Unconventional Giant "Magnetoresistance" in Bosonic Semiconducting Diamond Nanorings

The emergence of superconductivity in doped insulators such as cuprates and pnictides coincides with their doping-driven insulator-metal transitions. Above the critical doping threshold, a metallic state sets in at high temperatures, while superconductivity sets in at low temperatures. An unanswered question is whether the formation of Cooper pairsin a well-established metal will inevitably transform the host material into a superconductor, as manifested by a resistance drop. Here, this question is addressed by investigating the electrical transport in nanoscale rings (full loops) and half loops manufactured from heavily boron-doped diamond. It is shown that in contrast to the diamond half-loops (DHLs) exhibiting a metal-superconductor transition, the diamond nanorings (DNRs) demonstrate a sharp resistance increase up to 430% and a giant negative "magnetoresistance" below the superconducting transition temperature of the starting material. The finding of the unconventional giant negative "magnetoresistance", as distinct from existing categories of magnetoresistance, that is, the conventional giant magnetoresistance in magnetic multilayers, the colossal magnetoresistance in perovskites, and the geometric magnetoresistance in semiconductor-metal hybrids, reveals the transformation of the DNRs from metals to bosonic semiconductors upon the formation of Cooper pairs. DNRs like these could be used to manipulate Cooper pairs in superconducting quantum devices.

Tunable Noninteger Flux Quantum of Vortices in Superconducting Strips

Flux quantization has been widely regarded as the hallmark of the macroscopic quantum state of superconductivity. However, practical design of superconductor devices exploiting finite size confinement effects may induce exotic phenomena, including nonquantized vortices. In our research, the magnetic flux of vortices has been studied in a series of superconducting strips as a function of the strip width and the penetration depth. In both circumstances, the observation using scanning Hall probe microscope (SHPM) displays a controlled evolution from singly quantized vortices to nonquantized ones. It is also found that the magnetic flux is immune to the flowing supercurrent. The simulations based on Ginzburg-Landau theory agree well with experimental results. The observed behavior of the vortex flux may open new perspectives for fundamental research and applications based on vortex matter, such as vortex-memory devices and magnetic field traps for ultracold atoms.

Nonlocal response of plasmonic core-shell nanotopologies excited by dipole emitters

In light of the emergence of nonclassical effects, a paradigm shift in the conventional macroscopic treatment is required to accurately describe the interaction between light and plasmonic structures with deep-nanometer features. Towards this end, several nonlocal response models, supplemented by additional boundary conditions, have been introduced, investigating the collective motion of the free electron gas in metals. The study of the dipole-excited core-shell nanoparticle has been performed, by employing the following models: the hard-wall hydrodynamic model; the quantum hydrodynamic model; and the generalized nonlocal optical response. The analysis is conducted by investigating the near and far field characteristics of the emitter-nanoparticle system, while considering the emitter outside and inside the studied topology. It is shown that the above models predict striking spectral features, strongly deviating from the results obtained via the classical approach, for both simple and noble constitutive metals.

Long-Range Ordered Amorphous Atomic Chains as Building Blocks of a Superconducting Quasi-One-Dimensional Crystal

Crystalline and amorphous structures are two of the most common solid-state phases. Crystals having orientational and periodic translation symmetries are usually both short-range and long-range ordered, while amorphous materials have no long-range order. Short-range ordered but long-range disordered materials are generally categorized into amorphous phases. In contrast to the extensively studied crystalline and amorphous phases, the combination of short-range disordered and long-range ordered structures at the atomic level is extremely rare and so far has only been reported for solvated fullerenes under compression. Here, a report on the creation and investigation of a superconducting quasi-1D material with long-range ordered amorphous building blocks is presented. Using a diamond anvil cell, monocrystalline (TaSe₄ )₂ I is compressed and a system is created where the TaSe₄ atomic chains are in amorphous state without breaking the orientational and periodic translation symmetries of the chain lattice. Strikingly, along with the amorphization of the atomic chains, the insulating (TaSe₄ )₂ I becomes a superconductor. The data provide critical insight into a new phase of solid-state materials. The findings demonstrate a first ever case where superconductivity is hosted by a lattice with periodic but amorphous constituent atomic chains.

