Altering the Multimodal Resonance in Ultrathin Silicon Ring for Tunable THz Biosensing

An optimized metastructure switchable between ultra-wideband angle-insensitive absorption and transmissive polarization conversion: a theoretical study

An optimized metastructure (MS) switchable between ultra-wideband (UWB) angle-insensitive absorption, and transmissive linear-to-circular (LTC) polarization conversion (PC), is proposed, which is a theoretical study. The structural parameters of this MS are optimized by the thermal exchange optimization algorithm. By modulating the chemical potential (μc) of the graphene-based hyperbolic metamaterial embedded in the MS, the MS can achieve UWB absorption in the absorption state and LTC PC in the transmission state. At normal incidence, in the absorption state, the MS exhibits absorptivity exceeding 0.9 within 7-15.45 THz, with a relative bandwidth (RBW) of 75.28%. By elevating μc, an UWB LTC PC is realized, with a RBW of 118.8%, achieving transmittance above 0.9 and the axial ratio below 3 dB. When prioritizing the angular stability, in the absorption state, the MS secures the angular stability of 75° for TE waves and 65° for TM ones. In the transmission state, the angular stability of PC reaches 60°, with RBW = 100.7%. Moreover, by manipulating μc, the tunability of UWB absorption is realized. The optimized MS provides a reference for designing multifunctional intelligent terahertz modulators, with promising application potential in domains like electromagnetic shielding, communication systems, and THz modulation.

CPA-induced Hz to THz broadband absorber with switchable perfect absorption between radio-microwave and THz frequency spectrum

The coherent perfect absorption (CPA) occurring in the graphene sheet suspended in air can be utilized to develop an ultrathin, ultra-broadband absorber working in the frequency range from a few hertz (Hz) to terahertz (THz) with perfect absorption. A graphene sheet is studied to induce the CPA to cover radio, microwave and lower THz frequency ranges. A graphene resonator able to provide the surface plasmon resonance (SPR) is combined with the graphene sheet to provide CPA at either side of a thin dielectric layer forms metamaterial structure with the cavity and enhances the absorption bandwidth in the THz region by creating a resonance near quasi-CPA frequency. A dielectric silicon resonator is embedded in the structure, which creates dipolar resonances between the resonances obtained by the formed cavity between the graphene sheet and resonator. This enhances the absorption level in the THz region. The absorption bandwidth is further enhanced to 7 THz by including a graphene disc at the top of the silicon resonator. Thus, the multiple multi-order resonances occurring in the silicon dielectric and SPR of graphene resonators are merged with the phenomena of CPA occurring in the graphene sheets to extend the CPA bandwidth in the THz regime. The doping level of graphene or its tunable Fermi energy based on the applied DC electric field provides the tunability in the total obtained absorption bandwidth. The symmetric structure provides polarization-insensitive behavior with an allowed incident angle of more than 45° with more than 90% absorption.

Multiband terahertz metamaterial perfect absorber for microorganisms detection

We report a multi-resonant terahertz (THz) metamaterial perfect absorber (MPA)-based biosensor in the working frequency range of [Formula: see text] for sensing of microorganisms (such as fungi, yeast) and wheat pesticides. Nearly [Formula: see text] absorption is realized at [Formula: see text] and [Formula: see text]. We designed our THz MPA sensor making resonators' gap area compatible with the microorganisms' size. To obtain optimum performance of the MPA, a mapping of amplitudes and shifts in the absorption resonance peaks with different structural parameters of the resonators is carried out. A very high-frequency shift is obtained for microorganisms such as Penicillium chrysogenum (fungi), yeast, and pesticides (Imidacloprid, N, N-Diethyldithiocarbamate sodium salt trihydrate, Daminozide, N, N-Diethyldithiocarbamate sodium salt hydrate, and Dicofol). An equivalent circuit model using Advance Design System (ADS) software is developed. The calculated results through the model show similar trends as obtained in the simulations using CST. Investigations of the effect of incidence angle of THz wave on the absorption spectra of the MPA are also carried out. It is found that incidence angle does not impact the stability of the lower resonance absorption peak (1.79THz). Due to the wide working frequency range, the proposed sensor is extremely suitable for the detection of all range of pesticides because their specific absorption fingerprint lies in the frequency range of 0-3.8THz. We believe that our sensor could be a potential detection tool for detecting pesticide residues in agriculture and food products. The THz MPA-based biosensor is capable of detecting a very small change in the effective dielectric constant of the MPA environment. Therefore, it can also offer huge opportunities in label-free biosensing for future biomedical applications.

Resonant-Cavity-Enhanced Electrochromic Materials and Devices

With rapid advances in optoelectronics, electrochromic materials and devices have received tremendous attentions from both industry and academia for their strong potentials in wearable and portable electronics, displays/billboards, adaptive camouflage, tunable optics, and intelligent devices, etc. However, conventional electrochromic materials and devices typically present some serious limitations such as undesirable dull colors, and long switching time, hindering their deeper development. Optical resonators have been proven to be the most powerful platform for providing strong optical confinement and controllable lightmatter interactions. They generate locally enhanced electromagnetic near-fields that can convert small refractive index changes in electrochromic materials into high-contrast color variations, enabling multicolor or even panchromatic tuning of electrochromic materials. Here, resonant-cavity-enhanced electrochromic materials and devices, an advanced and emerging trend in electrochromics, are reviewed. In this review, w e will focus on the progress in multicolor electrochromic materials and devices based on different types of optical resonators and their advanced and emerging applications, including multichromatic displays, adaptive visible camouflage, visualized energy storage, and applications of multispectral tunability. Among these topics, principles of optical resonators, related materials/devices and multicolor electrochromic properties are comprehensively discussed and summarized. Finally, the challenges and prospects for resonant-cavity-enhanced electrochromic materials and devices are presented.

