In this work, a quasi-D-shaped photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) biosensor is proposed and numerically investigated using the finite element method (FEM) implemented in COMSOL Multiphysics version 6.2 for the detection of cancer cells with different refractive indices. The biosensor has a coating of plasmonic material gold (Au) and a polymer coat of polymethyl methacrylate (PMMA). The effects of plasmonic material thickness and air hole dimensions on key sensing parameters, including confinement loss (CL), wavelength sensitivity (WS), and amplitude sensitivity (AS), are systematically analyzed. The results revealed that increasing plasmonic thickness beyond its optimum value significantly raises CL while reducing sensitivity due to reduced penetration depth of the evanescent field. Similarly, variations in the geometrical dimensions of the air holes (±10%) also affect the sensor response, emphasizing the importance of precise structural optimization. For the optimized design the proposed biosensor exhibits high performance with a maximum WS of 31,000 nm/RIU for MDA-MB-231 cells under x-polarization and 29,500 nm/RIU under y-polarization. The corresponding resolutions achieved are as low as 3.22 × 10⁻⁶ RIU and 3.38 × 10⁻⁶ RIU, respectively, with AS exceeding 9000 RIU⁻¹. The WS, AS, and other sensing parameters obtained from our sensor are relatively higher than some of the PCF–SPR sensors reported recently. These numerical results demonstrate that the optimized quasi-D-shaped PCF–SPR biosensor exhibits enhanced sensitivity to refractive index (RI) variations associated with cancerous cells, suggesting its suitability for early detection.

Photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensors have emerged as powerful groundbreaking platforms for smart biomedical sensing due to their exceptional light–matter interaction, structural tunability, and high sensitivity. This review presents a concise and critical overview of recent advances in PCF-SPR sensing technologies with a primary focus on biomedical applications, including disease biomarkers, proteins, DNA, glucose, and pathogenic microorganisms’ detection. Smart sensing capabilities such as real-time monitoring, miniaturization, multiplexed detection, and enhanced sensor selectivity have been systematically analyzed. Key sensor performance parameters, including sensitivity, resolution, limit of detection (LOD), specificity, response time, refractive index (RI)-based detection, internal metal deposition (IMD), external metal deposition (EMD), biocompatibility, and hybrid models in sensor optimization, are comprehensively discussed. Across recent reports, wavelength sensitivity (WS) reaches up to 130 000 nm/RIU, sensor resolution (SR) is reported on the order of 10−7 RIU, LOD can be as low as $1.19 \times 10^{-5}$ RIU, and figures of merit (FOM) up to 6000 have been demonstrated. Additionally, this review briefly addresses temperature, chemical, and pressure sensing to provide a broader perspective. This work further highlights current challenges, research gaps, hybrid techniques, use of artificial intelligence (AI), and future opportunities in the development of next-generation smart PCF-SPR biomedical sensors.

The wide band at extremely high frequencies (EHF) above 30 GHz is applicable for high resolution directive radars, resolving the lack of free frequency bands within the lower part of the electromagnetic spectrum. Utilization of ultra-wideband signals in this EHF band is of interest, since it covers a relatively large spectrum, which is free of users, resulting in better resolution in both the longitudinal and transverse dimensions. Noting that frequencies in the millimeter band are subjected to high atmospheric attenuation and dispersion effects, a study of the degradation in the accuracy and resolution is presented. The fact that solid-state millimeter and sub-millimeter radiation sources are producing low power, the method of continuous-wave wideband frequency modulation becomes the natural technique for remote sensing and detection. Millimeter wave radars are used as complementary sensors for the detection of small radar cross-section objects under bad weather conditions, when small objects cannot be seen by optical cameras and infrared detectors. Theoretical analysis for the propagation of a wide “chirped” Frequency-Modulated Continuous-Wave (FMCW) radar signal in a dielectric medium is presented. It is shown that the frequency-dependent (complex) refractivity of the atmospheric medium causes distortions in the phase of the reflected signal, introducing noticeable errors in the longitudinal distance estimations, and at some frequencies may also degrade the resolution.

In this work, we investigate the effects of two distinct physical mechanisms, the antenna effect and plasmon resonance, on electric field enhancement in representative geometrical structures and at the apex of a sharp tip. The three-dimensional (3D) apertureless surface near-field optical microscopy (a-SNOM) configuration is numerically investigated using a finite element method (FEM). Our results show good agreement with analytical predictions and prior numerical benchmarks. Accounting for both antenna effects and plasmon resonances is crucial for accurate modeling of local field amplification in nanostructured probes. Through a systematic parametric numerical study, we show that field enhancement is strongly influenced by illumination conditions, polarization angle, and tip geometry. Finally, these results clarify how intrinsic material properties, extrinsic geometric features, and optical excitation govern field enhancement in nanostructures.

The subject of radio wave propagation in tunnels has gathered attention in recent years, mainly regarding the fading phenomena caused by internal reflections. Several methods have been suggested to describe the propagation inside a tunnel. This work is based on the ray tracing approach, which is useful for structures where the dimensions are orders of magnitude larger than the transmission wavelength. Using image theory, we utilized a multi-ray model to reveal non-dimensional parameters, enabling measurements in down-scaled experiments. We present the results of field experiments in a small concrete pedestrian tunnel with smooth walls for radio frequencies (RF) of 1, 2.4, and 10 GHz, as well as in a down-scaled model, for which millimeter waves (MMWs) were used, to demonstrate the roles of the frequency, polarization, tunnel dimensions, and dielectric properties on the wave propagation. The ray tracing method correlated well with the experimental results measured in the tunnel as well as in a scale model.