Yu-Shiba-Rusinov bands in ferromagnetic superconducting diamond

The combination of different exotic properties in materials paves the way for the emergence of their new potential applications. An example is the recently found coexistence of the mutually antagonistic ferromagnetism and superconductivity in hydrogenated boron-doped diamond, which promises to be an attractive system with which to explore unconventional physics. Here, we show the emergence of Yu-Shiba-Rusinov (YSR) bands with a spatial extent of tens of nanometers in ferromagnetic superconducting diamond using scanning tunneling spectroscopy. We demonstrate theoretically how a two-dimensional (2D) spin lattice at the surface of a three-dimensional (3D) superconductor gives rise to the YSR bands and how their density-of-states profile correlates with the spin lattice structure. The established strategy to realize new forms of the coexistence of ferromagnetism and superconductivity opens a way to engineer the unusual electronic states and also to design better-performing superconducting devices.

Direct Observation of Nanoscale Light Confinement without Metal

Manipulation of light below the diffraction limit forms the basis of nanophotonics. Metals can confine light at the subwavelength scale but suffer from high loss of energy. Recent reports have theoretically demonstrated the possibility of light confinement below the diffraction limit using transparent dielectric metamaterials. Here, nanoscale light confinement (<λ/20) in transparent dielectric materials is shown experimentally through a luminescent nanosystem with rationally designed dielectric claddings. Theoretically, green light with a wavelength of 540 nm has a transmission of 98.8% when passing through an ultrathin NaYF₄ /NaGdF₄ superlattice cladding (thickness: 6.9 nm). Unexpectedly, the complete confinement of green emission (540 nm) by such an ultrathin dielectric cladding is directly observed. FDTD calculations are used to confirm that the ultrathin dielectric cladding has negligible influence on the transmission of propagating light, but extraordinary confinement of evanescent waves. This will provide new opportunities for nanophotonics by completely averting the use of metals.

In situ tailoring of superconducting junctions via electro-annealing

We demonstrate the in situ engineering of superconducting nanocircuitry by targeted modulation of material properties through high applied current densities. We show that the sequential repetition of such customized electro-annealing in a niobium (Nb) nanoconstriction can broadly tune the superconducting critical temperature T_c and the normal-state resistance R_n in the targeted area. Once a sizable R_n is reached, clear magneto-resistance oscillations are detected along with a Fraunhofer-like field dependence of the critical current, indicating the formation of a weak link but with further adjustable characteristics. Advanced Ginzburg-Landau simulations fully corroborate this picture, employing the detailed parametrization from the electrical characterization and high resolution electron microscope images of the region within the constriction where the material has undergone amorphization by electro-annealing.

Electrically Driven Unidirectional Optical Nanoantennas

Directional antennas revolutionized modern day telecommunication by enabling precise beaming of radio and microwave signals with minimal loss of energy. Similarly, directional optical nanoantennas are expected to pave the way toward on-chip wireless communication and information processing. Currently, on-chip integration of such antennas is hampered by their multielement design or the requirement of complicated excitation schemes. Here, we experimentally demonstrate electrical driving of in-plane tunneling nanoantennas to achieve broadband unidirectional emission of light. Far-field interference, as a result of the spectral overlap between the dipolar emission of the tunnel junction and the fundamental quadrupole-like resonance of the nanoantenna, gives rise to a directional radiation pattern. By tuning this overlap using the applied voltage, we record directivities as high as 5 dB. In addition to electrical tunability, we also demonstrate passive tunability of the directivity using the antenna geometry. These fully configurable electrically driven nanoantennas provide a simple way to direct optical energy on-chip using an extremely small device footprint.

Near-Field Mapping of Optical Fabry-Perot Modes in All-Dielectric Nanoantennas

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. In this work, we take a deeper look into the unique interaction of light with rod-shaped amorphous silicon scatterers by tapping into their resonant modes with a localized subwavelength light source-an aperture scanning near-field probe. Our experimental configuration essentially constitutes a dielectric antenna that is locally driven by the aperture probe. We show how leaky transverse electric and magnetic modes can selectively be excited and form specific near-field distribution depending on wavelength and antenna dimensions. The probe's transmittance is furthermore enhanced upon coupling to the Fabry-Perot cavity modes, revealing all-dielectric nanorods as efficient transmitter antennas for the radiation of subwavelength emitters, in addition to constituting an elementary building block for all-dielectric metasurfaces and flat optics.

