Direct Observation of Early-stage Quantum Dot Growth Mechanisms with High-temperature Ab Initio Molecular Dynamics

Colloidal quantum dots (QDs) exhibit highly desirable size- and shape-dependent properties for applications from electronic devices to imaging. Indium phosphide QDs have emerged as a primary candidate to replace the more toxic CdSe QDs, but production of InP QDs with the desired properties lags behind other QD materials due to a poor understanding of how to tune the growth process. Using high-temperature ab initio molecular dynamics (AIMD) simulations, we report the first direct observation of the early stage intermediates and subsequent formation of an InP cluster from separated indium and phosphorus precursors. In our simulations, indium agglomeration precedes formation of In-P bonds. We observe a predominantly intercomplex pathway in which In-P bonds form between one set of precursor copies while the carboxylate ligand of a second indium precursor in the agglomerated indium abstracts a ligand from the phosphorus precursor. This process produces an indium-rich cluster with structural properties comparable to those in bulk zinc-blende InP crystals. Minimum energy pathway characterization of the AIMD-sampled reaction events confirms these observations and identifies that In-carboxylate dissociation energetics solely determine the barrier along the In-P bond formation pathway, which is lower for intercomplex (13 kcal/mol) than intracomplex (21 kcal/mol) mechanisms. The phosphorus precursor chemistry, on the other hand, controls the thermodynamics of the reaction. Our observations of the differing roles of precursors in controlling QD formation strongly suggests that the challenges thus far encountered in InP QD synthesis optimization may be attributed to an overlooked need for a cooperative tuning strategy that simultaneously addresses the chemistry of both indium and phosphorus precursors.Comment: 40 pages, 9 figures, submitted for publicatio

An Analysis of the Effects of Low Energy Electron Radiation of Al\u3csub\u3ex\u3c/sub\u3eGa\u3csub\u3e1-x\u3c/sub\u3eN/GaN Modulation-Doped Field-Effect Transistors

The effects of radiation on AlxGa1-xN/GaN MODFETs is an area of increasing interest to the USAF as these devices become developed and integrated in satellite-based systems Irradiation is also a valuable tool for analyzing the quantum-level characteristics and properties that are responsible for device operation AlxGa1-xN/GaN MODFETs were fabricated and irradiated at liquid nitrogen temperatures by 0,45-1,2MeV electrons up to doses of 6*1016 e/cm2. Following irradiation, low temperature I-V measurements were recorded providing dose-dependent measurements Temperature-dependent I-V measurements were also made during room temperature annealing following irradiation I-V measurements indicate radiation-induced changes occur in these devices creating increased gate and drain currents These increased currents are only maintained at low temperatures (T \u3c 300 K), It is believed that the increase in gate current is caused by an increase in the electron trap concentration of the AlxGa1-xN/GaN layer, This increase in trap concentration directly increases the trap-assisted tunneling current resulting in the observed increase in gate current The mechanism causing the increase in drain current is unknown, Several theories explaining this increase are presented along with the additional research necessary to illuminate the correct theory, This is the first experiment involving electron radiation of AlxGa1-xN/GaN MODFETs

Magnetic trapping of an ultracold (^87)Rb -(^133)Cs atomic mixture

This thesis reports on the realisation and characterisation of a magnetically trapped ultracold atomic mixture of (^87)Rb and (^133)Cs in the F = 1, m(_F) = -1 and F = 3, m(_F) = - 3 hyperfine states respectively. A compact two-species double magneto-optical trapping (MOT) apparatus is constructed in which a pyramid MOT acts to provide an independent flux of both atomic species for capture in the ultra-high vacuum science region of the apparatus. For the two-species science MOT in which this atom flux is captured, interspecies light assisted inelastic collisions are found to be a highly significant loss mechanism. A novel optical pressure spatial displacement technique is developed to minimise such losses, allowing near independent simultaneous loading of up to ~ 8 x 10(^8) (^87)Rb and ~ 3 x 10(^8) (^133)Cs atoms into an Ioffe-Pritchard 'baseball' magnetic trap at magnetic biasfields of 166.70(6) and 165.50(6) G respectively. At the loaded 87Rb and 133Cs atom number densities of 1.78(6) x 10(^9) and 2.53(6) X 10(^9) cm-3 respectively the magnetic trap lifetime of each atomic species is shown to be 100(10) s and independent of the presence of the second atomic species. Radio-frequency evaporative cooling trajectories for (^87)Rb and (^133)Cs of 129 s duration are separately optimised under single species magnetic trap operation to achieve phase-space densities of 6(1) x 10(^-7) and 3(1) X 10(^-4) respectively at temperatures of 7.6(1) μK and 520(10) nK.(^133)Cs Feshbach resonances at 118.06(8) and 133.4(1) G are characterised through the measurement of magnetic field dependent losses at the increased phase-space density. Implementation of simultaneous evaporative cooling following the single species trajectories is found to be ineffective below ~10 μK due to the increased thermal load imposed upon the (^133)Cs atoms as the(^87)Rb single species elastic collision cross section approaches the low energy limit. Following simultaneous evaporation to ~ 15 μK thermalisation of the mixtures axial and radial temperature components suggests a (^87)Rb-(^133)Cs interspecies elastic collision rate 3(1) and 7(1) times greater than the calculated single species (^133)Cs and (^87)Rb elastic collision rates respectively. An interspecies Feshbach resonance search is undertaken by measuring the number of atoms of each species remaining in the magnetic trap as a function of applied magnetic field following simultaneous evaporation. The absence of magnetic field dependent losses in conjunction with analysis of the measurement sensitivity demonstrates that no interspecies Feshbach resonances wider than 1 G with two-body inelastic collision rate constants greater than 5 X 10(^-10) cm(^3) s(^-1) are present over the magnetic field range 166 < B < 370 G in the trapped states. The sensitivity of this measurement is found to be highly dependent upon the magnetic field induced differential gravitational sag of the mixtures components

