Nonlinear Shift Research Articles

Negative Refraction in Mechanical Rotator Lattices

This paper presents numerical and experimental investigations of negative refraction at the interface between two mechanical rotator lattices, adding to the recent body of anomalous wave behavior reported for such simple lattices. Each lattice implements easily reconfigured interrotator coupling which determines whether the lattice's passband is acoustic (with positive group velocity) or optic (with negative group velocity). Since the group velocities have opposite signs over the entirety of both lattices' Brillouin zones, the negative refraction at their interface is inherently broadband. The numerical study constructs a large lattice structure for full-wave simulation and quantitatively analyzes the linear and nonlinear negative refraction. The experimental study documents robust negative refraction in a smaller-scale fabricated system and serves as validation for the numerical findings. We observe frequency-dependent transmission in both simulations and experiments. A linear analysis captures the observed phenomenon at low amplitude. At larger amplitudes, numerical simulations are used to document amplitude-dependent transmission. This phenomenon is explained by a nonlinear dispersion shift and transitional evanescent waves analyzed using a perturbation method. A sensitivity test demonstrates the robustness of negative refraction in the proposed rotator lattice structure.

Learning to restore multiple image degradations simultaneously

Image corruptions are common in the real world, for example images in the wild may come with unknown blur, bias field, noise, or other kinds of non-linear distributional shifts, thus hampering encoding methods and rendering downstream task unreliable. Image upgradation requires a complicated balance between high-level contextualised information and spatial specific details. Existing approaches to solving the problems are designed to focus on single corruption, which unavoidably results in poor performance when the acquisitions suffer from multiple degradations. In this study, we investigate the possibility of handling multiple degradations and enhancing the quality of images via deblurring, bias field correction, and denoising. To tackle the problems with propagating errors caused by independent learning, we propose a unified and scalable framework, which consists of three special decoders. Two decoders learn artifact attention from provided images thereby generating realistic individual artifact and multiple artifacts on single image; the third decoder is trained towards removing artifact on the synthetic image with multiple corruptions thereby generating high quality image. We additionally provide improvements over previous image degradation synthesis approaches by modelling multiple image degradations directly from data observations. We first create a toy MNIST dataset and investigate the properties of the proposed algorithm. We then use brain MRI datasets to demonstrate our method’s robustness, including both simulated (where necessary) and real-world artifacts. In addition, our method can be used for single/or multiple degradation(s) synthesis by implementing the learned degradation operators in a new domain from a given dataset. The code will be released upon acceptance of the paper.

Numerical Modelling of Dynamic Electromagnetic Problems Based on the Time-Domain Finite Integration Technique

Developing numerical methods to solve dynamic electromagnetic problems has broad application prospects. In computational electromagnetics, traditional numerical methods are commonly used to deal with static electromagnetic problems. However, they can hardly be applied in the modeling of time-varying materials and moving objects. So far, the studies on numerical methods that can efficiently solve dynamic electromagnetic problems are still very limited. In this paper, a numerical method called the time-domain finite integration technique (TDFIT) is extended to tackle this problem via the introduction of time-varying iterative coefficients. In order to validate the effectiveness of the proposed algorithm, three numerical examples are demonstrated, including two microstrip structures with a time-varying medium and a rapidly rotating structure. The numerical results reveal that the time-varying medium can induce a nonlinear spectrum shift, and the radar cross section (RCS) of a rapidly rotating structure is highly dependent on the rotating speed. The proposed algorithm opens a new avenue for the exploration of many intriguing phenomena in fundamental physics, including frequency conversion and magnetless nonreciprocity. Meanwhile, it can also lead to a wide range of promising practical applications, such as active electron devices, space-time metamaterials, and hypersonic vehicles.

A new idea in response to fast correlation attacks on small-state stream ciphers

In the conference “Fast Software Encryption 2015”, a new line of research was proposed by introducing the first small-state stream cipher (SSC). The goal was to design lightweight stream ciphers for hardware applications by going beyond the rule that the internal state size must be at least twice the intended security level. Fast correlation attack (FCA) was successfully applied to all proposed SSCs which can be implemented by less than 1000 gate equivalents in hardware. It is possible to increase the security of stream ciphers against FCA by exploiting more complicated functions for the nonlinear feedback shift register and the output function, but we use lightweight functions to design the lightest SSC in the world while providing more security against FCA. Our proposed cipher provides 80-bit security against all types of Time-memory-data trade-off (TMDTO) attacks, while Lizard and Plantlet provide only 60-bit and 58-bit security against TMDTO distinguishing attacks, respectively. Our main contribution is to propose a lightweight round key function with a very long period that increases the security of SSCs against FCA.

