Control and localisation of light with engineered nano-structures

Abstract

In this thesis I present my research on nano-scale light control using several novel approaches. I have demonstrated a planar metal nano-structure with cylindrical symmetry that is designed to create a super-oscillation of electromagnetic waves to focus light down to sizes smaller than the Abbe diffraction limit. For the first time this super-oscillatory lens was experimentally used for imaging of nano-structures. A pair of 0.3 λ diameter nano-holes with 0.16 λ edge-to-edge separation were resolved. I have demonstrated a novel type of super-oscillatory lens which produces a continuous distribution of sub-wavelength light localisations extending over several wavelengths along the optical axis. This 'optical needle' is also characterised by a large field of view. I have experimentally demonstrated a optical-needle-lens with 7µm depth of focus and 16% narrower than a diffraction-limited focal spot.

I have characterised the point spread function of the above-mentioned super-oscillatory lenses, i.e., their ability to accurately image a point source. The images of the point source generated by these super-oscillatory lenses are at least 24% smaller than that produced by an ideal glass lens restrained by the Abbe diffraction limit. I have experimentally verified the imaging characteristics of the optical-needle-lens and demonstrated its ability to detect the off-axis placement of a point-like source. I have developed the nano-fabrication processes for manufacturing the super-oscillatory lenses on thin films of metals (Au, Al, Ti) using gallium focused-ion-beam milling technology. The focusing characteristics of the fabricated structures showed very good agreement with computational predictions.

I have computationally shown that objects placed within the field of viewfocfocus of the optical-needle-lens can be imaged with super-resolution quality. This is a significant improvement over the sub-wavelength-step scanning imaging technique reported in this thesis for the other kind of super-oscillatory lens. For example, a super-oscillatory lens can resolve a 'random' cluster of 0.15 λ diameter nano-holes with the smallest edge-to-edge separation of 0.28 λ. I have experimentally demonstrated the first prototype of a solid-immersion superoscillatory lens that promises to achieve a 50 nm hotspot with 405 nm illumination for applications in heat-assisted magnetic recording technology.

I have demonstrated for the first time a planar diffraction grating for visible light designed by arranging meta-molecules to produce a periodic phase ramp. I have also demonstrated the first ever metamaterial-based planar lens-array that produced a 2D array of sub-wavelength foci. Finally, I have provided the first experimental evidence that photoluminescence of gold can be substantially enhanced by patterning the film with designed 2D nano-structured array (or, metamaterials). When resonant two-photon excitation is used the metamaterial enhances the photoluminescence by more than 76 times. I have also observed that the photoluminescence emission peaks are linked to the frequencies of absorption resonances in the metamaterials.

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Identifiers

ORCID for Nikolay I. Zheludev: orcid.org/0000-0002-1013-6636

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