Atom photon interfaces using fabricated spherical mirrors

Ph.DDOCTOR OF PHILOSOPH

Light-Assisted Collisions and Quantum State Tomography of Single-Atom Motion in Optical Tweezers

This thesis presents experiments with single ${}^{87}\text{Rb}$~atoms trapped in arrays of tightly confining optical tweezers. Optical tweezer arrays are a pioneering new platform for studying ultracold atomic physics and quantum computation. I begin by presenting our recent work in the preparation of large atom arrays and the quantum motional control of single atoms. Then, I present a new optical tweezer loading technique and our studies of the processes by which single atoms can be loaded near-deterministically into the microtraps. These processes depend sensitively on the long-range molecular physics of pairs of atoms, so I additionally discuss our theoretical calculations of the molecular potential landscapes relevant for such collisions. Finally, I present our all-mechanical time-of-flight-based quantum tomography of atoms in carefully prepared excited motional states of the optical tweezers

Advances in photostop

This thesis is an expansion on previous work using the photostop technique for the production of near-zero velocity atoms and molecules. The goal is to produce stopped SH molecules and trap them in a permanent magnetic trap and the aim of this project was to construct a new experimental apparatus to accomplish this. During initial tests of the apparatus, the Rayleigh scattering cross-section of N2 was measured to provide a reference point for future experiments. The uncertainty and systematic errors in the measurements was such that denitive quantitative results of this were not be obtained at this stage. The emerging technique of cavity-enhanced laser-induced uorescence (CELIF) was used to perform absolute number density measurements of a molecular beam of SO2. CELIF was then applied to measuring the photostop of SD/SH. This showed that CELIF would not have the required sensitivity to measure the trapped SD/SH molecules due to issues of stray light from the lasers. As a result of this we elected to use resonance-enhanced multi-photon ionisation (REMPI) as an alternative. We devised and constructed a novel ion extraction system for use in performing REMPI, which was based on a time-of- ight mass spectroscopy system, but utilising the magnets themselves as electrodes, as well as some ion lensing components. This was initially tested using Xe, showing a strong signal and good mass resolution. Using this, the photostop of SH and S was measured showing that the detection apparatus is able to distinguish signal over a range of 9 orders of magnitude. However, despite this sensitivity, the trapping of these stopped molecules could not initially be demonstrated as the signal from these stopped molecules was obscured by signal from the inadvertent dissociation of the background parent molecules by the probe laser. More recent measurements in the group have directly addressed this issue with background subtraction and the results have now demonstrated the trapping of SH. Signicant headway has been made in the demonstration of the trapping of SH produced by photostop. From the results produced using REMPI the detection limit has improved signicantly over the prior experiments and very recent measurements have successfully demonstrated the trapping of SH

Contactless quantum non-linear optics with cold Rydberg atoms

Rydberg quantum optics achieves optical non-linearities at the single-photon level by mapping the strong dipolar interactions between Rydberg atoms in cold atomic gases onto light fields using electromagnetically-induced transparency and photon storage. The non-linearities are a direct consequence of the long-range character of the interaction which allows a single photon to modify the optical response in a volume containing many atoms. In this thesis, the long-range character of the resulting effective photon-photon interaction is directly observed as photons propagating in non-overlapping optical modes are stored as collective Rydberg excitations in adjacent and non-overlapping microscopic clouds of 87Rb atoms. While stored, van-der-Waals interactions imprint spatially non-uniform phase shifts in the collective excitations. These distort the photons' retrieval modes resulting in anti-correlated retrieval between the original modes.In this first demonstration of contactless effective interactions between photons, these effects are observed between photons separated by more than 15 times their wavelength, well above the optical diffraction limit.This represents a promising step towards the implementation of scalable, multichannel quantum optical devices such as quantum gates. The experiments are enabled by a new, specialised experimental setup centred around a pair of in-vacuo aspheric lenses. These provide optical resolution of order 1 µm to optically trap and address the ensembles separated by distances well below the range of Rydberg interactions. The ensembles are prepared in approximately 100 ms thanks to efficient loading of a magneto-optical trap (MOT) from an atomic beam produced by a 2D MOT. Combined with the ability to recycle the ensembles > 20000 times, effective cycle times exceeding 100 kHz enable the acquisition of large datasets for the analysis of photon statistics within a matter of minutes