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.

Optical metasurfaces have shown unprecedented capabilities in the local manipulation of the light's phase, intensity, and polarization profiles, and represent a new viable technology for applications such as high-density optical storage, holography and display. Here, a novel metasurface platform is demonstrated for simultaneously encoding color and intensity information into the wavelength-dependent polarization profile of a light beam. Unlike typical metasurface devices in which images are encoded by phase or amplitude modulation, the color image here is multiplexed into several sets of polarization profiles, each corresponding to a distinct color, which further allows polarization modulation-induced additive color mixing. This unique approach features the combination of wavelength selectivity and arbitrary polarization control down to a single subwavelength pixel level. The encoding approach for polarization and color may open a new avenue for novel, effective color display elements with fine control over both brightness and contrast, and may have significant impact for high-density data storage, information security, and anticounterfeiting.

We propose the concept and design of terahertz (THz) phase shifters for phased antenna arrays based on integrally-gated graphene parallel-plate waveguides (GPPWGs). We show that an active transmission-line may be realized by combining GPPWGs with double-gate electrodes, in which the applied gate voltage can control the guiding properties of the gated sections. This may enable the realization of THz electronic switches and tunable loaded-lines for sub mm-wave antenna systems. Based on these active components, we theoretically and numerically demonstrate several digital and analog phase shifter designs for THz frequencies, with a wide range of phase shifts and small return loss, insertion loss and phase error. The proposed graphene-based phase shifters show significant advantages over other available technology in this frequency range, as they combine the low-loss and compact-size features of GPPWGs with electrically-programmable phase tuning. We envision that these electronic phase shifters may pave the way to viable phased-arrays and beamforming networks for THz communications systems, as well as for high-speed, low-RC-delay, inter/intra-chip communications.

Pencils and papers are ubiquitous in our society and have been widely used for writing and drawing, because they are easy to use, low-cost, widely accessible, and disposable. However, their applications in emerging skin-interfaced health monitoring and interventions are still not well explored. Herein, we report a variety of pencil–paper-based on-skin electronic devices, including biophysical (temperature, biopotential) sensors, sweat biochemical (pH, uric acid, glucose) sensors, thermal stimulators, and humidity energy harvesters. Among these devices, pencil-drawn graphite patterns (or combined with other compounds) serve as conductive traces and sensing electrodes, and office-copy papers work as flexible supporting substrates. The enabled devices can perform real-time, continuous, and high-fidelity monitoring of a range of vital biophysical and biochemical signals from human bodies, including skin temperatures, electrocardiograms, electromyograms, alpha, beta, and theta rhythms, instantaneous heart rates, respiratory rates, and sweat pH, uric acid, and glucose, as well as deliver programmed thermal stimulations. Notably, the qualities of recorded signals are comparable to those measured with conventional methods. Moreover, humidity energy harvesters are prepared by creating a gradient distribution of oxygen-containing groups on office-copy papers between pencil-drawn electrodes. One single-unit device (0.87 cm2) can generate a sustained voltage of up to 480 mV for over 2 h from ambient humidity. Furthermore, a self-powered on-skin iontophoretic transdermal drug-delivery system is developed as an on-skin chemical intervention example. In addition, pencil–paper-based antennas, two-dimensional (2D) and three-dimensional (3D) circuits with light-emitting diodes (LEDs) and batteries, reconfigurable assembly and biodegradable electronics (based on water-soluble papers) are explored.

We report a contact engineering method to minimize the Schottky barrier height (SBH) and contact resistivity of MoS2 field-effect transistors (FETs) by using ultrathin 2D semiconductors as contact interlayers. We demonstrate that the addition of a few-layer MoSe2 between the MoS2 channel and Ti electrodes effectively reduces the SBH at the contacts from ∼100 to ∼25 meV, contact resistivity from ∼6 × 10–5 to ∼1 × 10–6 Ω cm2, and current transfer length from ∼425 to ∼60 nm. The drastic reduction of SBH can be attributed to the synergy of Fermi-level pinning close to the conduction band edge of the MoSe2 interlayer and favorable conduction-band offset between the MoSe2 interlayer and MoS2 channel. As a result of the improved contacts, MoS2 FETs with Ti/MoSe2 contacts also demonstrate higher two-terminal mobility.

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We report a new approach to integrating high-κ dielectrics in both bottom- and top-gated MoS2 field-effect transistors (FETs) through thermal oxidation and mechanical assembly of layered two-dimensional (2D) TaS2. Combined x-ray photoelectron spectroscopy (XPS), optical microscopy, atomic force microscopy (AFM), and capacitance–voltage (C–V) measurements confirm that multilayer TaS2 flakes can be uniformly transformed to Ta2O5 with a high dielectric constant of ~15.5 via thermal oxidation, while preserving the geometry and ultra-smooth surfaces of 2D TMDs. Top-gated MoS2 FETs fabricated using the thermally oxidized Ta2O5 as gate dielectric demonstrate a high current on/off ratio approaching 106, a subthreshold swing (SS) down to 61 mV/dec, and a field-effect mobility exceeding 60 cm2 V−1 s−1 at room temperature, indicating high dielectric quality and low interface trap density.

In this paper, we review and discuss how nanoantennas may be used to largely enhance the nonlinear response of optical materials. For single nanoantennas, there have been tremendous advancements in understanding how to exploit the local fi eld enhancement to boost the nonlinear susceptibility at the surface or sharp edges of plasmonic metals. After an overview of the work in this area, we discuss the possibility of controlling the optical nonlinear response using nanocircuit concepts and of signifi cantly enhancing various nonlinear optical processes using planar arrays of plasmonic nanoantennas loaded with χ (2) or χ (3) nonlinear optical materials, forming ultrathin, nanometer-scale nonlinear metasurfaces, as optical nanodevices. We describe how this concept may be used to boost the effi ciency of nonlinear wave mixing and optical bistability, due to the large local fi eld enhancement at the nonlinear nanoloads associated with the plasmonic features of suitably tailored nanoantenna designs. We fi nally discuss three exciting applications of the proposed nonlinear metasurface: dramatically-enhanced frequency conversion effi ciency, effi cient phase-conjugation for super-resolution imaging and large optical bistabilities.