Bosonic Confinement and Coherence in Disordered Nanodiamond Arrays

In the presence of disorder, superconductivity exhibits short-range characteristics linked to localized Cooper pairs which are responsible for anomalous phase transitions and the emergence of quantum states such as the bosonic insulating state. Complementary to well-studied homogeneously disordered superconductors, superconductor-normal hybrid arrays provide tunable realizations of the degree of granular disorder for studying anomalous quantum phase transitions. Here, we investigate the superconductor-bosonic dirty metal transition in disordered nanodiamond arrays as a function of the dispersion of intergrain spacing, which ranges from angstroms to micrometers. By monitoring the evolved superconducting gaps and diminished coherence peaks in the single-quasiparticle density of states, we link the destruction of the superconducting state and the emergence of bosonic dirty metallic state to breaking of the global phase coherence and persistence of the localized Cooper pairs. The observed resistive bosonic phase transitions are well modeled using a series-parallel circuit in the framework of bosonic confinement and coherence.

Benchmarking of software tools for the characterization of nanoparticles

Although many commercially available electromagnetic tools are conveniently used in RF and microwave applications, only a few of them provide the capability to analyze the optical response of nanometric radiators and scatterers. The assessment of their performance in the visible to near ultraviolet part of the electromagnetic (EM) spectrum becomes more and more important, considering the exponential rise of nanoscale systems. Since the accuracy of these numerical tools has not been fully investigated in literature, in this paper we essentially demonstrate a comparative study of the most widely used EM field solvers in the area of nano-plasmonics: COMSOL, CST and Lumerical. This is done through the investigation of the near and far field characteristics of basic canonical nanoparticles such as spheres, shells, cubes and cuboids, varying their sizes and constituting materials. The benchmarking results clearly show that at this moment not all EM field solvers offer the same accuracy.

Controlled Generation of Quantized Vortex-Antivortex Pairs in a Superconducting Condensate

Quantized vortices, as topological defects, play an important role in both physics and technological applications of superconductors. Normally, the nucleation of vortices requires the presence of a high magnetic field or current density, which allow the vortices to enter from the sample boundaries. At the same time, the controllable generation of individual vortices inside a superconductor is still challenging. Here, we report the controllable creation of single quantum vortices and antivortices at any desirable position inside a superconductor. We exploit the local heating effect of a scanning tunneling microscope (STM) tip: superconductivity is locally suppressed by the tip and vortex-antivortex pairs are generated when supercurrent flows around the hot spot. The experimental results are well-explained by theoretical simulations within the Ginzburg-Landau approach.

Dendritic optical antennas: scattering properties and fluorescence enhancement

With the development of nanotechnologies, researchers have brought the concept of antenna to the optical regime for manipulation of nano-scaled light matter interactions. Most optical nanoantennas optimize optical function, but are not electrically connected. In order to realize functions that require electrical addressing, optical nanoantennas that are electrically continuous are desirable. In this article, we study the optical response of a type of electrically connected nanoantennas, which we propose to call “dendritic” antennas. While they are connected, they follow similar antenna hybridization trends to unconnected plasmon phased array antennas. The optical resonances supported by this type of nanoantennas are mapped both experimentally and theoretically to unravel their optical response. Photoluminescence measurements indicate a potential Purcell enhancement of more than a factor of 58.

Superconducting Ferromagnetic Nanodiamond

Superconductivity and ferromagnetism are two mutually antagonistic states in condensed matter. Research on the interplay between these two competing orderings sheds light not only on the cause of various quantum phenomena in strongly correlated systems but also on the general mechanism of superconductivity. Here we report on the observation of the electronic entanglement between superconducting and ferromagnetic states in hydrogenated boron-doped nanodiamond films, which have a superconducting transition temperature T_c ∼ 3 K and a Curie temperature T_Curie > 400 K. In spite of the high T_Curie, our nanodiamond films demonstrate a decrease in the temperature dependence of magnetization below 100 K, in correspondence to an increase in the temperature dependence of resistivity. These anomalous magnetic and electrical transport properties reveal the presence of an intriguing precursor phase, in which spin fluctuations intervene as a result of the interplay between the two antagonistic states. Furthermore, the observations of high-temperature ferromagnetism, giant positive magnetoresistance, and anomalous Hall effect bring attention to the potential applications of our superconducting ferromagnetic nanodiamond films in magnetoelectronics, spintronics, and magnetic field sensing.

