KEYWORDS: Near field optics, Near field scanning optical microscopy, Nanostructures, Visual optics, Visualization, Dielectrics, Light, Optical resonators, Photonics, Control systems
Subwavelength optical resonators and scatterers are dramatically expanding the toolset of the optical sciences and photonics engineering. By offering the opportunity to control and shape light waves in nanoscale volumes, recent developments using high-refractive-index dielectric scatterers gave rise to efficient flat-optical components such as lenses, polarizers, phase plates, color routers, and nonlinear elements with a subwavelength thickness. Here, we take a deeper look into the unique interaction of light with amorphous silicon scatterers by tapping into their resonant modes with a localized subwavelength light source—an aperture scanning near-field probe [1,2]. Scanning near-field optical microscopy (SNOM) is a powerful tool to image the near-field distribution of resonant optical modes supported by nanophotonic structures with sub-diffraction resolution. Our experimental configuration essentially constitutes a dielectric antenna that is locally driven by the aperture probe [3].
In stark contrast to the mostly uneventful far-field extinction response, a rich variety of distinct patterns of bright spots—corresponding to enhanced transmittance of the probe excitation—is observed in the near-field scans. Various transverse magnetic (TM) and transverse electric (TE) Fabry-Perot-like modes of different mode parities in a variety of nanostructure geometries can be revealed using full field 3D finite difference time domain simulations and group theory.
[1] A. Frolov et al. Nano Letters 2017, 17 (12), 7629–7637
[2] A. Frolov et al. in preparation 2018
[3] D. Denkova et al., ACS Nano 2013, 7, 3168–3176
Through the effect of Purcell enhancement, nanoantennas strongly modify the local density of optical states and control the emission of coupled emitters. These antennas determine, in addition to the emission spectrum and polarization, also the angular distribution of the emitted photons, i.e., the radiation pattern. Nearly all directional nanoantennas reported so far, rely on external excitation schemes such as a laser or scanning tunneling microscope (STM) tip, which severely hamper on-chip integration. Here, we experimentally demonstrate for the first time, unidirectional light emission from electrically driven in-plane two-wire nanoantennas in the shape of the letter V. The antenna wires are connected with narrow electrical leads which support electrical currents while preserving the resonant properties of the antenna [1]. A nanoscopic tunneling gap is formed at the feed point of the antenna through a controlled electromigration procedure. Strong far-field interference between the spontaneous dipolar light emission of the tunnel junction and the fundamental quadrupolar resonance of the antenna gives rise to a directional radiation pattern [2]. We show that this directivity can be actively tuned with the applied voltage, and passively tuned with the antenna geometry. The experimental findings are analyzed in detail through electro-optical characterization and extensive numerical simulations. These fully configurable ultra-fast tunneling nanoantennas seamlessly exploit light-matter interactions at the nanoscale and set a new paradigm for directing optical energy on chip using an extremely small footprint. [1] Kern et al. Nature Photonics (2015) 9, 582–586; [2] Vercruysse et al. ACS Nano (2014) 8(8), 8232−8241.
The development of innovative photonic devices and metamaterials with tailor-made functionalities depends critically on our capability to characterize them and understand the underlying light-matter interactions. Thus, imaging all components of the electromagnetic light field with nanoscale resolution is of paramount importance in this area. Nowadays, the electric and the vertical magnetic field components of light can be measured with sub-wavelength resolution. This is achieved by scanning the sample surface with specific probes in a method known as scanning near-field optical microscopy (SNOM). However, within this toolbox, an unambiguous way of visualizing the horizontal magnetic field component has been missing.
We have answered this challenge by demonstrating experimentally that a hollow-pyramid circular aperture probe SNOM can directly image the horizontal magnetic field of light in simple plasmonic antennas – rod, disk and ring. These results are also confirmed by numerical simulations, showing that the probe can be approximated, in the first order, by a magnetic point-dipole source. This approximation substantially reduces the simulation time and complexity and facilitates the otherwise controversial interpretation of near-field images. Further, we use the validated technique to study complex plasmonic antennas and to explore new opportunities for their engineering and characterization. The applicability of this methodology is currently being extended beyond plasmonics structures.
Thus, the presented hollow-pyramid circular aperture based SNOM approach complements the existing techniques for imaging the different electromagnetic field components, by providing an opportunity to explore the tangential magnetic field of light with sub-wavelength resolution.
