Tunable Enhancement of Light-Matter Interaction in Graphene Heterostructures

Optoelectronic applications and the exploration of 2D light-matter interactions often require an increase on the bare light absorption of ultra-thin 2D materials (2DM)s, such as graphene. Light absorption by these 2DMs can be enhanced by their incorporation in planar heterostructures which can act as optical interferometric cavities. Furthermore, by fabricating an optical cavity with a suspended 2DM, we can tune its light absorption by tuning the thickness of the air spacer (h_air). We report the development of an all-dry deterministic transfer technique that is capable of suspending ultra-thin 2DMs in order to fabricate suspended optical cavities, and also the ability to tune the light absorption of said cavities in-situ by the electrostatic tuning of h_air. We studied the absorption of visible light in suspended bilayer graphene (BLG) through Raman spectroscopy, which can distinguish the light scattering and absorption of the graphene layer from the rest of the heterostructure. We first adapted models for the exclusive light absorption and the Raman scattering enhancement (Raman factor) of arbitrary planar 2DM heterostructures using Fresnel equations. In order to fabricate pristine, suspended heterostructures, we developed a transfer method which capitalizes on the softness and adhesion of a nitrocellulose micro-stamp to dry-pickup and deterministically transfer 2DMs. This transfer method proved capable of fabricating on-substrate (95% success) and suspended (93%) optical cavities. Raman measurements on a partially-suspended BLG optical cavity agreed with our Raman factor model and demonstrated our ability to tune Raman factor (3.8x) and light absorption (6.3x) over a cavity thickness varying from 0 - 150 nm. We then sought to further enhance and tune Raman factor in-situ through the addition of an aluminium (Al) back-plane mirror, which can also act as a gate electrode to electrostatically tune h_air for suspended devices. On-substrate BLG optical cavity devices with an Al back-plane mirror demonstrated that Raman scattering can be tuned by a factor of 19 with cavity thickness differences on the order of 75 nm. We next assembled an optical cavity by suspending BLG over Al, whose Raman measurements at V_g = 0 V demonstrated that Raman factor can be tuned 1.8x over a 10 nm change of h_air in the same device. Once applying a gate voltage, our final measurements demonstrated an ability to electrostatically tune Raman factor by 20% and h_air by 3 nm with the application of 500 mV. These results pave the way for further research into the in-situ tunability of light-matter interactions for enhanced light absorption at higher gate voltages

Debris discs around nearby Solar analogues

An unbiased search for debris discs around nearby Sun-like stars is reported. Thirteen G-dwarfs at 12-15 parsecs distance were searched at 850 \umum wavelength, and a disc is confirmed around HD 30495. The estimated dust mass is 0.008 M$_{\oplus}$ with a net limit \la 0.0025 M$_{\oplus}$ for the average disc of the other stars. The results suggest there is not a large missed population of substantial cold discs around Sun-like stars -- HD 30495 is a bright rather than unusually cool disc, and may belong to a few hundred Myr-old population of greater dust luminosity. The far-infared and millimetre survey data for Sun-like stars are well fitted by either steady state or stirred models, provided that typical comet belts are comparable in size to that in the Solar System.Comment: published in MNRA

Practical investigations in robot localization using ultra-wideband sensors

Robot navigation is rudimentary compared to the capabilities of humans and animals to move about their environments. One of the core processes of navigation is localization, the problem of answering where one is at the present time. Robot localization is the science of using various sensors to inform a robot of where it is within its environment. Ultra-wideband (UWB) radio is one such sensor technology that can return absolute position information. The algorithm to accomplish this is known as multilateration, which uses a collection of distance measurements between multiple robot tag and environment anchor pairs to calculate the tag’s position. UWB is especially suited to the task of returning precise distance measurements due to its capabilities of short duration, high amplitude pulse generation and detection. Decawave Ltd. has created an UWB integrated circuit to perform ranging and a suite of products to support this technology. Claimed and verified accuracies using this implementation are on the order of 10cm. This thesis describes various experiments carried out using Decawave technology for robot localization. The progression of the chapters starts with commercial product verification before moving into development and testing in various environments of an open-source driver package for the Robot Operating System (ROS), then the development of a novel phase difference of arrival (PDoA) sensor for three-dimensional robot localization without an UWB anchor mesh, before concluding with future research directions and commercialization potential of UWB. This thesis is designed as a compilation of all that the author has learned through primary and secondary research over the past three years of investigation. The primary contributions are: 1. A modular ROS UWB driver framework and series of ROS bags for offline experimentation with multilateration algorithms. 2. A robust ROS framework for comparing motion capture system (MoCap) ground truth vs sensor data for rigorous statistical analysis and characterization of multiple sensors. 3. Development of a novel UWB PDoA sensor array and data model to allow 3D localization of a target from a single point without the deployment of an antenna mesh