Uncovering ecological regime shifts in the Sea of Marmara and reconsidering management strategies

Ecosystem regime shifts can alter ecosystem services, affect human well-being, and trigger policy conflicts due to economic losses and reductions in societal and environmental benefits. Intensive anthropogenic activities make the Sea of Marmara ecosystem suffer from nearly all existing available types of ecosystem pressures such as biological degradation, exposure to hydrological processes, nutrient and organic matter enrichment, plastic pollution, ocean warming, resulting in deterioration of habitats. In this study, using an integrated ecosystem assessment, we investigated for the first time the historical development and ecosystem state of the Sea of Marmara. Multivariate analyses were applied to the most comprehensive and unique long-term data sets of 9 biotic and 15 abiotic variables for ecosystem state and drivers respectively, from 1986 to 2020. Observed changes were confirmed by detecting shifts in the datasets. The Sea of Marmara ecosystem was classified into three regimes: i) an early initial state regime under the top-down control of predatory medium pelagic fish and fisheries exploitation until mid-1990s, ii) a transitional regime between mid-1990s and mid-2010s as from ecosystem restructuring, and iii) an alternate state late regime with prevailing impacts of climate change from mid-2010s until 2020. During the 20 years transitional regime, three different phases were also characterized; i) the 1st phase between mid-1990s and early 2000s with its gradual change in ecosystem state from a decrease in predators and significant shift in physical drivers of the ecosystem, ii) the 2nd phase between 2000 and mid-2000s with a strong shift in ecosystem state, an ongoing increase in climate indices and fishing mortality, and a gradual decrease in water quality; and iii) the 3rd phase between mid-2000s and mid-2010s with the reorganization of the ecosystem dominated by small pelagic fish and ameliorated water quality. During late regime, we observed that most of the biotic variables, mainly fish biomass, and climate variables did not return to their initial state despite the improvement in some abiotic variables such as water quality. We identify these observed changes in the SoM ecosystem as a non-linear regime shift. Finally, we also developed concrete suggestions for improved regional management.

Thermal squeezing and nonlinear spectral shift of magnons in antiferromagnetic insulators

We investigate the effect of magnon–magnon interactions on the dispersion and polarization of magnon modes in collinear antiferromagnetic (AF) insulators at finite temperatures. In two-sublattice AF systems with uniaxial easy-axis and biaxial easy-plane magneto-crystalline anisotropies, we implement a self-consistent Hartree–Fock mean-field approximation to explore the nonlinear thermal interactions. The resulting nonlinear magnon interactions separate into two-magnon intra- and interband scattering processes. Furthermore, we compute the temperature dependence of the magnon bandgap and AF resonance modes due to nonlinear magnon interactions for square and hexagonal lattices. In addition, we study the effect of magnon interactions on the polarization of magnon modes. We find that although the noninteracting eigenmodes in the uniaxial easy-axis case are circularly polarized, but in the presence of nonlinear thermal interactions the U(1) symmetry of the magnon Hamiltonian is broken. The attractive nonlinear interactions squeeze the low energy magnon modes and make them elliptical. In the biaxial easy-plane case, on the other hand, the bare eigenmodes of low energy magnons are elliptically polarized but thermal nonlinear interactions squeeze them further. Direct measurements of the predicted temperature-dependent AF resonance modes and their polarization can be used as a tool to probe the nonlinear interactions. Our findings establish a framework for exploring the effect of thermal magnon interactions in technologically important magnetic systems, such as magnetic stability of recently discovered two-dimensional magnetic materials, coherent transport of magnons, Bose–Einstein condensation of magnons, and magnonic topological insulators.

Weak-keys and key-recovery attack for text{ TinyJAMBU }

In this paper, we study NIST lightweight 3rd round candidate text{ TinyJAMBU }. The core component of text{ TinyJAMBU } is the keyed permutation mathcal {P}_n, which is based on a non-linear feedback shift register. By analysing this permutation carefully, we are able to find good cubes that are used to build distinguishers in the weak-key setting. In particular, we show that there are at least 2^{108} keys for which TinyJAMBU can be distinguished from a random source for up to 476 rounds. These distinguishers outperform the best-known distinguishers, which were proposed in ‘Scientific Reports - Nature’ by Teng et al. We are the first to study the exact degree of the feedback polynomial mathcal {P}_n in the nonce variables. This helped us in concluding that text{ TinyJAMBU } with more than 445 rounds is secure against distinguishers using 32 sized cubes in the normal setting. Finally, we give new key-recovery attacks against text{ TinyJAMBU } using the concepts of monomial trail presented by Hu et al. at ASIACRYPT 2020. Our attacks are unlikely to jeopardise the security of the entire 640 rounds text{ TinyJAMBU }, but we strongly anticipate that they will shed new lights on the cipher’s security.

