The fundamental constants of physics and the International System of Units

Air Canada managed to have a passenger aircraft run out of fuel in mid-air due to confusion about metric units (Stephenson in Mars climate orbiter mishap investigation board phase I report, NASA, 1999), and NASA lost an entire spacecraft due to a misunderstanding amongst engineers about the units used in the propulsion system design (Witkin in Jet's fuel ran out after metric conversion errors, The New York Times, 1983). Measurements only make sense if the units are correct and well-defined. A unit of measurement is a definite magnitude of a quantity, defined by convention or law. Any other quantity of that kind can then be expressed as a multiple or submultiple of the unit of measurement. The Egyptians used the Farao as definite magnitude, while many years later, the french revolutionists introduced the earth as a reference and laid the foundations for the modern decimal system. Since recently, we have a truly universal and stable system that uses physics's natural constants and laws to define the base units of measurement. This paper explains how this new concept works and how it is implemented in practice

CELES: CUDA-accelerated simulation of electromagnetic scattering by large ensembles of spheres

CELES is a freely available MATLAB toolbox to simulate light scattering by many spherical particles. Aiming at high computational performance, CELES leverages block-diagonal preconditioning, a lookup-table approach to evaluate costly functions and massively parallel execution on NVIDIA graphics processing units using the CUDA computing platform. The combination of these techniques allows to efficiently address large electrodynamic problems ($>10^4$ scatterers) on inexpensive consumer hardware. In this paper, we validate near- and far-field distributions against the well-established multi-sphere $T$-matrix (MSTM) code and discuss the convergence behavior for ensembles of different sizes, including an exemplary system comprising $10^5$ particles

Biomimetic Polymer Film with Brilliant Brightness Using a One‐Step Water Vapor–Induced Phase Separation Method

The scales of the white Cyphochilus beetles are endowed with unusual whiteness arising from the exceptional scattering efficiency of their disordered ultrastructure optimized through millions of years of evolution. Here, a simple, one‐step method based on water vapor–induced phase separation is developed to prepare thin polystyrene films with similar microstructure and comparable optical performance. A typical biomimetic 3.5 µm PS film exhibits a diffuse reflectance of 61% at 500 nm wavelength, which translates into a transport mean free path below 1 µm. A complete optical characterization through Monte Carlo simulations reveals how such a scattering performance arises from the scattering coefficient and scattering anisotropy, whose interplay provides insight into the morphological properties of the material. The potential of bright‐white coatings as smart sensors or wearable devices is highlighted using a treated 3.5 µm film as a real‐time sensor for human exhalation

Quantifying the Sensitivity and Unclonability of Optical Physical Unclonable Functions

Due to their unmatched entropy, complexity, and security level, optical physical unclonable functions (PUFs) currently receive a lot of interest in the literature.Despite the large body of existing works, herein, one of their core features in detail is studied, namely, their physical unclonability. This article tackles this fundamental and yet largely unaddressed issue. In simulations and/or experiments, the sensitivity of diffraction-based optical responses is investigated with respect to various small alterations such as variation in position, size, and number of the scatterers, as well as perturbations in the spatial alignment between the PUF and the measurement apparatus. The analysis focuses on 2Doptical PUFs because of their relevance in integrated applications and the need to reply to security concerns that can be raised when the physical structure of the geometry is accessible. Among the results of this study, the sensitivity analysis shows that a positional perturbation of scatterers on the order of 30 nm, that is,far below the wavelength of the probing laser light of 632 nm wavelength, is sufficient to invalidate the PUF response and thus detect forgery attempt. These results support and quantify the high adversarial efforts required to clone optical PUFs, even for 2D layouts

Dynamically Tunable Optical Cavities with Embedded Nematic Liquid Crystalline Networks

Tunable metal–insulator–metal (MIM) Fabry–P\ue9rot (FP) cavities that can dynamically control light enable novel sensing, imaging\ua0and display applications. However, the realization of dynamic cavities incorporating stimuli-responsive materials poses a significant engineering challenge. Current approaches rely on refractive index modulation and suffer from low dynamic tunability, high losses, and limited spectral ranges, and require liquid and hazardous materials for operation. To overcome these challenges, a new tuning mechanism employing reversible mechanical adaptations of a polymer network is proposed, and dynamic tuning of optical resonances is demonstrated. Solid-state temperature-responsive optical coatings are developed by preparing a monodomain nematic liquid crystalline network (LCN) and are incorporated between metallic mirrors to form active optical microcavities. LCN microcavities offer large, reversible and highly linear spectral tuning of FP resonances reaching wavelength-shifts up to 40\ua0nm via thermomechanical actuation while featuring outstanding repeatability and precision over more than 100 heating–cooling cycles. This degree of tunability allows for reversible switching between the reflective and the absorbing states of the device over the entire visible and near-infrared spectral regions, reaching large changes in reflectance with modulation efficiency ΔR\ua0=\ua079%

Flexible Physical Unclonable Functions based on non-deterministically distributed Dye-Doped Fibers and Droplets

The development of new anti-counterfeiting solutions is a constant challenge and involves several research fields. Much interest is devoted to systems that are impossible to clone, based on the Physical Unclonable Function (PUF) paradigm. In this work, new strategies based on electrospinning and electrospraying of dye-doped polymeric materials are presented for the manufacturing of flexible free-standing films that embed different PUF keys. Films can be used to fabricate anticounterfeiting labels having three encryption levels: i) a map of fluorescent polymer droplets, with non deterministic positions on a dense yarn of polymer nanofibers; ii) a characteristic fluorescence spectrum for each label; iii) a challenge-response pairs (CRPs) identification protocol based on the strong nature of the physical unclonable function. The intrinsic uniqueness introduced by the deposition techniques encodes enough complexity into the optical anti-counterfeiting tag to generate thousands of cryptographic keys. The simple and cheap fabrication process as well as the multilevel authentication makes such colored polymeric unclonable tags a practical solution in the secure protection of merchandise in our daily life

Photoresponsive Adhesives Based on Arylazoisoxazoles‐Containing Polymers

Abstract Adhesives exist in many forms and are essential in the daily life, in industrial processes, and in healthcare, calling for continuous improvements in terms of application fields, strength, biocompatibility, recyclability, and reversibility. Regarding the last two points, embedding photoresponsive molecules into the adhesive allows the use of light to attach or detach reversibly different surfaces. Photoresponsive adhesives can be removed without the use of high temperature or solvents, which is ideal for delicate environments. To date, azobenzenes are the most explored chromophore to prepare smart adhesive formulations. Here, the synthesis of polymer containing arylazoisoxazoles (AIZs) — azobenzene analogs containing a heterocycle — and their application as adhesives is described. Mechanical tests demonstrate an enhancement of the adhesion strength from 0.39 to 2.79 MPa, passing from low molecular weight monomer to selected AIZs acrylate polymer. Irradiation with UV induces the formation of the cis isomer with a consequent drop of more than 70% in the adhesion strength. The different compounds result as possible candidates as adhesives on the basis of the increased load that can be held, together with the possibility to obtain joints that can be either stable or detachable under light irradiation