Statistics of localized phase slips in tunable width planar point contacts

The main dissipation mechanism in superconducting nanowires arises from phase slips. Thus far, most of the studies focus on long nanowires where coexisting events appear randomly along the nanowire. In the present work we investigate highly confined phase slips at the contact point of two superconducting leads. Profiting from the high current crowding at this spot, we are able to shrink in-situ the nanoconstriction. This procedure allows us to investigate, in the very same sample, thermally activated phase slips and the probability density function of the switching current I_sw needed to trigger an avalanche of events. Furthermore, for an applied current larger than I_sw, we unveil the existence of two distinct thermal regimes. One corresponding to efficient heat removal where the constriction and bath temperatures remain close to each other, and another one in which the constriction temperature can be substantially larger than the bath temperature leading to the formation of a hot spot. Considering that the switching current distribution depends on the exact thermal properties of the sample, the identification of different thermal regimes is of utmost importance for properly interpreting the dissipation mechanisms in narrow point contacts.

Revealing Nanostructures through Plasmon Polarimetry

Polarized optical dark-field spectroscopy is shown to be a versatile noninvasive probe of plasmonic structures that trap light to the nanoscale. Clear spectral polarization splittings are found to be directly related to the asymmetric morphology of nanocavities formed between faceted gold nanoparticles and an underlying gold substrate. Both experiment and simulation show the influence of geometry on the coupled system, with spectral shifts Δλ = 3 nm from single atoms. Analytical models allow us to identify the split resonances as transverse cavity modes, tightly confined to the nanogap. The direct correlation of resonance splitting with atomistic morphology allows mapping of subnanometre structures, which is crucial for progress in extreme nano-optics involving chemistry, nanophotonics, and quantum devices.

Local destruction of superconductivity by non-magnetic impurities in mesoscopic iron-based superconductors

The determination of the pairing symmetry is one of the most crucial issues for the iron-based superconductors, for which various scenarios are discussed controversially. Non-magnetic impurity substitution is one of the most promising approaches to address the issue, because the pair-breaking mechanism from the non-magnetic impurities should be different for various models. Previous substitution experiments demonstrated that the non-magnetic zinc can suppress the superconductivity of various iron-based superconductors. Here we demonstrate the local destruction of superconductivity by non-magnetic zinc impurities in Ba0.5K0.5Fe2As2 by exploring phase-slip phenomena in a mesoscopic structure with 119 × 102 nm(2) cross-section. The impurities suppress superconductivity in a three-dimensional 'Swiss cheese'-like pattern with in-plane and out-of-plane characteristic lengths slightly below ∼1.34 nm. This causes the superconducting order parameter to vary along abundant narrow channels with effective cross-section of a few square nanometres. The local destruction of superconductivity can be related to Cooper pair breaking by non-magnetic impurities.

A radio-frequency coil for the microwave characterization of vortex dynamics in thin film superconductors

A radio-frequency coil for the experimental investigation of the magnetic properties of thin superconducting films under microwave fields at different values of temperature and dc magnetic field has been developed. The system has been used for low-temperature microwave frequency-dependent magnetization measurements in a Pb thin film with an engineered periodical antidot array. The characteristic frequencies and the electric and magnetic fields of the resonant system formed by a multi-turn coil with a sample loaded in its core are estimated using the helical approach. A good agreement of the calculated values with those recorded in swept-frequency spectra is obtained. The relation between the characteristics of the resonant structure and the frequency-driven magnetic response of the sample at different nominal microwave powers documents the capability and sensitivity of the layout.

Disrupting the wall accumulation of human sperm cells by artificial corrugation

Many self-propelled microorganisms are attracted to surfaces. This makes their dynamics in restricted geometries very different from that observed in the bulk. Swimming along walls is beneficial for directing and sorting cells, but may be detrimental if homogeneous populations are desired, such as in counting microchambers. In this work, we characterize the motion of human sperm cells ∼60 μm long, strongly confined to ∼25 μm shallow chambers. We investigate the nature of the cell trajectories between the confining surfaces and their accumulation near the borders. Observed cell trajectories are composed of a succession of quasi-circular and quasi-linear segments. This suggests that the cells follow a path of intermittent trappings near the top and bottom surfaces separated by stretches of quasi-free motion in between the two surfaces, as confirmed by depth resolved confocal microscopy studies. We show that the introduction of artificial petal-shaped corrugation in the lateral boundaries removes the tendency of cells to accumulate near the borders, an effect which we hypothesize may be valuable for microfluidic applications in biomedicine.

