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At least 757 records · Page 42

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DOE OSTI2014

High power terahertz generation using 1550 nm plasmonic photomixers

We present a 1550 nm plasmonic photomixer operating under pumping duty cycles below 10%, which offers significantly higher terahertz radiation power levels compared to previously demonstrated photomixers. The record-high terahertz radiation powers are enabled by enhancing the device quantum efficiency through use of plasmonic contact electrodes, and by mitigating thermal breakdown at high optical pump power levels through use of a low duty cycle optical pump. The repetition rate of the optical pump can be specifically selected at a given pump duty cycle to control the spectral linewidth of the generated terahertz radiation. At an average optical pump power of 150 mW with a pump modulation frequency of 1 MHz and pump duty cycle of 2%, we demonstrate up to 0.8 mW radiation power at 1 THz, within each continuous wave radiation cycle.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Plasmon resonance enhanced large third-order optical nonlinearity and ultrafast optical response in Au nanobipyramids

The third-order optical nonlinearity and response time of Au nanobipyramids have been investigated by using optical Kerr effect technique. Large third-order nonlinear optical susceptibility (χ{sup (3)}) and ultrafast optical response time have been obtained at the longitudinal surface plasmon resonance (LSPR) wavelength. As the excitation laser wavelength varies from non-resonance wavelength of 780 nm to the LSPR wavelength of 825 nm, χ{sup (3)} increases from 7.4 × 10{sup −14} to 3.9 × 10{sup −13} esu, the fast component of the response time decreases from 141 ± 23 to 83 ± 8 fs and the ratio of the fast component remains around 48%, while the slow response time decreases from 3200 ± 200 to 2310 ± 158 fs. The large enhancement of χ{sup (3)} is due to the large local field enhancement caused by the surface plasmon resonance, and the shortening of the response times are induced by the increased probability of the electron-electron and electron-phonon scatterings in the nanosystem. This significant ultrafast optical property in Au nanobipyramids has great application in future ultrafast information processors.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2014

Graphene-based terahertz tunable plasmonic directional coupler

We propose and numerically analyze a terahertz tunable plasmonic directional coupler which is composed of a thin metal film with a nanoscale slit, dielectric grating, a graphene sheet, and a dielectric substrate. The slit is employed to generate surface plasmon polaritons (SPPs), and the metal-dielectric grating-graphene-dielectric constructs a Bragg reflector, whose bandgap can be tuned over a wide frequency range by a small change in the Fermi energy level of graphene. As a graphene-based Bragg reflector is formed on one side of the slit, the structure enables SPP waves to be unidirectionally excited on the other side of the slit due to SPP interference, and the SPP waves in the Bragg reflector can be efficiently switched on and off by tuning the graphene's Fermi energy level. By introducing two optimized graphene-based Bragg reflectors into opposite sides of the slit, SPP waves can be guided to different Bragg reflectors at different Fermi energy levels, thus achieving a tunable bidirectional coupler.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Localized surface plasmon assisted contrast microscopy for ultrathin transparent specimens

We demonstrate a high contrast imaging technique, termed localized surface plasmon assisted contrast microscopy, by combining localized surface plasmon resonances (LSPR) and dark-field microscopy technique. Due to the sensitive response of LSPR to the refractive index of the surrounding media, this technique is capable of converting a small refractive index difference to a change in scattering intensity, resulting in a high-contrast, diffraction limited image of a thin unstained specimen with small, gradual refractive-index variation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Strong coupling in hybrid plasmon-modulated nanostructured cavities

The understanding of strong coupling between local restricted electromagnetic field of cavity and surface-plasmon (SP) on the metal surface lays the groundwork for many photonic applications. However, much of the fundamental photophysical properties underlying this performance such as how this strong coupling is induced by these two components, namely, the SP modes and cavity modes have remained unknown. Here, we present a study of a hybrid plasmon-modulated coupled system with Rabi splitting energy at 336 meV, and the coupled hybrid states are highly sensitive to the prosperity of SP mode which is induced by morphology of inlaid grating surfaces.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2014

Composite modulation of Fano resonance in plasmonic microstructures by electric-field and microcavity

Composite modulation of Fano resonance by using electric-field and microcavity simultaneously is realized in a plasmonic microstructure, which consists of a gold nanowire grating inserted into a Fabry-Perot microcavity composited of a liquid crystal layer sandwiched between two indium tin oxide layers. The Fano resonance wavelength varies with the applied voltage and the microcavity resonance. A large shift of 48 nm in the Fano resonance wavelength is achieved when the applied voltage is 20 V. This may provide a new way for the study of multi-functional integrated photonic circuits and chips based on plasmonic microstructures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Down-conversion photoluminescence sensitizing plasmonic silver nanoparticles on ZnO nanorods to generate hydrogen by water splitting photochemistry

Silver nanoparticles fabricated onto the surface of the ZnO nanorods form the photoanode and generate photoelectric current due to surface plasmon resonance, which serves as anode electrodes in photoelectrochemical hydrogen production. In order to increase the absorption spectrum of photoanode, organic pigments were utilized as photo-sensitizers to generate down-conversion photoluminescence to excite surface plasmon resonances of silver nanoparticles. The way of using light to carry the energy in electronic scattering regime runs the system for the enhancement of solar water splitting efficiency. It was significantly tuned in environmentally sustainable applications for power generation and development of alternative energy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Terahertz plasmonic Bessel beamformer

