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Ultrathin and two-dimensional (2D) metals can support strong plasmons, with concomitant tight field confinement and large field enhancement. Accordingly, 2D-metal nanostructures exhibiting plasmonic resonances are highly sensitive to the environment and intrinsically suitable for optical sensing. Here, based on a proof-of-concept numerical study, nano-engineered ultrathin 2D-metal films that support infrared plasmons are demonstrated to enable highly responsive refractive index (RI) sensing. For 3 nm-Au nanoribbons exhibiting plasmonic resonances at wavelengths around 1600 nm, a RI sensitivity of SRI > 650 nm per refractive index unit (RIU) is observed for a 100 nm-thick analyte layer. A parametric study of the 2D-Au system indicates the strong dependence of the RI sensitivity on the 2D-metal thickness. Furthermore, for an analyte layer as thin as 1 nm, a RI sensitivity up to 110 (90 nm/RIU) is observed in atomically thin 2D-In (2D-Ga) nanoribbons exhibiting highly localized plasmonic resonances at mid-infrared wavelengths. Our results not only reveal the extraordinary sensing characteristics of 2D-metal systems but also provide insight into the development of 2D-metal-based plasmonic devices for enhanced IR detection. [ABSTRACT FROM AUTHOR]

Sub-wavelength chiral resonators formed from artificial structures exhibit exceedingly large chiroptical responses compared to those observed in natural media. Owing to resonant excitation, chiral near fields can be significantly enhanced for these resonators, holding great promise for developing enantioselective photonic components such as biochemical sensors based on circular dichroism (CD) and spin-dependent nonlinear imaging. In the present work, strong linear and nonlinear chiroptical responses (scattering CD > 0.15 and nonlinear differential CDs > 0.4) at visible and near infrared frequencies are reported for the first time for individual micrometer-scale plasmonic and dielectric helical structures. By leveraging dark-field spectroscopy and nonlinear optical microscopy, the circular-polarization-selective scattering behavior and nonlinear optical responses (e.g., second harmonic generation and two-photon photoluminescence) of 3D printed micro-helices with feature sizes comparable to the wavelength (total length is ∼5λ) are demonstrated. These micro-helices provide potential for readily accessible photonic platforms, facilitating an enantiomeric analysis of chiral materials. One such example is the opportunity to explore ultracompact photonic devices based on single, complex meta-atoms enabled by state-of-the-art 3D fabrication techniques. [ABSTRACT FROM AUTHOR]

The resonant nature and geometric scalability make metamaterials an ideal platform for an enhanced light–matter interaction over a broad frequency range. The mid-infrared (IR) spectral range is of great importance for vibrational spectroscopy of molecules, while IR metamaterials created from lithography-based planar nanostructures have been used to demonstrate enhanced molecular detection. Compared with visible and near-infrared, the relative long wavelengths of IR light make it possible to achieve three-dimensional (3D) IR metamaterials via the state-of-the-art 3D fabrication techniques. Here, we design and fabricate a 3D printed plasmonic metamaterial absorber (MMA), and by performing Fourier-transform IR spectroscopy, we demonstrate that a series of molecular fingerprint vibrations of glycine can be significantly enhanced by the high absorption mode supported by the 3D meta-atoms of the MMA. The observed enhanced IR detection can also be partially attributed to the improved accessibility offered by the 3D architecture of the MMA. In particular, due to capillary forces during the drying process, the microscale 3D printed features lead to selective analyte deposition in high-field regions, which provides another degree of freedom in the design of the 3D printed structures for surface-enhanced IR detection. Our study shows the flexibility of metastructures based on advanced 3D printing technology in tailoring the interaction between IR light and materials on a subwavelength scale. [ABSTRACT FROM AUTHOR]

