Engineering the phase front of light with phase-change material based planar lenses.

A novel hybrid planar lens is proposed to engineer the far-field focusing patterns. It consists of an array of slits which are filled with phase-change material Ge2Sb2Te5 (GST). By varying the crystallization level of GST from 0% to 90%, the Fabry-Pérot resonance supported inside each slit can be spectrally shifted across the working wavelength at 1.55 µm, which results in a transmitted electromagnetic phase modulation as large as 0.56π. Based on this geometrically fixed platform, different phase fronts can be constructed spatially on the lens plane by assigning the designed GST crystallization levels to the corresponding slits, achieving various far-field focusing patterns. The present work offers a promising route to realize tunable nanophotonic components, which can be used in optical circuits and imaging applications.

Dip-pen patterning of poly(9,9-dioctylfluorene) chain-conformation-based nano-photonic elements.

Metamaterials are a promising new class of materials, in which sub-wavelength physical structures, rather than variations in chemical composition, can be used to modify the nature of their interaction with electromagnetic radiation. Here we show that a metamaterials approach, using a discrete physical geometry (conformation) of the segments of a polymer chain as the vector for a substantial refractive index change, can be used to enable visible wavelength, conjugated polymer photonic elements. In particular, we demonstrate that a novel form of dip-pen nanolithography provides an effective means to pattern the so-called ß-phase conformation in poly(9,9-dioctylfluorene) thin films. This can be done on length scales ≤500 nm, as required to fabricate a variety of such elements, two of which are theoretically modelled using complex photonic dispersion calculations.

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.

Experimental proof of concept of nanoparticle-assisted STED.

We imaged core-shell nanoparticles, consisting of a dye-doped silica core covered with a layer of gold, with a stimulated emission depletion, fluorescence lifetime imaging (STED-FLIM) microscope. Because of the field enhancement provided by the localized surface plasmon resonance of the gold shell, we demonstrate a reduction of the STED depletion power required to obtain resolution improvement by a factor of 4. This validates the concept of nanoparticle-assisted STED (NP-STED), where hybrid dye-plasmonic nanoparticles are used as labels for STED in order to decrease the depletion powers required for subwavelength imaging.

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.

Plasmonic nanoclusters with rotational symmetry: polarization-invariant far-field response vs changing near-field distribution.

Flexible control over the near- and far-field properties of plasmonic nanostructures is important for many potential applications, such as surface-enhanced Raman scattering and biosensing. Generally, any change in the polarization of the incident light leads to a change in the nanoparticle's near-field distribution and, consequently, in its far-field properties as well. Therefore, producing polarization-invariant optical responses in the far field from a changing near field remains a challenging issue. In this paper, we probe experimentally the optical properties of cruciform pentamer structures--as an example of plasmonic oligomers--and demonstrate that they exhibit such behavior due to their symmetric geometrical arrangement. We demonstrate direct control over hot spot positions in sub-20 nm gaps, between disks of 145 nm diameter at a wavelength of 850 nm, by means of scattering scanning near-field optical microscopy. In addition, we employ the coupled dipole approximation method to define a qualitative model revealing the relationship between the near and far field in such structures. The near-field profiles depend on particular mode superpositions excited by the incident field and, thus, are expected to vary with the polarization. Consequently, we prove analytically that the far-field optical properties of pentamers have to be polarization-independent due to their rotational symmetry.

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.

Plasmonic nanogap tilings: light-concentrating surfaces for low-loss photonic integration.

Owing to their ability to concentrate light on nanometer scales, plasmonic surface structures are ideally suited for on-chip functionalization with nonlinear or gain materials. However, achieving a high effective quantum yield across a surface requires not only strong light localization but also control over losses. Here, we report on a particular class of tunable low-loss metasurfaces featuring dense arrangements of nanometer-sized focal points on a photonic chip with an underlying waveguide channel. Guided within the plane, the photonic wave evanescently couples to the nanogaps, concentrating light in a lattice of hot-spots. In studying the energy transfer between photonic and plasmonic channels of single trimer molecules and triangular nanogap tilings in dependence on element size, we identify different regimes of operation. We show that the product of field enhancement, propagation length, and element size is close to constant in both the radiative and subwavelength regimes, opening pathways for device designs that combine high-field enhancements with large propagation lengths.

