Comparison of Metal Parameters Obtained from Prism and Grating Coupling to Surface Plasmon Polaritons

Abstract

Surface plasmon polariton resonances obtained from both prism coupling (in the Otto configuration) and grating coupling to the same metallic surface have been fitted using appropriate theories to give the dielectric constant of the metal at 632·8 nm. Results from silver, aluminium and gold samples have confirmed that the dielectric constants obtained from fitting the data obtained using grating coupling are the same (to within experimental errors) as those obtained from prism-coupled data.

This confirms for the first time the validity of the differential diffraction grating theory by direct comparison with known data for a metallic surface.     

Enhanced Raman Scattering from Benzene Condensed on a Silver Grating

Abstract

We study the Raman intensity from a benzene overlayer on a silver grating as a function of layer thickness both in theory and experiment. Numerical results are obtained using matrix formalism based on the Rayleigh method for a coated grating. The electromagnetic enhancement due to surface plasmon polariton resonances and multiple beam interferences for a p-polarized incident wave is discussed.     

The influence of grating profile on surface plasmon polariton resonances recorded in different diffracted orders

Abstract

Nonlinear processes involved in the manufacture of nominally sinusoidal surface relief diffraction gratings can introduce distortions into the profile of these surfaces. Such distortions may dramatically affect both the specular reflectivity and diffracted efficiencies from such a surface, particularly if it is metallic. To illustrate this a comprehensive numerical modelling study of the optical coupling to surface plasmon polaritons on silver gratings has been undertaken. The grating surface profile is represented in terms of a truncated Fourier series, the effect of varying the amplitude, and then the phase, of each Fourier component in turn, is explored. This illustrates the sensitivity of individual features to specific harmonic components of the surface, for surface plasmon resonances recorded in both the zeroth and higher diffracted orders.     

A surface plasmon study of the optical dielectric function of indium

Abstract

The optical dielectric function of indium is measured by optical excitation of surface plasmon polaritons on an indium-coated silica grating for a range of wavelengths in the visible region of the spectrum. By exciting the surface plasmon polariton at the buried indium-grating interface, the indium surface that supports the surface plasmon polariton is kept free from oxidation. Comparison of angle-dependent reflectivities with a grating modelling theory gives both the real and imaginary parts of the dielectric function of indium. These results are compared with free-electron models to obtain an estimate of the plasma frequency and relaxation time.     

Surface plasmon-polariton study of the optical dielectric function of zinc

Abstract

The optical dielectric function of zinc is measured by excitation of surface plasmon-polaritons on a silica prism coated in MgF₂ and zinc in the Kretchmann configuration and on a zinc-covered silica grating for a range of wavelengths in the visible spectrum. By exciting the surface plasmon at the zinc-MgF₂ or zinc-grating interfaces, the zinc is protected from oxidization and contamination. Comparison of angle-dependent reflectivities with either Fresnel's theory or grating modelling theory gives both the real and imaginary parts of the dielectric function of the zinc over the visible spectrum. The results compare very favourably with those obtained by other workers using more conventional methods for samples held in vacuum.     

Examination of the +1, −1 Surface Plasmon Mini-gap on a Gold Grating

Abstract

An energy gap in the excitation of surface plasmons is found for light at normal incidence to a gold grating. This gap occurs at the crossing of the plus and minus first order surface plasmons. It arises directly as a consequence of distortion of the grating from sinusoidality, the first harmonic of the grating providing coupling between the plus and minus one orders. Experiments have been performed using both wavelength scans, where at a fixed angle of incidence the wavelength of excitation is varied, and angle of incidence scans, where for a fixed wavelength the angle of incidence is varied a few degrees either side of normal to the grating. By fitting the angular dependent reflectivity scans using grating modelling theory the gold grating is characterized at all wavelengths. This then allows a detailed comparison of the theoretical dispersion curve with that obtained experimentally. The agreement for both p-polarized light (for angle dependence with the plane of incidence normal to the grating grooves) and for s-polarized light (angle dependence with the plane of incidence perpendicular to the grating grooves) is excellent. An apparent momentum gap in the lower energy branch of the dispersion curve, attributed to the loss of coupling strength, is found to move to the upper branch if the grating profile is inverted.     

