Spin-Selective Angular Dispersion Control in Dielectric Metasurfaces for Multichannel Meta-Holographic Displays.

Angle-dependent next-generation displays have potential applications in 3D stereoscopic and head-mounted displays, image combiners, and encryption for augmented reality (AR) and security. Metasurfaces enable such exceptional functionalities with groundbreaking achievements in efficient displays over the past decades. However, limitations in angular dispersion control make them unfit for numerous nanophotonic applications. Here, we propose a spin-selective angle-dependent all-dielectric metasurface with a unique design strategy to manifest distinct phase information at different incident angles of light. As a proof of concept, the phase masks of two images are encoded into the metasurface and projected at the desired focal plane under different angles of left circularly polarized (LCP) light. Specifically, the proposed multifunctional metasurface generates two distinct holographic images under LCP illumination at angles of +35 and -35°. The presented holographic displays may provide a feasible route toward multifunctional meta-devices for potential AR displays, encrypted imaging, and information storage applications.

Bio-net dataset: AI-based diagnostic solutions using peripheral blood smear images.

Peripheral blood smear examination is one of the basic steps in the evaluation of different blood cells. It is a confirmatory step after an automated complete blood count analysis. Manual microscopy is time-consuming and requires professional laboratory expertise. Therefore, the turn-around time for peripheral smear in a health care center is approximately 3-4 hours. To avoid the traditional method of manual counting under the microscope a computerized automation of peripheral blood smear examination has been adopted, which is a challenging task in medical diagnostics. In recent times, deep learning techniques have overcome the challenges associated with human microscopic evaluation of peripheral smears and this has led to reduced cost and precise diagnosis. However, their application can be significantly improved by the availability of annotated datasets. This study presents a large customized annotated blood cell dataset (named the Bio-Net dataset from healthy individuals) and blood cell detection and counting in the peripheral blood smear images. A mini-version of the dataset for specialized WBC-based image processing tasks is also equipped to classify the healthy and mature WBCs in their respective classes. An object detection algorithm called You Only Look Once (YOLO) with a refashion disposition has been trained on the novel dataset to automatically detect and classify blood cells into RBCs, WBCs, and platelets and compare the results with other publicly available datasets to highlight the versatility. In short the introduction of the Bio-Net dataset and AI-powered detection and counting offers a significant potential for advancement in biomedical research for analyzing and understanding biological data.

A High-Performance, Low-Cost, and Integrated Hairpin Topology RF Switched Filter Bank for Radar Applications.

Switched filter banks find widespread application in frequency-hopping radar systems and communication networks with multiple operating frequencies, especially in situations demanding elevated filter element isolation. In this paper, the design and implementation of a highly isolated switchable narrow-bandpass filter bank architecture using hairpin microstrip topology is presented. The filter bank has four discrete bandpass filters with passbands of 2.0-2.2 GHz, 2.3-2.5 GHz, 3.1-3.3 GHz, and 3.9-4.1 GHz. These filters span the radar S-frequency band (2.0-4.0 GHz). In order to switch between channels with a switching speed of nanoseconds, low-loss and highly isolated SP4T switches are implemented. Advanced design system (ADS) software is used to design the various filter functionalities, and the entire system is tested on a vector network analyzer (VNA). The proposed architecture makes it much easier to put the filter bank into practice and switch it to the desired frequency, which is useful for radar receiver applications.

Ultraviolet-Visible Multifunctional Vortex Metaplates by Breaking Conventional Rotational Symmetry.

Metasurfaces have shown remarkable potential to manipulate many of light's intrinsic properties, such as phase, amplitude, and polarization. Recent advancements in nanofabrication technologies and persistent efforts from the research community result in the realization of highly efficient, broadband, and multifunctional metasurfaces. Simultaneous control of these characteristics in a single-layered metasurface will be an apparent technological extension. Here, we demonstrate a broadband multifunctional metasurface platform with the unprecedented ability to independently control the phase profile for two orthogonal polarization states of incident light over dual-wavelength spectra (ultraviolet to visible). In this work, multiple single-layered metasurfaces composed of bandgap-engineered silicon nitride nanoantennas are designed, fabricated, and optically characterized to demonstrate broadband multifunctional light manipulation ability, including structured beam generation and meta-interferometer implementation. We envision the presented metasurface platform opening new avenues for broadband multifunctional applications including ultraviolet-visible spectroscopy, spatially modulated illumination microscopy, optical data storage, and information encoding.

