Far-field resolution imaging

• Main Author: Fleming, Matthew James
• Published: Imperial College London 2008
• Subjects: 621.36
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.497730

Wave based imaging methods aim to build an accurate reconstruction of the physical properties of an object by recording the scattered field caused by illumination from multiple directions. Classically the minimum distance between the characteristics of the object that can be resolved by an imaging method is limited by the wavelength, λ, of the interrogating field. In order to improve the resolution shorter wavelengths can be propagated; however, due to material absorption, this limits the penetration depth of the wave which consequently reduces the potential imaging range. Any imaging technique which can overcome the resolution limit is of great practical and academic interest and represents the subject of this thesis. Subwavelength characterisation has become well established in the field of Near- field Scanning Optical Microscopy which requires part of the probing system to be within λ of the object being illuminated (near field), in order to detect the nonpropagating evanescent waves. The super-oscillatory properties of the evanescent waves are subsequently used to achieve subwavelength resolution. However, access to the near field of an object is not always feasible and since evanescent waves decay exponentially they cannot be directly detected in the far field (greater than λ from the object). The aim of this thesis is to define and investigate an imaging strategy that will allow super resolution to be achieved from the far field. Conventional imaging techniques, which are constrained by the resolution limit, neglect the distortion of the scattered field caused by the internal structure of the object. This thesis will show that a more accurate description of the interaction of the incident field with the object, which includes the multiple scattering of evanescent waves, can lead to subwavelength resolution from the far field.