness of the vacuum structure and evolve in novel ways that are forbidden in free space. These unusual strong-coupling effects are described using a "vacuum structure term" of the Bloch equation, combined with field-dependent relaxation rates experienced by the QD Bloch vector. This leads to alternation between coherent evolution and enhanced relaxation. As a result, dynamic high-contrast switching of QD populations can be realized with a single beam of picosecond pulses. During enhanced relaxation to a slightly inverted steady state at the pulse peak, the Bloch vector rapidly switches from anti-parallel to parallel alignment with the pulse torque vector. This then leads to a highly inverted state through subsequent coherent "adiabatic following" near the pulse tail, providing a robust mechanism for picosecond, femto-Joule all-optical switching. The simultaneous input of a second, weaker (signal) driving beam at a different frequency on top of the stronger (holding) beam enables rich modulation effects and unprecedented coherent control over the QD population. This occurs through resonant coupling of the signal pulse with the Mollow sideband transitions created by the holding pulse, leading to either augmentation or negation of the final QD population achieved by the holding pulse alone. This effect is applied to ultrafast all-optical logic AND, OR and NOT gates in the presence of significant (0.1 THz) nonradiative dephasing and (about 1%) inhomogeneous broadening. Further numerical studies of pulse evolutions inside the proposed devices demonstrate satisfactory population contrast within a PBG waveguide length of about 10 micro meter. These results provide the building blocks for low-power, ultrafast, multi-wavelength channel, on-chip, all-optical computing.

Quantum Dot (QD) nanobeacons distinguish themselves from molecular beacons with the promise of non-linear activation, tunability, and multi-functionality. These unique features make them highly attractive for cancer detection imaging with opportunities for increased signal-to-background ratio and tunable sensitivity. In this thesis, a nanobeacon was designed to target matrix metalloproteinase-7 (MMP-7), known to be over-expressed by a wide array of tumours. The nanobeacon is normally dark until specifically activated by MMP-7. The overall design strategy links single QDs to multiple energy acceptors by GPLGLARK peptides that can be cleaved specifically by MMP-7. However, design details such as the choice of energy acceptor and conjugation method was found to drastically alter the function of the nanobeacon. Studies of nanobeacons synthesized with Black Hole Quencher-1 or Rhodamine Red by either covalent conjugation or electrostatic self-assembly revealed that peptide conformation and bonding flexibility are both important considerations in nanobeacon design due to QD sterics.

The commercially available solar cells suffer from low conversion efficiency due to the thermalization and transmission losses arising from the mismatch between the band gap of the semiconductor materials and the solar spectrum. Advanced device architectures based on nanomaterial have been proposed and being successfully used to enhance the efficiency of the solar cells. Quantum dots (QDs) and nanowires (NWs) are the nanosclae structures that have been exploited for the development of the third generation solar cell devices and nanowire based solar cells, respectively. The optical and electrical properties of these materials can be tuned by their size and geometry; hence they have great potential for the production of highly efficient solar cell. Application of QDs and NWs with enhanced optoelectronic properties and development of low-cost fabrication processes render a new generation of economic highly efficient PV devices. The most significant contribution of this PhD study is the development of simple and cost effective methods for fabrication of nanowires and quantum dots for advanced solar cell architectures. In advanced silicon nanowires (SiNWs) array cell, SiNWs have been widely synthesised by the well-known vapor-liquid-solid method. Electron beam lithography and deep reactive ion etching have also been employed for fabrication of SiNWs. Due to the high price and complexity of these methods, simple and cost effective approaches are needed for the fabrication of SiNWs. In another approach, to enhance the cell efficiency, organic dyes and polymers have been widely used as luminescent centers and host mediums in the luminescent down shifting (LDS) layers. However, due to the narrow absorption band of the dyes and degradation of the polymers by moisture and heat, these materials are not promising candidates to use as LDS. Highly efficient luminescent materials and transparent host materials with stable mechanical properties are demanded for luminescent down shifting applications. In this project, simple fabrication processes were developed to produce SiNWs and QDs for application in advanced cell architectures. The SiNWs array were successfully fabricated, characterized and deployed in new cell architectures with radial p-n junction geometry. The luminescence down shifting of layers containing QDs in oxide and glass mediums was verified. The silica coated quantum dots which are suitable for luminescence down shifting, were also fabricated and characterized for deployment in new design architectures. Silicon nanowires were fabricated using two simplified methods. In the first approach, a maskless reactive ion etching process was developed to form upright ordered arrays of the SiNWs without relying on the complicated nano-scale lithography or masking methods. The fabricated structures were comprehensively characterized. Light trapping and photoluminescence properties of the medium were verified. In the second approach, combination of the nanosphere lithography and etching techniques were utilized for wire formation. This method provides a better control on the wire diameters and geometries in a very simple and cost effective way. The fabricated silicon nanowires were used for formation of the radial p-n junction array cells. The functionality of the new cell structures were confirmed through experimental and simulation results. Quantum dots are promising candidates as luminescent centers due to their tunable optical properties. Oxide/glass matrices are also preferred as the host medium for QDs because of their robust mechanical properties and their compatibility with standard silicon processing technology. Besides, the oxide layers are transparent mediums with good passivation and anti-reflection coating properties. They can also be used to encapsulate the cell. In this work, ordered arrays of QDs were incorporated in an oxide layer to form a luminescent down shifting layer. This design benefits from the enhanced absorption of a periodic QD structure in a transparent oxide. The down shifting properties of the layer after deployment on a crystalline silicon solar cell were examined. For this purpose, crystalline silicon solar cells were fabricated to use as test platform for down shifting. In order to examine the down-shifting effect, different approaches for formation of a luminescence down shifting layer were developed. The LDS layer consist of cadmium selenide- zinc sulfide (CdSe/ZnS) quantum dots in oxide and glass layers to act as luminescent centers and transparent host medium, respectively. The structural and optical properties of the fabricated layers were studied. The concept of spectral engineering was proved by the deployment of the layer on the solar cell. To further benefit from the LDS technique, quantum efficiency of the QDs and optical properties of the layer must be improved. Demand for the high quantum efficiency material with desired geometry leaded us to synthesis quantum dots coated with a layer of grown oxide. As the luminescence quantum efficiency of the QDs is correlated to the surface defects, one advantage of having oxide on the outer shell of the QDs, is to passivate the surface non-radiative recombination centers and produce QDs with high luminescent quantum yield. In addition, nanoparticles with desired size can be obtained only by changing the thickness of the oxide shell. This method also simplifies the fabrication of QD arrays for luminescence down shifting application, since it is easier to form ordered arrays from larger particles. QD superlattices in an oxide medium can be fabricated on a large area by a simple spin-coating or dip coating methods. The photonic crystal properties of the proposed structure can greatly increase the absorption in the QDs layer and enhance the effect of down shifting.