Research Status of Nonlinear Feedback Shift Register Based on Semi-Tensor Product

Nonlinear feedback shift registers (NFSRs) are the main components of stream ciphers and convolutional decoders. Recent years have seen an increase in the requirement for information security, which has sparked NFSR research. However, the NFSR study is very imperfect as a result of the lack of appropriate mathematical tools. Many scholars have discovered in recent years that the introduction of semi-tensor products (STP) of matrices can overcome this issue because STP can convert the NFSR into a quasi-linear form. As a result of STP, new NFSR research has emerged from a different angle. In view of this, in order to generalize the latest achievements of NFSRs based on STP and provide some directions for future development, the research results are summarized and sorted out, broadly including the modeling of NFSRs, the analysis of the structure of NFSRs, and the study of the properties of NFSRs.

Characterizing ecosystem change in wetlands using dense earth observation time series

Wetlands in drylands are vulnerable to degradation and disappearance due to the combined effects of increasing anthropogenic disturbances and climatic extremes. Such influences may drive non-linear shifts in surface responses that require long-term monitoring approaches for their study. Here, we used a piece-wise regression model to characterize long-term Ecosystem Change Types (ECT) in the surface water and vegetation dynamics of the Inner Niger Delta wetlands in Mali between 2000 and 2019. We also examined the added benefits of using a dense Landsat time series for such segmented trend analysis in comparison with MODIS products that are regularly used for ecosystem trends assessment. Our approach has found statistically significant (p < 0.05) long-term changes in wetland ecosystems, as calculated from Modified Normalized Difference Water Index (MNDWI) and Normalized Difference Vegetation Index (NDVI) image series on both the MODIS and the Landsat scales. The class-specific accuracies of the detected ECTs were evaluated through the validation of temporal trajectories based on the TimeSync logic at selected probability sample locations. Results showed higher user's, producer's, and overall accuracies (OA) when using a dense Landsat time series (OA = 0.89 ± 0.01), outperforming the MOD09A1 time series (OA = 0.37 ± 0.03). Our study provides a robust framework for long-term wetland monitoring that demonstrates the benefits of applying dense Landsat time-series imagery for accurate quantifications of linear and non-linear ecosystem responses in vast highly dynamic floodplain systems. Delivering such an improved assessment, in a spatial resolution that better resolves the characteristics of wetlands ecosystems, has the potential to support the information needs of global conservation and restoration efforts.

Microbial network complexity drives non-linear shift in biodiversity-nutrient cycling in a saline urban reservoir

Aquatic biodiversity is important in mediating ecosystem functioning, contributing to ecosystem sustainability and human wellbeing. However, how microbial network complexity affects the biodiversity-nutrient cycling relationship in saline freshwater ecosystems remains underexplored. Using high-resolution time-series data, we examined the relationships between microeukaryotic-bacterial community network complexity, biodiversity and multi-nutrient cycling in an urban reservoir undergoing a freshwater salinization-desalinization cycle. We found that low microbial diversity enhanced ecosystem multi-nutrient cycling under high salinity stress. In addition, multi-nutrient cycling declined with increased network complexity. Further, we found a non-linear relationship between salinity-induced shifts in the complexity of the microbial network and biodiversity-nutrient cycling (BNC) relationship of keystone taxa, i.e. the strength of the BNC relationship first became weak and then strong with increased network complexity. Together, these results highlighted the significant insight that there is not always positive relationship between biodiversity/network complexity and multi-nutrient cycling, even between network complexity and BNC relationship in real-world ecosystems, suggesting that preserving microbial association is important in aquatic health managing and evaluating the freshwater salinization problem.

Scattering-medium-based optical image encryption by chaos and digital optical phase conjugation

Scattering-medium-based optical image encryption provides an intrinsically tamper-resistant physical key. In this paper, we utilize a scattering medium with ideal optical properties as the physical key. The plaintext is transformed into four noise-like holograms with the help of the four-step phase-shifting digital holography. Low-frequency components of the four holograms are extracted and combined into a new image by the lifting wavelet transform (LWT). Then the combined image is scrambled by the nonlinear 3D chaos-based cyclic shift. In decryption, the digital optical phase conjugation (DOPC) technique is utilized to extract plaintext. Numerical simulations prove the feasibility of the proposed method, and the security analysis shows that our method can resist different attacks.

