By reducing the number of dimensions that light can propagate in from three to two, control over the properties of propagation can be achieved. The plasmonic modes of planar metal-dielectric heterostructures will confine light in one dimension, enhancing the electromagnetic fields within the structure. This thesis focuses on two particular aspects of active nanoplasmonics in planar systems, stopped light lasing and plasmons with gain in nonequilibrium graphene. For stopped-light lasing, a plasmonic waveguide mode is designed to have two points of zero group velocity in a narrow frequency range, in order to increase the local density of optical states that a gain medium can emit into. The two stopped light points form a band of slow light that supports a wide range of wavevectors, allowing localisation over a sub-wavelength gain medium and providing the feedback required for lasing. This results in a new type of laser that does not rely on predefined cavity modes, in fact is cavity-free in 2D, dynamically forming its lasing mode about a locally pumped region of carrier inversion. Graphene, a single-atom thick semimetal, provides the ultimate miniaturisation as a truly 2D material. It is shown that graphene can support plasmons with gain, under realistic conditions of collision loss, temperature, doping, and carrier relaxation via amplified spontaneous emission. This is made possible by developing a scheme to evaluate polarisabilities for nonequilibrium carrier distributions, allowing the calculation of the exact RPA complex-frequency plasmon dispersion solutions. The rates of spontaneous emission are calculated and are critically dependant on the exact dispersion relation. The instantaneous rates are found to be 5 times faster than previously reported and, when coupled with phonons, lead to carrier relaxations on 100 fs timescales. The polarisability and relaxation rates must form the basis of any active graphene device, where electromagnetic energy is coupled to an evolving electronic system.

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No description available.

This thesis describes experiments involving the coherent manipulation of a single ⁸⁸88Sr⁺ ion that is confined in a microfabricated trap. A stable and agile laser is constructed and used to perform rotations on an optical qubit that is encoded in the D₁/₂ (mj = -1/2) -D₅/₂(mj = -5/2) transition at 674 nm. After the excess micromotion is fully compensated in the microtrap, frequency-resolved optical pumping to the S₁/₂(mj = 1/2) qubit level is demonstrated with 99.8 % efficiency and resolved sideband cooling is used to reduce the mean vibrational number of the axial mode to n̄z~1. Deterministic ion transport between segments of the microtrap is also performed at frequencies up to 1 kHz. The agile laser permits fast, accurate and precise modulation of the optical phase, amplitude and frequency on a sub-microsecond timescale. Amplitude-shaped pulses are produced in near-perfect agreement with the desired functional form for durations ranging over 10⁶, which allows the frequency spectrum to be tailored in order to reduce off - resonant excitation. The agile system is stabilised in frequency by injection locking to an ultra-stable laser to yield a linewidth of < 10 Hz. Also, interrogation pulses are stabilised over a range of 10⁵ in optical power using a cascaded series of avalanche photodiodes, and the beam position at the ion is stabilised to < 0.4 % of the beam diameter. The complete system is capable of performing single qubit gate operations with an infidelity below a fault-tolerant limit. The agile laser is used to demonstrate that using Blackman rather than square shaped interrogation pulses offers an 11 dB reduction in off -resonant excitation. Also the phase agility of the system is verifed using Rabi and Ramsey spectroscopy. Finally, a spin-echo sequence is used to enhance the storage time of the optical qubit, and high resolution scans over the quadrupole transition are performed to measure a system coherence time of 1 ms.

CuInSe₂, CuInS₂ and CuGaSe₂ are I-III-VI₂ compound semiconductors with a chalcopyrite structure. These ternary compounds exhibit favourable properties, such as direct band gaps and high absorption coefficients, for application as absorber layers in thin-film solar cells. Recently Cu(In,Ga)Se₂ based photovoltaic devices have demonstrated conversion efficiencies of 20.3 % which is the highest amongst polycrystalline thin-film solar cell technologies. This thesis describes a study of excitonic recombination processes in high quality CuInSe₂, CuInS₂ and CuGaSe₂ single crystals using photoluminescence (PL) spectroscopy as a function of excitation power, temperature and applied magnetic field. Excitation power dependent measurements confirm the identification of the free excitons in the PL spectra of the three chalcopyrite semiconductor compounds. Additional sharp lines in the PL spectra appear to be due to the radiative recombination of excitons bound to shallow hydrogenic defects. PL lines due to excitons bound to more complex defects with a low concentration of defects are also found in CuInSe₂ and CuInS₂. Analysis of the temperature dependent PL spectra lead to activation energies of the free and bound excitons in CuInSe₂, CuInS₂ and CuGaSe₂. In addition, phonon energies have been obtained from the temperature dependence of the free exciton spectral positions and of the full width at half maximum. PL spectra measured in applied magnetic fields allow estimation of the diamagnetic shift rates for CuInSe₂, CuInS₂ and CuGaSe₂. A first-order perturbation model leads to values for the excitonic reduced masses and the effective hole masses can be estimated. For CuInSe₂ a theoretically predicted anisotropy of the effective hole masses is demonstrated. The study of the excitonic states in CuInSe₂, CuInS₂ and CuGaSe₂ provides a deeper understanding of the electronic material properties which can facilitate further improvements in solar cell efficiencies.

