Low Loss Plasmonic Bragg Gratings with a Trench Plasmonic Waveguide

Plasmonics - We designed plasmonic Bragg gratings based on a plasmonic trench waveguide and calculated the characteristics of the designed structure. Conventional plasmonic Bragg gratings are...     

Self-Referencing Plasmonic Array Sensors

A self-referencing plasmonic platform is proposed and analyzed. By introducing a thin gold layer below a periodic two-dimensional nano-grating, the structure supports multiple modes including localized surface plasmon resonance (LSPR), surface plasmon resonance (SPR), and Fabry-Perot resonances. These modes get coupled to each other creating multiple Fano resonances. A coupled mode between the LSPR and SPR responses is spatially separated from the sensor surface and is not sensitive to refractive index changes in the surrounding materials or surface attachments. This mode can be used for self-referencing the measurements. In contrast, the LSPR dominant mode shifts in wavelength when the refractive index of the surrounding medium is changed. The proposed structure is easy to fabricate using conventional lithography and electron beam deposition methods. A bulk sensitivity of 429 nm/RIU is achieved. The sensor also has the ability to detect nanometer thick surface attachments on the top of the grating.

     

Boosting and Localizing Near-Field in Plasmonic Mirror-Image Nanoepsilon

A novel plasmonic mirror-image nanoepsilon (MINE) structure is studied to achieve significantly enhanced and localized near-field. It is well-known that the nanorod dimer is able to gain a high local field at the center of the structure by adjusting its rod width, rod length, and gap distance. When adopting an auxiliary nanoring structure electron reservoirs surrounding the nanorod-dimer to form the MINE structure, the local field can be further enhanced owing to a large amount of charges into sharp dimer structure which can confine the accumulation of charges to apexes. Thus, better synergistic interaction of gap effect and lightning-rod effect can be achieved for high field enhancement around nanoscale gap. The symmetric mode in the MINE structure enables strong near-field with a concentrated distribution around the gap. The influences of rod-tip angle and gap distance on the optical properties of plasmonic MINE structures are numerically investigated. With reducing the rod-tip angle and gap distance, considerable enhancements on the field intensities at the rod tips and the gap center can be attributed to the improved lightning-rod and gap effects. The near-field intensities at the rod tip and gap center are dramatically enlarged ~1.94 × 10⁴ and ~1.41 × 10⁴ times with the rod-tip angle of 41° and gap distance of 10 nm, and the near-field is localized within an extremely small range. These features are very beneficial for various plasmonic applications.     

Optimization of Optoelectronic Plasmonic Structures

We discuss the interplay between surface plasmon polaritons (SPPs) and localized shape resonances (LSRs) in a plasmonic structure working as a photo-coupler for a GaAs quantum well photodetector. For a targeted electronic inter-subband transition inside the quantum well, maximum photon absorption is found by compromising two effects: the mode overlapping with incident light and the lifetime of the resonant photons. Under the optimal conditions, the LSR mediates the coupling between the incident light and plasmonic structure while the SPP provides long-lived resonance which is limited ultimately by metal loss. The present work provides insight to the design of plasmonic photo-couplers in semiconductor optoelectronic applications.     

Polarization-Controlled Tunable Multi-focal Plasmonic Lens

A polarization-controlled tunable plasmonic lens which can generate different multi-focal combinations with exciting sources of left and right circular polarizations is proposed in this paper. Both position and intensity of each focal point can be adjusted by modulating the structure of the plasmonic lens. It is believed that the polarization-controlled tunable plasmonic multi-focal lens can be potentially used for optical switches and multi-channel couplers in future logic photonic and plasmonic systems.     

Highly Efficient Ultrathin Plasmonic Insulator-Metal-Insulator-Metal Solar Cell

Nano-porous ultrathin plasmonic insulator-metal-insulator-metal (IMIM) solar cell with high power conversion efficiency up to 7% in broad wavelength range from 300 to 750 nm was theoretically studied. The proposed IMIM design allows to choose various bottom insulators with desired barrier height of metal-insulator interface due to independence of the total absorbance on the bottom insulator. IMIM structure shows 73.8% difference in the average absorbance between the top and bottom metal layers with 1-nm bottom insulator. Moreover, the incident light decreases the absorbance negligibly up to 35 degrees for both TE and TM modes and by 17.5% at 70 degrees. Furthermore, the absorption between TE and TM modes differs by less than 5%, which indicates the structure as polarization independent. Our results indicate IMIM design benefit in plasmonic solar cells demanding low thickness, flexibility, low-cost, and polarization independence. Moreover, this structure can be implemented for integrated optical circuits as well as for solar thermoelectric generator.     