Atmospheric propagation of space-fractional Gaussian-beam waves in a FSO communication system.

We present a novel, self-consistent analytical model of Gaussian-beam propagation through the atmospheric turbulence by solving the paraxial wave equation in a fractional-dimension space of dimension D, in the range 2 < D ≤ 3, corresponding to the effective spatial dimension experienced by the beam under given turbulent conditions in a free space optical (FSO) communication system. The well-known refractive index structure parameter (C n2) has been mapped from D = 2.668 (C n2≈10-13, strong fluctuations) to D = 2.999 (C n2≈10-16, weak fluctuations) in our simple analytical model, whereas D = 3 corresponds to the ideal case of free-space propagation under zero turbulence. Finally, an optimization problem is developed to mitigate the effects of atmospheric turbulence, leading to efficient transceiver design for the FSO communication system to ensure the reliability of links under varying atmospheric turbulence.

Facile Pressure-Sensitive Capacitive Touch Keypad for a Green Intelligent Human-Machine Interface.

There is a great demand for human-machine interfaces (HMIs) in emerging electronics applications. However, commercially available plastic-based HMIs are primarily rigid, application-specific, and hard to recycle and dispose of due to their non-biodegradability. This results in electronic and plastic waste, potentially damaging the environment by ending up in landfills and water resources. This work presents a green, capacitive pressure-sensitive (CPS), touch sensor-based keypad as a disposable, wireless, and intelligent HMI to mitigate these problems. The CPS touch keypads were fabricated through a facile green fabrication process by direct writing of graphite-on-paper, using readily available materials such as paper and pencils, etc. The interdigitated capacitive (IDC) touch sensors were optimized by analyzing the number of electrode fingers, dimensions, and spacing between the electrode fingers. The CPS touch keypad was customized to wirelessly control a robotic arm's movements based on the touch input. A low-pressure touch allows slow-speed robotic arm movement for precision movements, and a high-pressure touch allows high-speed robotic arm movement to cover the large movements quickly. The green CPS touch keypad, as a disposable wireless HMI, has the potential to enforce a circular economy by mitigating electronic and plastic waste, which supports the vision of a sustainable and green world.

Garage-Fabricated, Ultrasensitive Capacitive Humidity Sensor Based on Tissue Paper.

The role of humidity sensors in different industries and field applications, such as agriculture, food monitoring, biomedical equipment, heating, and ventilation, is well known. However, most commercially available humidity sensors are based on polymers or electronic materials that are not degradable and thus contribute to electronic waste. Here, we report a low-cost, flexible, easy-to-fabricate, and eco-friendly parallel-plate capacitive humidity sensor for field applications. The sensor is fabricated from copper tape and tissue paper, where copper tape is used to create the plates of the capacitor, and tissue paper is used as a dielectric sensing layer. Along with the low cost, the high sensitivity, better response and recovery times, stability, and repeatability make this sensor unique. The sensor was tested for relative humidity (RH), ranging from 40% to 99%, and the capacitance varied linearly with RH from 240 pF to 720 pF, as measured by an Arduino. The response time of the sensor is ~1.5 s, and the recovery time is ~2.2 s. The experiment was performed 4-5 times on the same sensor, and repeatable results were achieved with an accuracy of ±0.1%. Furthermore, the sensor exhibits a stable response when tested at different temperatures. Due to the above advantages, the presented sensor can find ready applications in different areas.

Exploiting zirconium nitride for an efficient heat-resistant absorber and emitter pair for solar thermophotovoltaic systems.

A perfect absorber in the visible-infrared regime maintaining its performance at elevated temperatures and under a harsh environment is needed for energy harvesting using solar-thermophotovoltaic (STPV) systems. A near-perfect metasurface absorber based on lossy refractory metal nitride, zirconium-nitride (ZrN), having a melting-point of 2,980°C, is presented. The numerically proposed design with metal-insulator-metal configuration exhibits an average of > 95% for 400-800 nm and 86% for 280-2200 nm. High absorption is attributed to magnetic resonance leading to free-space impedance matching. The subwavelength structure is polarization- and angle-insensitive and is highly tolerant to fabrication imperfections. An emitter is optimized for bandgap energy ranging from 0.7 eV-1.9 eV.

Single-layered meta-reflectarray for polarization retention and spin-encrypted phase-encoding.