Sources of Spring and Fall Hop Aphid, Phorodon humuli (Schrank), (Homoptera: Aphididae) Migrants in South Central Washington

The hop aphid, Phorodon humuli (Schrank), flies from hop, Humulus lupulus L., to its overwintering Prunus spp. hosts in the fall. The sources of these aphids were not known because much of the aphid flight occurs after hop plants are removed from fields during harvest. We found that the bottoms of hop plants remaining alive in harvested hop yards averaged 1.7 to 5.8 hop aphids per leaf in three years of sampling. Unharvested hop plants remaining after harvest averaged 32.8 to 127.1 aphids per leaf in two years. Feral hops were also infested with hop aphids in late summer and early fall. Sources for the spring aphid flight from Prunus spp. to hop included Prunus cerasifera Ehrhart, which averaged 44.0 to 105.1 aphids per shoot in two years of sampling. Fruit-type Prunus spp. trees growing on residential properties averaged 0.9 and 11.3 aphids per shoot in the same years but few of these trees were found. Plum and prune orchards averaged 0 to 5.5 aphids per shoot in two years and estimates indicate that orchard trees are much more numerous than other hop aphid host trees. Potential alternative management strategies for hop aphid control are discussed

Improved statistical methods for estimating infestation rates in quarantine research when hosts are naturally infested

Trading partners often require phytosanitary or quarantine treatments for fresh horticultural produce to ensure no economically important pest species are moved with the imported product. When developing such treatments, it is essential that the level of treatment efficacy can be determined. This is often based on the mortality of the total number of target pests exposed to treatment, but in naturally infested products this number is not always known. In such cases, the infestation rate and subsequently an estimate of the number of pests are obtained directly from a set of untreated control samples of the host product. The International Plant Protection Convention (IPPC) Secretariat has provided 2 formulas for these situations that place an interval around the point estimate obtained from the control samples to obtain an estimate of the infestation rate. However, these formulas do not allow a confidence level to be assigned to the estimate, and there are concerns with the assumptions regarding the distribution and the measure of variability used in the formulas. In this article, we propose 2 alternative formulas. We propose that the lower one-sided confidence limit should be applied to all infestation datasets that are approximately normally distributed. As infestation data are sometimes skewed, it is proposed the lower one-sided modified Cox confidence limit is applied to data approximately log-normal distributed. These well-recognized formulas are compared to the formulas recommended by the IPPC and applied to 3 datasets involving natural infestation

Debris disc candidates in systems with transiting planets

Debris discs are known to exist around many planet-host stars, but no debris dust has been found so far in systems with transiting planets. Using publicly available catalogues, we searched for infrared excesses in such systems. In the recently published Wide-Field Infrared Survey Explorer (WISE) catalogue, we found 52 stars with transiting planets. Two systems with one transiting "hot Jupiter" each, TrES-2 and XO-5, exhibit small excesses both at 12 and 22 microns at a > 3 sigma level. Provided that one or both of these detections are real, the frequency of warm excesses in systems with transiting planets of 2-4 % is comparable to that around solar-type stars probed at similar wavelengths with Spitzer's MIPS and IRS instruments. Modelling suggests that the observed excesses would stem from dust rings with radii of several AU. The inferred amount of dust is close to the maximum expected theoretically from a collisional cascade in asteroid belt analogues. If confirmed, the presence of debris discs in systems with transiting planets may put important constraints onto formation and migration scenarios of hot Jupiters.Comment: Accepted for publication in MNRAS Letter