Broadband Excitation and Emission of Surface Plasmons

Abstract

The well known advantages of using surface plasmons, in particular the high sensitivity to surface adsorbates, are nearly always compromised in practice by the use of monochromatic excitation and the consequent lack of proper spectroscopic information. This limitation arises from the angle/wavelength selective nature of the surface plasmon resonance. The work described here uses an elegant broadband excitation/decay scheme in a substrate(silica)-grating profiled photoresist-Ag film geometry. Laser radiation of wavelength 488 nm, incident through the silica substrate, excites by near-field coupling a broad band of surface plasmons at the photoresist-Ag interface within the spectral range of the photoresist fluorescence. With a judicious choice of grating period this mode can cross-couple to the mode supported at the Ag-air interface. This latter mode can, in turn, couple out to light by virtue of the same grating profile. The spectral distribution of the light emitted due to this three-step process has been studied as a function of the angle of emission and depth of the grating profiled surface for each polarization. It is found that the optimum emission efficiency occurs with a groove depth in the region of 65 nm. This is considerably greater than the optimum depth of 40 nm required for surface plasmon-photon coupling at a Ag-air interface or, in other words, for the last step of the process in isolation.     

Surface Plasmon Enhanced Laser Ablation of Thin Metal Films

Abstract

Surface plasmon enhancement of laser ablation of thin Al films is examined with a view to its application in metal film patterning and nano-structuring. Al films, deposited on silica prisms, are first characterized by attenuated total reflection using a broadband UV source and appropriate interference filter. The films are subsequently subjected to excimer laser radiation of wavelength 248 nm under conditions both of direct incidence from the air side of the film, and of surface plasmon excitation in which light is incident through the prism at greater than critical angle. For a given level of ablation damage in a particular film the fluence required using the surface plasmon technique is 3–5 times less than that needed when direct incidence is used. This is roughly in line with the energy absorbed in the film. From a practical standpoint it is clear that ablation of metal films can be achieved with much lower fluences than has hitherto been possible, thus reducing the requirements on laser output and relaxing the power handling constraints on any input optical elements.     

Properties of TM resonances on metallic slit gratings

Electromagnetic resonances on metallic slit gratings induced by TM polarized incident light have been investigated and physically interpreted. We have developed an electromagnetic model imposing surface impedance boundary conditions on the metallic grating surface from which we derive simple formulas explaining all physical properties of these resonances. It is demonstrated that Fabry-Perot (or cavity) resonances are generated by the zeroth slit mode yielding extraordinary transmission. For very narrow slits, the resonant H-field is squeezed to the slit walls and causes enhanced power losses. The excitation of surface plasmon polaritons (SPPs), however, is generated by two mode coupling. SPPs are linked to sharp absorption peaks and dips in transmittance. It is shown that these phenomena are primarily caused by the interaction of the electromagnetic fields with the finite conducting slit walls. These findings have been confirmed by measured transmittance data of gold gratings with periods of 0.5 μm, 1 μm, and 2 μm.     

Surface Plasmon-polariton Study of the Optical Dielectric Function of Copper

Abstract

The optical dielectric function of copper is measured by excitation of surface plasmon-polaritons on a copper-covered silica grating for a range of wavelengths in the visible region of the spectrum. By exciting the surface plasmon-polariton at the grating/copper interface, the copper is protected from oxidation and contamination. Comparison of angle-dependent reflectivities to grating modelling theory gives both the real and imaginary parts of the dielectric function of the copper. The results compare very favourably with those obtained by other workers using more conventional methods for samples held in a vacuum.     

