Magnetic spheres in microwave cavities

We apply Mie scattering theory to study the interaction of magnetic spheres with microwaves in cavities beyond the magnetostatic and rotating wave approximations. We demonstrate that both strong and ultra-strong coupling can be realized for a stand alone magnetic spheres made from yttrium iron garnet (YIG), acting as an efficient microwave antenna. The eigenmodes of YIG spheres with radii of the order mm's display distinct higher angular momentum character that has been observed in experiments.Comment: 7 pages, 5 figure

Action recognition by graph embedding and temporal classifiers

University of Technology, Sydney. Faculty of Engineering and Information Technology.With the improved accessibility to an exploding amount of video data and growing demand in a wide range of video analysis applications, video-based action recognition becomes an increasingly important task in computer vision. Unlike most approaches in the literature which rely on bag-of-feature methods that typically ignore the structural information in the data, in this monograph we incorporate the spatial relationship and the time stamps in the data in the recognition and classification processes. We capture the spatial relationships in the subject performing the action by representing the actor’s shape in each frame with a graph. This graph is then transformed into a vector of real numbers by means of prototype-based graph embedding. Finally, the temporal structure between these vectors is captured by means of sequential classifiers. The experimental results on a well-known action dataset (KTH) show that, although the proposed method does not achieve accuracy comparable to that of the best existing approaches, these embedded graphs are capable of describing the deformable human shape and its evolution over time. We later propose an extended hidden Markov model, called the hidden Markov model for multiple, irregular observations (HMM-MIO), capable of fusing spatial information provided by graph embedding and the textural information of STIP descriptors. Experimental results show that recognition accuracy can be significantly improved by combining the spatio-temporal features with the structural information obtaining higher accuracy than from either separately. Furthermore, HMM-MIO is applied to the task of joint action segmentation and classification over a concatenated version of the KTH action dataset and the challenging CMU multi-modal activity dataset. The achieved accuracies proved comparable to or higher than state-of-the-art approaches and show the usefulness of the proposed model also for this task. The next and most remarkable contribution of this dissertation is the creation of a novel framework for selecting a set of prototypes from a labelled graph set taking class discrimination into account. Experimental results show that such a discriminative prototype selection framework can achieve superior results, not only for the task of human action recognition, but also in the classification of various structured data such as letters, digits, drawings, fingerprints compared to other well-established prototype selection approaches. Lastly, we change our focus from the forementioned problems to the recognition of complex event, which is a recent area of computer vision expanding the traditional boundaries of visual recognition. For this task, we have employed the notion of concept as an alternative intermediate representation with the aim of improving event recognition. We model an event by a hidden conditional random field and we learn its parameters by a latent structural SVM approach. Experimental results over video clips from the challenging TRECVID MED 2011 and MED 2012 datasets show that the proposed approach achieves a significant improvement in average precision at a parity of features and concepts

Long-range interactions between an atom in its ground S state and an open-shell linear molecule

Theory of long-range interactions between an atom in its ground S state and a linear molecule in a degenerate state with a non-zero projection of the electronic orbital angular momentum is presented. It is shown how the long-range coefficients can be related to the first and second-order molecular properties. The expressions for the long-range coefficients are written in terms of all components of the static and dynamic multipole polarizability tensor, including the nonadiagonal terms connecting states with the opposite projection of the electronic orbital angular momentum. It is also shown that for the interactions of molecules in excited states that are connected to the ground state by multipolar transition moments additional terms in the long-range induction energy appear. All these theoretical developments are illustrated with the numerical results for systems of interest for the sympathetic cooling experiments: interactions of the ground state Rb($^2$S) atom with CO($^3\Pi$), OH($^2\Pi$), NH($^1\Delta$), and CH($^2\Pi$) and of the ground state Li($^2$S) atom with CH($^2\Pi$).Comment: 30 pages, 3 figure

Optimal mode matching in cavity optomagnonics

Inelastic scattering of photons is a promising technique to manipulate magnons but it suffers from weak intrinsic coupling. We theoretically discuss an idea to increase optomagnonic coupling in optical whispering gallery mode cavities, by generalizing previous analysis to include the exchange interaction. We predict that the optomagnonic coupling constant to surface magnons in yttrium iron garnet (YIG) spheres with radius $300\,\mathrm{\mu}$m can be up to $40$ times larger than that to the macrospin Kittel mode. Whereas this enhancement falls short of the requirements for magnon manipulation in YIG, nanostructuring and/or materials with larger magneto-optical constants can bridge this gap.Comment: Comments welcom

Strain-induced topological phase transition in phosphorene and phosphorene nanoribbons

Using the tight-binding (TB) approximation with inclusion of the spin-orbit interaction, we predict a topological phase transition in the electronic band structure of phosphorene in the presence of axial strains. We derive a low-energy TB Hamiltonian that includes the spin-orbit interaction for bulk phosphorene. Applying a compressive biaxial in-plane strain and perpendicular tensile strain in ranges where the structure is still stable leads to a topological phase transition. We also examine the influence of strain on zigzag phosphorene nanoribbons (zPNRs) and the formation of the corresponding protected edge states when the system is in the topological phase. For zPNRs up to a width of 100 nm the energy gap is at least three orders of magnitude larger than the thermal energy at room temperature.Comment: 10 pages, 6 figure

Compressible flow structures interaction with a two-dimensional ejector: a cold-flow study

An experimental study has been conducted to examine the interaction of compressible flow structures such as shocks and vortices with a two-dimensional ejector geometry using a shock-tube facility. Three diaphragm pressure ratios ofP4 =P1 = 4, 8, and 12 have been employed, whereP4 is the driver gas pressure andP1 is the pressure within the driven compartment of the shock tube. These lead to incident shock Mach numbers of Ms = 1:34, 1.54, and 1.66, respectively. The length of the driver section of the shock tube was 700 mm. Air was used for both the driver and driven gases. High-speed shadowgraphy was employed to visualize the induced flowfield. Pressure measurements were taken at different locations along the test section to study theflow quantitatively. The induced flow is unsteady and dependent on the degree of compressibility of the initial shock wave generated by the rupture of the diaphragm

Application of AA-PSP to hypersonic flows: the double ramp model

Anodized Aluminium Pressure Sensitive Paint (AA-PSP) is known for its rapid response characteristics, making it a highly desirable technique when studying high-speed phenomenon on a global scale. The current study examines the efficacy of the AA-PSP technique, which is prepared with a more practical approach than that reported in literature, in analysing the flow characteristics of a double ramp model placed in hypersonic flow of M = 5. Three different flow angles of 0°, −2°, and −4° are studied. Two-dimensional colour schlieren visualisation, using a colour wheel, is employed alongside high sensitivity Kulite pressure tap data to corroborate the AA-PSP findings. The AA-PSP results show good correlation between the qualitative schlieren and ±8.9% discrepency with the quantitative pressure tap data. The more practical AA-PSP preparation proposed in the current study, which uses aluminium alloy 6-series rather than pure aluminium, is proved to have the response time and the accuracy to be applied to unsteady high-speed flows