Broadband communication with high data rates is a dire need for state-of-the-art wireless technologies. For achieving efficient wireless communication (particularly in an indoor environment), the electromagnetic (EM) waves should maintain their state of polarization despite encountering multiple reflections. Metasurfaces provide a unique platform to design subwavelength-featured meta-reflectarrays which enable the desired retention of the polarization state of an EM wave upon reflection. We present a single-layered broadband meta-reflectarray, simultaneously breaking n-fold (n > 2) rotational and mirror symmetry, which exhibits an unprecedented control over the phase, amplitude, and polarization of a reflected EM wave. This unique control enables the retention of polarization state and recording of spin-encrypted information for the reflected EM waves. Such novel multifunctional meta-reflectarray can be crucial to building an indoor setup for high data rate wireless communications. Meanwhile, the meta-array's ability to encode phase information provides an extra degree of freedom to structure and control (via incident spin) the reflected EM beam in the desired way. For the proof of concept, we have experimentally demonstrated a spin-encrypted holographic display which reconstructs the recorded holographic image at an image plane for the left circularly polarized (LCP) illumination and exhibits circular dichroism for the right circularly polarized (RCP) incident waves. The proposed meta-array can find applications in 5G indoor wireless communication, chiral sensing, spin-selective imaging, holography, and encryption.

Chiroptical Metasurfaces: Principles, Classification, and Applications.

Chiral materials, which show different optical behaviors when illuminated by left or right circularly polarized light due to broken mirror symmetry, have greatly impacted the field of optical sensing over the past decade. To improve the sensitivity of chiral sensing platforms, enhancing the chiroptical response is necessary. Metasurfaces, which are two-dimensional metamaterials consisting of periodic subwavelength artificial structures, have recently attracted significant attention because of their ability to enhance the chiroptical response by manipulating amplitude, phase, and polarization of electromagnetic fields. Here, we reviewed the fundamentals of chiroptical metasurfaces as well as categorized types of chiroptical metasurfaces by their intrinsic or extrinsic chirality. Finally, we introduced applications of chiral metasurfaces such as multiplexing metaholograms, metalenses, and sensors.

Engineering multimodal dielectric resonance of TiO2 based nanostructures for high-performance refractive index sensing applications.

Optical metasurface based refractive index (RI) sensors find applications in chemical, environmental, biomedical, and food processing industries. The existing RI sensors based on metals suffer from the plasmonic loss in the optical regime; in contrast, those based on Fano-type resonances generated by dielectric materials are either polarization-sensitive or are based on complex geometrical structures prone to fabrication imperfections that can lead to severe performance degradation. Here, we demonstrate that careful engineering of resonance modes in dielectric metasurfaces based on simple symmetric meta-atoms can overcome these limitations. More specifically, we have designed low-loss high-performance RI sensors using all-dielectric metasurfaces composed of TiO2 based nanostructures of three different shapes (i.e., cylindrical, square and elliptical) operating at near-infrared (NIR) wavelengths, which are robust against the perturbations of geometric parameters. In terms of physics, this work reports sensor structures achieving sharp resonant dips of high Q-factor in the transmission spectra corresponding to multiple dielectric resonance modes (i.e., electric quadrupole, magnetic dipole, and electric dipole) with superior performance as compared to the state-of-the-art. Four absolute liquids (water, ethanol, pentanol, and carbon tetrachloride) with a refractive index ranging from 1.333 to 1.453 are used to numerically validate the performance, and a maximum sensitivity of 798 nm/RIU with FOM up to 732 has been achieved.

Highly efficient generation of Bessel beams with polarization insensitive metasurfaces.

We present a generic approach for the generation of pseudo non-diffracting Bessel beams using polarization insensitive metasurfaces with high efficiency. Cascaded unit cells, which are fully symmetric, are designed for the complete 2π phase control in the transmission mode. Based on the topological arrangements of such unit cells, two metasurfaces for the generation of zero-order (i.e., single phase profile) and first-order (i.e., merger of two distinct phase profiles) Bessel beams are designed and characterized. Both numerical simulations and experimental measurements are in agreement with each other, confirming the electromagnetic characteristics of the reported Bessel beams. Owing to the isotropy of the unit cells and the rotational symmetry of the arrangements, the proposed metasurfaces are polarization insensitive, providing a promising avenue for achieving such wave manipulations with any linear or circular polarization.

Dual field-of-view step-zoom metalens.

