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Exceptional points in optics and photonics
Many complex systems operate with loss. Mathematically, these systems can be described as non-Hermitian. A property of such a system is that there can exist certain conditions—exceptional points—where gain and loss can be perfectly balanced and exotic behavior is predicted to occur. Optical systems generally possess gain and loss and so are ideal systems for exploring exceptional point physics. Miri and Alù review the topic of exceptional points in photonics and explore some of the possible exotic behavior that might be expected from engineering such systems. Science , this issue p. eaar7709 Exceptional points are branch point singularities in the parameter space of a system at which two or more eigenvalues, and their corresponding eigenvectors, coalesce and become degenerate. Such peculiar degeneracies are distinct features of non-Hermitian systems, which do not obey conservation laws because they exchange energy with the surrounding environment. Non-Hermiticity has been of great interest in recent years, particularly in connection with the quantum mechanical notion of parity-time symmetry, after the realization that Hamiltonians satisfying this special symmetry can exhibit entirely real spectra. These concepts have become of particular interest in photonics because optical gain and loss can be integrated and controlled with high resolution in nanoscale structures, realizing an ideal playground for non-Hermitian physics, parity-time symmetry, and exceptional points. As we control dissipation and amplification in a nanophotonic system, the emergence of exceptional point singularities dramatically alters their overall response, leading to a range of exotic optical functionalities associated with abrupt phase transitions in the eigenvalue spectrum. These concepts enable ultrasensitive measurements, superior manipulation of the modal content of multimode lasers, and adiabatic control of topological energy transfer for mode and polarization conversion. Non-Hermitian degeneracies have also been exploited in exotic laser systems, new nonlinear optics schemes, and exotic scattering features in open systems. Here we review the opportunities offered by exceptional point physics in photonics, discuss recent developments in theoretical and experimental research based on photonic exceptional points, and examine future opportunities in this area from basic science to applied technology.
Journal Article
A flexible piezoresistive strain sensor based on laser scribed graphene oxide on polydimethylsiloxane
Flexible strain sensors are an important constituent in soft robotics, health care devices, and in the defence industry. Strain sensors are characterized by their sensitivity (gauge factor-GF) and sensing range. In flexible strain sensors, simultaneously achieving consistency and high sensitivity has always been challenging. A number of materials and their derivatives have been explored to achieve balanced sensitivity with respect to sensing range with limited results. In this work, a low-cost flexible piezoresistive strain sensor has been developed using reduced graphene oxide (rGO) on polydimethylsiloxane (PDMS). The reduction has been performed using laser scribing, which enables the fabrication of arbitrary structures. After lead-out, the devices were again sandwiched in a layer of PDMS to secure the structures before performing their testing using a locally developed testing rig. Compared to previously reported graphene strain sensors, the devices fabricated in this work show relatively high GF with respect to sensing range. The GF calculated for stretching, bending and torsion was 12.1, 3.5, and 90.3 respectively, for the strain range of 0–140%, 0–130%, and 0–11.1%. A hand test was performed for the detection of joint movement. Change of resistance has been observed indicating muscle motion.
Journal Article
Parity-time–symmetric microring lasers
The ability to control the modes oscillating within a laser resonator is of fundamental importance. In general, the presence of competing modes can be detrimental to beam quality and spectral purity, thus leading to spatial as well as temporal fluctuations in the emitted radiation. We show that by harnessing notions from parity-time (PT) symmetry, stable single–longitudinal mode operation can be readily achieved in a system of coupled microring lasers. The selective breaking of PT symmetry can be used to systematically enhance the maximum attainable output power in the desired mode. This versatile concept is inherently self-adapting and facilitates mode selectivity over a broad bandwidth without the need for other additional intricate components. Our experimental findings provide the possibility to develop synthetic optical devices and structures with enhanced functionality.
Journal Article
Recommendations for accurate reporting in medical research statistics
In particular, large p values should not be interpreted as showing no association or no effect: absence of evidence is not evidence of absence.8 Only a very narrow interval estimate near the null value (0 for differences, 1 for ratios) warrants inferring that the study found no important association or effect. Better methods include inverse probability weighting and multiple imputation, although these still depend on missingness being conditionally random.14,15 An important source of bias in logistic or Cox regression is sparse data—ie, a low number of events in some combinations of levels of variables. Unrealistically large ratio measures with wide interval estimates (eg, an odds ratio >10 with limits of 2 and 50) indicate sparse-data bias, which can be reduced with penalised or Bayesian methods.16,17 When the dependent variable is an indicator of a common outcome, adjusted risk ratios are preferable to odds ratios for assessing clinical relevance, due to their ease of proper interpretation and resistance to sparse-data bias.
Journal Article
Internet of Things intrusion detection systems: a comprehensive review and future directions
The Internet of Things (IoT) is a paradigm that connects objects to the Internet as a whole and enables them to work together to achieve common objectives, such as innovative home automation. Potential attackers see the scattered and open IoT service structure as an appealing target for cyber-attacks. So, security cannot be dealt with independently. Security must be designed and built-in to every layer of the IoT system. IoT security concerns not only network and data security but also human health and life attacks. Therefore, the development of the loT system to provide security through resistance to attacks is a de facto requirement to make the loT safe and operational. Protecting these things is very important for system security. Plus, it is important to integrate the Intrusion Detection System (IDS) with IoT systems. IDS intends to track and analyze network traffic from different resources and detect malicious activities. It is a significant part of cybersecurity technology. In short, IDS is a process used to detect malicious activities against victims by several methods. Besides, the method of Systematic Literature Review (SLR) is used to classify, review, and incorporate results from all similar research that answers one or more IDS research topics and perform a detailed empirical research analysis on IDS techniques. Furthermore, depending on the detection technique, we classify IDS approaches in IoT as signature-based, anomaly-based, specification-based, and hybrid. Also, for the IDS approaches, the authors give a parametric comparison. The benefits and drawbacks of the chosen mechanisms are then addressed. Eventually, there is an analysis of open problems as well as potential trend directions.
Journal Article