Synchronization of spatiotemporal semiconductor lasers and its application in color image encryption

Optical chaos is a topic of current research characterized by high-dimensional nonlinearity which is attributed to the delay-induced dynamics, high bandwidth and easy modular implementation of optical feedback. In light of these facts, which adds enough confusion and diffusion properties for secure communications, we explore the synchronization phenomena in spatiotemporal semiconductor laser systems. The novel system is used in a two-phase colored image encryption process. The high-dimensional chaotic attractor generated by the system produces a completely randomized chaotic time series, which is ideal in the secure encoding of messages. The scheme thus illustrated is a two-phase encryption method, which provides sufficiently high confusion and diffusion properties of chaotic cryptosystem employed with unique data sets of processed chaotic sequences. In this novel method of cryptography, the chaotic phase masks are represented as images using the chaotic sequences as the elements of the image. The scheme drastically permutes the positions of the picture elements. The next additional layer of security further alters the statistical information of the original image to a great extent along the three-color planes. The intermediate results during encryption demonstrate the infeasibility for an unauthorized user to decipher the cipher image. Exhaustive statistical tests conducted validate that the scheme is robust against noise and resistant to common attacks due to the double shield of encryption and the infinite dimensionality of the relevant system of partial differential equations.Comment: 20 pages, 11 figures; Article in press, Optics Communications (2011

Enhanced image security using residue number system and new Arnold transform

This paper aims to improve the image scrambling and encryption effect in traditional two-dimensional discrete Arnold transform by introducing a new Residue number system (RNS) with three moduli and the New Arnold Transform. The study focuses on improving the classical discrete Arnold transform with quasi-affine properties, applying image scrambling and encryption research. The design of the method is explicit to three moduli set {2n, 2n+1+1, 2n+1-1}. These moduli set includes equalized and shapely moduli leading to the effective execution of the residue to binary converter. The study employs an arithmetic residue to the binary converter and an improved Arnold transformation algorithm. The encryption process uses MATLAB to accept a digital image input and subsequently convert the image into an RNS representation. The images are connected as a group. The resulting encrypted image uses the Arnold transformation algorithm. The encrypted image is used as input at decryption using the anti-Arnold (Reverse Arnold) transformation algorithm to convert the picture to the original RNS (original pixel value). Then the RNS was used to retransform the original RNS to its binary form. Security analysis tests, like histogram analysis, keyspace, key sensitivity, and correlation coefficient analysis, were administered on the encrypted image. Results show that the hybrid system can use the improved Arnold transform algorithm with better security and no constraint on image width and size

Large-scale physically accurate modelling of real proton exchange membrane fuel cell with deep learning

Proton exchange membrane fuel cells, consuming hydrogen and oxygen to generate clean electricity and water, suffer acute liquid water challenges. Accurate liquid water modelling is inherently challenging due to the multi-phase, multi-component, reactive dynamics within multi-scale, multi-layered porous media. In addition, currently inadequate imaging and modelling capabilities are limiting simulations to small areas (<1 mm2) or simplified architectures. Herein, an advancement in water modelling is achieved using X-ray micro-computed tomography, deep learned super-resolution, multi-label segmentation, and direct multi-phase simulation. The resulting image is the most resolved domain (16 mm2 with 700 nm voxel resolution) and the largest direct multi-phase flow simulation of a fuel cell. This generalisable approach unveils multi-scale water clustering and transport mechanisms over large dry and flooded areas in the gas diffusion layer and flow fields, paving the way for next generation proton exchange membrane fuel cells with optimised structures and wettabilities

Tracing back the source of contamination

From the time a contaminant is detected in an observation well, the question of where and when the contaminant was introduced in the aquifer needs an answer. Many techniques have been proposed to answer this question, but virtually all of them assume that the aquifer and its dynamics are perfectly known. This work discusses a new approach for the simultaneous identification of the contaminant source location and the spatial variability of hydraulic conductivity in an aquifer which has been validated on synthetic and laboratory experiments and which is in the process of being validated on a real aquifer

Next-generation single-photon sources using two-dimensional hexagonal boron nitride

