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Fano resonance with an asymmetric and ultrasharp resonant line shape has been extensively studied in various light scattering scenes, unlocking several applications for sensing, information processing, and optical identification. Fano resonance appearing in multilayered nanoparticles (NPs) is particularly intriguing as its sharp and comb-like resonant line shape may enable optical identification at the nanoscale. We herein propose the concept of the optical physical unclonable function (PUF) based on the scattering responses of core–shell (plasmonic-dielectric) NPs. Specifically, the sharp, asymmetric spectral responses near the Fano resonance frequency, which are highly sensitive to perturbations (e.g., nanomanufacturing imperfections), can be exploited as a unique electromagnetic fingerprint for PUF-based identification and anti-counterfeiting applications. Here, we theoretically and statistically demonstrate that scattering from Fano-resonant multilayered NPs can be regarded as a perfect entropy source for the generation of PUF encryption keys, with outstanding performance in terms of uniqueness, randomness, encoding capacity, and NIST randomness test results. The proposed optical PUF opens pathways to implement nano-tags for optical identification, authentication, and anti-counterfeiting applications. 

Physically unclonable functions (PUFs) are a class of hardware-specific security primitives based on secret keys extracted from integrated circuits, which can protect important information against cyberattacks and reverse engineering. Here, we put forward an emerging type of PUF in the electromagnetic domain by virtue of the self-dual absorber-emitter singularity that uniquely exists in the non-Hermitian parity-time (PT)–symmetric structures. At this self-dual singular point, the reconfigurable emissive and absorptive properties with order-of-magnitude differences in scattered power can respond sensitively to admittance or phase perturbations caused by, for example, manufacturing imperfectness. Consequently, the entropy sourced from inevitable manufacturing variations can be amplified, yielding excellent PUF security metrics in terms of randomness and uniqueness. We show that this electromagnetic PUF can be robust against machine learning–assisted attacks based on the Fourier regression and generative adversarial network. Moreover, the proposed PUF concept is wavelength-scalable in radio frequency, terahertz, infrared, and optical systems, paving a promising avenue toward applications of cryptography and encryption. 

Parity-time-reciprocal scaling (PTX)-symmetry has been recently proposed to tailor the resonance linewidth and gain threshold of non-Hermitian systems with new exhilarating applications, such as coherent perfect absorber-laser (CPAL) and exceptional point (EP)-based devices. Here, we put forward a nearly-lossless, low-index metachannel formed byPTX-symmetric metasurfaces operating at the CPAL point, supporting the undamped weakly-guided fast wave (leaky mode) and thus achieving ultradirective leaky-wave radiation. Moreover, this structure allows for a reconfigurable and tunable radiation angle as well as beamwidth determined by the reciprocally scaled gain-loss parameter. We envision that the proposedPTX-symmetric metasurfaces will shed light on the design of antennas and emitters with ultrahigh directionality, as well as emerging applications enabled by extreme material properties, such as epsilon-near-zero (ENZ) and beyond. 

Abstract We introduce cascaded parity-time (PT)-symmetric artificial sheets (e.g. metasurfaces or frequency selective surfaces) that may exhibit multiple higher-order laser-absorber modes and bidirectional reflectionless transmission resonances within the PT-broken phase, as well as a unidirectional reflectionless transmission resonance associated with the exceptional point (EP). We derive the explicit expressions of the gain–loss parameter required for obtaining these modes and their intriguing physical properties. By exploiting the cascaded PT structures, the gain–loss threshold for the self-dual laser-absorber operation can be remarkably lowered, while the EP remains unaltered. We further study interferometric sensing based on such a multimodal laser-absorber and demonstrate that its sensitivity may be exceptionally high and proportional to the number of metasurfaces along the light propagation direction.