School of Physics - Research Publications
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ItemOptical Chirality from Dark-Field Illumination of Planar Plasmonic Nanostructures(WILEY-V C H VERLAG GMBH, 2017-11)Abstract Dark‐field illumination is shown to make planar chiral nanoparticle arrangements exhibit circular dichroism in extinction, analogous to true chiral scatterers. Single oligomers, consisting rotationally symmetric arrangements of gold nanorods, are experimentally observed to exhibit circular dichrosim at their maximum scattering with strong agreement to numerical simulation. A dipole model is developed to show that this effect is caused by a difference in the projection of a nanorod onto the handed orientation of electric fields created by a circularly polarized dark‐field normally incident on a glass‐air interface. Owing to this geometric origin, the wavelength of the peak chiral response is experimentally shown to shift depending on the separation between nanoparticles. All presented oligomers have physical dimensions less than the operating wavelength, and the applicable extension to closely packed planar arrays of oligomers is demonstrated to amplify the magnitude of circular dichroism. This realization of strong chirality in these oligomers demonstrates a new path to engineer optical chirality from planar devices using dark‐field illumination.
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ItemNo Preview AvailableMesoscale surface plasmons: modelling and imaging using near-field scanning optical microscopy(OPTICAL SOC AMER, 2018-09-03)Meso-scale plasmons are supported by structures with dimensions on the order of tens of plasmon wavelengths. Metal structures at this length-scale are promising for the design and engineering of structures to direct the flow of optical energy and generate high intensity, localized electric fields. The near-field optical properties of mesoscale crystalline gold plates were examined using near-field scanning optical microscopy with a focus on the effects of modifying morphology and excitation conditions. Excitation of surface plasmon polaritons (SPPs) at plate edges and their subsequent propagation and interference as radial waves across the surface results in nodes of enhancement of the near-field on the plate surface at specific positions within the plate. The spatial position of the near-field enhancement may be directed by controlling either, or both, the boundary conditions (plate shape) and polarization of the excitation light.
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ItemNo Preview AvailableChiral Electromagnetic Fields Generated by Arrays of Nanoslits(AMER CHEMICAL SOC, 2012-07)Using a modal matching theory, we demonstrate the generation of short-range, chiral electromagnetic fields via the excitation of arrays of staggered nanoslits that are chiral in two dimensions. The electromagnetic near fields, which exhibit a chiral density greater than that of circularly polarized light, can enhance the chiroptical interactions in the vicinity of the nanoslits. We discuss the features of nanostructure symmetry required to obtain the chiral fields and explicitly show how these structures can give rise to detection and characterization of materials with chiral symmetry.
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ItemHighly compact refractive index sensor based on stripe waveguides for lab-on-a-chip sensing applications(BEILSTEIN-INSTITUT, 2016-05-25)In this paper we report the design and experimental realisation of a novel refractive index sensor based on coupling between three nanoscale stripe waveguides. The sensor is highly compact and designed to operate at a single wavelength. We demonstrate that the sensor exhibits linear response with a resolution of 6 × 10(-4) RIU (refractive index unit) for a change in relative output intensity of 1%. Authors expect that the outcome of this paper will prove beneficial in highly compact, label-free and highly sensitive refractive index analysis.
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ItemShort-range surface plasmonics: Localized electron emission dynamics from a 60-nm spot on an atomically flat single-crystalline gold surface(AMER ASSOC ADVANCEMENT SCIENCE, 2017-07)We experimentally and theoretically visualize the propagation of short-range surface plasmon polaritons using atomically flat single-crystalline gold platelets on silicon substrates. We study their excitation and subfemtosecond dynamics via normal-incidence two-photon photoemission electron microscopy. By milling a plasmonic disk and grating structure into a single-crystalline gold platelet, we observe nanofocusing of the short-range surface plasmon polariton. Localized two-photon ultrafast electron emission from a spot with a smallest dimension of 60 nm is observed. Our novel approach opens the door toward reproducible plasmonic nanofocusing devices, which do not degrade upon high light intensity or heating due to the atomically flat surface without any tips, protrusions, or holes. Our nanofoci could also be used as local emitters for ultrafast electron bunches in time-resolved electron microscopes.
