Nanoscale investigation of light-matter interactions mediated by magnetic and electric coupling

In nano-optics, light is controlled at length scales smaller than the wavelength. Consequently, investigations of photonic nanostructures require a resolution beyond the diffraction limit. Near-field microscopy has been one of the pillars of nano-optics since 1980s, as it can provide the necessary subwavelength resolution. This thesis provides a careful study of the electro-magnetic response of the coated probe which forms the heart of a near-field microscope. With the resulting insights we succeed in performing a new type of nanoscale investigation which involves both magnetic and electric fields at the nanoscale. \ud Firstly, we show that an aperture probe can simultaneously map the two in-plane electric field components of light in a photonic nanostructure. By performing phase-sensitive near-field measurements of both components, we reconstruct the highly structured in-plane polarization state of light in a photonic crystal waveguide, leading to the observation of polarization singularities at the nanoscale.\ud Secondly, we found that a coated probe is sensitive to the out-of-plane component of a magnetic field at optical frequency. Although this magnetic coupling does not lead to a direct detection of the magnetic field, it gives rise to new a type of interaction between probe and sample. By controlling the probe position near a maximum of a rapidly varying magnetic field component of light trapped in a photonic crystal nanocavity, we induce a novel blue-shift of the resonance frequency. In addition, we are able to increase the photon lifetime of the cavity through magnetic interaction. \ud Thirdly, by engineering the geometry of an aperture probe at the nanoscale, we succeed in unambiguously mapping the magnetic field of propagating light in a photonic structure. By using metamaterials concepts, we simultaneously visualize the electric and magnetic component of light with subwavelength resolution and phase sensitivity

Weak localization of light in superdiffusive random systems

L\'evy flights constitute a broad class of random walks that occur in many fields of research, from animal foraging in biology, to economy to geophysics. The recent advent of L\'evy glasses allows to study L\'evy flights in controlled way using light waves. This raises several questions about the influence of superdiffusion on optical interference effects like weak and strong localization. Super diffusive structures have the extraordinary property that all points are connected via direct jumps, meaning that finite-size effects become an essential part of the physical problem. Here we report on the experimental observation of weak localization in L\'evy glasses and compare results with recently developed optical transport theory in the superdiffusive regime. Experimental results are in good agreement with theory and allow to unveil how light propagates inside a finite-size superdiffusive system

Light Transport and localization in two-dimensional correlated disorder

Structural correlations in disordered media are known to affect significantly the propagation of waves. In this Letter, we theoretically investigate the transport and localization of light in 2D photonic structures with short-range correlated disorder. The problem is tackled semianalytically using the Baus-Colot model for the structure factor of correlated media and a modified independent scattering approximation. We find that short-range correlations make it possible to easily tune the transport mean free path by more than a factor of 2 and the related localization length over several orders of magnitude. This trend is confirmed by numerical finite-difference time-domain calculations. This study therefore shows that disorder engineering can offer fine control over light transport and localization in planar geometries, which may open new opportunities in both fundamental and applied photonics research

Bright-white beetle scales optimise multiple scattering of light.

Whiteness arises from diffuse and broadband reflection of light typically achieved through optical scattering in randomly structured media. In contrast to structural colour due to coherent scattering, white appearance generally requires a relatively thick system comprising randomly positioned high refractive-index scattering centres. Here, we show that the exceptionally bright white appearance of Cyphochilus and Lepidiota stigma beetles arises from a remarkably optimised anisotropy of intra-scale chitin networks, which act as a dense scattering media. Using time-resolved measurements, we show that light propagating in the scales of the beetles undergoes pronounced multiple scattering that is associated with the lowest transport mean free path reported to date for low-refractive-index systems. Our light transport investigation unveil high level of optimisation that achieves high-brightness white in a thin low-mass-per-unit-area anisotropic disordered nanostructure.The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement n [291349] and USAF grant FA9550-10-1-0020.This is the final published version, also available from Nature Publishing at http://www.nature.com/srep/2014/140815/srep06075/full/srep06075.html

Anisotropic Light Transport in White Beetle Scales

open6sìThe extremely brilliant whiteness shown by the Cyphochilus beetle is generated by multiple scattering of light inside the ultrathin scales that cover its body, whose interior is characterized by an anisotropic nanostructured network of chitin filaments. It is demonstrated that the structural anisotropy of the network is crucial in order to achieve high broadband reflectance from such a thin, low‐refractive‐index system.openCortese, L; Pattelli, L; Utel, F; Vignolini, S; Burresi, M; Wiersma, DSCortese, L; Pattelli, L; Utel, F; Vignolini, S; Burresi, M; Wiersma, D