The physics and technology of the heterojunction infrared photodetectors having different material systems have been studied extensively. Devices used in this study have been characterized by using mainly optical methods, and electrical measurements have been used as an auxiliary method. The theory of internal photoemission in semiconductor heterojunctions has been investigated and the existing model has been extended by incorporating the effects of the difference in the effective masses in the active region and the substrate, nonspherical-nonparabolic bands, and the energy loss per collisions. The barrier heights (correspondingly the cut-off wavelengths) of SiGe/Si samples have been found from their internal photoemission spectrums by using the complete model which has the wavelength and doping concentration dependent free carrier absorption parameters. A qualitative model describing the mechanisms of photocurrent generation in SiGe/Si HIP devices has been presented. It has been shown that the performance of our devices depends significantly on the applied bias and the operating temperature. Properties of internal photoemission in a PtSi/Si Schottky type infrared detector have also been studied. InGaAs/InP quantum well photodetectors that covers both near and mid-infrared spectral regions by means of interband and intersubband transitions have been studied. To understand the high responsivity values observed at high biases, the gain and avalanche multiplication processes have been investigated. Finally, the results of a detailed characterization study on a systematic set of InAs/GaAs self-assembled quantum dot infrared photodetectors have been presented. A simple physical picture has also been discussed to account for the main observed features.

Semiconductors and related low-dimensional nanostructures are extremely important in the modern world. They have been extensively studied and applied in industry/military areas such as ultraviolet optoelectronics, light emitting diodes, quantum-dot photodetectors and lasers. The knowledge of growth dynamics of semiconductor nanostructures by metalorganic chemical vapour deposition (MOCVD) is very important then. MOCVD, which is widely applied in industry, is a kind of chemical vapour deposition method of epitaxial growth for compound semiconductors. In this method, one or several of the precursors are metalorganics which contain the required elements for the deposit materials. Theoretical studies of growth mechanism by MOCVD from a realistic reactor dimension down to atomic dimensions can give fundamental guidelines to the experiment, optimize the growth conditions and improve the quality of the semiconductor-nanostructure-based devices. Two main types of study methods are applied in the present thesis in order to understand the growth dynamics of semiconductor nanostructures at the atomic level: (1) Kinetic Monte Carlo method which was adopted to simulate film growths such as diamond, Si, GaAs and InP using the chemical vapor deposition method; (2) Computational fluid dynamics method to study the distribution of species and temperature in the reactor dimension. The strain energy is introduced by short-range valence-force-field method in order to study the growth process of the hetero epitaxy. The Monte Carlo studies show that the GaN film grows on GaN substrate in a two-dimensional step mode because there is no strain over the surface during homoepitaxial growth. However, the growth of self-assembled GaSb quantum dots (QDs) on GaAs substrate follows strain-induced Stranski-Krastanov mode. The formation of GaSb nanostructures such as nanostrips and nanorings could be determined by the geometries of the initial seeds on the surface. Furthermore, the growth rate and aspect ratio of the GaSb QD are largely determined by the strain field distribution on the growth surface. / QC 20100713

