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It is well-known that gold nanoparticle (AuNP) clusters generate strong surface-enhanced Raman scattering (SERS). In order to produce spatially uniform Raman-enhancing substrates at a large scale, we synthesized vertically perforated three-dimensional (3D) AuNP stacks. The 3D stacks were fabricated by first hydrothermally synthesizing ZnO nanowires perpendicular to silicon wafers followed by repetitively performing liquid-phase deposition of AuNPs on the tops and side surfaces of the nanowires. During the deposition process, the nanowires were shown to gradually dissolve away, leaving hollow vestiges or perforations surrounded by stacks of AuNPs. Simulation studies and experimental measurements reveal these nanoscale perforations serve as light paths that allow the excitation light to excite deeper regions of the 3D stacks for stronger overall Raman emission. Combined with properly sized nanoparticles, this feature maximizes and saturates the Raman enhancement at 1-pM sensitivity across the entire wafer-scale substrate, and the saturation improves the wafer-scale uniformity by a factor of 6 when compared to nanoparticle layers deposited directly on a silicon wafer substrate. Using the 3D-stacked substrates, quantitative sensing of adenine molecules yielded concentrations measurements within 10% of the known value. Understanding the enhancing mechanisms and engineering the 3D stacks have opened a new method of harnessing the intense SERS observed in nanoparticle clusters and realize practical SERS substrates with significantly improved uniformity suitable for quantitative chemical sensing.

A meta-atom platform providing decoupled tuning for the constitutive wave parameters remains as a challenging problem, since the proposition of Pendry. Here we propose an electromagnetic meta-atom design of internal anisotropy (εr ≠ εθ), as a pathway for decoupling of the effective- permittivity εeff and permeability μeff. Deriving effective parameters for anisotropic meta-atom from the first principles, and then subsequent inverse-solving the obtained decoupled solution for a target set of εeff and μeff, we also achieve an analytic, top-down determination for the internal structure of a meta-atom. To realize the anisotropy from isotropic materials, a particle of spatial permittivity modulation in r or θ direction is proposed. As an application example, a matched zero index dielectric meta-atom is demonstrated, to enable the super-funneling of a 50λ-wide flux through a sub-λ slit; unharnessing the flux collection limit dictated by the λ-zone.

Surface Enhanced Raman Scattering (SERS) has a great potential to serve as a monitoring technology for biomolecules, but sensing biomolecules for practical purposes have remained challenging for two reasons. One of the challenges is securing SERS substrates with uniform spatial enhancement that is crucial for quantitative measurements, and the other is finding proper linker molecules that will promote the surface enhancement. To address these challenges, we have been developing a new approach of using highly sensitive surface enhanced Raman scattering (SERS) platform for glucose sensing. In the presentation, I will discuss the fabrication of high performance 3D SERS substrate based on straightforward, two successive wet chemical processes, with experimentally proven strong enhancement and excellent spatial uniformity as well as the use of new linker molecules for making glucose-specific SERS substrates and their use in performing quantitative glucose measurements. Glucose sensing results from different development stages will be discussed.

Unusual performances of metamaterials such as negative index of refraction, memory effect, and cloaking originate from the resonance features of the metallic composite atom(1-6). Indeed, control of metamaterial properties by changing dielectric environments of thin films below the metallic resonators has been demonstrated(7-11). However, the dynamic control ranges are still limited to less than a factor of 10,(7-11) with the applicable bandwidth defined by the sharp resonance features. Here, we present ultra-broad-band metamaterial thin film with colossal dynamic control range, fulfilling present day research demands. Hybridized with thin VO(2) (vanadium dioxide) (12-18) films, nanoresonator supercell arrays designed for one decade of spectral width in terahertz frequency region show an unprecedented extinction ratio of over 10000 when the underlying thin film experiences a phase transition. Our nanoresonator approach realizes the full potential of the thin film technology for long wavelength applications.

Recently, much research has been done for to realizeing nano-scale photonic circuits based on photonic crystal, plasmonics and silicon photonics in order to overcome fundamental limits of electronic circuits. These limits include such as bottleneck of speed, and size that cannot be reduced. Even though several kinds of coupling schemes have been reported, coupling structures are still large when it is compared with the nano-scale optical circuit. In this paper, we proposed using a very thin Fresnel lens while shortening the focal length of the Fresnel lens as much as possible. We proposed, for the first time, to utilize metal slits that are able to use the optical coupling system between a nano-scale optical circuit and the standard single mode optical fiber for overcoming the limitation of focal length shortening of the Fresnel lens. Comparative study has been carried out with a FDTD simulation between normal and metal slit assisted Fresnel lens. From the result of simulation, we can achieve 65% coupling efficiency for the metal-slit Fresnel lens when the focal length of metal-slit Fresnel lens is just . On the other hand, the coupling efficiency of the normal Fresnel lens is about 43%.