A conventional optical zoom system is bulky, expensive, and complicated for real-time adjustment. Recent progress in metasurface research has provided a new solution to achieve innovative compact optical systems. In this Letter, we propose a highly integrated step-zoom lens with dual field of view (FOV) based on double-sided metasurfaces. With silicon nanobrick arrays of spatially varying orientations sitting on both sides of a transparent substrate, this ultrathin step-zoom metalens can be designed to focus an incident circular polarized beam with handedness-dependent FOVs without varying the focal plane, which is important for practical applications. The proposed dual FOV step-zoom metalens, with advantages such as ultracompactness, flexibility, and replicability, can find applications in fields that require ultracompact zoom imaging and beam focusing.

A numerical approach to optimize the performance of HTL-free carbon electrode-based perovskite solar cells using organic ETLs.

Carbon electrode-based perovskite solar cells (c-PSCs) without a hole transport layer (HTL) have obtained a significant interest owing to their cost-effective, stable, and simplified structure. However, their application is limited by low efficiency and the prevalence of high-temperature processed electron transport layer (ETL), e.g. TiO2, which also has poor optoelectronic properties, including low conductivity and mobility. In this study, a series of organic materials, namely PCBM ((Park et al., 2023; Park et al., 2023) [6,6]-phenyl-C61-butyric acid methyl ester, C72H14O2), Alq3 (Al(C9H6NO)3), BCP (2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline, C26H20N2), C60, ICBA (indene-C60 bisadduct, C78H16) and PEIE (poly (ethylenimine) ethoxylated, (C37H24O6N2)n) have been numerically analyzed in SCAPS-1D solar simulator to explore alternative potential ETL materials for HTL-free c-PSCs. The presented device has FTO/ETL/CH3NH3PbI3/carbon structure, and its performance is optimized based on significant design parameters. The highest achieved PCEs for PCBM, Alq3, BCP, C60, ICBA, and PEIE-based devices are 22.85%, 19.08%, 20.99%, 25.51%, 23.91%, and 22.53%, respectively. These PCEs are obtained for optimum absorber thickness for each case, with an acceptor concentration of 1.0 × 1017 cm-3 and defect density of 2.5 × 1013 cm-3. The C60-based cell has been found to outperform with device parameters as Voc of 1.29 V, Jsc of 23.76 mA/cm2, and FF of 82.67%. As the design lacks stability when only organic materials are employed, each of the presented devices have been analyzed by applying BiI3, LiF, and ZnO as protective layers with the performances not compromised. We believe that our obtained results will be of great interest in developing stable and efficient HTL-free c-PSCs.

Emerging Two-Dimensional Materials-Based Electrochemical Sensors for Human Health and Environment Applications.

Two-dimensional materials (2DMs) have been vastly studied for various electrochemical sensors. Among these, the sensors that are directly related to human life and health are extremely important. Owing to their exclusive properties, 2DMs are vastly studied for electrochemical sensing. Here we have provided a selective overview of 2DMs-based electrochemical sensors that directly affect human life and health. We have explored graphene and its derivatives, transition metal dichalcogenide and MXenes-based electrochemical sensors for applications such as glucose detection in human blood, detection of nitrates and nitrites, and sensing of pesticides. We believe that the areas discussed here are extremely important and we have summarized the prominent reports on these significant areas together. We believe that our work will be able to provide guidelines for the evolution of electrochemical sensors in the future.

Invisible touch sensors-based smart and disposable door locking system for security applications.

Nowadays, security is one of the living essentials, and there is a dire need for reliable, secure, and smarter locking systems. The stand-alone smart security systems are of great interest as they do not involve keys, cards, or unsecured communication in order to prevent carrying, loss, duplication, and hacking. Here, we report an invisible touch sensors-based smart door locking system (DLS). The passive transducer-based touch sensors are fabricated through a facile do-it-yourself (DIY) based fabrication process by pasting the hybrid geometry copper electrodes on cellulose paper. The employment of biodegradable, and non-toxic materials like paper and copper tape makes this configuration a good candidate for green electronics. For additional security, the keypad in the DLS is made invisible by covering it with paper and spray paint. One can only open the door by knowing the password as well as the location of each key on the sensor keypad. The system can efficiently recognize the exact pattern of passwords without any false values. Invisible touch sensors-based locking systems can easily contribute to the security applications in homes, banks, automobiles, apartments, lockers, and cabinets.

Dielectric chiral metasurfaces for enhanced circular dichroism spectroscopy at near infrared regime.