Material simulation using molecular dynamics (MD) at the quantum mechanical (QM) accuracy level has gained great interest in the community. However, the bottleneck arising from the O(N3) scaling of QM calculation has enormously limited its investigation scope. As an approach to address this issue, in this thesis, I proposed a machine-learning (ML) MD scheme based on Bayesian inference from CPU-intensive QM force database. In this scheme, QM calculations are only performed when necessary and used to augment the ML database for more challenging prediction case. The scheme is generally transferable to new chemical situations and database completeness is never required. To achieve the maximal ML eciency, I use a symmetrically reduced internal-vector representation for the atomic congurations. Signicant speed-up factor is achieved under controllable accuracy tolerance in the MD simulation on test case of Silicon at dierent temperatures. As the database grows in conguration space, the extrapolative capability systematically increases and QM calculations are nally not needed for simple chemical processes. In the on-the-y ML force calculation scheme, sorting/selecting out the closest data congurations is used to enhance the overall eciency to scale as O(N). The potential application of this methodology for large-scale simulation (e.g. fracture, amorphous, defect), where chemical accuracy and computational eciency are required at the same time, can be anticipated. In the context of fracture simulations, a typical multi-scale system, interesting events happen near the crack tips beyond the description of classical potentials. The simulation results by machine-learning potential derived from a xed database with no enforced QM accuracy inspire a theoretical model which is further used to investigate the atomic bond breaking process during fracture propagation as well as its relation with the initialised vibration modes, crack speed, and bonding structure.

No description available.

The binding energy of helium is calculated by the Ritz variational method using phenomenological two body interaction -containing a central and tenor forces of different ranges. The calculation is performed for all the sets of interaction parameters found by Feshbach and Schwinger (1951) to give the experimental deuteron binding energy end quadrupole moment; some of the sets of parameters are also consistent with the other low-energy to-body data. The wave function adopted is linear combination of a 1So wave function and six 5Do wave functions. Charge exchange and spin-orbit coupling are considered, and the present position of the" consistency" problem is examined.

Through the application of vertically oriented probes (VOPs) in a scattering evanescent wave (SEW) detection system the molecular resolution of SPM has been returned to the high temporal resolution, non-invasive regime of its optical microscopy forebear. The potential for VOP miniaturisation in the SEW system has been investigated using silvergallium needles and specifically designed, high-sensitivity cantilevers. These have spring constants as low 0.007 N/m and resonance frequencies up to 918 kHz in ambient conditions and 197 kHz in water. An optical feedback mechanism creates a hybrid microscopy, which presents a scanning plane at constant separation from the sample substrate with sub-nanometre precision in the vertical axis. Utilising this hybrid microscopy and high-sensitivity cantilevers, scan rates up to 8 frames per second in liquid environments were attained and lambda-DNA weakly bound to a mica substrate was resolved. The optical control presented is predicted to have a wide application in the measurement of surface and sample forces at the nano-scale. Indeed, it has already become the standard operating mode of all VOP experimentation within the Nano science group of Bristol University, due to its high degree of vertical control. Hybrid microscopy has answered questions about the formation and structure of self assembled cage-like structures (SAGEs), which had not been answerable by other microscopies currently available. The size of SAGEs formed in situ is characterized, their hollow structure confirmed and most dramatically, a predicted hexagonal ultra-structure of the particles is resolved in their natural liquid formation environment.

A hot gas jet was produced by an Argon plasma torch and was played on a water cooled rotating cylinder mounted 30 mm above the nozzle of the torch. The power and gas flow rate to the torch were varied. A Mach-Zehnder interferometer was used to form the interference fringe shift patterns. A version of the Abel Transformation was used to derive radial temperature distribution results from collimated measurements of fringe shift along parallel chords at points long the axis of the jet. The axial temperatures of the jet were found to be between 800 K and-8000-K. No results were possible in the area of the jet close to the cool surface due to turbulence in the fringe pattern. Relationships between input conditions to the torch and the temperatures in the jet were sought but no conclusions could be drawn due-to the severe limitations found in the analysis. In the range of power, 1.3 to 3 kW, and flow rate, 1.4 to 4.3 1/min, supplied to the torch, the number of fringe shifts observed in the interferograms was small, usually less than 2.5, making the fitting of fringe shift curves to the experimental data points uncertain. Tests undertaken fitting different shaped curves to a sample set of data caused 50-100% variations in the resulting radial temperature distribution. Slight variations in the radius of the jet measured from the interferograms caused large changes in on-axis temperature, though the distribution-towards the outer edge of the jet was unaffected. A similar-phenomenon was observed by changing the number of iteration points used in the numerical analysis. Areas for further work are identified and discussed, in the thesis.