Althuwayb A, Ibrahim IM, Saeidi T, Dayo ZA, Ahmad S. A Miniaturized and Highly Sensitive Microwave Sensor Based on CSRR for Characterization of Liquid Materials

In this work, a miniaturized and highly sensitive microwave sensor based on a complementary split-ring resonator (CSRR) is proposed for the detection of liquid materials. The modeled sensor was designed based on the CSRR structure with triple rings (TRs) and a curve feed for improved measurement sensitivity. The designed sensor oscillates at a single frequency of 2.5 GHz, which is simulated using an Ansys HFSS simulator. The electromagnetic simulation explains the basis of the mode resonance of all two-port resonators. Five variations of the liquid media under tests (MUTs) are simulated and measured. These liquid MUTs are as follows: without a sample (without a tube), air (empty tube), ethanol, methanol, and distilled water (DI). A detailed sensitivity calculation is performed for the resonance band at 2.5 GHz. The MUTs mechanism is performed with a polypropylene tube (PP). The samples of dielectric material are filled into PP tube channels and loaded into the CSRR center hole; the E-fields around the sensor affect the relationship with the liquid MUTs, resulting in a high Q-factor value. The final sensor has a Q-factor value and sensitivity of 520 and 7.032 (MHz)/εr) at 2.5 GHz, respectively. Due to the high sensitivity of the presented sensor for characterizing various liquid penetrations, the sensor is also of interest for accurate estimations of solute concentrations in liquid media. Finally, the relationship between the permittivity and Q-factor value at the resonant frequency is derived and investigated. These given results make the presented resonator ideal for the characterization of liquid materials.

Fragmented graphene synthesized on a dielectric substrate for THz applications

Fragmented multi-layered graphene films were directly synthesized via chemical vapor deposition (CVD) on dielectric substrates with a pre-deposited copper catalyst. We demonstrate that the thickness of the sacrificial copper film, process temperature, and growth time essentially influence the integrity, quality, and disorder of the synthesized graphene. Atomic force microscopy and Kelvin probe force microscopy measurements revealed the presence of nano-agglomerates and charge puddles. The potential gradients measured over the sample surface confirmed that the deposited graphene film possessed a multilayered structure, which was modelled as an ensemble of randomly oriented conductive prolate ellipsoids. THz time domain spectroscopy measurements gave theacconductivity of the graphene flakes and homogenized graphitic films as being around 1200 S cm^-1and 1000 S cm^-1, respectively. Our approach offers a scalable fabrication of graphene structures composed of graphene flakes, which have effective conductivity sufficient for a wide variety of THz applications.

SARS-CoV-2 Detection using Colorimetric Plasmonic Sensors: A Proof-of-Concept Computational Study

Traditional molecular techniques for SARS-CoV-2 viral detection are time-consuming and can exhibit a high probability of false negatives. In this work, we present a computational study of SARS-CoV-2 detection using plasmonic gold nanoparticles. The resonance wavelength of a SARS-CoV-2 virus was recently estimated to be in the near-infrared region. By engineering gold nanospheres to specifically bind with the outer surface of the SARS-CoV-2 virus, the resonance frequency can be shifted to the visible range (380 nm - 700 nm). Moreover, we show that broadband absorption will emerge in the visible spectrum when the virus is partially covered with gold nanoparticles at a specific coverage percentage. This broadband absorption can be used to guide the development of an efficient and accurate colorimetric plasmon sensor for COVID-19 detection. Our observation also suggests that this technique is unaffected by the number of protein spikes present on the virus outer surface, hence can pave a potential path for a label-free COVID-19 diagnostic tool independent of the number of protein spikes.

Graphene-based dual-functional chiral metamirror composed of complementary 90° rotated U-shaped resonator arrays and its equivalent circuit model

An equivalent circuit model (ECM) using a MATLAB code to analyze a tunable two-layered graphene-based chiral dual-function metamirror, is proposed in this work. The investigated metastructure is composed of complementary U-shaped graphene resonator arrays in the terahertz (THz) region. The ECM analysis could be used for any two-layered chiral metastructure for any frequencies, containing resonators with a thickness less than λ/50. The characteristics of the proposed tunable metamirror were analyzed numerically using the finite element method (FEM) in CST Software to verify the ECM analysis. The proposed metamirror can be used in polarization-sensitive devices in the THz region with simpler biasing without a need for ion gels or similar. It works as a broadband TE and multiband (four bands) TM mirror in the 0.3-4.5 THz bandwidth with a strong linear dichroism (LD) response (up to 96%). The designed mirror is a dynamically tunable, dual-functional structure, requiring only 90° rotation of the incident electromagnetic fields to switch between broadband and multiband spectral behavior making it a promising candidate for future THz intelligent systems. The proposed ECM is in agreement with the FEM results. The ECM analysis provides a simple, fast, and effective way to understand the metamirror's behavior and guides for the design and analysis of graphene-based chiral metastructures in the THz region.