Fabrication and Characterization of Large, Perfectly Periodic Arrays of Metallic Nanocups

Fabrication of plasmonic resonance devices composed of large arrays of highly ordered gold nanocups is presented. The nanostructures are generated from periodic photoresist templates created by interference lithography and subsequent reflow, deposition, and dislodging. The nanocups are hemispherical in shape and arranged in both rectangular and hexagonal arrays with periods of ~500?nm. Their ability to support surface plasmonic resonances is manifested experimentally by reflectance spectroscopy. Theoretical modeling to ascertain the plasmonic spectra of these nanostructures is performed. The computed spectra of the rectangular structure are in qualitative agreement with the measurements. A weaker correlation observed for the hexagonal structure is explained by its more intricate symmetry which complicates the spectral response.     

Tunable,Mid-Infrared Ultra-Narrowband Filtering Effect Induced by Two Coplanar Graphene Strips

The two coplanar graphene strips coupling system supported on substrates is proposed and constructed on a monolayer graphene by spatially varying gate voltages. It is investigated numerically by using the finite-difference time-domain method. Simulation results reveal that despite of no traditional ring, disk, and rectangular geometry resonators used usually in metallic plasmonic filters, this structure based on the edge mode propagation exhibits an original, ultra-narrowband band-stop filtering effect in the mid-infrared region. This filtering effect results from the novel side-coupled resonator formed by the parallel graphene strips. The transmission spectrum is tuned and modified not only by engineering the locations of gate voltages without re-fabricating structures but also via changing substrates. Simulation results are consistent with the theoretical analysis. Our studies hence support the fabrication of ultra-compact planar plasmonic devices in nano-integrated circuits.     

Plasmonic Coupling Effect in Ag Nanocap–Nanohole Pairs for Surface-Enhanced Raman Scattering

A plasmonic coupling structure composed of Ag nanocap–nanohole pairs was fabricated through a novel and facile method. Both surface-enhanced Raman scattering (SERS) measurements and numerical simulations show that the cap-hole system produces much larger electric field enhancement and SERS signal than the isolated structures, which is due to the plasmonic coupling effect between the gap of the cap and the hole. Additionally, the plasmonic enhancement is sensitive to the gap size, which can be controlled by the Ag layer thickness during the evaporation process. A maximum enhancement factor of 1.1×10⁸ can be obtained with optimized gap size.     

Plasmonic Lens Based on Rectangular Holes

A compact plasmonic lens is proposed in this paper. This plasmonic lens consists of rectangular holes etched on the silver film and arranged on one straight line and possesses the characteristics of short focus length, ultrathin thickness, and strong focus ability. The theoretical design for the plasmonic lens abides by the constructive interference theorem, and the surface plasmon polaritons excited by the holes with linearly polarized light illumination focuses effectively. The plasmonic lenses with single and double focus spots are provided, and the simulation experiment gives the powerful verification. The distinct structure feature and the excellent focusing characteristic of this plasmonic lens are benefit for its applications in optical integration.     

Design and Theoretical Analyses of Tip–Insulator–Metal Structure with Bottom–Up Light Illumination: Formations of Elongated Symmetrical Plasmonic Hot Spot at Sub-10 nm Resolution

Ag tip–insulator–metal structure with bottom–up light illumination is proposed and theoretically analyzed. It shows that there is a strong plasmonic coupling between Ag tip and metallic surface. Different from oblique light illumination, this novel design possesses unique advantages of symmetrical hot spot profile and enlarged depth of focus at a sub-10-nm spatial resolution. Influences of tip size, insulator, and metallic layer thickness are studied. It is found that the metallic layer thickness greatly affects the plasmonic hot spot quality. Meanwhile, the thickness of photoresist plays a major role in controlling light spot size, indicating that much higher resolution can be achieved for the Ag tips with large curvature radius.     

Simulation and Analytical Study of Optical Complex Field in Nano-corral Slits Plasmonic Lens

Although spiral plasmonic lens has been proposed as circular polarization analyzer, there is no such plasmonic nanostructure available for linear polarization. In the current work, we have designed nano-corral slits (NCS) plasmonic lens, which focuses the x- and y-polarized light into spatially distinguished plasmonic fields. We have calculated analytically and numerically the electric field intensity and phase of the emission from nano-corral slits plasmonic lens with different pitch lengths under various polarizations of the illumination. It has been shown that one can control the wave front of the output beam of these plasmonic lenses by manipulating the illumination of both circular and linear polarization. Our theoretical study in correlation with FDTD simulation has shown that NCS plasmonic lens with pitch length equal to λ_spp produces scalar vortex beam having optical complex fields with helical wave front and optical singularity at the center under circular polarization of light. When NCS lens (pitch = λ_spp) is illuminated with linearly polarized light, it exhibits binary distribution of phase with same electric field intensity around the center. However, with pitch length of 0.5λ_spp, NCS shows linear dichroism under linearly polarized illumination unlike spiral plasmonic lens (SPL) eliminating the use of circularly polarized light. Optical complex fields produced by these NCS plasmonic lenses may find applications for faster quantum computing, data storage, and telecommunications.