300 mm Wafer-level, ultra-dense arrays of Au-capped nanopillars with sub-10 nm gaps as reliable SERS substrates

The 193 nm deep UV immersion lithography is leveraged to fabricate highly dense and uniform arrays of Au-capped Si nanopillars on a 300 mm wafer level, and the substrates are applied in surface enhanced Raman spectroscopy for reliable molecule detection. Due to the sub-10 nm gap sizes and ultra-high array density with the lattice constant less than 100 nm, our nanopillar based substrates outperform the current commercial products in terms of the signal intensity, reproducibility and fabrication scale.

Directional fluorescence emission by individual V-antennas explained by mode expansion

Specially designed plasmonic antennas can, by far-field interference of different antenna elements or a combination of multipolar antenna modes, scatter light unidirectionally, allowing for directional light control at the nanoscale. One of the most basic and compact geometries for such antennas is a nanorod with broken rotational symmetry, in the shape of the letter V. In this article, we show that these V-antennas unidirectionally scatter the emission of a local dipole source in a direction opposite the undirectional side scattering of a plane wave. Moreover, we observe high directivity, up to 6 dB, only for certain well-defined positions of the emitter relative to the antenna. By employing a rigorous eigenmode expansion analysis of the V-antenna, we fully elucidate the fundamental origin of its directional behavior. All findings are experimentally verified by measuring the radiation patterns of a scattered plane wave and the emission pattern of fluorescently doped PMMA positioned in different regions around the antenna. The fundamental interference effects revealed in the eigenmode expansion can serve as guidelines in the understanding and further development of nanoscale directional scatterers.

Nonlinear superchiral meta-surfaces: tuning chirality and disentangling non-reciprocity at the nanoscale

Circularly polarized light is incident on a nanostructured chiral meta-surface. In the nanostructured unit cells whose chirality matches that of light, superchiral light is forming and strong optical second harmonic generation can be observed.

Lateral magnetic near-field imaging of plasmonic nanoantennas with increasing complexity

The design of many promising, newly emerging classes of photonic metamaterials and subwavelength confinement structures requires detailed knowledge and understanding of the electromagnetic near-field interactions between their building blocks. While the electric field distributions and, respectively, the electric interactions of different nanostructures can be routinely measured, for example, by scattering near-field microscopy, only recently experimental methods for imaging the magnetic field distributions became available. In this paper, we provide direct experimental maps of the lateral magnetic near-field distributions of variously shaped plasmonic nanoantennas by using hollow-pyramid aperture scanning near-field optical microscopy (SNOM). We study both simple plasmonic nanoresonators, such as bars, disks, rings and more complex antennas. For the studied structures, the magnetic near-field distributions of the complex resonators have been found to be a superposition of the magnetic near-fields of the individual constituting elements. These experimental results, explained and validated by numerical simulations, open new possibilities for engineering and characterization of complex plasmonic antennas with increased functionality.

Mode parity-controlled Fano- and Lorentz-like line shapes arising in plasmonic nanorods

We present the experimental observation of spectral lines of distinctly different shapes in the optical extinction cross-section of metallic nanorod antennas under near-normal plane wave illumination. Surface plasmon resonances of odd mode parity present Fano interference in the scattering cross-section, resulting in asymmetric spectral lines. Contrarily, modes with even parity appear as symmetric Lorentzian lines. Finite element simulations are used to verify the experimental results. The emergence of either constructive or destructive mode interference is explained with a semianalytical 1D line current model. This simple model directly explains the mode-parity dependence of the Fano-like interference. Plasmonic nanorods are widely used as half-wave optical dipole antennas. Our findings offer a perspective and theoretical framework for operating these antennas at higher-order modes.

