Gyroscopes are inertial sensors whose field of application is rapidly expanding. The growing interest in autonomous systems, CubeSats as well as their constellations, and unmanned vehicles is stimulating a strong interest in miniaturized gyroscopes for integration in inertial units with a volume smaller than 100 cm3. Some of these applications require gyroscopes that are highly immune to disturbances, especially vibrations, mechanical shocks, and radiation. In these application contexts, the use of microelectromechanical gyroscopes is often avoided in favor of sensors without moving parts, such as optical gyroscopes based on the Sagnac effect. The technology of interferometric optical gyroscopes is currently considered to be potentially capable of enabling the development of highly innovative angular velocity sensors, which are simultaneously inertial grade and miniaturized. This paper critically examines the scientific and R&D activity aimed at miniaturization of interferometric optical gyroscopes, focusing on recent results, perspectives, and physical limitations. Interferometric optical gyroscopes with dimensions similar to those of microelectromechanical gyroscopes and better performance than the latter in terms of immunity to disturbances have not yet been demonstrated, but this paper shows how this goal could be realistically achieved in the medium term through the use of integrated microphotonics. This paper compares the technology being analyzed with the competitive technologies, highlighting their strengths and limitations.
Label-free sensing is a promising approach for point-of-care testing devices. Among optical transducers, photonic crystal slabs (PCSs) have positioned themselves as an inexpensive yet versatile platform for label-free biosensing. A spectral resonance shift is observed upon biomolecular binding to the functionalized surface. Commonly, a PCS is read out by a spectrometer. Alternatively, the spectral shift may be translated into an intensity change by tailoring the system response. Intensity-based camera setups (IBCS) are of interest as they mitigate the need for postprocessing, enable spatial sampling, and have moderate hardware requirements. However, they exhibit modest performance compared with spectrometric approaches. Here, we show an increase of the sensitivity and limit of detection (LOD) of an IBCS by employing a sharp-edged cut-off filter to optimize the system response. We report an increase of the LOD from (7.1 ± 1.3) × 10−4 RIU to (3.2 ± 0.7) × 10−5 RIU. We discuss the influence of the region of interest (ROI) size on the achievable LOD. We fabricated a biochip by combining a microfluidic and a PCS and demonstrated autonomous transport. We analyzed the performance via refractive index steps and the biosensing ability via diluted glutathione S-transferase (GST) antibodies (1:250). In addition, we illustrate the speed of detection and demonstrate the advantage of the additional spatial information by detecting streptavidin (2.9 µg/mL). Finally, we present the detection of immunoglobulin G (IgG) from whole blood as a possible basis for point-of-care devices.
The ion-sensitive field-effect transistor is a well-established electronic device typically used for pH sensing. The usability of the device for detecting other biomarkers in easily accessible biologic fluids, with dynamic range and resolution compliant with high-impact medical applications, is still an open research topic. Here, we report on an ion-sensitive field-effect transistor that is able to detect the presence of chloride ions in sweat with a limit-of-detection of 0.004 mol/m3. The device is intended for supporting the diagnosis of cystic fibrosis, and it has been designed considering two adjacent domains, namely the semiconductor and the electrolyte containing the ions of interest, by using the finite element method, which models the experimental reality with great accuracy. According to the literature explaining the chemical reactions that take place between the gate oxide and the electrolytic solution, we have concluded that anions directly interact with the hydroxyl surface groups and replace protons previously adsorbed from the surface. The achieved results confirm that such a device can be used to replace the traditional sweat test in the diagnosis and management of cystic fibrosis. In fact, the reported technology is easy-to-use, cost-effective, and non-invasive, leading to earlier and more accurate diagnoses.