Numerous applications of chiro-optical effects can be found in nanophotonics, including imaging and spin-selective absorption, particularly in sensing for separating and detecting chiral enantiomers. Flat single-layer metasurfaces composed of chiral or achiral sub-wavelength structures offer unique properties to manipulate the light due to their extraordinary light-matter interaction. However, at optical wavelengths, the generation of strong chirality is found to be challenging via conventional chiral metasurface approaches. This work intends to design and optimize a dielectric chiral meta-nano-surface based on a diatomic design strategy to comprehend giant chiro-optical effects in the near-infrared (NIR) regime for potential application in circular dichroism (CD) spectroscopy. Instead of using a single chiral structure that limits the CD value at optical wavelengths, the proposed metasurface used a diatomic (two meta-atoms with distinct geometric parameters) chiral structure as a building block to significantly enhance the chiro-optical effect. Combining both meta-atoms in a single periodicity of the building block introduces constructive and destructive interferences to attain the maximum circular dichroism value exceeding 75%. Moreover, using multipolar resonance theory, the physics behind the generation of giant chiro-optical effects have also been investigated. The proposed dielectric chiral meta-platform based on the extra degree of freedom can find application in compact integrated optical setups for CD spectroscopy, enantiomer separation and detection, spin-dependent color filters, and beam splitters.

Dynamic control of hybrid grafted perfect vector vortex beams.

Perfect vector vortex beams (PVVBs) have attracted considerable interest due to their peculiar optical features. PVVBs are typically generated through the superposition of perfect vortex beams, which suffer from the limited number of topological charges (TCs). Furthermore, dynamic control of PVVBs is desirable and has not been reported. We propose and experimentally demonstrate hybrid grafted perfect vector vortex beams (GPVVBs) and their dynamic control. Hybrid GPVVBs are generated through the superposition of grafted perfect vortex beams with a multifunctional metasurface. The generated hybrid GPVVBs possess spatially variant rates of polarization change due to the involvement of more TCs. Each hybrid GPVVB includes different GPVVBs in the same beam, adding more design flexibility. Moreover, these beams are dynamically controlled with a rotating half waveplate. The generated dynamic GPVVBs may find applications in the fields where dynamic control is in high demand, including optical encryption, dense data communication, and multiple particle manipulation.

Paper-based facile capacitive touch arrays for wireless mouse cursor control pad.

Wireless devices have become extremely inexpensive and popular in recent years. The two most significant advantages of wireless devices over wired ones are convenience and flexibility. Considering this, a wireless mouse pad prototype for access has been developed in this study. A capacitive sensors-based mouse pad with basic operations and conventional features has been developed using sensing arrays on paper. A facile, do-it-yourself fabrication process was utilized to develop a cost-effective, thin, wearable, and cleanroom-free wireless mouse cursor control (MCC) pad. The ablation process was used to cut the traces of conductive tape and paste them onto the paper to develop the MCC pad. The pad was connected with Espressif Systems (ESP)32 to wirelessly control the cursor of mobile and laptop. The capacitive touch sensor array-based pad is easy to reproduce and recycle. This pad can contribute to future advancements in thin human-machine interfaces, soft robotics, and medical and healthcare applications.

Fast Response Facile Fabricated IDE-Based Ultra-sensitive Humidity Sensor for Medical Applications.

An eco-friendly, biodegradable, flexible, and facile fabricated interdigital electrode-based capacitive humidity sensor with applications in health and medicine has been reported here. Several sensors use copper tape as electrodes on the polyethylene terephthalate (PET) substrate, with non-woven paper as the sensing layer. Two different configurations of sensors were tested, i.e., with and without pores in the PET substrate. The sensing performance of both sensors has been tested for relative humidity ranging from 35 to 100% at temperatures ranging from 20 to 50 °C. The capacitance of the sensor varies linearly in response to the change in humidity. The sensor with pores shows a response from 28 to 630 pF as the humidity varied from 35 to 100%, whereas the sensor without pores responded from 22 to 430 pF. The response and recovery times of the fabricated sensor are observed as ∼2.4, and ∼1.8 s, respectively, and the sensitivity is 9.67 pF/% RH. The sensors are tested multiple times, and repeatable results are achieved each time with an accuracy of ±0.22%. Further, the sensor's response is also stable for different ranges of temperatures. Finally, to demonstrate an application of the proposed sensor, it has been utilized to monitor respiration through nose and mouth breathing. The low-cost, stable, repeatable, and highly sensitive response makes our fabricated sensor a promising candidate for practical field applications.