Spectral interferometric microscopy reveals absorption by individual optical nanoantennas from extinction phase

Optical antennas transform light from freely propagating waves into highly localized excitations that interact strongly with matter. Unlike their radio frequency counterparts, optical antennas are nanoscopic and high frequency, making amplitude and phase measurements challenging and leaving some information hidden. Here we report a novel spectral interferometric microscopy technique to expose the amplitude and phase response of individual optical antennas across an octave of the visible to near-infrared spectrum. Although it is a far-field technique, we show that knowledge of the extinction phase allows quantitative estimation of nanoantenna absorption, which is a near-field quantity. To verify our method we characterize gold ring-disk dimers exhibiting Fano interference. Our results reveal that Fano interference only cancels a bright mode’s scattering, leaving residual extinction dominated by absorption. Spectral interference microscopy has the potential for real-time and single-shot phase and amplitude investigations of isolated quantum and classical antennas with applications across the physical and life sciences.

Absorption by an optical nanoantenna determines its interaction strength with light, yet this quantity is hidden from conventional spectroscopy. Gennaro et al. now demonstrate a spectroscopic technique that reveals a nanoantenna’s absorption by recovering its amplitude and phase response.

Global and local superconductivity in boron-doped granular diamond

Strong granularity-correlated and intragrain modulations of the superconducting order parameter are demonstrated in heavily boron-doped diamond situated not yet in the vicinity of the metal-insulator transition. These modulations at the superconducting state (SC) and at the global normal state (NS) above the resistive superconducting transition, reveal that local Cooper pairing sets in prior to the global phase coherence.

Dynamic visualization of nanoscale vortex orbits

Due to the atomic-scale resolution, scanning tunneling microscopy is an ideal technique to observe the smallest objects. Nevertheless, it suffers from very long capturing times in order to investigate dynamic processes at the nanoscale. We address this issue, for vortex matter in NbSe2, by driving the vortices using an ac magnetic field and probing the induced periodic tunnel current modulations. Our results reveal different dynamical modes of the driven vortex lattices. In addition, by recording and synchronizing the time evolution of the tunneling current at each pixel, we visualize the overall dynamics of the vortex lattice with submillisecond time resolution and subnanometer spatial resolution.

Geometrical guidance and trapping transition of human sperm cells

The guidance of human sperm cells under confinement in quasi-2D microchambers is investigated using a purely physical method to control their distribution. Transport property measurements and simulations are performed with diluted sperm populations, for which effects of geometrical guidance and concentration are studied in detail. In particular, a trapping transition at convex angular wall features is identified and analyzed. We also show that highly efficient microratchets can be fabricated by using curved asymmetric obstacles to take advantage of the spermatozoa specific swimming strategy.

Interacting plasmonic nanostructures beyond the quasi-static limit: a "circuit" model

The interaction between individual plasmonic nanoparticles plays a crucial role in tuning and shaping the surface plasmon resonances of a composite structure. Here, we demonstrate that the detailed character of the coupling between plasmonic structures can be captured by a modified "circuit" model. This approach is generally applicable and, as an example here, is applied to a dolmen-like nanostructure consisting of a vertically placed gold monomer slab and two horizontally placed dimer slabs. By utilizing the full-wave eigenmode expansion method (EEM), we extract the eigenmodes and eigenvalues for these constituting elements and reduce their electromagnetic interaction to the structures' mode interactions. Using the reaction concept, we further summarize the mode interactions within a "coupling" matrix. When the driving voltage source imposed by the incident light is identified, an equivalent circuit model can be constructed. Within this model, hybridization of the plasmonic modes in the constituting nanostructure elements is discussed. The proposed circuit model allows the reuse of powerful circuit analysis techniques in the context of plasmonic structures. As an example, we derive an equivalent of Thévenin's theorem in circuit theory for nanostructures. Applying the equivalent Thévenin's theorem, the well-known Fano resonance is easily explained.

Ag nanocluster functionalized glasses for efficient photonic conversion in light sources, solar cells and flexible screen monitors

An ever growing demand for efficient energy conversion, for instance in luminescent lamps, flexible screens and solar cells, results in the current significant growth of research on functionalized nanomaterials for these applications. This paper reviews recent developments of a new class of optically active nanostructured materials based on glasses doped with luminescent Ag nanoclusters consisting of only a few Ag atoms, suitable for mercury-free white light generation and solar down-shifting. This new approach, based solely on Ag nanocluster doped glasses, is compared to other alternatives in the field of Ag and rare-earth ion co-doped materials.