Plasmonic Optical Properties and Applications of Metal Nanostructures

In this review article, we provide an overview of recent research activities in the study of plasmonic optical properties of metal nanostructures with emphasis on understanding the relation between surface plasmon absorption and structure. Both experimental results and theoretical calculations have indicated that the plasmonic absorption strongly depends on the detailed structure of the nanomaterials. Examples discussed include spherical nanoparticles, nanorods, nanowires, hollow nanospheres, aggregates, and nanocages. Plasmon–phonon coupling measured from dynamic studies as a function of particle size, shape, and aggregation state is also reviewed. The fascinating optical properties of metal nanostructures find important applications in a number of technological areas including surface plasmon resonance, surface-enhanced Raman scattering, and photothermal imaging and therapy. Their novel optical properties and emerging applications are illustrated using specific examples from recent literature. The case of hollow nanosphere structures is highlighted to illustrate their unique features and advantages for some of these applications.     

All-Optical Surface Plasmonic Universal Logic Gate Devices

We have introduced optically controlled two-stage cascaded surface plasmonic two-mode interference waveguide structure (having silicon core and silver upper and lower cladding) as universal gates. GaAsInP cladding is used in left and right side of core for optical pulse controlled cladding refractive index modulation which controls propagation of excited modes. The universal logic gate operations have been shown with this structure. These universal gates have potential in development of large-scale integrated optical processor due to its compactness and high fabrication tolerance.     

Novel Dielectric-Loaded Plasmonic Waveguide for Tight-Confined Hybrid Plasmon Mode

In this paper, a novel metal-dielectric waveguide structure is proposed to support hybrid long range surface plasmon polaritons (LRSPPs) with a highly confined mode field. The simulation results showed that our proposed structure has better mode confinement and propagation length compared to that of conventional dielectric-loaded surface plasmon polaritons (DLSPPs) waveguides. This structure offers greater flexibility for the design of surface plasmon polaritons (SPPs) waveguides by altering the trade-off between mode confinement and propagation length. The proposed structure has significant potential for application in highly integrated photonic circuits.     

A Plasmonic Wavelength-Selected Intersection Structure

A subwavelength intersection structure with wavelength filtering functionality is proposed by using an open loop which consists of four slot cavities and four metal-insulator-metal (MIM) drop waveguides. The slot cavities are used as the resonators while the MIM waveguides serve as the input/output ports. The SPPs propagations and the transmission wavelengths in two pairs of matched input/output ports are immune to each other, which satisfy the intersection characters. The results based on finite difference time domain (FDTD) method demonstrate that high transmittance for the expected output port and high isolation for other ports have been achieved. Specifically, the transmittance and isolation is larger than 0.64 and 14 dB, respectively. In addition, a transmission peak, whose wavelength can be modulated by adjusting the length of the slot cavities, is available for the output port. Such a device can find applications in various optical systems, such as the wavelength filtering area and the optical intersection area.     

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.     

Plasmonic Ladder-Like Structure for NIR Polarizer

Polarization-dependent light transmission property is investigated in two-dimensional plasmonic ladder-like structure in the Near-infrared (NIR) regime of 900 to 1600 nm. The plasmonic ladder-like structures are fabricated using cost-effective laser interference lithography. Optical transmission studies reveal that in the stated NIR regime, the structure has nearly 30 % absolute transmission with respect to air when the long axis is aligned parallel to the polarization axis of the incident excitation and has negligible transmission at the crossed polarization state. The findings have potential implications in designing large area flat NIR polarizers.     

A Three-Dimensional Plasmonic Nanostructure with Extraordinary Optical Transmission

We report a 3D plasmonic nanostructure having an extraordinary optical transmission due to localized surface plasmon (LSP) coupling between nanoholes and nanodisks. The nanostructure contains a free-standing gold nanohole array (NHA) film above a cavity and an array of nanodisks at the bottom of the cavity that is aligned with the NHA. For the device, the LSP-mediated resonance position was dependent on the hole and nanodisk diameter as well as the separation distance. Also, the effect of LSP coupling between each hole and corresponding nanodisk became negligible for cavities deeper than 200 nm as observed as a disappearance of the LSP resonance. The greatest LSP resonance transmission and the highest electric field intensity were observed for the structure with the shallowest cavity. In addition, the structure had high surface plasmon resonance sensitivity and may have potential for surface-enhanced Raman spectroscopy and optical trapping applications.     

