Progress towards the realization of an optical Far-Field Superlens

Abstract

<p>Conventional optics suffer from a fundamental resolution limit due to the nature of light. The near-field superlens concept was introduced two decades ago, and its theory for enabling high resolution imaging is well-established now. Initially, this superlens, which has a simple setup, became a hot topic given the proposition of overcoming the diffraction limit. It has been demonstrated that a near-field superlens can reconstruct images using evanescent waves emanating from small objects by means of resonant excitations on the surface of the superlens. A modified version of the superlens named the far-field superlens is theorized to be able to project the near-field subwavelength information to the far-field region. By design, the far-field superlens is a near-field superlens with nanostructures added on top of it. These nanostructures, referred to as diffraction gratings help couple object information available in the evanescent waves to the far-field. Work reported in this thesis is divided to two major sections. The first describes the modelling technique that investigates the performance of a far-field superlens. This section focuses on evaluating the impact of the diffraction gratings geometry and the object size on the far-field superlens performance as well as the resulting far-field pattern. It was shown that a far-field superlens with a nanograting having a duty cycle of 40% to 50% produces the maximum intensity and contrast in the far-field interactions. For periodic rectangular objects, an inverse-trapezoidal nanograting was shown to provide the best contrast and intensity for far-field interactions. The minimal simulation domain to model a symmetric far-field superlens design was determined both in 2D and 3D. This input reduced the required modelling time and resources. Finally, a 3D far-field superlens model was proposed, and the effect of light polarization on the far-field pattern was studied. The second section of this thesis contains the experimental study that explores a new material as a potential candidate for the construction of far-field superlens. The material conventionally used for superlens design is silver, as its plasmonic properties are well-established. However, scaling down silver features to the nanoscale introduces fundamental fabrication challenges. Furthermore, silver oxidizes due to its reactions with sulphur compounds at ambient conditions, which means that operating a silver far-field superlens is only possible in a well-controlled environment. This disagrees with our proposed concept of a low-cost and robust superlens imaging device. On the other hand, highly doped semiconductors are emerging candidates for plasmonic applications due to the possibility of tuning their optical and electrical properties during the fabrication process. While the working principle of a superlens is independent of the plasmonic material of choice, every plasmonic material has a particular range of operating wavelengths. The pros and cons of each plasmonic material are usually identified once used experimentally. In this work, aluminium-doped zinc oxide was the proposed material of choice for the far-field superlens design. The second part of this thesis details the characterization results of the optical, electrical and structural properties of this proposed alternative. Our aluminium-doped zinc oxide samples were highly transparent for large parts of the spectrum. Their carrier concentration was of the order of 10+20 cm-3, and a resistivity of about 10-3 Ω.cm was achieved. The modelled dielectric permittivity for the studied samples showed a cross-over frequency in the near-infrared region, with the highest plasma frequency achieved in this study being 4710 cm-1.</p>

Silver Surfaces and Nanostructures for Imaging and Sensing Applications.

“…Progress has also been made towards the realisation of an FSL with a 2D diffraction grating. Using a 2D diffraction grating that allows spatial frequencies in multiple directions to be acquired has been proposed [83,84]. Figure 2.11b illustrates the concept, where the k x and k y spatial frequencies are transformed into the propagating band.…”

“…Figure 2.11b illustrates the concept, where the k x and k y spatial frequencies are transformed into the propagating band. Electromagnetics finite element modelling (FEM) has been conducted, which suggested that by adjusting the illumination polarisation and using a 2D grating design, spatial features in the xand ydirection could be recovered in the far-field [84]. The proposed 2D FSL was illuminated by 365 nm wavelength light.…”

“…This design would remove the need to rotate the object relative to the FSL and instead rotate the illumination source relative to the combined 2D FSL and object. Although it was concluded that there would be significant fabrication difficulties in creating the 2D FSL, the optimised grating structure was for a 150 nm pitch grating structure with a 0.40 duty cycle ratio and a height of 30 nm [84].…”

“…It, however, requires advanced mathematics and potentially proprietary software such as COMSOL multiphysics. FEM has been used in the literature to model the far-field electromagnetic field of a 2D FSL [84], the optical transfer function of a LPSIM substrate [88].…”

“…For a proposed 2D FSL, it has been suggested that a nanopillar height of 45 nm and width of 75 nm with a repeating array pitch of 150 nm would be required [84,91]. The modelling of non-ideal structures for FSL fabrication has also been explored.…”