Usant de certaines conditions, la croissance épitaxiale de GaN sur un large panel de substrats donne lieu à une assemblée de nanofils. Cette géométrie filaire peut permettre la croissance d'hétérostructures libres de tous défauts cristallins étendus, ce qui les rendent attractives pour créer des dispositifs de hautes performances. En premier lieu, mon travail de thèse a visé à clarifier le mécanisme de nucléation auto-organisé des nanofils de GaN sur substrat de silicium. Dans ce but, une étude approfondie de la couche tampon d'AlN, déposée préalablement à la nucléation des nanofils, a été réalisée, mettant en évidence une inattendue forte réactivité de l'Al avec le substrat. La nécessité de la polarité azote pour la croissance des nanofils de GaN a été mise en lumière, bien que des nanofils contenant dans leur cœur un domaine de polarité Ga ont également été observés. Dans ces nanofils, une paroi d'inversion de domaine est présente et a été démontrée être optiquement active, exhibant une photoluminescence à 3.45 eV. Ensuite des hétérostuctures filaires GaN/AlN ont été synthétisée pour des caractérisations structurales et optiques. Il a été montré que le mode de croissance de l'hétérostructure peut être changé en fonction du diamètre du nanofil. En dernier lieu, en prenant avantage de la géométrie cylindrique des nanofils, des mesures de diffusion de porteurs de charge ont été réalisées dans des nanofils de GaN et d'AlN. / Using specific conditions, GaN can be epitaxially grown on a large variety of substrates as a nanowire (NW) array. This geometry allows the subsequent growth of wire-like heterostructures likely free of extended defects, which makes them promising for increasing device controllability and performance. First, my PhD work has been devoted to the understanding of self-organized nucleation of GaN NWs on silicon substrates. For this purpose, a deep characterization of the growth mechanism of the AlN buffer deposited prior to NW nucleation has been done, emphasizing an unexpected large reactivity of Al with the substrate. The requirement of the N polarity to nucleate GaN NWs has been evidenced, although the possible existence of NWs hosting a Ga polar core has been observed as well. In these NWs, an inversion domain boundary is present and has been demonstrated to be optically active, having a photoluminescence signature at 3.45 eV. Next, GaN/AlN wire heterostructures have been grown for structural and optical characterization. It has been shown that by changing the wire diameter, different growth mode for the heterostructure could be reached.At last, thanks to the cylindrical geometry of NWs, the measurement of diffusion length for charge carriers in GaN and AlN NWs have been performed.

Quantum networks lie at the heart of distributed quantum computing and secure quantum communication - research areas that have seen a strong increase of interest over the last decade. Their basic architecture consist of stationary nodes composed of quantum processors which are linked via photonic channels. The key requirement, and at the same time the most demanding challenge, is the efficient distribution of entanglement between distant nodes. The two ground states of single spins confined in self-assembled InGaAs quantum dots provide an effective two-level system for the implementation of quantum bits. Moreover, they offer strong transition dipole moments with outstanding photonic properties allowing for the realisation of close to ideal, high-bandwidth spin-photon interfaces. These properties are combined with the benefits of working in the solid state, such as scalability and integrability of devices, to form a promising candidate for the implementation of fast entanglement distribution. In this dissertation we provide the first implementation of a unit cell of a quantum network based on single electron spins in InGaAs. We use a probabilistic scheme based on spin-photon entanglement and the erasure of which path information to project the two distant spins into a maximally entangled Bell state. The successful generation of entanglement is verified through a reconstruction of the final two-spin state and we achieve an average fidelity of $61.6\pm2.3\%$ at a record-high generation rate of $5.8\,\mathrm{kHz}$. One of the main constraints to the achieved fidelity is the limited coherence of the electron spin. We show that it can be extended by three orders of magnitude through decoupling techniques and develop a new measurement technique, allowing us to investigate the origins of the decoherence which has previously been obscured by nuclear feedback processes. Our results evidence that further extension of coherence is ultimately limited by intrinsic mechanisms closely related to local strain due to the growth method of self-assembled quantum dots. After establishing the intrinsic limits to the electron coherence we investigate the coherence properties of the single hole spin as an alternative two-level system with the potential for higher coherence times. We show that the hole spin coherence is indeed superior to the one of the electron and realise the first successful dynamic decoupling scheme implemented in these systems. We find that the decoherence at low external magnetic fields is still governed by coupling to the nuclear spins whereas it is dominated by electrical noise for fields exceeding a few Tesla. This noise source is extrinsic to the quantum dots and a better understanding offers the potential for further improvement of the coherence time. The findings of this work present a complete study of the coherence of the charge carriers in self-assembled quantum dots and provide the knowledge needed to improve the implementation of a quantum-dot based quantum network. In particular, the combination of spin-spin entanglement and the hole coherence times enable further research towards multidimensional photonic cluster states.