     

Terahertz Plasmonic Microcavity with High Quality Factor and Ultrasmall Mode Volume

In this paper, the characteristics of a novel terahertz plasmonic microcavity consisting of a circular hole and a coaxial (metallic) cylindrical core machined on a planar metal surface is theoretically investigated. It is shown that such a structure can sustain plasmonic modes, whose resonant wavelengths are much larger than the hole diameter and fields tightly localized within the cavity. For this cavity, both high quality factor and ultrasmall mode volume can be achieved in the terahertz range. As this type of microcavity is particularly compatible with planar technology, it has promising applications in the miniaturization and integration of terahertz optical components.     

Influence of Substrate on Plasmon-Induced Absorption Enhancements

A set of periodic plasmonic nanostructures is designed and fabricated as a means to investigate light absorption in single-crystal silicon thin-film structures with silicon-on-insulator (SOI) wafers as a model system. It is shown both computationally and experimentally that plasmon-induced absorption enhancement is remarkably higher for such devices than for thick or semi-infinite structures or for the thin-film amorphous silicon solar cells reported in the literature. Experimental photocurrent enhancements of the orders of 12 and 20 are demonstrated for non-optimized 2200-nm-thick photoconductive and 300-nm-thick photovoltaic test structures, respectively. Theoretical absorption enhancements as high as 80 are predicted to be achievable for the similar structures. The features of the spectral enhancements observed are attributed to several interacting resonance phenomena: not just to the favourable scattering of light by the periodic plasmonic nanoparticle arrays into the SOI device layer and coupling to the waveguide modes interacting with the plasmonic array but also to the Fabry-Pérot type interferences in the layered structure. We show that the latter effect gives a significant contribution to the spectral features of the enhancements, although frequently ignored in the discussions of previous reports.

     

Plasmonic Lenses: A Review

Four types of plasmonic lenses for the purpose of superfocusing designed on the bases of approximate negative refractive index concept, subwavelength metallic structures, waveguide mode were introduced, and curved chains of nanoparticles, respectively, were introduced. Imaging mechanism, fabrication, and characterization issues were presented. Theoretical analyses of the illumination with different polarization states on focusing performance of the plasmonic lenses were given also. In addition, a hybrid Au-Ag plasmonic lens with chirped slits for the purpose of avoiding oxidation of Ag film was presented.     

Analytical Methods of Equivalent Circuit Model and Transmission Matrix for a Plasmonic Switch with Sensing Capability in Near-Infrared Region

In this paper, the simultaneous switching and sensing capabilities of a compact plasmonic structure based on a conventional rectangular hole in a silver film are proposed and investigated. The proposed structure has ultrahigh sensitivity up to 3000 nm/RIU and high figure of merit of 170 RIU⁻¹. Also, the simulation results show the potential of the presented refractive index sensor to detect malaria infection, cancer cells, bacillus bacteria, and solution of glucose in water. Simultaneously, by changing the incident lightwave polarization, the structure behaves like a plasmonic switch, which has high extinction ratios of 15.81, 31.20, and 25.03 dB at three telecommunication wavelengths of 850, 1310, and 1550 nm, respectively. The ultrafast response time of 20 fs is achieved for the wideband application of the switching capability at the wavelength range of 1056 to 1765 nm. Moreover, the equivalent circuit model and transmission (ABCD) matrix methods are derived to validate the simulated results. Simple design, good agreement between the numerical and analytical results, biomedical applications, ultrahigh sensitivity, and ultrafast performance of the proposed structure help this idea to open up paths for design and implementation of other multi-application plasmonic devices in near-infrared region. To the best of our knowledge, the mentioned analytical methods have not been studied former at near-infrared wavelengths. Therefore, the achievements could pave the way for verifying the simulation results of plasmonic nanostructures in future investigations.

     

Absorption Circular Dichroism Induced by Contorted Electrical Oscillations in Rectangular Nanoholes

The chiroptical response of plasmonic chiral nanostructures can be tuned by combining different structures. In this paper, an L-shaped metal strip is introduced into a rectangular metal nanohole to generate absorption circular dichroism (ACD). The results of a finite-element method calculation show that ACD effects result from contorted electrical oscillations in the L-shaped strip. Additional calculations show that the ACD effects of the proposed metasurface depend strongly on the structure parameters. These findings provide not only a mechanism for enhancing chiroptical responses in planar structures but also a general strategy for the chiral manipulation of light and creation of chiroptical devices.