Use of a gold reflecting-layer in optical antenna substrates for increase of photoluminescence enhancement.

We report on a straightforward way to increase the photoluminescence enhancement of nanoemitters induced by optical nanotantennas. The nanoantennas are placed above a gold film-silica bilayer, which produces a drastic increase of the scattered radiation power and near field enhancement. We demonstrate this increase via photoluminescence enhancement using an organic emitter of low quantum efficiency, Tetraphenylporphyrin (TPP). An increase of the photoluminescence enhancement by a factor larger than three is observed compared to antennas without the reflecting-layer. In addition, we study the possibility of influencing the polarization of the light emitted by utilizing asymmetry of dimer antennas.

Probing the dielectric response of graphene via dual-band plasmonic nanoresonators.

In this article, we use optical transmission spectroscopy to measure the changes in the resonance features of a Au plasmonic nanoresonator array consisting of concentric ring/disc cavity elements, when graphene is introduced as an encapsulating medium. We show that by using finite element modelling to best reproduce our experimental results the dielectric response of the graphene film can be determined. We discuss the potential of such structures for chemical sensing applications.

Nanoparticle-assisted stimulated-emission-depletion nanoscopy.

We show that metal nanoparticles can be used to improve the performance of super-resolution fluorescence nanoscopes based on stimulated-emission-depletion (STED). Compared with a standard STED nanoscope, we show theoretically a resolution improvement by more than an order of magnitude, or equivalently, depletion intensity reductions by more than 2 orders of magnitude and an even stronger photostabilization. Our scheme may allow improvement of existing STED nanoscopes and assist in the development of low-power, low-cost nanoscopes. This has the potential to increase the availability of STED nanoscopes and lead to a significant expansion of our understanding of biological and biochemical phenomena occurring on the nanoscale.

Directional excitation of surface plasmon polaritons via nanoslits under varied incidence observed using leakage radiation microscopy.

Surface Plasmon Polaritons (SPPs) are excited at the interface between a thin gold film and air via the illumination of nanoslits etched into the film. The coupling efficiency to the two propagation directions away from the slits is determined by leakage radiation microscopy, when the angle of incidence of the pump beam is changed from 0° to 20°. We find that preferential coupling of SPPs into one direction can be achieved for non-normal incidence in the case of single slits and slit pairs. The proportion of SPP excited into one direction can be in excess of 90%. We further provide a simple model of the process, and directly compare the performances of the two approaches.

Quantum statistics of surface plasmon polaritons in metallic stripe waveguides.

Heralded single surface plasmon polaritons are excited using photons generated via spontaneous parametric down conversion. The mean excitation rates, intensity correlations, and Fock state populations are studied. The observed dependence of the second-order coherence in our experiment is consistent with a linear uncorrelated Markovian environment in the quantum regime. Our results provide important information about the effect of loss for assessing the potential of plasmonic waveguides for future nanophotonic circuitry in the quantum regime.

Revealing plasmonic gap modes in particle-on-film systems using dark-field spectroscopy.

Polarization-controlled excitation of plasmonic modes in nanometric Au particle-on-film gaps is investigated experimentally using single-particle dark-field spectroscopy. Two distinct geometries are explored: nanospheres on top of and inserted in a thin gold film. Numerical simulations reveal that the three resonances arising in the scattering spectra measured for particles on top of a film originate from highly confined gap modes at the interface. These modes feature different azimuthal characteristics, which are consistent with recent theoretical transformation optics studies. On the other hand, the scattering maxima of embedded particles are linked to dipolar modes having different orientations and damping rates. Finally, the radiation properties of the particle-film gap modes are studied through the mapping of the scattered power within different solid angle ranges.