We demonstrate a facile method for fabrication of colloidal crystals containing a planar defect by using
PS@SiO2 core-shell spheres as building blocks. A monolayer of solid spheres was embedded in
core-shell colloidal crystals serving as the defect layer, which formed by means of self-assembly at the
air/water interface. Compared with previous methods, this fabrication method results in pronounced
passbands in the band gaps of the colloidal photonic crystal. The FWHM of the obtained passband is only
~16nm, which is narrower than the previously reported results. The influence of the defect layer
thickness on the optical properties of these sandwiched structures was also investigated. No high-cost
processes or specific equipment is needed in our approach. Inverse opals with planar defects can be
obtained via calcination of the PS cores, without the need of infiltration. The experimental results are in
good agreement with simulations performed using the FDTD method.
Mikhail Shestakov, Maria Meledina, Stuart Turner, Wouter Baekelant, Niels Verellen, Xianmei Chen, Johan Hofkens, Gustaaf Van Tendeloo, Victor Moshchalkov
Ag nanocluster-doped glasses have been prepared by a conventional melt-quenching method. The effect of melt
temperature and dwell time on the formation of Ag nanoclusters and Ag nanoparticles in simple host oxyfluoride glasses
has been studied. The increase of melt temperature and dwell time results in the dissolution of Ag nanoparticles and
substantial red-shift of absorption and photoluminescence spectra of the prepared glasses. The quantum yield of the
glasses is ~ 5% and does not depend on melt temperature and dwell time. The prepared glasses may be used as red
phosphors or down-conversion layers for solar-cells.
We report mapping of the lateral magnetic near-field distribution of plasmonic resonant modes in different nanostructure geometries by hollow-pyramid probe aperture-SNOM. Using full-field simulations we investigate how the near-field probe acts as a confined light source and how it efficiently excites surface plasmons. This excitation occurs at lateral magnetic field maxima, enabling the visualization of the lateral magnetic near-field distribution with subwavelength spatial resolution. Our approach complements the available methods for imaging the different field components of light.
[1] D. Denkova, N. Verellen et al., ACS nano 7(4), 3168-3176 (2013).
[2] D. Denkova, N. Verellen et al., Small, accepted (2013).
We report our latest results on second harmonic generation (SHG) microscopy from arrays of G-shaped
chiral gold nanostructures. The nanostructures are arranged in unit cells composed of four Gs, each rotated
at 90° with respect to its neighbors. As it has already been demonstrated, for linearly polarized light, these
unit cells yield a pattern of four SHG hotspots. However, upon increasing the pitch of the nanostructured
arrays, extra hotspots can be observed at the edges of the unit cells. While the origin of these extra hotspots
remains to be elucidated, their position indicates a relationship to coupling behavior between the unit cells.
Scanning second harmonic generation (SHG) microscopy is becoming an important tool for characterizing
nanopatterned metal surfaces and mapping plasmonic local field enhancements. Here we study G-shaped
and mirror-G-shaped gold nanostructures and test the robustness of the experimental results versus the
direction of scanning, the numerical aperture of the objective, the magnification, and the size of the laser
spot on the sample. We find that none of these parameters has a significant influence on the experimental
results.
We report that in the absence of electric dipole contributions, upon azimuthal sample rotation, the
corresponding SHG response was found to be chiral, i.e. it shows the presence of asymmetries with a sense
of rotation (lack of mirror symmetry). It was found that this sense of rotation reverses with the handedness
configuration (G and mirror-G, see Fig. 1). While it is apparent that the property originates in local field
enhancements of electric and/or magnetic multipoles, its explanation invites further theoretical research.
A systematic study of the geometry dependent nucleation of superconductivity in nanoscaled superconductors is presented in this paper. The experimental Τc(Η) phase boundary is compared to theoretical calculations obtained in the framework of the linearized Ginzburg-Landau theory for different geometries (square, triangle, disk). The influence of the transformation of a square into a rectangle on the Τc(Η) phase boundary is analyzed. In elongated rectangles, a crossover from a linear to a parabolic field dependence of Τc has been observed. The evolution of the superconducting state is studied in a perforated disk by varying the size of the hole. A transition from a one-dimensional to a two-dimensional regime is seen when increasing the magnetic field for disks with small holes.
