Ph.D Thesis

Ph.D StudentDavid Asaf
SubjectPhotonic Structures for Nanoscale Light Manipulation
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROF. Guy Bartal
Full Thesis textFull thesis text - English Version


The wave nature of light imposes some fundamental limitations on conventional optical systems. In particular, the diffraction limit prevents direct observation by optical microscopes of two objects spatially separated by less than half the wavelength. Moreover, light cannot be confined to dimensions that are below the order of magnitude of its wavelength. While the former limitation presents an inherent resolution barrier for direct optical imaging, the latter restrict the use of optics as a means of energy and information delivery to above a certain scale. In addition to the scientific interest of breaking fundamental limitations of physics, a growing interest in sub-wavelength systems for imaging, spectroscopy, lithography and focusing has emerged due to the various applications they can provide: biological imaging for the research of single molecules, protein complexes or viruses, optical lithography of nanometric components, fast and miniaturized photonic integrated circuits and devices that apply ultra-small light focusing for cancer therapy and nonlinear spectroscopy. The demand for high spatial resolution encouraged large scientific interest in the last decades, where various super-resolution techniques have been demonstrated and successfully implemented. While all of these techniques suffer from some disadvantages by being slow, destructive and expensive, optical imaging tools are generally simple, reliable, instantaneous and noninvasive.

In this thesis, we develop and characterize novel nanometric optical platforms that enable the realization of unique photonic capabilities in visible light such as focusing light into a nano-sized spot and optical vortices, and also ultra-small super-oscillatory features. To do so we develop layered structures of metallic and dielectric materials to achieve short-wavelength guided modes that scale the intrinsic diffraction limit of the system and thus enhance its focusing abilities. The high resolution platforms developed are all homogeneous systems that do not depend on specific structuring or resonances. Namely, it is achieved by scaling the diffraction limit rather than breaking it. We show 2-fold wavelength shortening using short wavelength surface-plasmons on a SiN-Ag interface. We further enhance this diffraction scaling to almost 4-fold by developing a Si-based platform to support hybrid plasmonic-photonic modes with short wavelength. We measure directly the point-spread-function of this platform which is in the order of 60nm full-width at half-maximum (FWHM). We show the tunability and flexibility of the platform to support one or more guided modes where the characteristics of each individual mode (wavelength and propagation length) can be tuned separately. We use this tuning ability to create interference patterns that can be designed to give rise to super oscillations, as we show by direct measurements. On similar platforms we focus circularly polarized light into optical vortices with on-demand topological charge in unpreceded size of tens of nano-meters. Additionally, we show that utilizing such platforms of short-wavelength, we can achieve background free near-field detection. All of the developed structures and devices are experimentally demonstrated using scattering type scanning near-field optical microscopy (s-NSOM) that provides high resolution phase-resolved direct measurements.