With the second quantum revolution unfolding, the realization of optical quantum technologies will transform future information processing, communication, and sensing. One of the crucial building blocks of quantum information architectures is a single-photon source. Promising candidates for such quantum light sources are quantum dots, trapped ions, color centers in solid-state crystals, and sources based on heralded spontaneous parametric down-conversion. The recent discovery of optically active defects hosted by 2D materials has added yet another class to the solid-state quantum emitters. Stable quantum emitters have been reported in semiconducting transition metal dichalcogenides (TMDs) and in hexagonal boron nitride (hBN). Owing to the large band gap, the energy levels of defects in hBN are well isolated from the band edges. In contrast to TMDs, this allows for operation at room temperature and prevents non-radiative decay, resulting in a high quantum yield. Unlike NV centers in diamond and other solid-state quantum emitters in 3D systems, the 2D crystal lattice of hBN allows for an intrinsically ideal extraction efficiency. In this thesis, advances in developing this new type of emitter are described. In the first experiment, quantum emitters hosted by hBN are attached by van der Waals force to the core of multimode fibers. The system features a free space and fiber-coupled single-photon generation mode. The results can be generalized to waveguides and other on-chip photonic quantum information processing devices, thus providing a path toward integration with photonic networks. Next, the fabrication process, based on a microwave plasma etching technique, is substantially improved, achieving a narrow emission linewidth, high single-photon purity, and a significant reduction of the excited state lifetime. The defect formation probability is influenced by the plasma conditions, while the emitter brightness correlates with the annealing temperature. Due to their low size, weight and power requirements, the quantum emitters in hBN are promising candidates as light sources for long-distance satellite-based quantum communication. The next part of this thesis focuses on the feasibility of using these emitters as a light source for quantum key distribution. The necessary improvement in the photon quality is achieved by coupling an emitter with a microcavity in the Purcell regime. The device is characterized by a strong increase in spectral and single-photon purity and can be used for realistic quantum key distribution, thereby outperforming efficient state-of-the-art decoy state protocols. Moreover, the complete source is integrated on a 1U CubeSat, a picoclass satellite platform encapsulated within a cube of length 10cm. This makes the source among the smallest, fully self-contained, ready-to-operate single-photon sources in the world. The emitters are also space-qualified by exposure to ionizing radiation. After irradiation with gamma-rays, protons and electrons, the quantum emitters show negligible change in photophysics. The space certification study is also extended to other 2D materials, suggesting robust suitability for use of these nanomaterials for space instrumentation. Finally, since the nature of the single-photon emission is still debated and highly controversial, efforts are made to locate the defects with atomic precision. The positions at which the defects form correlate with the fabrication method. This allows one to engineer the emitters to be close to the surface, where high-resolution electron microscopy can be utilized to identify the chemical defect. The results so far prove that quantum emitters in hBN are well suited for quantum information applications and can also be integrated on satellite platforms. A device based around this technology would thus provide an excellent building block for a worldwide quantum internet, where metropolitan fiber networks are connected through satellite relay stations

Assessing coupled mechanical behavior and environmental degradation at submicron scales

Mechanical and electromechanical properties, deformation and fracture mechanisms, and environmental resistance of materials at submicron scales have been investigated through the combination of nanomechanical testing, high resolution microscopy, diffraction, and electrochemical testing. Nanomechanical techniques were used to isolate environmental, orientation, and size effects. Material evaluation focuses on metals, both model and engineering alloys, in bulk and thin-film form as well as oxide-substrate systems. Yield behavior of Ni 200, a model material, depends on sampled volume size, orientation, and surface preparation. Exposure to high-pressure hydrogen gas is also found to impact incipient plasticity and mechanical properties of commercially pure Ni 201. Nanomechanical testing of oxide-substrate systems can be used to study coupling of environment and size effects. Investigation of films grown on 304L stainless steel and commercially pure grade II Ti via nanosecond pulsed laser irradiation has enabled isolation of film fracture behavior and the effect of processing on mechanical and electromechanical properties. Additionally, laser processing causes substrate composition gradients that limit environmental stability. Combining techniques provides a unique approach for understanding and improving materials reliability in harsh environments

Effects of Plastic Deformation From Ultrasonic Additive Manufacturing

Nuclear energy technology can be exponentially advanced using advanced manufacturing, which can drastically transform how materials, structures, and designs can be built. Ultrasonic Additive Manufacturing (UAM) represents one of the four main additive manufacturing methods, although it is also the newest. As UAM technology and applications develop, a fundamental understanding of the bonding mechanism is crucial to fully realize its potential. Currently UAM bonding is considered to occur through breaking down surface asperities and removing surface oxides. Plastic deformation occurs although its role is currently unclear. This research analyzes material configurations in a variety of geometries, with similar and dissimilar material interfaces, and with pure metals and complex engineered materials. A variety of characterization techniques were used to develop a general description that UAM bonding requires plastic deformation. First, we analyzed various dissimilar material interfaces created between UAM foils and the coating of embedded optical fibers. Enhanced interdiffusion of elements was found beyond that expected from the thermal profile experienced during bonding. This interdiffusion was rationalized based on enhanced point defect vacancies creating additional diffusion pathways. Following on this study, we analyzed the local strengthening at one of these interfaces. These interfaces strengthened through a complex interaction dominated by dislocation forest hardening, reduced grain sizes, and vacancy clusters created by the agglomeration of vacancies. UAM bonding of pre-treated Al 6061 was also performed and analyzed using multi-length scale characterization. Macroscale strengthening was observed as well as foil-foil interface strengthening. This was a result of dynamic recrystallization, dynamic recovery, adiabatic heating, and precipitate dissolution (as the vacancies allowed enhanced diffusion of elements). Finally, UAM bonding of titanium was analyzed. The HCP phase of titanium significantly resisted plastic deformation, which resulted in a phase transformation to the BCC phase, which was stabilized by the introduction of certain stabilizing elements. The strain induced phase transformation and enhanced vacancy driven interdiffusion were utilized to demonstrate a viable method of improving UAM bonding by focusing on the plastic deformation requirement. The phenomena outlined in this research demonstrates an improvement in our understanding of the fundamental bonding requirements of UAM, and deformation induced vacancy formation