We theoretically and experimentally investigate a metasurface supporting a silicon-slot quasi-bound state in the continuum (qBIC) mode resonating in the near-infrared spectrum. The metasurface is composed of circular slots etched in a silicon layer on a sapphire substrate. The symmetry of the metasurface unit cell is reduced in order to provide access to the symmetry-protected mode, whose properties are investigated by finite-element full-wave and eigenfrequency analysis. The measured transmittance spectra verify the excitation of the investigated qBIC mode with experimental quality factors exceeding 700. The near-field distribution of the resonant qBIC mode shows strong field confinement in the slots, leading to high sensitivity values for refractometry.
The recent improvements in diagnosis enabled by advances in liquid biopsy and oncological imaging significantly better cancer care. Both these complementary approaches, which are used for early tumor detection, characterization, and monitoring, can benefit from applying techniques based on surface-enhanced Raman scattering (SERS). With a detection sensitivity at the single-molecule level, SERS spectroscopy is widely used in cell and molecular biology, and its capability for the in vitro detection of several types of cancer biomarkers is well established. In the last few years, several intriguing SERS applications have emerged, including in vivo imaging for tumor targeting and the monitoring of drug release. In this paper, selected recent developments and trends in SERS applications in the field of liquid biopsy and tumor imaging are critically reviewed, with a special emphasis on results that demonstrate the clinical utility of SERS.
Purpose: The diagnosis of obstructive sleep apnea (OSA) is instrument, operator, and time-dependent and therefore requires long waiting times. In recent decades, technological development has produced useful devices to monitor the health status of the population, including sleep. Therefore, the aim of this study was to evaluate a wearable device (WD) in a group of individuals at high risk of OSA. Methods: The study was conducted on consecutive subjects with high risk of OSA assessed by sleep questionnaires and clinical evaluation. All subjects performed cardio-respiratory monitoring (CRM) and WD simultaneously on a single night, after which the parameters of the two sleep investigations were compared. Results: Of 20 individuals enrolled, 60% were men and mean age was 57.3 ± 10.7 years. The apnea–hypopnea index (AHI) for the CRM was 23.1 ± 19.6 events·h−1 while it was 10.3 ± 8.3 events·h−1 for the WD. Correlation analysis between the results of the two investigations showed r = 0.19 (p = 0.40) for AHI and r = 0.4076 (p = 0.07) for sO2%. The accuracy for different stages of OSA severity was 70% in OSA cases and 60% in moderate to severe cases with sensitivity and specificity varying a great deal. Conclusion: Small and low-cost devices may prove to be a valuable resource to reduce costs and waiting times for a sleep investigation in suspected OSA. However, diagnosis of sleep apnea requires valid and reliable instruments, so validation tests are necessary before a device can be commercialized.
In this work, a silicon metasurface designed to support electromagnetically induced transparency (EIT) based on quasi-bound states in the continuum (qBIC) is proposed and theoretically demonstrated in the near-infrared spectrum. The metasurface consists of a periodic array of square slot rings etched in a silicon layer. The interruption of the slot rings by a silicon bridge breaks the symmetry of the structure producing qBIC stemming from symmetry-protected states, as rigorously demonstrated by a group theory analysis. One of the qBIC is found to behave as a resonance-trapped mode in the perturbed metasurface, which obtains very high quality factor values at certain dimensions of the silicon bridge. Thanks to the interaction of the sharp qBIC resonances with a broadband bright background mode, sharp high-transmittance peaks are observed within a low-transmittance spectral window, thus producing a photonic analogue of EIT. Moreover, the resonator possesses a simple bulk geometry with channels that facilitate the use in biosensing. The sensitivity of the resonant qBIC on the refractive index of the surrounding material is calculated in the context of refractometric sensing. The sharp EIT-effect of the proposed metasurface, along with the associated strong energy confinement may find direct use in emerging applications based on strong light-matter interactions, such as non-linear devices, lasing, biological sensors, optical trapping, and optical communications.