We experimentally demonstrate terahertz Bessel beamforming based on the concept of plasmonics. The proposed planar structure is made of concentric metallic grooves with a subwavelength spacing that couple to a point source to create tightly confined surface waves or spoof surface plasmon polaritons. Concentric scatterers periodically incorporated at a wavelength scale allow for launching the surface waves into free space to define a Bessel beam. The Bessel beam defined at 0.29 THz has been characterized through terahertz time-domain spectroscopy. This approach is capable of generating Bessel beams with planar structures as opposed to bulky axicon lenses and can be readily integrated with solid-state terahertz sources.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Surface-plasmon-enhanced lasing emission based on polymer distributed feedback laser

Optical losses associated with the metallic contacts necessary for charge injection are an obstacle to the development of electrically pumped organic lasers. In this work, we show that it is possible to overcome these losses by introducing surface plasmons (SPs) in a distributed feedback laser to enhance the lasing emission. We perform a detailed study of the SPs influence on the lasing emission. We experimentally show that enhanced lasing emission has been successfully achieved in the presence of a metal electrode. The laser emission is strongly dependent on the thickness of Ag layer. By optimizing the thickness of Ag layer, surface-plasmon-enhanced lasing emission has been achieved with much reduced thresholds and higher intensity. When the thickness of the Ag layer increases to 50 nm, the device exhibits ten-fold emission intensity and a fifth of excitation threshold comparing with Ag-free one. The finite-difference time-domain (FDTD) results show that large field intensity is built at the 4-(dicyanomethylene)-2-i-propyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl) -4H-pyran:/poly(9-vinylcarbazole)Ag interface, which could lead to a strong coupling between lasing and SPs, and consequently a much enhanced laser emission at the photon energy of around 2.02 eV (615 nm). Our FDTD simulations gave an explanation of the effects of the SPs on lasing operation in the periodic structures. The use of SPs would lead to a new class of highly efficient solid-state laser sources and provide a new path to achieve electrically pumped organic lasers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

All-optical switching of localized surface plasmon resonance in single gold nanosandwich using GeSbTe film as an active medium

Localized surface plasmon resonance (LSPR) switching was investigated in a Au/GeSbTe/Au nanosandwich as a key active element for plasmonic integrated circuits and devices. Near-infrared single-particle spectroscopy was conducted to examine the interaction of a Au nanorod (AuNR) and Au film, between which a GeSbTe layer was incorporated as an active phase-change media. Numerical calculation revealed that hybridized modes of the AuNR and Au film exhibit a significant change of scattering intensity with the phase change. In particular, the antisymmetric (magnetic resonance) mode can be modulated effectively by the extinction coefficient of GST, as well as its refractive index. Experimental demonstration of the switching operation was performed by alternate irradiation with a picosecond pulsed laser for amorphization and a continuous wave laser for crystallization. Repeatable modulation was obtained by monitoring the scattering light around the LSPR peak at λ = 1070 nm.

36 MATERIALS SCIENCE↗

DOE OSTI2015

An efficient plasmonic photovoltaic structure using silicon strip-loaded geometry

We show that a silicon thin-film photovoltaic structure with silicon strips on the top and grooves on the silver back contact layer can absorb incident solar energy over a broad spectral range. The silicon strips on the top scatter the incident light and significantly help couple to the photonic modes in the smaller wavelength range. The grooves on the silver back contact layer both scatter the incident light and help couple to the photonic modes and resonant surface plasmon polaritons. We find an increase of ∼46% in total integrated solar absorption in the proposed strip-loaded structure compared to that in a planar thin film structure of same dimensions. The proposed structure offers simpler fabrication compared to similar plasmonic-inspired designs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

A hybrid plasmonic waveguide terahertz quantum cascade laser

We present the realization of a quantum cascade laser emitting at around 2.85 THz, based on a hybrid plasmonic waveguide with a low refractive index dielectric cladding. This hybrid waveguide design allows the performance of a double-metal waveguide to be retained, while improving the emission far-field. A set of lasers based on the same active region material were fabricated with different metal layer thicknesses. A detailed characterization of the performance of these lasers revealed that there is an optimal trade-off that yields the best far-field emission and the maximum temperature of operation. By exploiting the pure plasmonic mode of these waveguides, the standard operation conditions of a double-metal quantum cascade laser were retrieved, such that the maximum operating temperature of these devices is not affected by the process. These results pave the way to realizing a class of integrated devices working in the terahertz range which could be further exploited to fabricate terahertz on-chip circuitry.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Localized surface plasmon and exciton interaction in silver-coated cadmium sulphide quantum dots