Characterization of low conductivity wide band gap semiconductors

This thesis covers research on low electric conductivity wide band gap semiconductors of the group-III nitride material system. The work presented focussed on using multi-mode scanning electron microscope (SEM) techniques to investigate the luminescence properties and their correlation with surface effects, doping concentration and structure of semiconductor structures.The measurement techniques combined cathodoluminescence (CL) for the characterization of luminescence properties, secondary electron (SE) imaging for imaging of the morphology and wavelength dispersive X-ray (WDX) spectroscopy for compositional analysis. The high spatial resolution of CL and SE-imaging allowed for the investigation of nanometer sized features, whilst environmental SEM allowed the characterisation of low conductivity samples.The investigated AlₓGa₁₋ₓN samples showed a strong dependence on the miscut of the substrate, which was proven to influence the surface morphology and the compositional homogeneity. Studying the influence of the AlₓGa₁₋ₓN sample thickness displayed a reduced strain in the samples with increasing thickness as well as an increasing crystalline quality. The analysis of AlₓGa₁₋ₓN:Si samples showed the incorporation properties of Si in AlₓGa₁₋ₓN, the correlation between defect luminescence, Si concentration and resistivity as well as the influence of threading dislocations on the luminescence properties and incorporation of point defects.The characterization of UV-LED structures demonstrated that a change in the band structure is one of the main reasons for a decreasing output power in AlₓGa₁₋ₓN based UV-LEDs. In addition the dependence of the luminescence properties and crystalline quality of InₓAl₁₋ₓN based UV-LEDs on various growth parameters (e.g. growth temperature, quantum well thickness) was investigated.The study of nanorods revealed the influence of the template on the compositional homogeneity and luminescence of InₓAl₁₋ₓN nanorod LEDs. Furthermore,the influence of optical modes in these structures was studied and found to provide an additional engineering parameter for the design of nanorod LEDs.This thesis covers research on low electric conductivity wide band gap semiconductors of the group-III nitride material system. The work presented focussed on using multi-mode scanning electron microscope (SEM) techniques to investigate the luminescence properties and their correlation with surface effects, doping concentration and structure of semiconductor structures.The measurement techniques combined cathodoluminescence (CL) for the characterization of luminescence properties, secondary electron (SE) imaging for imaging of the morphology and wavelength dispersive X-ray (WDX) spectroscopy for compositional analysis. The high spatial resolution of CL and SE-imaging allowed for the investigation of nanometer sized features, whilst environmental SEM allowed the characterisation of low conductivity samples.The investigated AlₓGa₁₋ₓN samples showed a strong dependence on the miscut of the substrate, which was proven to influence the surface morphology and the compositional homogeneity. Studying the influence of the AlₓGa₁₋ₓN sample thickness displayed a reduced strain in the samples with increasing thickness as well as an increasing crystalline quality. The analysis of AlₓGa₁₋ₓN:Si samples showed the incorporation properties of Si in AlₓGa₁₋ₓN, the correlation between defect luminescence, Si concentration and resistivity as well as the influence of threading dislocations on the luminescence properties and incorporation of point defects.The characterization of UV-LED structures demonstrated that a change in the band structure is one of the main reasons for a decreasing output power in AlₓGa₁₋ₓN based UV-LEDs. In addition the dependence of the luminescence properties and crystalline quality of InₓAl₁₋ₓN based UV-LEDs on various growth parameters (e.g. growth temperature, quantum well thickness) was investigated.The study of nanorods revealed the influence of the template on the compositional homogeneity and luminescence of InₓAl₁₋ₓN nanorod LEDs. Furthermore,the influence of optical modes in these structures was studied and found to provide an additional engineering parameter for the design of nanorod LEDs