The common behaviour of a wave is determined by wave parameters of its medium, which are generally associated with the characteristic oscillations of its corresponding elementary particles. In the context of metamaterials, the decoupled excitation of these fundamental oscillations would provide an ideal platform for top–down and reconfigurable access to the entire constitutive parameter space; however, this has remained as a conceivable problem that must be accomplished, after being pointed out by Pendry. Here by focusing on acoustic metamaterials, we achieve the decoupling of density ρ, modulus B−1 and bianisotropy ξ, by separating the paths of particle momentum to conform to the characteristic oscillations of each macroscopic wave parameter. Independent access to all octants of wave parameter space (ρ, B−1, ξ)=(+/−,+/−,+/−) is thus realized using a single platform that we call an omni meta-atom; as a building block that achieves top–down access to the target properties of metamaterials., For meta-atoms, achieving full access to the wave parameter space remains a problem. Here, Koo et al. outline the criteria for the decoupling of wave parameter from first principles, and realize an omni meta-atom that achieves independent, broad-range access to all octants of acoustic parameter space.

In this work, a whispering gallery mode (WGM) microdisk resonator based on Ge1-xSnx grown by molecular beam epitaxy (MBE) was fabricated and characterized. Various process conditions and different Sn contents (4% and 1%) were explored to confirm the feasibility of Ge1-xSnx for microcavity device operation. Optical modes with wavelengths in the infrared (IR) range beyond 1550 nm were successfully confined in the devices fabricated with different diameters, and free-spectral ranges (FSRs) near 20 nm were obtained.

We propose a novel trench-assisted circular metal nano-slit (CMNS) structure implementable on a fiber platform for the generation of a low-noise cylindrical surface plasmon (CSP) hotspot. We design trench structures based on a multi-pole cancellation method in order that a converging surface plasmon signal is well separated from co-propagating non-confined diffracted light (NCDL) at the hotspot location. In fact, the secondary radiation by the quasi-pole oscillation at the edge of the trench cancels the primary NCDL, thereby enhancing the signal-to-noise ratio (SNR) of the CSP hotspot. In particular, we investigate two types of trench structures: a rectangular-trench (RT) structure and an asymmetric-parabolic-trench (APT) structure, which are considered for the sake of the simplicity of fabrication and of the maximal enhancement of the SNR, respectively. In comparison with a conventional CMNS having no trenches, we highlight that the mean SNR of the CSP hotspot is enhanced by 6.97 and 11.89 dB in case of the optimized RT and APT CMNSs, respectively. The proposed schemes are expected to be useful for increasing the SNR of plasmonic devices that are interfered by NCDL, such as various types of nano-slits for generating high-resolution plasmonic signals, for example. (C)2014 Optical Society of America

lsothermal vapor liquid equilibrium data for two binary mixtures of alternative refrigerants were determined by using an apparatus applying recirculating vapor and liquid. The difluoromethane (HFC-32) + 1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea) and 1,1,1,2-tetrafluoroethane (HFC-134a) + 1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea) systems were studied at 298.15 and 312.65 K. The pressure and vapor and liquid compositions were measured at each temperature. The experimental data were correlated with the Peng Robinson equation of state using the van der Waals one-fluid mixing rule. Calculated results show that this equation yields good agreement with the experimental data.

Local distribution of the optical magnetic field is a critical parameter in developing materials with artificially engineered optical properties. Optical magnetic field characterization in nano-scale remains a challenge, because of the weak matter-optical magnetic field interactions. Here, we demonstrate an experimental visualization of the optical magnetic field profiles by raster scanning circular apertures in metal film or in a conical probe. Optical magnetic fields of surface plasmon polaritons and radially polarized beam are visualized by measuring the transmission through metallic apertures, in excellent agreements with theoretical predictions. Our results show that Bethe-Bouwkamp aperture can be used in visualizing optical magnetic field profiles. (C) 2013 Optical Society of America

We use coupled mode theory (CMT) to analyze a metal-insulator-metal (MIM) plasmonic stub structure, to reveal the existence of asymmetry in its transmittance spectra. Including the effect of the near field contribution for the stub structure, the observed asymmetry is interpreted as Fano-type interference between the quasi-continuum T-junction-resonator local-modes and discrete stub eigenmodes. Based on the asymmetry factor derived from the CMT analysis, methods to control transmittance asymmetry are also demonstrated.