Joint approximation by non-linear shifts of Dirichlet L-functions

In the paper, a theorem on the simultaneous approximation of a collection of analytic functions by non-linear shifts of Dirichlet L-functions (L(s+itτα1,χ1),…,L(s+itταr,χr)) is obtained. Here tτ is the Gram function, α1,…,αr are fixed different positive numbers, and χ1,…,χr are arbitrary Dirichlet characters. Also, an example of approximation by a certain composition of the above shifts is given.

Loaded Slot Cavity Induced Sensing Enhancement and Transparency Based on Plasmonic Structure

Biosensors have proved immensely useful in numerous vital areas for detection and testing. Here, a novel plasmonic nanosensor, based on loaded slot cavity, is proposed and studied. We conduct a detailed analysis of the influence of the load’s parameters on the transmission characteristics based on the finite element method and surface charge and current model. Simulation results reveal that the existence of the load can cause the resonant wavelength to have a linear or nonlinear red shift, and yield an enhanced plasmonic nanosensor with sensitivity about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}={2900}$ </tex-math></inline-formula> nm/RIU and a detection limit about 1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times \,\,10^{\text {-3}}$ </tex-math></inline-formula> . In addition, the proposed structure is well used in actual biosensing for blood plasma concentration, glucose concentration, ethanol temperature and diseased cell detection with high sensitivity. Finally, an extended structure with detuned loaded slot cavities is proposed to realize PIT/multi-PIT and slow light effect. The special features of our proposed structure are applicable in the realization of various integrated components for the development of high-performance plasmonic biosensor.

Gap solitons in heterostructure magnonic crystal/semiconductor

The main features of the nonlinear spin wave propagating in the magnonic crystal/semiconductor (SC) layered heterostructure are studied. We demonstrate the possibility of double electric and magnetic control of the gap solitons parameters (their number, threshold of formation, velocity), bistability of spin waves, switching powers and nonlinear band gap shift in investigated structure. Our results show the possibility of nonlinear magnonics and SC electronics integration based on the proposed structure.

Improved conditional differential attacks on lightweight hash family QUARK

Nonlinear feedback shift register (NFSR) is one of the most important cryptographic primitives in lightweight cryptography. At ASIACRYPT 2010, Knellwolf et al. proposed conditional differential attack to perform a cryptanalysis on NFSR-based cryptosystems. The main idea of conditional differential attack is to restrain the propagation of the difference and obtain a detectable bias of the difference of the output bit. QUARK is a lightweight hash function family which is designed by Aumasson et al. at CHES 2010. Then the extended version of QUARK was published in Journal of Cryptology 2013. In this paper, we propose an improved conditional differential attack on QUARK. One improvement is that we propose a method to select the input difference. We could obtain a set of good input differences by this method. Another improvement is that we propose an automatic condition imposing algorithm to deal with the complicated conditions efficiently and easily. It is shown that with the improved conditional differential attack on QUARK, we can detect the bias of output difference at a higher round of QUARK. Compared to the current literature, we find a distinguisher of U-QUARK/D-QUARK/S-QUARK/C-QUARK up to 157/171/292/460 rounds with increasing 2/5/33/8 rounds respectively. We have performed the attacks on each instance of QUARK on a 3.30 GHz Intel Core i5 CPU, and all these attacks take practical complexities which have been fully verified by our experiments. As far as we know, all of these results have been the best thus far.

Multistable excitonic Stark effect

The optical Stark effect is a tell-tale signature of coherent light--matter interaction in excitonic systems, wherein an irradiating light beam tunes exciton transition frequencies. Here we show that, when excitons are placed in a nanophotonic cavity, the excitonic Stark effect can become highly nonlinear, exhibiting multivalued and hysteretic Stark shifts that depend on the history of the irradiating light. This multistable Stark effect (MSE) arises from feedback between the cavity mode occupation and excitonic population mediated by the Stark-induced mutual tuning of the cavity and excitonic resonances. Strikingly, the MSE manifests even for very dilute exciton concentrations and can yield discontinuous Stark shift jumps of order meV. We expect that the MSE can be realized in readily available transition metal dichalcogenide excitonic systems placed in planar photonic cavities at modest pump intensities. This phenomenon can provide new means to engineer coupled states of light and matter that can persist even in the single exciton limit.

