Cylindrical vector modes with azimuthal polarization and low transmission loss are proposed for the first time to be utilized in a novel design of a surface plasmon resonance (SPR) sensor based on a circular photonic crystal fiber (C-PCF). A C-PCF with a ring of air holes in the cladding is designed where a gold layer with a thickness of 44 nm is coated on the outer cladding surface. The optimal geometric parameters are determined using the finite-element method (FEM) for a high-quality TE₀₁ mode and high sensitivity of the sensor. The proposed SPR sensor shows high sensitivity for analyte refractive index (RI) ranging from n_a = 1.29 to 1.34 over the wavelength range of 1400–2000 nm. It is expected that the proposed sensor can sense low concentrations of hemoglobin, lymphocytes and monocytes of red and white blood cells which are effective in diagnosing the progress of cancer tumors. The maximum sensitivity of 13,800 nm/RIU is obtained in the refractive index environment of 1.33–1.34. The sensor resolution is of the order of 10⁻⁶ and the amplitude sensitivity reaches its maximum of 2380 RIU⁻¹ at n_a = 1.30 which is the highest value ever reported. Our proposed sensor shows high sensitivity and simultaneously simple design with high performance.

Terahertz (THz) radiation has recently received a lot of interests in science and technology. The potential of orbital angular momentum (OAM) of light in the THz region is a novel idea for researchers with increasing applications in broadband communications. THz waveguides have rarely been reported to generate and guide OAM modes. In this paper, we propose for the first time hollow-core optical fibers consist of circular air-hole arrays for guiding THz OAM modes with the topological charge order up to l=3. Using numerical simulations, two low-loss transmission windows over 0.47 0.65 THz (window1) and 0.95 1.20 THz (window2) are recognized to guide OAM modes. Results show that both windows of the THz fiber exhibit the flat near-zero dispersion as well as low confinement loss (~ 10-3 dB/cm) with simultaneously high purity of the OAM modes (higher than 90%). Finally, two designs of THz optical fibers consist of one and two rings of air-hole arrays in their cladding are examined and it is shown that difference of the confinement loss and the modal purity between two designs are negligible. This implies that hollow-core optical fibers consist of one ring of air-hole array in their cladding are very promising for efficient transmission of OAM modes in the THz region.

A novel helically twisted photonic crystal fiber (PCF) is designed and proposed for sensing toxic gases with refractive indices ranging from 1.00 to 1.08. The PCF consists of twelve hollow pipes arranged circularly around the hollow core to support THz radiation propagation. Low-loss polymer Topas is used as the background material of cladding. The fiber is twisted 360° over 50 cm to enhance anti-resonance in the THz region. The fundamental LP₀₁ mode is analyzed using the finite-difference eigenmode (FDE) method. The sensor operates across four frequency bands (0.2 to 3.0 THz) with minimal transmission loss (~ 10⁻⁴ 1/cm). Key parameters such as refractive index sensitivity, relative sensitivity, resolution, and figure of merit (FOM) are evaluated. The average refractive index sensitivities are 1450, 2250, 3000, and 2550 for Bands 1 to 4, respectively, with 100% relative sensitivity across all bands. The sensor detects refractive index changes as small as 10⁻⁴. The FOM, defined as the inverse of the full width at half maximum, exceeds 30 1/RIU, reaching up to 250 1/RIU due to sharp resonance peaks. Compared to other THz sensors, this design offers enhanced performance in sensing gases like SOx, NOx, and CO, while maintaining a simple structure.

A simple design of a D-shaped photonic crystal fiber sensor with 10 layers of graphene on the surface plasmon of gold is proposed for tuning refractive index (RI) sensitivity in the visible and IR regions. The proposed structure has been simulated by optimizing the mesh size and the method of finite-difference eigenmode. By tuning the chemical potential of graphene ( μ c ), we realize tunable and enhanced RI sensing for a wide RI range from 1.35 to 1.42. The maximum wavelength sensitivities of the proposed sensor are 5200 nm/RIU, 6000 nm/RIU, and 7500 nm/RIU for μ c = 0.8 e v , 1.0 ev, and 1.2 ev, respectively. Further, sensor characteristic parameters such as amplitude sensitivity, resolution, and figure of merit have been investigated for different chemical potentials. All dramatic changes in sensor parameters show an advancement plan to control and tune RI-based optical sensors assisted by graphene.