Exploiting High-Index Dielectric Structures on Optical Fibres to Achieve High-Spatial Resolution via Photonic Nanojets

Abstract

In this thesis, we propose a mechanism for achieving high-resolution spatial focusing of electromagnetic waves at telecommunication wavelengths (λ0 = 1.55 µm) using high-refractive index dielectrics. Our approach involves the production of photonic nanojets (PNJs) with a high intensity and spatial resolution. We use cylindrical and spherical dielectrics with radii ranging from R = 0.15λ0 to 1.55λ0, placed at the end of an optical fibre to achieve PNJ focusing. We evaluate the response of the device in 2D and 3D configurations using cylindrical and spherical dielectrics, respectively. By truncating the output profile of the dielectric, we are able to shift the PNJs towards the output surface of the high-index dielectric, resulting in a high transversal resolution. The Full-Width at Half-Maximum (FWHM) achieved is FWHM = 0.28λ0 (2D truncated dielectric), and FWHMy = 0.17λ0 and FWHMx = 0.21λ0 (3D truncated dielectric). We also explore the potential of a specific structure for applications requiring high spatial resolution working at λ0 = 2 µm. Specifically, we report preliminary results from an experiment involving a full and truncated Germanium (Ge) sphere surrounded by cladding material, placed on top of the core of an optical fibre. Our findings demonstrate that this structure is able to generate PNJs with a high transversal resolution at the output surface of the truncated dielectric sphere, measuring FWHMy = 0.13λ0 and FWHMx = 0.16λ0. Interestingly, we also observe the PNJs beyond the output surface of the truncated dielectric sphere (outside) at a distance of 0.1λ0, with a resolution of FWHMy = 0.43λ0 and FWHMx = 0.41λ0. Our proposed structure consisting of an optical fibre and a truncated high index dielectric may have applications in sensing and imaging systems that require near-field high spatial resolution.