The nature of light as an electromagnetic wave with transverse components has been confirmed using optical polarizers, which are sensitive to the orientation of the electric field. Recent advances in nanoscale optical technologies demand their magnetic counterpart, which can sense the orientation of the optical magnetic field. Here we report that subwavelength metallic apertures on infinite plane predominantly sense the magnetic field of light, establishing the orientation of the magnetic component of light as a separate entity from its electric counterpart. A subwavelength aperture combined with a tapered optical fibre probe can also serve as a nanoscale polarization analyser for the optical magnetic field, analogous to a nanoparticle sensing the local electric polarization. As proof of its functionality, we demonstrate the measurement of a magnetic field orientation that is parallel to the electric field, as well as a circularly polarized magnetic field in the presence of a linearly polarized electric field., Determining the direction of the magnetic field of light is important for optical applications. Here, scattering of light from a subwavelength aperture in a metal plane is shown to be governed by its magnetic vector, providing the magnetic field orientation independently of the electric field.

We observe that rectangular holes fabricated in a gold film with a sub-skindepth thickness show blue-shifted resonance frequencies as the width increases. For rectangular holes on a finite substrate its thickness and the hole width are crucial factors determining the resonance frequency.

We report on an order of magnitude enhanced nonlinear response of vanadium dioxide thin film patterned with nanoresonators--nano slot antennas fabricated on the gold film. Transmission of terahertz radiation, little affected by an optical pumping for the case of bulk thin film, can now be completely switched-off: DeltaT/T approximately -0.9999 by the same optical pumping power. This unprecedentedly large optical pump-terahertz probe nonlinearity originates from the insulator-to-metal phase transition drastically reducing the antenna cross sections of the nanoresonators. Our scheme enables nanoscale-thin film technology to be used for all-optical switching of long wavelength light.

We propose a design method of structured grating for out of plane conversion, focusing and manipulation of Surface Plasmon Polariton waves. Advanced functional devices, such as balanced power splitters, and wavelength demultiplexers are demonstrated with high conversion efficiency.

We propose a rigorous design method of structured gratings for out of plane mode conversion, line focusing and manipulation of Surface Plasmon Polariton (SPP) waves. Employing a blazed grating to incorporate the directionality of SPP launch, and at the same time controlling grating depth and chirp to account for the radiation loss and diffraction angle, it was possible to achieve high efficiency and flexible SPP to freespace mode conversion. Devices with advanced functionalities, such as balanced SPP power splitter, and SPP wavelength demultiplexer are demonstrated with over 75% of power efficiencies at reasonable working distances of less than several wavelengths.

We have measured transmission properties of a composite structure consisting of nano-rods on a long (a y =300 micron) nano gap (70 nm) on Au film in broad frequency range of 0.1 THz to 1.0 THz using THz time-domain spectroscopy. The normalized transmittance with no nano-bridge or nano-rod structure in the middle shows a half-wavelength resonance: the resonance frequency is ~c/(2na y ) where n is the index of refraction of the substrate. The nano-size bridge at the center of the nano gap gives changes the resonance characteristics profoundly, because in essence, the length a y now halves. Mostly the same resonance-changing behavior is expected with a nano-rod structure fabricated by Pt-deposition method using a focused ion beam (FIB). This small rod also acts as a bridge dividing the length of the rectangle. The structure dependent resonance allows to detect nano-size particles and to tailor resonance characteristics with feature sizes of λ/10,000.

We report on the terahertz modulation based on the micro- and nano- sized apertures with metal-insulator phase transition of VO 2 thin film. Terahertz wave transmission through subwavelength apertures is controlled by temperature driven refractive index change of VO 2 substrate.

We proposed, for the first time, to utilize metal-slits array Fresnel lens for the optical coupling form free space into silicon slab waveguide while overcoming near focal length limit of the conventional dielectric Fresnel lens. Physical limits of the conventional Fresnel lens were revealed and it was proposed to employ metal-slit array to overcome these limitations. Comparative study has been carried out with a FDTD simulation between normal Fresnel lens coupling and metal-slit array Fresnel lens coupling.

