Imaging and engineering optical localized modes at the nanoscale

Two-dimensional photonic crystal cavities fabricated on semiconductor slab are the state of the art devices to strongly localize electromagnetic fields in volumes below a cubic optical wavelength of light, thus acting as effcient nano-resonators. The high quality factor of two-dimensional photonic crystal cavities allows these systems to reach the strong coupling regime between the photonic localized mode and a two level quantum emitter. However, a fundamental request to maximize these effects is the precise positioning of the light source in the exact maximum spot of the localized modes. Moreover, when systems of interacting resonators are conceived, a basic condition to be satisfied is the precise control on the overlap between adjacent elements to induce an effective coupling. The sub-wavelength imaging of the electromagnetic fields localized in optical nano-resonators is therefore of the utmost relevance in the road map to integrate photonic nano-resonators in chipset architectures. In order to experimentally achieve this goal we need to overcome the resolution limitations that are imposed by light diffraction to any standard microscope, as reported in Chapter (1). In this Chapter we also investigate and review the more commonly methods employed to achieve a sub-wavelength spatial resolution. In particular, we describe the principle of operation of the scanning near-field optical microscope. In Chapter (2) we describe the basic properties of light confinement in photonic crystal nanocavities and also in optical nano-resonators based on disordered arrangements of light scatterers, where multiple scattering regime gives rise to randomly placed nanocavities. In particular, we illustrate how, on one hand, near-field techniques can experimentally probe the e.m. optical fields localized at the nanoscale and, on the other hand, how finite elements numerical calculations can simulate the behaviour of light in such nano-resonators. In Chapter (3), by exploiting the scanning near-field probe perturbation imaging technique, we report the sub-wavelength mapping of the electric or magnetic localized field component of light using dielectric tapered probes or aperture metal coated probes, respectively. The advent of artificial metamaterials operating at optical frequencies, in which the magnetic interaction with light can be as relevant as the electric response, makes it straightforward to simultaneously detect both the e.m. field components. Therefore, here we develop a plasmonic-based near-field probe to strongly enhance the collection efficiency from light-emitting nano-structures and, more interestingly, to achieve an ultra-bright and sub-wavelength simultaneous detection of both the resonant electric and magnetic fields. Photoluminescence based imaging methods, however, require optically active samples with the constrain of a spectral and spatial matching between the photonic mode and the light sources, which may suer of bleaching or blinking. Therefore, a pure optical method that can be applied on any kind of high quality factor nano-resonators to retrieve the confined modes distributions is actually missing. In order to achieve this goal we investigate the localized nanophotonic modes by developing a different approach. The presented experimental method, as reported in Chapter (4), combines scanning near-field optical microscopy with resonant scattering spectroscopy, and it is called Fano-imaging. This technique largely extends the investigation of nanoscale localized light states, since it is applicable to nano-resonators based on any kind of material, even where light sources cannot be embedded. Moreover, resonant scattering experiments exhibit spectral Fano resonances, which correspond to the interference between light directly scattered from the sample and light scattered after being resonantly coupled tothe localized mode. From the detailed analysis of Fano lineshapes it is possible to retrieve a deep sub-wavelength imaging of both the electric field intensity and the electric field spatial phase distribution, polarization resolved. Thus, we obtain unprecedented local information about the resonant light states. In Chapter (5) we deeply investigate, both theoretically and experimentally, systems composed by coupled nano-resonators, called “photonic molecules”. In fact, light behaviour in system based on multiple aligned photonic crystal nanocavities resembles the molecular interaction where the resulting normal modes exhibit energy splitting and spatial delocalization. This condition is achieved by an evanescent photon tunnelling, which occurs whenever the resonant wavelength matching condition and the electromagnetic field spatial overlap between them are fulfilled. These structures represent a large research topic also for quantum optics. However, a fundamental requirement to create proper quantum-optics devices is the design and control of adjacent nanocavity modes at the target wavelengths, within an accuracy which is not directly obtainable due to the fabrication tolerances. The compensation of the fluctuations related to the structural disorder and, more generally, the control of the resonance wavelength of each resonator and also of the tunnelling coefficient between adjacent nanocavities is a primary task to be achieved for developing efficient operating devices. In our analysis, we compare the interaction strength and the mode symmetry character of photonic molecules aligned along different lattice symmetry directions and composed by two or three photonic crystal nanocavities. Moreover, we theoretically evaluate the proper set of parameters to efficiently act on the coupling strength at the fabrication level or even with post-growing techniques. In particular, we develop a laser-assisted local oxidation of the dielectric environment in which the photonic cavities are fabricated. This oxidation induces a smooth and irreversible spectral shift of the resonant modes confined at the laser spot location. Therefore, the spatial selectivity of the postfabrication technique is exploited not only to adjust the resonant wavelength of a given nano-resonator to a target value, but, more strikingly, to modify the coupling strength in photonic molecules. Finally, by comparing the case of two and three nano-resonators we investigate the nearest-neighbour and next-nearest-neighbour coupling in array of photonic molecules. The last part of the thesis deals with the engineering of light states localized in strongly scattering disordered media, as reported in Chapter (6). Light behaviour in complex disordered systems attracts a lot of attention by fundamental physics as well as by technological applications involved in imaging through turbid media such as fog, clouds or living tissues. The occurrence of localized states in disordered media is a well-established phenomenon traced back to Anderson localization for electrons. However, the interaction between adjacent light sates driven by disorder has still to be completely understood and experimentally investigated. In Chapter (6) we demonstrate the possibility to engineer the confinement and the mutual interaction of modes in a two-dimensional disordered photonic structure. On one hand, the strong light confinement is achieved at the fabrication stage by an optimization of the design parameters. On the other hand, exploiting the accurate and local post-fabrication laser oxidation, we probe the interaction between overlapping localized modes, thereby paving the way for the creation of open transmission channels in strongly scattering media.