Ultraviolet surface plasmons on aluminium and a non-contact technique for characterizing short-pitch gratings

Abstract

In this work we present grating-coupled surface plasmon characterization of aluminium in the ultraviolet. Light of wavelength 325 nm from a helium-cadmium laser was used to excite surface plasmon-polaritons both at a buried aluminium-silica interface and on oxidized aluminium. By fitting the angle-dependent reflectivity data to model grating theory, the optical permittivity of uncontaminated aluminium at 325 nm is deduced. These results are compared with those extrapolated from prism-coupled surface plasmon excitation in the visible. The further characterization of the oxidized aluminium allows optical non-contact profilometry to be performed on short-pitch gratings that are opaque or are comprised of materials with ill-defined optical properties. This profile characterization is demonstrated for a short-pitch grating manufactured in photoresist.     

Wide-aperture detector of terahertz radiation based on GaAs/InGaAs transistor structure with large-area slit grating gate

Terahertz photoresponse of a GaAs/InGaAs transistor structure with large-area slit grating gate has been measured. Peaks in the photoresponse curve are assigned to plasmon resonances excited in the structure. More effective excitation of plasmon resonances is achieved in a grating gate structure with narrow slits, which increase the photoresponse amplitude by an order of magnitude.     

Tunable Fano Resonance and Plasmon–Exciton Coupling in Single Au Nanotriangles on Monolayer WS2 at Room Temperature

Tunable Fano resonances and plasmon–exciton coupling are demonstrated at room temperature in hybrid systems consisting of single plasmonic nanoparticles deposited on top of the transition metal dichalcogenide monolayers. By using single Au nanotriangles (AuNTs) on monolayer WS₂ as model systems, Fano resonances are observed from the interference between a discrete exciton band of monolayer WS₂ and a broadband plasmonic mode of single AuNTs. The Fano lineshape depends on the exciton binding energy and the localized surface plasmon resonance strength, which can be tuned by the dielectric constant of surrounding solvents and AuNT size, respectively. Moreover, a transition from weak to strong plasmon–exciton coupling with Rabi splitting energies of 100–340 meV is observed by rationally changing the surrounding solvents. With their tunable plasmon–exciton interactions, the proposed WS₂–AuNT hybrids can open new pathways to develop active nanophotonic devices.     

Diffraction Grating Characterization Using Multiple-wavelength Excitation of Surface Plasmon Polaritons

Abstract

In this work we have used the optical excitation of surface plasmon polaritons to characterize the profile of a diffraction grating with a large groove-depth-to-pitch ratio at wavelengths spanning the visible spectrum. This provides us with confirmation of the accuracy of the modelling theory used to predict the optical response of a deep grating. In addition it draws attention to a particular wavelength regime which proves sensitive to the higher harmonics of the grating profile.     

Transient-grating Studies in GaAs/GaAlAs Multiple Quantum Wells

Abstract

We report degenerate four-wave mixing experiments at room temperature, using a three-beam geometry and sub-picosecond laser to study GaAs/GaAlAs multiple quantum well material. Diffraction efficiencies and grating decay times for two samples of different carrier lifetimes have been measured in the spectral region corresponding to excitonic resonances. The measurements provide values of the nonlinear refractive index per carrier pair (n _eh) deriving from the excitonic saturation, and also the recombination times and diffusion coefficients for the samples.     

The inverted surface plasmon resonance: Phenomenological explanation

Abstract

A phenomenological explanation is made of different types of resonances in the reflectivity curves of a prism coupler in the region of surface plasmon excitation. The approach is based on the poles and zeros of the scattering operator and demonstrates that the existence of peaks instead of dips is natural and is determined by the positions of the pole and the zero in the complex plane.     

