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The ability to engineer the thermal conductivity of materials allows us to control the flow of heat and derive novel functionalities such as thermal rectification, thermal switching and thermal cloaking. While this could be achieved by making use of composites and metamaterials at bulk length-scales, engineering the thermal conductivity at micro- and nano-scale dimensions is considerably more challenging. In this work, we show that the local thermal conductivity along a single Si nanowire can be tuned to a desired value (between crystalline and amorphous limits) with high spatial resolution through selective helium ion irradiation with a well-controlled dose. The underlying mechanism is understood through molecular dynamics simulations and quantitative phonon-defect scattering rate analysis, where the behaviour of thermal conductivity with dose is attributed to the accumulation and agglomeration of scattering centres at lower doses. Beyond a threshold dose, a crystalline-amorphous transition was observed., Manipulating the flow of heat at the nanoscale is difficult because it requires the ability to tune the thermal properties of tiny structures. Here, the authors locally change the thermal conductivity of an individual silicon nanowire by irradiating it with helium ions.

Wavefront manipulation in metasurfaces typically relies on phase mapping with a finite number of elements. In particular, a discretized linear phase profile may be used to obtain a beam bending functionality. However, discretization limits the applicability of this approach for high angle bending due to the drastic efficiency drop when the phase is mapped by a small number of elements. In this work, we discuss a novel concept for energy redistribution in diffraction gratings and its application in the visible spectrum range, which helps overcome the constraints of ultrahigh angle (above 80°) beam bending. Arranging asymmetric dielectric nanoantennas into diffractive gratings, we show that one can efficiently redistribute the power between the grating orders at will. This is achieved by precise engineering of the scattering pattern of the nanoantennas. The concept is numerically and experimentally demonstrated at visible frequencies using several designs of TiO2 (titanium dioxide) nanoantennas for medium (...

Metasurfaces are planar arrangements of elements that are designed to present a particular response to an incident electromagnetic field. Due to their ability to control at will the phase, polarization and amplitude of the reflected and/or transmitted waves at a subwavelength scale they have gathered a great deal of attention among the research community. Although the first metasurface proposals were realized with plasmonic particles, the focus is now turning into all-dielectric approaches, in order to mitigate losses and increase the device efficiencies. Besides the obvious advantage of loss reduction, when high-index, subwavelength particles are considered a whole new family of resonant, magnetic-like modes is accessible. This new set of modes, which cannot be excited in simple metallic particles, brings additional functionalities for these metasurfaces, as will be shown in this talk. We will focus on the interesting effects that arise as a consequence of the far-field interference between electric and magnetic modes excited in the dielectric particles forming the metasurface and the strong modification of their scattering patterns as a consequence of this interference. In particular, we will show the possibility to realize so called ideal Huygens’ secondary sources to generate a perfectly transmissive metasurface with full phase control. We will also show that these metasurfaces support a generalized version of Brewster’s effect, in which the phenomenon is not restricted to a particular angle or polarization of incidence but can be tuned at will, and the different implications that this concept has.

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In this work, we propose a novel approach to efficient high-angle light beam bending and demonstrate it using TiO 2 and Si metasurfaces operating in the visible and near IR spectral ranges. The proposed approach allows large angle bending, with up to 82° experimentally demonstrated. Based on this concept we designed a lens with near-unity numerical aperture (NA∼0.99), exceeding all previously reported values.