Thanks to their lower losses and sharper resonances compared to their metallic counterparts, all-dielectric metasurfaces are attracting a quickly growing research interest. The application of such metasurfaces in the field of refractive index sensing is extremely attractive, especially due to the expected high performance and the simplicity of the sensing element excitation and readout. Herein, we report on an all-dielectric silicon metasurface based on complementary split-ring resonators (CSRRs) optimized for refractive index sensing. A quasi-bound state in the continuum (quasi-BIC) with an ultra-high quality factor can be excited in the near-infrared (NIR) window by violating the structure symmetry. By using the three-dimensional finite element method (3D-FEM), a refractive index sensor for biomedical applications with an ultra-high figure of merit (FoM > 100,000 RIU−1) has been designed, exploiting the quasi-BIC resonance. The proposed design strategy opens new avenues for developing flat biochemical sensors that are accurate and responsive in real time.
The recent development of sophisticated techniques capable of detecting extremely low concentrations of circulating tumor biomarkers in accessible body fluids, such as blood or urine, could contribute to a paradigm shift in cancer diagnosis and treatment. By applying such techniques, clinicians can carry out liquid biopsies, providing information on tumor presence, evolution, and response to therapy. The implementation of biosensing platforms for liquid biopsies is particularly complex because this application domain demands high selectivity/specificity and challenging limit-of-detection (LoD) values. The interest in photonics as an enabling technology for liquid biopsies is growing owing to the well-known advantages of photonic biosensors over competing technologies in terms of compactness, immunity to external disturbance, and ultrahigh spatial resolution. Some encouraging experimental results in the field of photonic devices and systems for liquid biopsy have already been achieved by using fluorescent labels and label-free techniques and by exploiting super-resolution microscopy, surface plasmon resonance, surface-enhanced Raman scattering, and whispering gallery mode resonators. The current state-of-the-art is critically reviewed here, starting from the requirements imposed by the detection of the most common circulating biomarkers. Open research challenges are considered together with competing technologies, and the most promising paths of improvement are discussed for future applications.
In this work, a novel all-dielectric metasurface made of arrayed circular slots etched in a silicon layer is proposed and theoretically investigated. The structure is designed to support both Mie-type multipolar resonances and symmetry-protected bound states in the continuum (BIC). Specifically, the metasurface consists of interrupted circular slots, following the paradigm of complementary split-ring resonators. This configuration allows both silicon-on-glass and free-standing metasurfaces and the arc length of the split-rings provides an extra tuning parameter. The nature of both BIC and non-BIC resonances supported by the metasurface is investigated by employing the Cartesian multipole decomposition technique. Thanks to the non-radiating nature of the quasi-BIC resonance, extremely high Q-factor responses are calculated, both by fitting the simulated transmittance spectra to an extended Fano model and by an eigenfrequency analysis. Furthermore, the effect of optical losses in silicon on quenching the achievable Q-factor values is discussed. The metasurface features a simple bulk geometry and sub-wavelength dimensions. This novel device, its high Q-factors, and strong energy confinement open new avenues of research on light-matter interactions in view of new applications in non-linear devices, biological sensors, and optical communications.
TM-pass polarizers are pivotal components of photonic integrated circuits (PICs), especially those intended for biosensing applications. In the literature, several silicon TM-pass polarizers have been proposed, designed and experimentally demonstrated, but their insertion loss is not compatible with the current trend of silicon photonics aimed at exponentially increasing the component density within PICs. Herein, we propose and design a TM-pass polarizer whose insertion loss is carefully minimized to 0.05 dB at wavelength 1.55 μm by utilizing a combination of an asymmetric directional coupler and a mode evolution section. The adoption of appropriate technical solutions makes this record insertion loss value compatible with a high extinction ratio equal to 38 dB. With a device footprint of only 2.5 × 20 μm2, the design exhibits an insertion loss less than 1.7 dB and extinction ratio better than 30 dB over a large bandwidth of 200 nm. The design assumes the constraints of a typical silicon photonics open-access technological process and a standard 220 nm silicon-on-insulator (SOI) wafer. A very low sensitivity of the achieved performance to reasonable fabrication inaccuracies is demonstrated, with a worst-case insertion loss of only 0.32 dB at wavelength 1.55 μm.