Surface-Assisted Carrier Excitation in Plasmonic Nanostructures

We present a quantum-mechanical model for surface-assisted carrier excitation by optical fields in plasmonic nanostructures of arbitrary shape. We derive an explicit expression, in terms of local fields inside the metal structure, for surface absorbed power and surface scattering rate that determine the enhancement of carrier excitation efficiency near the metal-dielectric interface. We show that surface scattering is highly sensitive to the local field polarization and can be incorporated into metal-dielectric function along with phonon and impurity scattering. We also show that the obtained surface scattering rate describes surface-assisted plasmon decay (Landau damping) in nanostructures larger than the nonlocality scale. Our model can be used for calculations of plasmon-assisted hot carrier generation rates in photovoltaics and photochemistry applications.     

Plasmonic Coupler and Multiplexer/Demultiplexer Based on Nano-Groove-Arrays

Plasmonics - A novel plasmonic unidirectional coupler and its extension to a multiplexer/demultiplexer are proposed and simulated. The proposed structure can be etched adjacent to...     

Hybrid Plasmonic Waveguide Based on Tapered Dielectric Nanoribbon: Excitation and Focusing

The nanofocusing of light source was proposed and simulated using the dielectric-loaded surface plasmon polariton (SPP) model with various laterally tapered planar dielectric architectures on the top surface of the metal. By using finite-difference time-domain method, enhancement factor for the local electric field under distinctive incident polarization was analyzed with different taper apexes under various incident wavelengths and incident angles of the excitation laser. The SPP dispersion and the effect of dissipation on adiabatic nanofocusing of SPP in a sharp taper structure were used to predict the optimal taper angles of the structure and to explain the phenomena of SPP wave slowing down as it propagating toward the taper end. This SPP nanofocusing process was also experimentally realized by illuminating the structure of a tapered CdS nanoribbon deposited on the Ag surface. As the emission of the focused SPP at the taper end, the proposed plasmonic structure can be severed as a light nanosource emitter in the future optical integrated circuits.     

Subwavelength Focusing Using Plasmonic Wavelength-Launched Zone Plate Lenses

We propose a plasmonic wavelength-launched Fresnel zone plate structure for subwavelength focusing. The plasmonic structure consists of a central circular groove surrounded by 12 transparent and opaque zones. All the zones with widths smaller than one half of the incident wavelength are used to enhance the field of evanescent waves in the transmission. Based on the finite-difference time-domain analysis, a focus spot with a full-width at half-maximum of 270 nm (= 0.4λ _in ) can be achieved, accompanied by a largely reduced depolarization effect. The sharp waistline indicates that the surface waves are largely converged in the region of focus.     

Plasmonic Electro-Optical Switches Operating at Telecom Wavelengths

Electro-optical (EO) switches with a subwavelength device length based on metal–dielectric–metal nanocavities waveguide combined with organic EO materials have been proposed and numerically investigated. The finite difference time domain (FDTD) method with perfectly matched layer absorbing boundary condition is adopted to simulate and study their properties. The FDTD simulation results reveal that these structures filled with EO materials can realize the function of switch with low-driving voltage. The wavelength conversion switch structure might become a choice for the design of integrated architectures for optical computing and communication, especially in WDM systems in the nanoscale.     

Adjustable Plasmonic Multi-channel Demultiplexer with Graphene Sheets and Ring Resonators

Plasmonics - A novel designing method of tunable plasmonic multi-channel demultiplexers is proposed by combining two kinds of graphene ring coupling systems, i.e., middle-coupling structure and...     

Complex Polarization Response in Plasmonic Nanospirals

Archimedean nanospirals exhibit many far-field resonances that result from the lack of symmetry and strong intra-spiral plasmonic interactions. Here, we present a computational study, with corroborating experimental results, on the plasmonic response of the 4π Archimedean spiral as a function of incident polarization, for spirals in which the largest linear dimension is less than 550 nm. We discuss the modulation of the near-field structure for linearly and circularly polarized light in typical nanospiral configurations. Computational studies of the near-field distributions excited by circularly polarized light illustrate the effects of chirality on plasmonic mechanisms, while rotation of linearly polarized light provides a detailed view of the effects of broken symmetry on nanospiral fields in any given direction in the plane of the spiral. The rotational geometry exhibits a preference for circular polarization that increases near-field enhancement compared to excitation with linearly polarized light and exchanges near-field configurations and resonant modes. By analyzing the effects of polarization and wavelength on the near-field configurations, we also show how the nanospiral could be deployed in applications such as tunable near-field enhancement of nonlinear optical signals from chiral molecules.