The scaling behavior of the normal state transport and magnetic properties of the underdoped cuprates as a function of T/T0 is now interpreted as an evidence for the opening of a pseudogap at T* approximately T0. To investigate the origin of the pseudogap, we have analyzed first the temperature dependence of the resistivity (rho) (T) of the novel Sr2.5Ca11.5Cu24O41 spin-ladder compound under hydrostatic pressure of up to 8 GPa. This (rho) (T) dependence has been successfully explained by assuming that the relevant length scale for electrical transport -- the inelastic length -- is given by the magnetic correlation length related to the opening of a spin-gap in a one- dimensional (1D) even-chain spin-ladder (1D-SL). As a next step, we compared the (rho) (T) curves of the SL compounds and underdoped cuprates. We have found that the (rho) (T) dependences of YBa2Cu4O8 and underdoped YBa2Cu3Ox demonstrate a remarkable scaling with the (rho) (T) of the 1D-SL compound Sr2.5Ca11.5Cu24O41. This scaling implies that the pseudo-gap below T* in underdoped YBa2Cu4O8 and YBa2Cu3Ox is the spin-gap in the even-chain 1D-SL formed at T less than T* in these materials. Therefore, at temperatures Tc(x) less than T less than T*(x), underdoped cuprates are effectively in a 1D ('stripe') regime. A Josephson-like coupling of these stripes leads to a recovery of a 2D regime at low temperatures T approximately Tc(x).
In order to study the interplay between superconductivity and magnetism in proximity systems, we measured the critical temperature Tc decreases continuously with increasing tFe up to tF3 equals 15 angstrom. At higher tFe values Tc suddenly drops due to a larger pair breaking effect when the Fe layers change from the nonmagnetic to the ferromagnetic to the ferromagnetic regime. Measurements of Hc2(T) show that the superconducting Nb layers are already decoupled for tFe equals 12 angstrom; magnetization measurements however indicate that the Fe layers are only ferromagnetic for tFe > 15 angstrom. Critical current measurements indicate the presence of a matching effect for fields applied parallel to the layer due to the penetration of a row of vortices in the middle of the multilayer. The interaction of flux lines with magnetic dots is studied in more detail in Pb films with a lateral superlattice of magnetic Co dots. Matching anomalies in the magnetization curves indicate the stabilization of specific stable vortex lattices at integer and rational multiples of the first matching field. The influence of the stray field of the magnetic dots on the pinning properties will be discussed.
We have studied the flux-line confinement by a triangular and square array of submicron holes (antidots) in superconducting films (Pb, WGe) and multilayers (Pb/Ge). For large antidots, sharp cusp-like magnetization anomalies appear at the matching fields Hm, caused by the formation of multi-quanta vortices at each subsequent Hm. Critical current density jc and pinning force fp are strongly enhanced due to the pinning of single- or multiple-quanta vortices by the antidots. For relatively small antidots, new composite flux-line lattices, consisting of single- or multi-quanta vortices at antidots and single vortices at the interstices are observed at temperatures close to the critical temperature. The measurements of jc and fp for several antidot radii demonstrate that pinning centers with a size considerably larger than the coherence length are very efficient.
The temperature and field dependence of the flux creep rate S(T,H)=M0dMJd(lnt) and critical current j(T,H) have been measured in low- Tc (Pb/Ge) and high- Tc (YBa2Cu3O7IPrBa2Cu3O7) (YBCOIPrBCO) multilayers with different coupling strength between the superconducting layers. By taking thicker insulating separator layers, the dimensional crossover from an anisotropic 3D to a quasi 2D behaviour is induced. The detailed analysis of the S(T,H) and j(T,H) dependence made it possible to identify the pinning regimes as predicted by the collective pinning theory. In YBCO/PrBCO multilayers with completely decoupled YBCO planes, a strong enhancement of the quantum flux creep rate S(T—O) has been observed. The value of S(T—O)3% in the 3D YBCO superconductor and increases up to S(T—*O) 2O% in the quasi 2D YBCOIPrBCO multilayers. We also used magnetic fields H<H3D÷2D to decouple the Cu02 planes in Bi2Sr2CaCu2O8 (BSCCO) single crystals. A great similarity between the S(T,H0) andjc(T,Ho) curves has been found in BSCCO, with the superconducting layers decoupled by the applied field, and in the YBCOIPrBCO with the YBCO layers decoupled by inserting thick insulating PrBCO layers.
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