: In this paper, a dual-core photonic crystal fiber (DC-PCF) based surface plasmon resonance (SPR) bio-compatible sensor is proposed for various bio-organic molecules and biochemical analytes refractive index (RI) detection in the visible to near-infrared region (0.5 to 2 μm). Two hexagonal ring lattice with all circular air-holes are used to simplify the sensor structure. To make the practical applications feasible, plasmonic material and analyte sensing layer both are employed at the outer surface of the fiber. Noble plasmonic material gold (Au) having a thickness of 30 nm is used to excite the surface plasmons. A thin layer of titanium oxide (TiO2) having a thickness of 5 nm is also considered as an adhesive layer between the Au and silica glass. The sensor response is investigated using the mode solver based finite element method (FEM). Numerical results indicate that the proposed sensor shows a maximum amplitude sensitivity (AS) of 6829 RIU-1, amplitude resolution (AR) of 5x10-6 RIU, maximum wavelength sensitivity (WS) of 28,000 nm/RIU, and wavelength resolution (WR) of 3.57x10-6 RIU, using the amplitude and wavelength interrogation methods, respectively. Moreover, a maximum figure of merit (FOM) of 2800 RIU-1 is obtained, which is the highest among the reported PCF-SPR sensor. Owing to the promising sensitivity and simple structure, the proposed sensor can be potentially applicable for the detection of biochemical solutions and biological samples.

In this paper, a low-loss, spiral lattice photonic crystal fiber (PCF)-based plasmonic biosensor is proposed for its application in detecting various biomolecules (i.e., sugar, protein, DNA, and mRNA) and biochemicals (i.e., serum and urine). Plasmonic material gold (Au) is employed externally to efficiently generate surface plasmon resonance (SPR) in the outer surface of the PCF. A thin layer of titanium oxide (TiO₂) is also introduced, which assists in adhering the Au layer to the silica fiber. The sensing performance is investigated using a mode solver based on the finite element method (FEM). Simulation results show a maximum wavelength sensitivity of 23,000 nm/RIU for a bio-samples refractive index (RI) detection range of 1.32–1.40. This sensor also exhibits a very low confinement loss of 0.22 and 2.87 dB/cm for the analyte at 1.32 and 1.40 RI, respectively. Because of the ultra-low propagation loss, the proposed sensor can be fabricated within several centimeters, which reduces the complexity related to splicing, and so on.

In this work, a highly sensitive dual-core configured microchannel-based plasmonic refractive index (RI) sensor was investigated, which can be used for low RI detection. Both the sensing layer and the plasmonic material layer were built outside of the fiber design to detect the surrounding medium’s RI changes. Additionally, the effects of different plasmonic materials gold (Au), silver (Ag), and copper (Cu) toward sensitivity were investigated for the same structure. An adhesive agent was used in this work, titanium dioxide (TiO₂), and was coated on top of the plasmonic material to prevent the oxidation of Ag and Cu. The coupling strength between the fundamental mode and the surface plasmon polariton (SPP) mode was observed to be very strong due to the TiO₂ adhesive agent. With a resolution of 7.41 × 10⁻⁷ RIU, maximum wavelength sensitivity (WS) of 135,000 nm/RIU and amplitude sensitivity (AS) of 3239 RIU⁻¹ were achieved using the proposed sensor while using Au as a plasmonic material for an analyte RI range of 1.29–1.39. A detailed study of relevant literature revealed that the achieved wavelength sensitivity for plasmonic material gold (Au) is the highest among reported photonic crystal fiber (PCF)-surface plasmon resonance (SPR) sensors to date.

An improved design of a D-shaped photonic crystal fiber (PCF)-based refractive index (RI) sensor is proposed that uses the surface plasmon resonance phenomenon. The sensor consists of a large semicircular open channel that is inserted at the upside of the D-shaped PCF. A thin plasmonic sensing layer is deposited on the interior surface of the channel to excite the surface plasmon wave that eliminates the requirement of additional effort to fabricate a well-polished sensing layer of the D-shaped sensor. The sensor’s optical properties are numerically explored by the finite element method. The sensor is optimized to detect the RI of biological and biochemical analytes in the range of 1.33 to 1.44, shows spectral sensitivity as high as 63,000 nm/RIU with a spectral resolution of 1.59 × 10⁻⁰⁶ RIU, and maximum amplitude sensitivity of 1439 RIU⁻¹ with a resolution of 6.94 × 10⁻⁰⁶ RIU. It is also found that the sensor’s linearity parameter is very high with a large figure of merit of about 839. Additionally, the sensor’s fabrication tolerance is studied by varying its structural parameters. Therefore, high sensing parameters with a wide detection range make this microchannel-based D-shaped PCF sensor an appropriate device for the application of biological and biochemical analyte detection.

In this paper, we present and compare two experimentally feasible photonic crystal fiber (PCF) designs (Type — I and Type — II) which ensure near-zero flattened dispersion with ultrahigh phase and group birefringence at THz frequencies. Both structures are based on a subset of a triangular array of circular air-holes, which define the cladding of the PCF and a central elliptical air-hole which breaks the symmetry of the structure, thus introducing high levels of birefringence. Additionally, we investigate the possibility of further enhancing the birefringence properties of Type — II structure by selectively filling the air-holes with Potassium Chloride (KCl) as strong Epsilon-Near-Zero (ENZ) material. Our investigation reveals that significant enhancement of birefringence can be achieved than its original counterpart with birefringence to be as high as 0.0627 at 6.2 THz and near-zero flat dispersion of −0.54 ± 0.04 ps/THz/cm over the frequency range of 6.2—6.3 THz.

We present a tunable terahertz metamaterial absorber featuring a fabrication-convenient structure that combines a vanadium dioxide (VO2) square patch and a graphene loop for multifunctional operation. This paper demonstrates an absorber that can selectively operate in narrow-band, dual-band, or broad-band modes. Combining graphene’s electrical tunability with VO2’s thermally induced phase transitions, this absorber realizes dynamically switchable and tunable absorption properties. When VO2 is in its metallic state with chemical potential of graphene, Ef = 0 eV, and relaxation time, $\tau $ = 0.1 ps, the structure achieves a broad-band absorption bandwidth of 6.31 THz (¿90% absorptance from 5.04 THz to 11.35 THz), with an average absorption of 97.24% and a relative absorption bandwidth of 77.12%. By adjusting graphene’s Ef to 1 eV and $\tau $ to 0.7 ps, the absorber transitions to a dual-band response, exhibiting peak absorptions of 91.13% at 2.62 THz and 95.48% at 10.81 THz. Conversely, when VO2 remains in insulating state and graphene’s Ef is tuned to 0.6 eV and $\tau $ to 0.2 ps, a narrow-band absorption peak of 98.18% emerges. Furthermore, we demonstrate the narrow-band configuration as an ultra-sensitive biosensor, where the resonance peak undergoes a significant redshift while maintaining high absorption efficiency with minimal changes in the surrounding medium’s refractive index. The design exhibits polarization insensitivity and wide-angle stability, validated through electric field analysis, impedance matching theory, and equivalent circuit modeling. To the best of our knowledge, this is the first reported structure to integrate such a wide absorption bandwidth of 6.31 THz with ultra-high biosensing sensitivity of 1929 GHz/RIU in a single fabrication-friendly configuration. These capabilities make the absorber highly promising for applications in energy harvesting, stealth technology, and biochemical sensing.