Achieving sufficient energy transmission over a wide frequency range is a challenge which has restricted the application of many types of air-coupled ultrasonic transducers. Conventional transducer configurations such as the piezoelectric micromachined or flexural ultrasonic transducers can be considered as narrowband. This study reports a type of ultrasonic transducer, the wideband electromagnetic dynamic acoustic transducer (WEMDAT), which operates through a combination of electromagnetic induction and Lorentz force with dynamic behaviour of a micro-scale-thick conductive film. WEMDAT prototypes have been designed, fabricated, and tested, showing their compatibility with both low and high power inputs, operating efficiently as a wideband transmitter from 46.4 kHz to 144.6 kHz with a good directivity. The WEMDAT has also been shown to operate effectively as a wideband ultrasonic receiver through the measurement in a pitch-catch configuration. The WEMDAT prototypes possess an adjustable drive coil lift-off distance from the active membrane, providing flexibility for optimizing the sensitivity of the transducers for different input levels. The performance of the WEMDATs can be optimized, showing significant potential for air-coupled ultrasonic applications. [ABSTRACT FROM AUTHOR]

As a wide bandgap semiconductor, SiC holds great importance for high temperature and high power devices. It is known that the intrinsic defects play key roles in determining the overall electronic properties of semiconductors; however, a comprehensive understanding of the intrinsic defect properties in the prototype 4H-SiC is still lacking. In this study, we have systematically investigated the electronic properties and kinetic behaviors of intrinsic point defects and defect complexes in 4H-SiC using advanced hybrid functional calculations. Our results show that all the point defects in 4H-SiC have relatively high formation energies, i.e., low defect concentrations even at high growth temperatures. Interestingly, it is found that the migration barriers are very high for vacancies (>3 eV) but relatively low for interstitial defects (∼1 eV) in SiC. Meanwhile, the diffusion energy barriers of defects strongly depend on their charge states due to the charge-state-dependent local environments. Furthermore, we find that VSi in SiC, a key defect for quantum spin manipulation, is unstable compared to the spin-unpolarized VC–CSi complex in terms of the total energy (under p-type conditions). Fortunately, the transformation barrier from VSi to VC–CSi is as high as 4 eV, which indicates that VSi could be stable at room (or not very high) temperature. [ABSTRACT FROM AUTHOR]

Spoof plasmonics, which can enable strong terahertz (THz) radiation–matter interactions, hold great promise for the advancement of THz science and technology. However, THz spoof plasmonic devices based on micro-structured metals are in general limited by lithography-based fabrication processes as well as metals' mechanical, chemical, and thermal stability, which hinders their applications in, for instance, flexible and wearable THz imaging and communications, molecular sensing, etc. Possessing high electrical conductivity and outstanding mechanical robustness, graphene-assembled films (GAFs) promise many benefits for electronics as an alternative to metals. Here, by studying the resonance-enhanced transmission properties of subwavelength GAF hole arrays, we demonstrate a GAF metasurface as a transformative platform for flexible THz spoof plasmonics. Based on a laser direct writing (LDW) patterning technique, the proposed micro-engineered GAF is expected to pave the way toward large-area, durable, and inexpensive THz metadevices with superior flexibility. [ABSTRACT FROM AUTHOR]

The resonance based strong light-matter interaction in metamaterials offers unprecedented opportunities to manipulate polarization of electromagnetic waves. In this work, we fabricate a three-dimensional (3D) metamaterial consisting of 90°-twisted split-tube resonators using a 3D printing technique and demonstrate the corresponding asymmetric transmission for linearly polarized electromagnetic waves in the Ku band with near-unity polarization conversion efficiency. Experimental results reveal a 90° polarization rotation and an incident polarization angle dependent asymmetric transmission at a frequency around 15.2 GHz. The experimental results are in good agreement with simulations. Possessing the merits of both flexibility of response tailoring and ease of fabrication, the proposed 3D-printed metamaterials have great potential for compact polarization-control devices exhibiting unidirectional transmission at both microwave and terahertz frequencies. [ABSTRACT FROM AUTHOR]