This paper reviews the-state-of-the-art of electromagnetic (EM) metasurfaces and emergent applications in advanced integrated devices and instruments from the design method to physical implementation. The design method includes the analytical coupled mode theory model and commonly used building blocks to construct functional metasurfaces. The modeling approach creates a common design basis of metasurface devices for optical beam steering, focusing, modulation, lasing, and detection. The proof of concept of metasurfaces has been established and is translating to practical applications. Previous studies demonstrated promising applications of metasurfaces including but not limited to optical imaging instruments, biochemical sensing devices, and multifunctional microoptoelectromechanical systems (MOEMS). Significant performance improvement of devices and instruments has been achieved due to the implementation of specially tailored metasurfaces. This review provides an alternative for researchers to step forward on the way of advancing devices and instruments by the deployment of metasurfaces.

Terahertz (THz) waves have exhibited promising applications in imaging, sensing, and communications, especially for the next-generation wireless communications due to the large bandwidth and abundant spectral resources. Modulators and waveguides to manipulate THz waves are becoming key components to develop the relevant technologies where metamaterials have exhibited extraordinary performance to control free-space and on-chip propagation, respectively. In this review, we will give a brief overview of the current progress in active metadevices and topological photonic crystals, for applications of terahertz free-space modulators and on-chip waveguides. In the first part, the most recent research progress of active terahertz metadevices will be discussed by combining metamaterials with various active media. In the second part, fundamentals of photonic topological insulations will be introduced where the topological photonic crystals are an emerging research area that would boost the development of on-chip terahertz communications. It is envisioned that the combination of them would find great potential in more advanced terahertz applications, such as reconfigurable topological waveguides and topologically-protected metadevices.

Metasurfaces are ultrathin metamaterials consisting of subwavelength scatterers (e.g., meta-atoms) arranged in a specific sequence that generates low radiation losses and fantastic optical resonances. According to the electromagnetic response properties, metasurfaces can be divided into two categories: metallic nanostructures based on the response of plasmonic excitations (e.g., noble metals and graphene) and all-dielectric nanostructures based on near-field scattering (e.g., Mie scattering). Metasurfaces supporting various optical modes possess optical localization and electromagnetic field enhancement capabilities on the subwavelength scale, making them a promising platform for label-free detection in biomedical sensing. Metasurface-based optical sensors offer several outstanding advantages over conventional spectroscopic detection solutions, such as planar structures, low loss, miniaturization, and integration. Recently, novel sensing and even imaging tools based on metasurfaces have widely loomed and been proposed. Given recent advances in the field of metasurface spectroscopic detection, this review briefly summarizes the main resonance mechanisms of metasurfaces and the notable achievements, including refractive index sensing, surface-enhanced Raman scattering, surface-enhanced infrared absorption, and chiral sensing in the ultraviolet to terahertz wavelengths. Ultimately, we draw a summary of the current challenges of metasurface spectroscopic detection and look forward to future directions for improving these techniques. As the subject is broad and growing, our review will not be comprehensive. Nevertheless, we will endeavor to describe the main research in this area and assess some of the relevant literature.

In recent years, metamaterials and metasurfaces have prospered in many fields of “science and technology,” covering the entire electromagnetic spectrum. Metasurface devices constituting of a set arrangement of meta-atoms translate into modern-day miniaturized means to achieve planar, ultrathin, multifunctional electromagnetic (EM) systems. Metasurfaces are ideal candidates to develop next-generation, lightweight, and fabrication-friendly optical components as they impart local and space-variant phase changes on incident EM waves, providing more comprehensive control over EM wavefronts. This attribute has been instrumental in realizing a variety of special beams for high-capacity data transmission and superresolution imaging. Furthermore, from the perspective of efficiency, the below-par performance of previously explored plasmonic-based metasurfaces can be enhanced by employing all-dielectric metasurfaces. All-dielectric metasurfaces with high refractive indices have high resonance quality factors, low cost, and CMOS fabrication compatibility. 2D materials-based metasurface design has succeeded in further reducing the device footprints for better integration in optoelectronic devices. The conventional, time- and computation-intensive EM solvers have largely been assisted by artificial intelligence techniques, resulting in quicker metasurface designing. This review focuses on the state-of-the-art meta-devices employed for wavefront manipulations of optical waves. The design variants and applications of metasurfaces constitute a prolific field for future research to meet existing challenges and make the devices more suitable for real-time applications.

Real-time monitoring of the gastrointestinal tract in a safe and comfortable manner is valuable for the diagnosis and therapy of many diseases. Within this realm, our review captures the trends in ingestible capsule systems with a focus on hardware and software technologies used for capsule endoscopy and remote patient monitoring. We introduce the structure and functions of the gastrointestinal tract, and the FDA guidelines for ingestible wireless telemetric medical devices. We survey the advanced features incorporated in ingestible capsule systems, such as microrobotics, closed-loop feedback, physiological sensing, nerve stimulation, sampling and delivery, panoramic imaging with adaptive frame rates, and rapid reading software. Examples of experimental and commercialized capsule systems are presented with descriptions of their sensors, devices, and circuits for gastrointestinal health monitoring. We also show the recent research in biocompatible materials and batteries, edible electronics, and alternative energy sources for ingestible capsule systems. The results from clinical studies are discussed for the assessment of key performance indicators related to the safety and effectiveness of ingestible capsule procedures. Lastly, the present challenges and outlook are summarized with respect to the risks to health, clinical testing and approval process, and technology adoption by patients and clinicians.

A Sagnac interferometer has been a powerful tool for gyroscope, spectroscopy, and navigation based on the Sagnac effects between counterpropagating twin fields in a closed loop, whose difference phase is caused by Einstein’s special relativity. Here, a nonclassical version of a Sagnac interferometer is presented using completely different physics of coherence de Broglie waves (CBW) in a cavity, where CBW is a nonclassical feature overcoming the standard quantum limit governed by classical physics.