Nanoscale Spectroscopic Characterization of InGaN/GaN Multiple Quantum Wells on GaN Nanorods

This thesis investigates the photophysics of InGaN/GaN multiple quantum wells (MQW) on top of GaN nanorods. InGaN/GaN MQW is a promising candidate material for high-performance light-emitting diodes and solar cells. In this thesis, InGaN/GaN MQW nanorods were fabricated by nanosphere lithography, a top-down method using reactive ion etching (RIE) of c-plane GaN with InGaN quantum wells. The nanorod arrays presents a hexagonal perodicity with uniform morphology. InGaN/GaN MQW nanorods demonstrate significantly improved optical and electronic properties compared to their planar counterparts. However, the exact nature of the processes whereby nanorod structures impact the carrier dynamics in InGaN quantum wells is not well understood. Using a confocal microscopy, associated with time-resolved spectroscopy, combining selective one- and two- photon excitations steady and time-resolved photoluminescence characterization provides detailed carrier dynamic analysis. The depth- and spatial-resolution at the nanoscale is helpful for understanding the optical properties of InGaN/GaN MQW nanorods. While nanostructured surfaces enhance luminescence performances of InGaN/GaN MQW, the increased surface defects impair the device performance, which is dealt with by surface treatments in this thesis. By studying the intensity-dependent PL of InGaN/GaN MQW, this thesis proves that photoexcited electrons and holes are strongly bound by Coulomb interactions (i.e., excitons) in planar MQWs due to the large exciton binding energy in InGaN quantum wells. In contrast, free electron-hole recombination becomes the dominant mechanism in nanorods, which is ascribed to efficient exciton dissociation. The nanorod sidewall provides an efficient pathway for exciton dissociation that significantly improves the optical performance of InGaN/GaN MQW. This thesis provides new insights into excitonic and charge carrier dynamics of quantum confined materials as well as the influence of surface states. The optical characterization techniques provide depth-resolved and time-resolved carrier dynamics with nanoscale spatially-resolved mapping, which is crucial for a comprehensive and thorough understanding of nanostructured materials

suCAQR: A Simplified Communication-Avoiding QR Factorization Solver Using the TBLAS Framework

The scope of this paper is to design and implement a scalable QR factorization solver that can deliver the fastest performance for tall and skinny matrices and square matrices on modern supercomputers. The new solver, named scalable universal communication-avoiding QR factorization (suCAQR), introduces a simplified and tuning-less way to realize the communication-avoiding QR factorization algorithm to support matrices of any shapes. The software design includes a mixed usage of physical and logical data layouts, a simplified method of dynamic-root binary-tree reduction, and a dynamic dataflow implementation. Compared with the existing communication avoiding QR factorization implementations, suCAQR has the benefits of being simpler, more general, and more efficient. By balancing the degree of parallelism and the proportion of faster computational kernels, it is able to achieve scalable performance on clusters of multicore nodes. The software essentially combines the strengths of both synchronization-reducing approach and communication-avoiding approach to achieve high performance. Based on the experimental results using 1,024 CPU cores, suCAQR is faster than DPLASMA by up to 30%, and faster than ScaLAPACK by up to 30 times

Multiple Sparse Measurement Gradient Reconstruction Algorithm for DOA Estimation in Compressed Sensing

A novel direction of arrival (DOA) estimation method in compressed sensing (CS) is proposed, in which the DOA estimation problem is cast as the joint sparse reconstruction from multiple measurement vectors (MMV). The proposed method is derived through transforming quadratically constrained linear programming (QCLP) into unconstrained convex optimization which overcomes the drawback that l1-norm is nondifferentiable when sparse sources are reconstructed by minimizing l1-norm. The convergence rate and estimation performance of the proposed method can be significantly improved, since the steepest descent step and Barzilai-Borwein step are alternately used as the search step in the unconstrained convex optimization. The proposed method can obtain satisfactory performance especially in these scenarios with low signal to noise ratio (SNR), small number of snapshots, or coherent sources. Simulation results show the superior performance of the proposed method as compared with existing methods