     

Applications of spectral hole burning spectroscopies to antenna and reaction center complexes

The underlying principles of spectral hole burning spectroscopies and the theory for hole profiles are reviewed and illustrated with calculated spectra. The methodology by which the dependence of the overall hole profile on burn wavelength can be used to reveal the contributions from site inhomogeneous broadening and various homogeneous broadening contributions to the broad Q_y-absorption bands of cofactors is emphasized. Applications to the primary electron donor states of the reaction centers of purple bacteria and Photosystems I and II of green plants are discussed. The antenna (light harvesting) complexes considered include B800–B850 and B875 of Rhodobacter sphaeroides and the base-plate complex of Prosthecochloris aestuarii with particular attention being given to excitonic interactions and level structure. The data presented show that spectral hole burning is a generally applicable low temperature approach for the study of excited state electronic and vibrational (intramolecular, phonon) structures, structural heterogeneity and excited state lifetimes.William E. Catron Fellow.     

Designable nanoplasmonic biomarkers for direct microscopy cytopathology diagnostics

Direct microscopy interpretation of fine‐needle biopsy cytological samples is routinely used by practicing cytopathologists. Adding possibility to identify selective and multiplexed biomarkers on the same samples and with the same microscopy technique can greatly improve diagnostic accuracy. In this article, we propose to use biomarkers based on designable plasmonic nanoparticles (NPs) with unique optical properties and excellent chemical stability that can satisfy the above‐mentioned requirements. By finely controlling the size and composition of gold‐silver alloy NPs and gold nanorods, the NPs plasmonic resonance properties, such as scattering efficiency and resonance peak spectral position, are adjusted in order to provide reliable identification and chromatic differentiation by conventional direct microscopy. Efficient darkfield NPs imaging is performed by using a novel circular side illumination adaptor that can be easily integrated into any microscopy setup while preserving standard cytopathology visualization method. The efficiency of the proposed technology for fast visual detection and differentiation of three spectrally distinct NP‐markers is demonstrated in different working media, thus confirming the potential application in conventional cytology preparations. It is worth emphasizing that the presented technology does not interfere with standard visualization with immunohistochemical staining, but should rather be considered as a second imaging modality to confirm the diagnostics.      

Gray Level Image Encoding in Plasmonic Metasurfaces

Plasmonic metasurfaces have been widely used for image and color representation using nanoscale structures. By designing the size or shape of nanostructures, the phase or amplitude of transmitted or reflected electromagnetic spectrum can be manipulated via control of resonant wavelength, which results in multiple colors and can be used for color and/or image generation. Instead of color printing, we introduce a new approach from which images with multiple gray scales can be generated and hidden under light with specific wavelengths. In this work, plasmonic structures of nanorods with identical geometries are fabricated in arrays on a glass substrate, but the nanorods comprise binary alloys of aluminum and titanium; therefore, grayscale image information is encoded with the material composition. Under white light illumination with polarization along the long axis of a nanorod, the response of the nanorods has essentially the same resonant frequency, but with varying magnitudes of transmission. Based on this, using nanorods with four different compositions, four gray level imaging has been achieved. Since nanorods are sensitive to the polarization and spectral composition of the incident light, under unpolarized white light illumination, the image disappears. Therefore, this technique is promising in image encryption. We also show that the manipulation of light at the resonant wavelength by this method occurs in amplitude, not phase.

     

Broadband Optical Response in Ternary Tree Fractal Plasmonic Nanoantenna

The ability to precisely tailor lineshapes, operational bandwidth, and localized electromagnetic field enhancements (“hot spots”) in nanostructures is currently of interest in advancing the performance of plasmonics-based chemical and biological sensing techniques. Fractal geometries are an intriguing alternative in the design of plasmonic nanostructures as they offer tunable multiband response spanning the visible and infrared spectral regions. A numerical study of the optical behavior of ternary tree fractal plasmonic nanoantenna is presented. Self-similar features are seen to emerge in the extinction spectra with the increase in fractal order N of the tree structure. Plasmon oscillations occurring at different length scales are shown to correspond to the multiple peaks and are compared with the spatial maps of electric field enhancement at the surface of the nanoantenna. The multiple peaks are shown to be independently tunable by structural variation. The robustness of the spectral response and polarization dependence arising due to various asymmetries is discussed.     

