Wireless sensors based on micromachined tunable resonators are important in a variety of applications, ranging from medical diagnosis to industrial and environmental monitoring. The sensitivity of these devices is, however, often limited by their low quality (Q) factor. Here, we introduce the concept of isospectral party-time-reciprocal scaling (PTX) symmetry and show that it can be used to build a new family of radiofrequency wireless microsensors exhibiting ultrasensitive responses and ultrahigh resolution, which are well beyond the limitations of conventional passive sensors. We show theoretically, and demonstrate experimentally using microelectromechanical-based wireless pressure sensors, that PTXsymmetric electronic systems share the same eigenfrequencies as their parity-time (PT)symmetric counterparts, but crucially have different circuit profiles and eigenmodes. This simplifies the electronic circuit design and enables further enhancements to the extrinsic Qfactor of the sensors.

We investigate here active metasurfaces obeying parity-time (PT) symmetry and their sensing applications, taking advantage of singularities unique to non-Hermitian systems, such as the spontaneous PT-symmetry-breaking point (exceptional point or EP) and the coherent perfect absorber-laser (CPAL) point. We show theoretically that a PT-symmetric metasurface sensor may provide enhanced sensitivities compared to traditional passive sensors based on metamaterial/ metasurface resonators, because the singular point of one-way zero reflection arising from the EP or the CPAL-related sharp resonance may result in dramatically modulated scattering responses or resonance offsets. We demonstrate the proposed concept with realistic metasurface sensors based on photopumped graphene metasurfaces that simultaneously offer terahertz optical gain and (bio) chemical sensing functions. The proposed PT-symmetric metasurfaces may impact not only loss compensation and extraordinary manipulation of electromagnetic waves, but also practical sensing and detection applications.

Standard exceptional points (EPs) are non-Hermitian degeneracies that occur in open systems.At an EP, the Taylor series expansion becomes singular and fails to converge-a feature that was exploited for several applications. Here, we theoretically introduce and experimentally demonstrate a new class of parity-time symmetric systems [implemented using radio frequency (rf) circuits] that combine EPs with another type of mathematical singularity associated with the poles of complex functions. These nearly divergent exceptional points can exhibit an unprecedentedly large eigenvalue bifurcation beyond those obtained by standard EPs. Our results pave the way for building a new generation of telemetering and sensing devices with superior performance.

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Hot-electron devices are emerging as promising candidates for the transduction of optical radiation into electrical current, as they enable photodetection and solar/infrared energy harvesting at sub-bandgap wavelengths. Nevertheless, poor photoconversion quantum yields and low bandwidth pose fundamental challenge to fascinating applications of hot-electron optoelectronics. Based on a novel hyperbolic metamaterial (HMM) structure, we theoretically propose a verticallyintegrated hot-electron device that can efficiently couple plasmonic excitations into electron flows, with an external quantum efficiency approaching the physical

We explore broadband, wide-angle mid-infrared rectification based on nanopatterned hyperbolic metamaterials (HMMs), composed of two dissimilar metals separated by a subnanometer tunnel barrier. The exotic slow-light modes supported by such periodically trenched HMMs efficiently trap incident radiation in massively parallel metal-insulator-metal tunnel junctions supporting ultrafast optical rectification induced by photon-assisted tunneling. This leads to highly efficient photon-to-electron conversion, orders of magnitude larger than conventional optical rectennas. Our results promise an impact on infrared energy harvesters and plasmonic photodetectors. PACS: 42.25.Bs, 42.65.An, 78.56.-a, 78.67.Pt, 85.60.Gz. Photon-assisted tunneling is an intrinsic quantum-size effect in plasmonic nanostructures, which has recently attracted growing interest because of its exotic nonlinear and nonlocal optical properties [1]-[7]. The excitation of surface plasmon polariton resonances, combined with J qV J qV J qV qV n J ω J

A fundamental challenge for the nonradiative wireless power transfer (WPT) resides in maintaining the stable power transmission with a consistently high efficiency under dynamic conditions. Here, we propose and experimentally demonstrate that a frequency-locked WPT system satisfying the higher-order parity-time (PT) symmetry can achieve a near-unity power transfer efficiency that is resilient to effects of distance variation and misalignment between coils, and impedance fluctuations in electric grids. In specific higher-order PT electronic systems, a purely real-valued and coupling-invariant (nonbifurcated) eigenfrequency would enable the robust and efficient wireless charging without frequency hopping or adaptive impedance matching, even for midrange operation. We envision that this WPT technique may provide reliable, fast and efficient power delivery for a variety of consumer electronics, electric vehicles, and medical devices.