Localized surface plasmon and exciton coupling has been investigated on colloidal solutions of silver-coated CdS nanoparticles (NPs), synthesized by gamma irradiation. Two broad photoluminescence (PL) bands (blue/red) corresponding to band to band and defect state transitions have been observed for the bare and coated samples. In case of bare CdS NPs, the intensity of the red PL peak is about ten times higher than the blue PL peak intensity. However, on coating the CdS NPs with silver, the peak intensity of the blue PL band gets enhanced and becomes equal to that of the red PL band. High-resolution transmission electron microscopic (HRTEM) images adequately demonstrate size distribution of these metal/semiconductor nanocomposites. UV-Vis absorption studies show quantum confinement effect in these semiconductor quantum dot (SQD) systems. Absorption spectrum of silver-coated SQDs shows signature of surface plasmon-exciton coupling which has been theoretically verified.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Plasmon modes of metallic nanowires including quantum nonlocal effects

The properties of electrostatic surface and bulk plasmon modes of cylindrical metallic nanowires are investigated, using the quantum hydrodynamic theory of plasmon excitation which allows an analytical study of quantum tunneling effects through the Bohm potential term. New dispersion relations are obtained for each type of mode and their differences with previous treatments based on the standard hydrodynamic model are analyzed in detail. Numerical results show by considering the quantum effects, as the value of wave number increases, the surface modes are slightly red-shifted first and then blue-shifted while the bulk modes are blue-shifted.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2015

Formation of a hybrid plasmonic waveguide mode probed by dispersion measurement

Hybrid waveguides, i.e., dielectric waveguides combined with plasmonic waveguides, have great potential for concomitantly exhibiting subwavelength confinement and long range propagation, enabling a highly integrated photonic circuit. We report the characterization of hybrid waveguide modes excited in Si/SiO{sub 2}/Al films, by dispersion measurement using angle-resolved electron energy-loss spectroscopy. This experiment directly verifies the formation of the hybrid waveguide mode with a strongly localized electromagnetic field in a 6-nm-thick SiO{sub 2} layer. The results clearly describe the characteristic behavior of the hybrid waveguide mode, which depends on the effective index of the constituent dielectric waveguide and the surface plasmon-polariton modes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Si/ZnO nanorods/Ag/AZO structures as promising photovoltaic plasmonic cells

The test structures for photovoltaic (PV) applications based on zinc oxide nanorods (NRs) that were grown using a low-temperature hydrothermal method on p-type silicon substrates (100) covered with Ag nanoparticles (NPs) were studied. The NPs of three different diameters, i.e., 5–10 nm, 20-30 nm, and 50–60 nm, were deposited using a sputtering method. The morphology and crystallinity of the structures were confirmed by scanning electron microscopy and Raman spectroscopy. It was found that the nanorods have a hexagonal wurtzite structure. An analysis of the Raman and photoluminescence spectra permitted the identification of the surface modes at 476 cm{sup −1} and 561 cm{sup −1}. The presence of these modes is evidence of nanorods oriented along the wurtzite c-axis. The NRs with Ag NPs were covered with a ZnO:Al (AZO) layer that was grown using the low-temperature atomic layer deposition technique. The AZO layer served as a transparent ohmic contact to the ZnO nanorods. The applicability of the AZO layer for this purpose and the influence of the Ag nanoparticles on the effectiveness of light acquisition by such prepared PV cells were checked by reflectance and transmittance measurements of the AZO/glass and AZO/NPs/glass reference structures. Based on these studies, the high-energy transmittance edge was assigned to the ZnO energy gap, although it is blueshifted with respect to the bulk ZnO energy gap because of Al doping. It was also shown that the most optimal PV performance is obtained from a structure containing Ag nanoparticles with a diameter of 20–30 nm. This result is confirmed by the current-voltage measurements performed with 1-sun illumination. The structures show a plasmonic effect within the short wavelength range: the PV response for the structure with Ag nanoparticles is twice that of the structure without the nanoparticles. However, the influence of the Ag nanoparticle diameters on the plasmonic effect is ambiguous.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2015

Excitation of plasmons in Ag/Fe/W structure by spin-polarized electrons

Using Spin-polarized Electron-Energy Loss Spectroscopy (SPEELS), the plasmon excitations were probed in a few atomic layers thick Ag film deposited on an Fe layer or on a single crystal of W(110). The measurements were performed at two specular geometries with either a 25° or 72° angle of incidence. On a clean Fe layer (10 atomic layers thick), Stoner excitation asymmetry was observed, as expected. Deposition of a silver film on top of the Fe layer dramatically changed the asymmetry of the SPEELS spectra. The spin-effect depends on the kinematics of the scattering: angles of incidence and detection. The spin-dependence of the plasmon excitations in the silver film on the W(110) surface and on the ferromagnetic Fe film is suggested to arise from the spin-active Ag/W or Ag/Fe interfaces.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

An ultrafast nanotip electron gun triggered by grating-coupled surface plasmons

We demonstrate multiphoton photoelectron emission from gold nanotips induced by nanofocusing surface plasmons, resonantly excited on the tip shaft by a grating coupler. The tip is integrated into an electron gun assembly, which facilitates control over the spatial emission sites and allows us to disentangle direct grating emission from plasmon-triggered apex emission. The nanoscale source size of this electron gun concept enables highly coherent electron pulses with applications in ultrafast electron imaging and diffraction.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