Unidirectional side scattering of light by a single-element nanoantenna

Unidirectional side scattering of light by a single-element plasmonic nanoantenna is demonstrated using full-field simulations and back focal plane measurements. We show that the phase and amplitude matching that occurs at the Fano interference between two localized surface plasmon modes in a V-shaped nanoparticle lies at the origin of this effect. A detailed analysis of the V-antenna modeled as a system of two coherent point-dipole sources elucidates the mechanisms that give rise to a tunable experimental directivity as large as 15 dB. The understanding of Fano-based directional scattering opens a way to develop new directional optical antennas for subwavelength color routing and self-referenced directional sensing. In addition, the directionality of these nanoantennas can increase the detection efficiency of fluorescence and surface enhanced Raman scattering.

Rendering dark modes bright by using asymmetric split ring resonators

We have studied both theoretically and experimentally symmetric and asymmetric planar metallic Split Ring Resonators. We demonstrate that introducing structural asymmetry makes it possible to excite several higher order modes of both even (l = 2) and odd (l = 3, 5) order, which are otherwise inaccessible for a normally incident plane wave in symmetric structures. Experimentally we observe that the even mode resonances of asymmetric resonators have a quality factor 5.8 times higher than the higher order odd resonances.

Nanostripe length dependence of plasmon-induced material deformations

Following the impact of a single femtosecond light pulse on nickel nanostripes, material deformations-or "nanobumps"-are created. We have studied the dependence of these nanobumps on the length of nanostripes and verified the link with plasmons. More specifically, local electric currents can melt the nanostructures in the hotspots, where hydrodynamic processes give rise to nanobumps. This process is further confirmed by independently simulating local magnetic fields, since these are produced by the same local electric currents.

Influence of swimming strategy on microorganism separation by asymmetric obstacles

It has been shown that a nanoliter chamber separated by a wall of asymmetric obstacles can lead to an inhomogeneous distribution of self-propelled microorganisms. Although it is well established that this rectification effect arises from the interaction between the swimmers and the noncentrosymmetric pillars, here we demonstrate numerically that its efficiency is strongly dependent on the detailed dynamics of the individual microorganism. In particular, for the case of run-and-tumble dynamics, the distribution of run lengths, the rotational diffusion, and the partial preservation of run orientation memory through a tumble are important factors when computing the rectification efficiency. In addition, we optimize the geometrical dimensions of the asymmetric pillars in order to maximize the swimmer concentration and we illustrate how it can be used for sorting by swimming strategy in a long array of parallel obstacles.

Mapping magnetic near-field distributions of plasmonic nanoantennas

We present direct experimental mapping of the lateral magnetic near-field distribution in plasmonic nanoantennas using aperture scanning near-field optical microscopy (SNOM). By means of full-field simulations it is demonstrated how the coupling of the hollow-pyramid aperture probe to the nanoantenna induces an effective magnetic dipole which efficiently excites surface plasmon resonances only at lateral magnetic field maxima. This excitation in turn affects the detected light intensity enabling the visualization of the lateral magnetic near-field distribution of multiple odd and even order plasmon modes with subwavelength spatial resolution.

Metal-bosonic insulator-superconductor transition in boron-doped granular diamond

In a variety of superconductors, mostly in two-dimensional (2D) and one-dimensional (1D) systems, the resistive superconducting transition R(T) demonstrates in many cases an anomalous narrow R(T) peak just preceding the onset of the superconducting state R=0 at T(c). The amplitude of this R(T) peak in 1D and 2D systems ranges from a few up to several hundred percent. In three-dimensional (3D) systems, however, the R(T) peak close to T(c) is rarely observed, and it reaches only a few percent in amplitude. Here we report on the observation of a giant (∼1600%) and very narrow (∼1  K) resistance peak preceding the onset of superconductivity in heavily boron-doped diamond. This anomalous R(T) peak in a 3D system is interpreted in the framework of an empirical model based on the metal-bosonic insulator-superconductor transitions induced by a granularity-correlated disorder in heavily doped diamond.