Role of defects in the phase transition of VO2 nanoparticles probed by plasmon resonance spectroscopy.

Defects are known to affect nanoscale phase transitions, but their specific role in the metal-to-insulator transition in VO(2) has remained elusive. By combining plasmon resonance nanospectroscopy with density functional calculations, we correlate decreased phase-transition energy with oxygen vacancies created by strain at grain boundaries. By measuring the degree of metallization in the lithographically defined VO(2) nanoparticles, we find that hysteresis width narrows with increasing size, thus illustrating the potential for domain boundary engineering in phase-changing nanostructures.

Single-particle plasmon resonance spectroscopy of phase transition in vanadium dioxide.

We demonstrate thermally controlled plasmon resonance modulation of single gold nanoparticles on vanadium dioxide thin films by performing dark-field spectroscopy measurements at different temperatures. The plasmon resonance of the nanoparticles exhibits a significant blueshift in the visible range when the vanadium dioxide film undergoes its insulator-to-metal phase transition around 67 °C. More importantly, the resonance shift shows a clear hysteresis, mirroring the behavior of the vanadium dioxide film. At a fixed wavelength, the scattering intensity of Au particles also shows a hysteretic behavior decorated with an overshoot before (after) the insulator-metal (metal-insulator) phase transition of the vanadium dioxide film, suggesting that the nanoparticle is probing local variations in the phase transition.

Controlling light localization and light-matter interactions with nanoplasmonics.

Nanoplasmonics is the emerging research field that studies light-matter interactions mediated by resonant excitations of surface plasmons in metallic nanostructures. It allows the manipulation of the flow of light and its interaction with matter at the nanoscale (10(-9) m). One of the most promising characteristics of plasmonic resonances is that they occur at frequencies corresponding to typical electronic excitations in matter. This leads to the appearance of strong interactions between localized surface plasmons and light emitters (such as molecules, dyes, or quantum dots) placed in the vicinity of metals. Recent advances in nanofabrication and the development of novel concepts in theoretical nanophotonics have opened the way to the design of structures aimed to reduce the lifetime and enhance the decay rate and quantum efficiency of available emitters. In this article, some of the most relevant experimental and theoretical achievements accomplished over the last several years are presented and analyzed.

Plasmonic light-harvesting devices over the whole visible spectrum.

On the basis of conformal transformation, a general strategy is proposed to design plasmonic nanostructures capable of an efficient harvesting of light over a broadband spectrum. The surface plasmon modes propagate toward the singularity of these structures where the group velocity vanishes and energy accumulates. A considerable field enhancement and confinement is thus expected. Radiation losses are also investigated when the structure dimension becomes comparable to the wavelength.

Experimental realization of subradiant, superradiant, and fano resonances in ring/disk plasmonic nanocavities.

Subradiant and superradiant plasmon modes in concentric ring/disk nanocavities are experimentally observed. The subradiance is obtained through an overall reduction of the total dipole moment of the hybridized mode due to antisymmetric coupling of the dipole moments of the parent plasmons. Multiple Fano resonances appear within the superradiant continuum when structural symmetry is broken via a nanometric displacement of the disk, due to coupling with higher order ring modes. Both subradiant modes and Fano resonances exhibit substantial reductions in line width compared to the parent plasmon resonances, opening up possibilities in optical and near IR sensing via plasmon line shape design.

Near-field optical microscopy with a nanodiamond-based single-photon tip.

We introduce a point-like scanning single-photon source that operates at room temperature and offers an exceptional photostability (no blinking, no bleaching). This is obtained by grafting in a controlled way a diamond nanocrystal (size around 20 nm) with single nitrogen-vacancy color-center occupancy at the apex of an optical probe. As an application, we image metallic nanostructures in the near-field, thereby achieving a near-field scanning single-photon microscopy working at room temperature on the long term. Our work may be of importance to various emerging fields of nanoscience where an accurate positioning of a quantum emitter is required such as for example quantum plasmonics.