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ItemOptical image processing with metasurface dark modes(OPTICAL SOC AMER, 2018-09-01)Here we consider image processing using the optical modes of metasurfaces with an angle-dependent excitation. These spatially dispersive modes can be used to directly manipulate the spatial frequency content of an incident field, suggesting their use as ultra-compact alternatives for analog optical information processing. A general framework for describing the filtering process in terms of the optical transfer functions is provided. In the case where the relevant mode cannot be excited with a normally incident plane wave (a dark mode), high-pass filtering is obtained. We provide examples demonstrating filtering of both amplitude and pure phase objects.
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ItemMetasurfaces, dark modes, and high NA illumination(OPTICAL SOC AMER, 2018-10-15)The interaction of a focused beam with a metasurface supporting dark modes is investigated. We show computationally and experimentally that the excitation of dark modes is accompanied by characteristic changes in the reflected Fourier spectrum. This spatial frequency filtering capability indicates an avenue for the all-optical, on-chip detection of phase gradients for biological and other imaging techniques.
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ItemCathodoluminescence as a probe of the optical properties of resonant apertures in a metallic film(BEILSTEIN-INSTITUT, 2018-05-18)Here we present the results of an investigation of resonances of azimuthal trimer arrangements of rectangular slots in a gold film on a glass substrate using cathodoluminescence (CL) as a probe. The variation in the CL signal collected from specific locations on the sample as a function of wavelength and the spatial dependence of emission into different wavelength bands provides considerable insight into the resonant modes, particularly sub-radiant modes, of these apertures. By comparing our experimental results with electromagnetic simulations we are able to identify a Fabry-Pérot mode of these cavities as well as resonances associated with the excitation of surface plasmon polaritons on the air-gold boundary. We obtain evidence for the excitation of dark (also known as sub-radiant) modes of apertures and aperture ensembles.
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ItemLuminescence of a Transition Metal Complex Inside a Metamaterial Nanocavity(John Wiley & Sons, 2017-08-25)Modification of the local density of optical states using metallic nanostructures leads to enhancement in the number of emitted quanta and photocatalytic turnover of luminescent materials. In this work, the fabrication of a metamaterial is presented that consists of a nanowire separated from a metallic mirror by a polymer thin film doped with a luminescent organometallic iridium(III) complex. The large spin–orbit coupling of the heavy metal atom results in an excited state with significant magnetic-dipole character. The nanostructured architecture supports two distinct optical modes and their assignment achieved with the assistance of numerical simulations. The simulations show that one mode is characterized by strong confinement of the electric field and the other by strong confinement of the magnetic field. These modes elicit drastic changes in the emitter’s photophysical properties, including dominant nanocavity-derived modes observable in the emission spectra along with significant increases in emission intensity and the total decay rate. A combination of simulations and momentum-resolved spectroscopy helps explain the mechanism of the different interactions of each optical mode supported by the metamaterial with the excited state of the emitter.
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ItemNo Preview AvailablePhotoinduced Electron Transfer in the Strong Coupling Regime: Waveguide-Plasmon Polaritons(AMER CHEMICAL SOC, 2016-04)Reversible exchange of photons between a material and an optical cavity can lead to the formation of hybrid light-matter states where material properties such as the work function [ Hutchison et al. Adv. Mater. 2013 , 25 , 2481 - 2485 ], chemical reactivity [ Hutchison et al. Angew. Chem., Int. Ed. 2012 , 51 , 1592 - 1596 ], ultrafast energy relaxation [ Salomon et al. Angew. Chem., Int. Ed. 2009 , 48 , 8748 - 8751 ; Gomez et al. J. Phys. Chem. B 2013 , 117 , 4340 - 4346 ], and electrical conductivity [ Orgiu et al. Nat. Mater. 2015 , 14 , 1123 - 1129 ] of matter differ significantly to those of the same material in the absence of strong interactions with the electromagnetic fields. Here we show that strong light-matter coupling between confined photons on a semiconductor waveguide and localized plasmon resonances on metal nanowires modifies the efficiency of the photoinduced charge-transfer rate of plasmonic derived (hot) electrons into accepting states in the semiconductor material. Ultrafast spectroscopy measurements reveal a strong correlation between the amplitude of the transient signals, attributed to electrons residing in the semiconductor and the hybridization of waveguide and plasmon excitations.