Fatty Acid‐Binding Proteins Respond Differently When Interacting with Endogenous Cannabinoids under High Pressure

Liver‐type and intestinal fatty acid‐binding proteins (LFABP and IFABP, respectively) are both located in enterocytes, but their different functions are demonstrated by phenotypic differences observed in knockout mice for either protein. Though the two proteins are structurally similar, they differ significantly in their ligand‐binding cavities. The region of a protein with the greatest conformational variability is often found in the area surrounding the ligand‐binding cavity to enable an induced fit to a specific substrate. We used solution NMR spectroscopy while applying increasing pressure to investigate the interaction of these two proteins with ligand, in the hopes of capturing transient intermediate conformations of the protein and to identify potential on‐pathway states for substrate binding that would not otherwise be seen under standard conditions.Our initial experiments showed apo‐ and ligand‐bound IFABP remains stable under increasing pressure, up to 2500 bar. In contrast, apo‐LFABP and LFABP with anandamide completely unfolds at 1750 bar. Interestingly, the presence of oleate and arachidonic acid stabilizes LFABP, making it resistant to unfolding at high pressure. Our results show that this stabilization is ligand specific, correlating with the ligand binding affinities we determined previously. This trend suggests that specific protein/ligand interactions within the cavity are required for stability.To make a more detailed analysis of which residues are involved in binding, we generated plots of the pressure‐dependence of backbone 15N and 1H chemical shifts of LFABP and IFABP in either apo‐ or holo‐ form. Two regions of the protein were most responsive to changes in pressure as assessed by the nonlinear chemical shifts changes: i) the conformationally flexible portal and ii) a region of the ligand‐binding cavity we had previously reported as involved in the specificity of binding endocannabinoid ligands. This finding is consistent with the requirement of the conformational variability in these regions for ligand binding. We are currently performing a comprehensive analysis of these effects for the interaction of both proteins with a range of structurally related endogenous cannabinoids.

The roles of kinematic constraint and diffusion in non-equilibrium solid state phase transformations of Ti-6Al-4V

A solid state phase transformation of Ti-6Al-4V was studied using high speed in situ x-ray diffraction measurements made during rapid cooling of a cold metal transfer arc weld bead deposited onto a water cooled substrate. Analysis of body centered cubic (BCC) and hexagonal close packed (HCP) lattices revealed an abrupt, nonlinear shift in the lattice parameters of both phases just after the HCP phase had nucleated. Postmortem transmission electron microscopy confirmed that V diffusion was mostly suppressed during cooling. Together, these results indicate that at this cooling rate of approximately 104 K/s, which is representative of cooling rates of many additive manufacturing and welding processes, kinematic coherency of the BCC–HCP interfaces gives rise to the anomalous lattice expansion and contraction behaviors of both phases during the initial nucleation and growth stages of (mostly) martensitic transformation from BCC to HCP; the role of diffusion in such lattice anomalies is shown to be minimal.

Inhomogeneous light shifts of coherent population trapping resonances

Coherent population trapping (CPT) in atomic vapors using all-optical interrogation has enabled the miniaturization of microwave atomic clocks. Light shifts induced by the CPT driving fields can impact the spectral profile of CPT resonances and are a common limit to the long-term stability of CPT clocks. Nonlinear light shifts have been observed in several CPT systems and have not been explored in detail. In this Letter, we demonstrate that nonlinear light shifts in CPT clocks can arise from spatially inhomogeneous CPT driving fields. We measure this effect using Gaussian laser beams in a buffer gas cell and show strong agreement with a four-level model describing the CPT Λ-system with a noninteracting “trap” state. We estimate the effect of this nonlinearity on recently developed light shift mitigation techniques and suggest improvements to existing techniques.

Nonlinear signal processing with magnonic superlattice with two periods

In this work, we provide investigations of nonlinear spin wave propagation in magnonic superlattice. Magnonic superlattice is a ferromagnetic film loaded on a dielectric ceramic substrate with the copper grating with two periods. It is shown that a superlattice with two periods allows the formation of narrower bandgaps as compared to magnonic crystal with one period. Nonlinear effects, such as the generation of spin wave excitations at half-frequency, caused by first-order parametric processes, and a nonlinear shift of the bandgap, caused by a change in the longitudinal component of magnetization with an increase in the input signal power, are revealed. For the experimental study, the Brillouin light scattering spectroscopy was used, and the theoretical model was built using the transfer matrix method.