Tunable Plasmon-Induced Transparency of Double Continuous Metal Films Sandwiched with a Plasmonic Array

Making a continuous metal film with near-unity transparency has received more and more attention in recent years because of its potential applications for various optoelectronic devices. Here, we theoretically show that a high tunable plasmon-induced transparency metal film structure can be performed by double continuous metal films inserted with a two-dimensional hexagonal lattice array of plasmonic nanopariticles. The proposed structure shows near-unity anti-reflection and intensively enhanced transmission via the cooperative effects of strong resonant near-field light input and output coupling by the plasmonic array and the excitation of surface electromagnetic waves of the metal films. The optical response can be efficiently mediated by varying the sizes of nanoparticles and the separated distance between the metal array and the metal films. With the merits of high transparency, sub-wavelength sizes and wholly retained metal characteristics including high conductivity via using the pure metallic materials, the structure proposed here suggests various potential applications in optoelectronic integrated circuits.     

Automatic registration of microarray images. II. Hexagonal grid

MOTIVATION: In the first part of this paper the author presented an efficient, robust and completely automated algorithm for spot and block indexing in microarray images with rectangular grids. Although the rectangular grid is currently the most common type of grouping the probes on microarray slides, there is another microarray technology based on bundles of optical fibers where the probes are packed in hexagonal grids. The hexagonal grid provides both advantages and drawbacks over the standard rectangular packing and of course requires adaptation and/or modification of the algorithm of spot indexing presented in the first part of the paper. RESULTS: In the second part of the paper the author presents a version of the spot indexing algorithm adapted for microarray images with spots packed in hexagonal structures. The algorithm is completely automated, works with hexagonal grids of different types and with different parameters of grid spacing and rotation as well as spot sizes. It can successfully trace the local and global distortions of the grid, including non-orthogonal transformations. Similar to the algorithm from part I, it scales linearly with the grid size, the time complexity is O(M), where M is total number of grid points in hexagonal grid. The algorithm has been tested both on CCD and scanned images with spot expression rates as low as 2%. The processing time of an image with about 50 000 hex grid points was less than a second. For images with high expression rates ( approximately 90%) the registration time is even smaller, around a quarter of a second. Supplementary information: http://fleece.ucsd.edu/~vit/Registration_Supplement.pdf     

Plasmonic Wood for High‐Efficiency Solar Steam Generation

Plasmonic metal nanoparticles are a category of plasmonic materials that can efficiently convert light into heat under illumination, which can be applied in the field of solar steam generation. Here, this study designs a novel type of plasmonic material, which is made by uniformly decorating fine metal nanoparticles into the 3D mesoporous matrix of natural wood (plasmonic wood). The plasmonic wood exhibits high light absorption ability (≈99%) over a broad wavelength range from 200 to 2500 nm due to the plasmonic effect of metal nanoparticles and the waveguide effect of microchannels in the wood matrix. The 3D mesoporous wood with numerous low‐tortuosity microchannels and nanochannels can transport water up from the bottom of the device effectively due to the capillary effect. As a result, the 3D aligned porous architecture can achieve a high solar conversion efficiency of 85% under ten‐sun illumination (10 kW m^?2). The plasmonic wood also exhibits superior stability for solar steam generation, without any degradation after being evaluated for 144 h. Its high conversion efficiency and excellent cycling stability demonstrate the potential of newly developed plasmonic wood to solar energy‐based water desalination.     

Efficient Plasmonic Au/CdSe Nanodumbbell for Photoelectrochemical Hydrogen Generation beyond Visible Region

In this communication, light harvesting and photoelectrochemical (PEC) hydrogen generation beyond the visible region are realized by an anisotropic plasmonic metal/semiconductor hybrid photocatalyst with precise control of their topology and heterointerface. Controlling the intended configuration of the photocatalytic semiconductor to anisotropic Au nanorods' plasmonic hot spots, through a water phase cation exchange strategy, the site‐selective overgrowth of a CdSe shell evolving from a core/shell to a nanodumbbell is realized successfully. Using this strategy, tip‐preferred efficient photoinduced electron/hole separation and plasmon enhancement can be realized. Thus, the PEC hydrogen generation activity of the Au/CdSe nanodumbbell is 45.29 µmol cm^?2 h^?1 (nearly 4 times than the core/shell structure) beyond vis (λ > 700 nm) illumination and exhibits a high faradic efficiency of 96% and excellent stability with a constant photocurrent for 5 days. Using surface photovoltage microscopy, it is further demonstrated that the efficient plasmonic hot charge spatial separation, which hot electrons can inject into CdSe semiconductors, leads to excellent performance in the Au/CdSe nanodumbbell.     

All-Optical Plasmonic Switches Based on Asymmetric Directional Couplers Incorporating Bragg Gratings

Plasmonics - In this paper, a novel technique for realization of all-optical plasmonic switches is presented. The proposed structure is based on an asymmetric metal-insulator-metal plasmonic...