Origin of visible photoluminescence from arrays of vertically arranged Si-nanopillars decorated with Si-nanocrystals

Arrays of vertically aligned Si-nanopillars, with average diameters of 100 nm and 5 μm length, have been prepared by wet chemical etching of crystalline silicon in a special manner. Samples with smooth- and porous-walled nanopillars have been studied. In the case of the latter, Si-nanocrystals, passivated with SiO(x), decorating the surface of the nanopillars are identified by the means of TEM and FTIR. When excited by UV-blue light, the porous-walled Si-nanopillars are found to have a strong broad visible emission band around 1.8 eV with a nearly perfect Gaussian shape, μs luminescence lifetimes, minor emission polarization and a non-monotonic temperature dependence of luminescence. The Si-nanocrystal surface is found to be responsible for the luminescence. The red-shift of the emission maximum and the luminescence quenching induced by oxidation in UV-ozone confirm this assumption. A model of luminescence involving UV photon absorption by Si-nanocrystals with subsequent exciton radiative recombination on defect sites in SiO(x) covering Si-nanocrystals has been proposed. Possible applications of the nanopillar arrays are discussed.

Method for the solution of the nucleation problem in arbitrary mesoscopic superconductors: theory and application

We present a method for finding the condensate distribution at the nucleation of superconductivity for arbitrary polygons. The method is based on conformal mapping of the analytical solution of the linearized Ginzburg-Landau problem for the disk and uses the superconducting gauge for the magnetic potential proposed earlier. As a demonstration of the method's accuracy, we calculate the distribution of the order parameter in regular polygons and compare the obtained solutions with available numerical results. As an example of an irregular polygon, we consider a deformed hexagon and prove that its calculation with the proposed method requires the same level of computational efforts as the regular ones. Finally, we extend the method over samples with arbitrary smooth boundaries. With this, we have made simulations for an experimental sample. They have shown perfect agreement with experimental data.

Polarization memory of white luminescence of Ag nanoclusters dispersed in glass host

A mechanism for white luminescence of Ag nanoclusters dispersed in oxyfluoride glass host has been revealed by studying a temperature dependence of its polarization memory. The spectral dependence of the polarization memory indicates the presence of a variety of Ag nanoclusters, particularly emitting in the blue, green and red. Temperature activated intercluster energy transfer has been found responsible for white luminescence. The means for increasing luminescence quantum yield have been suggested. This efficient white luminescence may be used in highly demanded devices, such as luminescent lamps, displays, color phosphors for LEDs, photovoltaic devices based on down shifting of solar spectrum.

Ultrafast polarization shaping with Fano plasmonic crystals

Femtosecond-scale polarization state shaping is experimentally found in optical response of a plasmonic nanograting by means of time-resolved Stokes polarimetry. Simultaneous measurements of the Stokes parameters as a function of time reveal a remarkable alteration of the polarization state inside a single femtosecond pulse reflected from a plasmonic crystal due to the excitation of time-delayed polarization-sensitive surface plasmons with a highly birefringent Fano-type spectral profile. Time-dependent depolarization, indicating the sub-130-femtosecond polarization change inside the pulse, is experimentally found and described within an analytical model which predicts the fivefold enhancement of the polarization conversion effect with the use of a narrower time gate.

Energy level diagram and kinetics of luminescence of Ag nanoclusters dispersed in a glass host

A site-selective spectroscopy study of Ag nanoclusters dispersed in oxyfluoride glass hosts has been carried out. The nano- to millisecond, essentially non-exponential, luminescence kinetics of Ag nanoclusters has been detected in the spectral range from 450 to 1000 nm, when excited at discrete wavelengths in the range 250 to 450 nm. Based on these experimental observations, the energy level configuration coordinate diagram for the involved ground and excited singlet/triplet states of the Ag nanoclusters has been proposed and confirmed by the density functional theory (DFT). The sites for the Ag nanoclusters are argued to be multiple. The structure/geometry of the involved Ag nanoclusters has been suggested to involve spin-paired dimers Ag²⁺, or tetramers Ag₄²⁺, with a varying elongation/distortion along the tetramer diagonals.

Chirality in nonlinear-optical response of planar G-shaped nanostructures

Chirality effects in optical second harmonic generation (SHG) are studied in periodic planar arrays of gold G-shaped nanostructures. We show that G-shaped structures of different handedness demonstrate different SHG efficiency for the left and right circular polarizations, as well as the opposite directions of the SHG polarization plane rotation. The observed effects are interpreted as the appearance of chirality in the SHG response which allows clear distinguishing of two enantiomers.