Photonics

Clathrates have gained considerable attention due to their potential impact on various industries, including oil and gas production, and more recently in the fields ranging from energy storage and transportation to environmental protection and gas separation processes, opening up new technological possibilities. Overall, the attention is focused on their spontaneous and uncontrolled formation/nucleation in offshore oil and gas pipelines, which can lead to numerous and serious operational problems. Accordingly, significant research efforts have focused on understanding the mechanisms of clathrate formation and inhibition or dissociation. Different approaches are being explored; some are ambitious and innovative, whereas others seek further validation. Among these, particular interest has emerged in the coupling of Terahertz (THz) radiation with the collective low-energy and/or vibrational modes of water, and/or other molecules, as well as their clusters. In this review, we summarize recent advances and findings in this promising research field, highlighting the potential applications of THz radiation and spectroscopy, future applications in the field of clathrates, and the technological progress toward the implementation of THz-based solutions in transportation and industrial processes.

This paper describes the development and characterization of surface-enhanced Raman spectroscopy (SERS) substrates that employ the excitation of surface plasmon polaritons (SPPs) on periodic metal diffraction gratings to amplify the Raman signal of an analyte. The gratings were fabricated via interference photolithography on As₄₀S₄₀Se₂₀ thin films. The resulting surface relief was subsequently coated with either aluminium (Al gratings) or aluminium followed by silver (Ag gratings). The ratio of the relief depth to the grating period (h/a) was optimized to maximize SPP excitation efficiency. For both types of gratings, a strong angular dependence of the Raman scattering intensity of the analyte molecule (Rhodamine 6G) was observed, which anticorrelates with the angular dependence of the specularly reflected light intensity. The enhancement factor is 2 × 10² for the aluminium grating and 1 × 10³ for the silver grating. This finding suggests that aluminium-based SERS substrates may serve as a cost-effective alternative to those coated with noble metals. Although the overall amplification is significantly lower than that achieved by SERS substrates based on localized surface plasmon (LSP) excitation, the grating-based SPP substrates offer a crucial advantage for quantitative measurements due to their uniform enhancement across the entire substrate area.

Hollow-core microstructured optical fibres exhibit excellent properties, such as a low loss, tuneable high birefringence, and low nonlinearity, finding extensive applications across communications, industry, agriculture, medicine, military, and sensing technologies. This paper designs two types of asymmetric hollow-core photonic bandgap fibres featuring a high birefringence and low confinement loss. Both feature a cladding structure of rounded hexagonal honeycomb lattice, while the core structures comprise elliptical hollow cores and rounded rhombic hollow cores, respectively. By adjusting the radius of the cladding air holes and the core structure parameters, this study aims to maximise the birefringence coefficient and minimise the confinement loss. The control variable method is employed to optimise the parameters of two fibres. The simulation results indicate that, at a wavelength of 1.55 μm, the birefringence coefficient of the rhombic core, after parameter optimisation, reaches 1.4 × 10⁻⁴, with the confinement loss achieving 4.4 × 10⁻³ dB/km. Its bending loss remains at the order of 10⁻³ dB/km, indicating that this fibre maintains an exceptionally high transmission efficiency even when wound with a small curvature radius (such as within the resonant cavity of a compact fibre optic gyroscope). The elliptical core’s birefringence coefficient also reaches 3 × 10⁻⁴, with the confinement loss achieving 1.9 × 10⁻¹ dB/km. Specifically, this paper employs bismuth tellurite glass as the substrate material to simulate the performance of elliptical cores. Within a specific refractive index range, the elliptical-core fibre with a bismuth tellurite glass substrate exhibits a confinement loss comparable to quartz glass, whilst its birefringence coefficient reaches as high as 5.8 × 10⁻⁴. Therefore, the hollow-core photonic bandgap fibres designed in this thesis provide valuable reference and innovative significance, both in terms of the performance of two asymmetric core structures and in the exploration of polarisation-maintaining hollow-core photonic bandgap fibres on novel material substrates.