Light-Induced Interactions using Optical Near-Field Devices

Optical near-fields are generated when light passes through components with wavelength, or subwavelength features. The near-fields generated at the surfaces of devices are often neglected, in part because the far-fields have more applications and are more readily accessible. Near-fields, as one might expect, occur very close to the surface of the material through which the light passes. However, near-fields present an interesting method of overcoming Rayleigh\u27s diffraction limit. For example, the evanescent field at the surface of a prism or ultrathin fibre rapidly decays, but can exist in sub-diffraction limited areas. Similarly, the field generated by a subwavelength aperture or a plasmonic particle can have local field distributions with minute dimensions, allowing one to confine light to areas otherwise unattainable, extremely close to the surface of the material in question. By exploiting this aspect of optical near-fields we apply them to problems in atom and particle trapping.Our main focus is on ultrathin optical fibres. These fibres differ from telecommunications fibre due to their lack of cladding material and their wavelength-scale dimensions. These two factors combine to produce a significant evanescent field at their waist. This field is readily accessible and can be used to trap particle or atoms through the optical forces which arise in such light-matter interactions. We can also use such devices to passively collect light which is emitted into the available guided mode. Here, we demonstrate how an ultrathin fibre can be used as a probe to determine the temperature of a cold atom cloud.Ultrathin fibres, while extremely useful, have some limiting factors related to the strength and distribution of their evanescent fields. To improve upon the design, we also investigated how one can nanostructure an optical fibre using focussed ion beam milling techniques or combine optical fibres with gold dimer arrays to produce localised field enhancements. We used nanostructured fibres to trap 100 and 200 nm dielectric spheres within the structured region. Various numerical techniques were employed to characterise both the nanostructured fibre and the plasmonic-enhanced fibre.Aside from optical fibres, we also briefy discuss how an array of Fresnel microlenses can be packaged with other atom chip designs to produce a device which could trap atoms microns away from a gold surface. We discuss the theory and fabrication technique for such a Fresnel microlens array atom chip.Okinawa Institute of Science and Technology Graduate Universit