Dynamic surface properties,wettability and mimic oil recovery of ethanediyl- α, β -bis(cetyldimethylammonium bromide) on dodecane modified silica powder

In order to increase petroleum oil recovery, the wettability of the interface between flooding aqueous solution and oil reservior plays an important role. A surfactant dissolved in the flooding solution can be adsorbed on the reservior surface and then the wettability of the interface can be changed. For the purpose of mimic oil recovery, a cationic gemini surfactant, ethanediyl-α, β -bis(cetyldimethylammonium bromide) (16-2-16), was synthesized and characterized. The dynamic surface and interface tensions have been measured using pendant drop method at the air-water surface and dodecane-water interface, and experimental curve is close to orientation model and interaction model of surface equations of state. Compared with conventional cationic surfactants, cetyldimethylammonium bromide (CTAB), 16-2-16 has very low cmc value (1.8 × 10^{− 5} mol/L to 1.0 × 10^{− 3} mol/L). The most hydrophilic condition between the surfactant aqueous solution and the silica powder surface appears near the cmc for both 16-2-16 and CTAB. The best mimic oil recovery is also reached around the cmc for the two surfactants, but the efficiency for the former (68%) is higher than the one for the latter (63%).     

Plasmons in nanoscale and atomic-scale systems

Abstract

Plasmons in metallic nanomaterials exhibit very strong size and shape effects, and thus have recently gained considerable attention in nanotechnology, information technology, and life science. In this review, we overview the fundamental properties of plasmons in materials with various dimensionalities and discuss the optical functional properties of localized plasmon polaritons in nanometer-scale to atomic-scale objects. First, the pioneering works on plasmons by electron energy loss spectroscopy are briefly surveyed. Then, we discuss the effects of atomistic charge dynamics on the dispersion relation of propagating plasmon modes, such as those for planar crystal surface, atomic sheets and straight atomic wires. Finally, standing-wave plasmons, or antenna resonances of plasmon polariton, of some widely used nanometer-scale structures and atomic-scale wires (the smallest possible plasmonic building blocks) are exemplified along with their applications.     

Lithography‐Free Fabrication of Silica Nanocylinders with Suspended Gold Nanorings for LSPR‐Based Sensing

Tunable plasmonic platforms are important for a variety of applications such as photovoltaics, LED's, optoelectronics, medical research, and biosensors. In particular, development of label‐free plasmonic biosensors is one of the key research areas that utilizes plasmonic nanostructures for detection of biologically relevant molecules at low concentrations. The authors have developed a cost‐effective, fast, and lithography‐free method to fabricate transparent fused silica nanocylinders. The technique allows tuning of nanocylinder height, diameter, and density and can be scaled to large surface areas, such as 8 in. wafers. The authors demonstrate that gold coated nanocylinders support localized surface plasmon resonances (LSPR) from visible to near infrared wavelengths. The plasmonic platform can be characterized as suspended gold nanorings and exhibits a sensitivity of 658 nm RIU^–1 with a figure‐of‐merit of 10, comparable to other state‐of‐the‐art LSPR sensing platforms that utilize more complex nanofabrication pathways. It was observed that the LSPR peak positions can be controlled by varying the geometry of the nanocylinders. The authors illustrate surface functionalization, biosensing, and surface regeneration properties of the platform using thiols and detection of bovine serum albumin (BSA). The observed LSPR shifts for 11‐mercaptoundecanoic acid and BSA was 12 and 26 nm, respectively.     

An Inverted Surface Plasmon Resonance

Abstract

An inverted surface plasmon resonance is observed by measuring the internal reflectivity of a bilayer chromium-gold thin film deposited on a high index glass prism. With incident transverse magnetic radiation and suitable choice of chromium and gold thicknesses a strong resonance maximum is recorded which is nearly as narrow as the more commonly observed surface plasmon resonance minimum for gold. A peak reflectivity of order 40% has been recorded which with fine adjustment of the film thickness may be increased to perhaps 80% although it never approaches 100% because of the intrinsic absorption by the chromium. The angle dependent reflectivity data obtained are analysed using a multilayer Fresnel equation model and optical field and power loss profiles through the resonance are presented to illustrate its nature.