We propose and analyze the "complementary" structure of a metallic nanogap, namely, the metallic nanowire for magnetic field enhancement. A huge enhancement of the field up to a factor of 300 was achieved. Introducing the surface impedance concept, we also develop and numerically confirm a new analytic theory which successfully predicts the field enhancement factors for metal nanostructures. Compared to the predictions of the classical Babinet principle applied to a nanogap, an order of magnitude difference in the field enhancement factor was observed for the sub-skin-depth regime nanowire.

We show that terahertz electromagnetic waves transmit through ?/10,000 nanogap devices. Both non-resonant and resonant field enhancement are observed in nanogap and nanoantenna. We discuss applications of these structures.

We report the effect of geometrical factors governing the polarization profiles of near-field scanning optical microscope (NSOM) probes. The most important physical parameter controlling the selective electric or magnetic field sensitivity is found to be the width of the metal rim surrounding aperture. Probes with metal rim width w < lambda/2 selectively senses the optical electric field, while those with w > lambda/2 selectively senses the optical magnetic field. Intensity variation of optical Hertz standing wave formed upon reflection at oblique incidence shows a phase difference of pi/2 between electric and magnetic probes: an analogue of the classical Wiener's experiment. Our work paves way towards electromagnetic engineering of nanostructures. (C) 2015 Optical Society of America

We compare single- and double-sided excitation methods of adiabatic surface plasmon polariton (SPP) wave superfocusing for scattering-type metallic near-field scanning optical microscopy (s-NSOM). Using the results of full 3D finite difference time domain analyses, the differences in field enhancement factors are explained and reveal the mode selectivity of a conical NSOM tip for adiabatic SPP superfocusing. Exploiting the mode-symmetric nature of the tip further, we also show that it is possible to selectively confine either the electric or magnetic field at the NSOM tip apex, by simply adjusting the relative phase between the SPP waves in the double-sided excitation approach.

We propose a novel metal slit array Fresnel lens for wavelength-scale optical coupling into a nanophotonic waveguide. Using the plasmonic waveguide structure in Fresnel lens form, a much wider beam acceptance angle and wavelength-scale working distance of the lens was realized compared to a conventional dielectric Fresnel lens. By applying the plasmon waveguide dispersion relation to a phased antenna array model, we also develop and analyze design rules and parameters for the suggested metal slit Fresnel lens. Numerical assessment of the suggested structure shows excellent coupling efficiency (up to 59%) of the 10 mum free-space Gaussian beam to the 0.36 mum Si waveguide within a working distance of a few mum.

A photonic analog-to-digital converter (PADC) utilizing a slow-light photonic crystal Mach-Zehnder interferometer (MZI) is proposed, to enable the optically coded output of a PADC with reduced device size and power consumption. Assuming an index modulation for the MZI on the Taylor’s PADC structure, limiting factors in device size, speed, and effective number of bits are derived considering the signal transition time of the light and the slow light dispersion effects. Details of the device design and results of a time domain assessment of the device performance is described with discussions on the feasibility of sub-mm size, 20GS/s operation of the device having the ENOB (effective number of bits) >5.

We proposed an all-optical logic system constructed by combining cross-waveguide geometries including nonlinear cavity enabling all-kinds of logic gates. Proposed logic system is reconfigurable into any kinds of Boolean operations by appropriate placing of defect rods and adjusting its refractive index.

In this paper, surface roughness effect on Q-factor on Ge whispering gallery mode microdisk resonator is thoroughly investigated by 2D and 3D FDTD simulations with variations on roughness indices.

Operation frequency tuning for an all-optical switch employing 2-dimensional cross-waveguide photonic crystal is demonstrated with an electric field bias control. Applying static electric field up to 600 V/mum perpendicular to the signal plane, up to 10 THz tuning of the switching frequency can be achieved, without any degradation in the switching characteristics of the device.

Junction structures provide the foundation of digital electronics and spintronics today. An equivalent, a photonic junction to achieve systematic and drastic control of photon flow is currently missing, but is mandatory for serious all-optical signal processing. Here we propose a photonic junction built upon mode-orthogonal hetero-structures, as a fundamental structural unit for photonic integrated circuits. Controlling the optical potential of mode-orthogonal junctions, the flow of photons can be dynamically manipulated, to complete the correspondence to the electronic junction structures. Of the possible applications, we provide examples of a photonic junction diode and a multi-junction half-adder, with exceptional performance metrics. Highly directional (41dB), nearly unity throughput, ultra-low threshold-power, high quality signal regeneration at 200Gb/s, and all-optic logic operations are successfully derived with the self-induced, bi-level dynamic mode-conversion process across the junction.