Experimental study on penetration of dental implants into the maxillary sinus in different depths

The exposing of dental implant into the maxillary sinus combined with membrane perforation might increase risks of implant failure and sinus complications. Objective: The purpose of this study was to investigate the effects of the dental implant penetration into the maxillary sinus cavity in different depths on osseointegration and sinus health in a dog model. Material and Methods: Sixteen titanium implants were placed in the bilateral maxillary molar areas of eight adult mongrel dogs, which were randomly divided into four groups according to the different penetrating extents of implants into the sinus cavities (group A: 0 mm; group B: 1 mm; group C: 2 mm; group D: 3 mm). The block biopsies were harvested five months after surgery and evaluated by radiographic observation and histological analysis. Results: No signs of inflammatory reactions were observed in any maxillary sinus of the eight dogs. The tips of the implants with penetrating depth of 1 mm and 2 mm were found to be fully covered with newly formed membrane and partially with new bone. The tips of the implants with penetrating depth over 3 mm were exposed in the sinus cavity and showed no membrane or bone coverage. No significant differences were found among groups regarding implant stability, bone-to-implant contact (BIC) and bone area in the implant threads (BA). Conclusions: Despite the protrusion extents, penetration of dental implant into the maxillary sinus with membrane perforation does not compromise the sinus health and the implant osseointegration in canine

Mining the Relationship between Emoji Usage Patterns and Personality

Emojis have been widely used in textual communications as a new way to convey nonverbal cues. An interesting observation is the various emoji usage patterns among different users. In this paper, we investigate the correlation between user personality traits and their emoji usage patterns, particularly on overall amounts and specific preferences. To achieve this goal, we build a large Twitter dataset which includes 352,245 users and over 1.13 billion tweets associated with calculated personality traits and emoji usage patterns. Our correlation and emoji prediction results provide insights into the power of diverse personalities that lead to varies emoji usage patterns as well as its potential in emoji recommendation tasks.Comment: To appear at The International AAAI Conference on Web and Social Media (ICWSM) 201

Actively controllable topological phase transition in phononic beam systems

Topological insulators, which allow edge or interface waves but forbid bulk waves, have revolutionized our scientific cognition of acoustic/elastic systems. Due to their nontrivial topological characteristics, edge (interface)waves are topologically protected against defects and disorders. This superior and unique characteristic could lead to a wealth of new opportunities in applications of quantum and acoustic/elastic information processing. However, current acoustic/elastic topological insulators are still at an infancy stage where the theory and prediction only work in laboratories and there are still many problems left open before promoting their practical applications. One of the apparent disadvantages is their narrow working frequency range, which is the main concern in this paper. We design a one-dimensional phononic beam system made of a homogeneous epoxy central beam sandwiched by two homogeneous piezoelectric beams, and covered with extremely thin electrodes, periodically and separately placed. These electrodes are connected to external electric circuits with negative capacitors. We show that a topological phase transition can be induced and tuned by changing the values of the negative capacitors. It follows that the working frequency of the topologically protected interface mode can be widely changed, such that the working frequency range of the topological insulator can be considerably `broadened'. This intelligent topological device may also find wide applications in intelligent technologies that need controllable information processing of high precision

Observing Parity Time Symmetry Breaking in a Josephson Parametric Amplifier

A coupled two-mode system with balanced gain and loss is a paradigmatic example of an open quantum system that can exhibit real spectra despite being described by a non-Hermitian Hamiltonian. We utilize a degenerate parametric amplifier operating in three-wave mixing mode to realize such a system of balanced gain and loss between the two quadrature modes of the amplifier. By examining the time-domain response of the amplifier, we observe a characteristic transition from real-to-imaginary energy eigenvalues associated with the Parity-Time-symmetry-breaking transition.Comment: 6 pages, 4 figure