Browsing by Subject "Surface plasmon resonance."
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Item Open Access Applications of plasmon enhanced emission and absorption(2009) Ayas, SencerThe term plasmon-polariton is used to describe coupled modes of electromagnetic waves with electronic plasma oscillations in conductors. Surface plasmon resonances have found profound interest over the last few decades in multiple fields ranging from nanophotonics to biological sensing. In this thesis, we study enhancement of absorption and emission of radiation due to the presence of a modified local electromagnetic mode density within the vicinity of metallic surfaces supporting plasmon modes. Various coupling schemes of freespace electromagnetic modes to plasmon modes are investigated theoretically and experimentally. Local mode densities and field enhancements due to plasmon modes in planar structures, gratings and optical antennas have been studied in their relation to the absorption and emission enhancement of dipoles positioned in various orientations and locations with respect to structures displaying plasmonic effects. Particularly, grating coupled plasmon resonances were analysed using Rigorously Coupled Wave Analysis and Finite Difference Time domain methods. Experimental demonstrations of absorption and emission enhancement of dielectric layers containing Rhodamine 6G on various plasmonic structures are given. Confocal Raman microscopy was used in characterization of fabricated structures. It is seen that the experimental measurements are in good agreement with theoretical predictions. Direction dependent luminescence enhancement is observed with dye molecules on grating structures. Potential applications of plasmon enhanced absorption and emission include high sensitivity absorption spectroscopy, performance enhancement in thin film solar cells and luminescent concentrators.Item Open Access Chiral metamaterial and high-contrast grating based polarization selective devices(2013) Mutlu, MehmetThe utilization of purposely designed artificial media with engineered electromagnetic responses enables the obtaining of intriguing features that are either impossible or difficult to realize using readily available natural materials. Here, we focus on two classes of artificial media: metamaterials and high-contrast gratings. Metamaterials and high-contrast gratings are designed within the subwavelength periodicity range and therefore, they are non-diffractive. We exploit the magnetoelectric coupling effect in chiral metamaterials to design several structures. Firstly, we design a linear to circular polarization convertor that operates for x-polarized normally incident plane waves. Then, we combine the chirality feature and the electromagnetic tunneling phenomenon to design a polarization insensitive 90◦ polarization rotator that exhibits unity transmission and crosspolarization conversion efficiencies. Subsequently, we combine this polarization rotator with a symmetric metallic grating with a subwavelength slit for the purpose of enabling the one-way excitation of spoof surface plasmons and achieving a reversible diodelike beaming regime. Then, we exploit the asymmetric transmission property of chiral metamaterials and show that a polarization angle dependent polarization rotation and a strongly asymmetric diodelike transmission is realizable. Afterwards, a brief waveguide theory is provided and eventually, the dispersion relations for a periodic dielectric waveguide geometry are derived. Then, using these relations and considering the finiteness of the waveguide length, we show the theoretical description of high-contrast gratings. Finally, we theoretically and experimentally show that the achievement of a broadband quarter-wave plate regime is possible by using carefully designed high-contrast gratings.Item Open Access Coupled surface plasmon structures and applications(2009) Gürel, KemalSurface plasmons have attracted great interest during past decades due to their unique physical properties. In this thesis, we study grating-coupled surface plasmons for sensing and filtering applications. We first present simple physical and chemical procedures that allow tuning and modification of the topography of gratings present in optical storage discs into geometries optimal for grating coupled plasmon resonance excitation. After proper metal coating, the tuned surfaces exhibit sharp plasmon resonances that can be excited at wavelengths ranging from 260 nm to over 2.7 µm with relatively high quality factors. As an immediate exemplary application, use of such optimized gratings in aqueous medium for refractive index measurement is demonstrated. We also report another plasmonic component based on a pair of surfaces displaying grating coupled plasmon enhanced transmission. We observe high quality factor transmission peaks as high as 100 through our plasmonic filter based on gratings obtained directly from optical storage disks. Wavelength and polarization dependent transmission is also demonstrated in the visible and infrared portions of the spectrum. The resonance wavelength of this filter can be tuned by simply changing the angle of incidence. Numerical calculations agree well with measurements. Our work can open up directions toward disposable optical components such as filters and polarizers. Morever, we investigate plasmonic force between two coupled metallic layers. We observe the mode splitting due to coupling between plasmonic surfaces by using finite difference time domain simulations.Item Open Access EBL fabricated plasmonic nanostructures for sensing applications(2013) Cinel, Neval APlasmonics is a major branch of photonics dealing with light-matter interactions in metallic nanostructures. Plasmonic devices provide extreme confinement of electromagnetic oscillations to very small volumes beyond diffraction limit at optical frequencies. Our aim in this thesis study is to demonstrate the utilization of plasmonics for several applications such as optical localized surface plasmon resonance (LSPR) biosensor design, enhancement of signal intensity in surface enhanced Raman spectroscopy (SERS) and absorption enhancement in photodetectors. Firstly, a sensor structure that detects the changes in the refractive index of the surrounding medium by optical transmission measurements was designed. Periodic silver nano-disk arrays on sapphire substrate written by Electron-Beam Lithography (EBL) were used for this aim. Optical characterization was done through transmission/reflection measurements and supported by finite difference time domain (FDTD) simulations. The sensor was first verified by a biotinavidin bioassay. Real time binding studies showed that the sensor response was saturated within the first 30 minutes of application. Concentration dependency of the sensor structure showed an adequate response at the 1 nM-100 nM range. The refractive index sensitivity of the sensor was determined as 354 nm/RIU. The idea was finally applied to the detection of heat killed E.Coli bacteria. Promising results that indicate the possibility of using the sensor for bacteria detection was obtained. Secondly, tandem truncated nano-cones composed of Au-SiO2-Au layers that exhibit highly tunable double resonance behavior were shown to increase SERS signal intensity, for the first time. Enhancement factor (EF) calculations indicated an enhancement factor of 3.86 x107 . The double resonance design showed a 10 fold better enhancement when compared to its single resonance counterpart. This enhancement is believed to be even more prominent for applications such as NIR-SERS and Surface Enhanced Hyper Raman Scattering (SEHRS). Another SERS substrate containing dual layer, periodic, “coupled” concentric rings, separated by a dielectric spacer provided Raman signal intensity 630 times larger than plain gold film and 8 times larger than an “etched” concentric ring structure. The design provided an enhancement factor of 1.67x107 . Finally, Al nanoparticles with plasmonic resonance at UV wavelengths fabricated in between the Schottky contacts of an MSM detector on semi-insulating GaN was shown to yield 1.5 fold enhancement in absorption and photocurrent collection. Plasmonic enhancement in UV was studied for the first time with this study. Another UV-MSM photodetector on GaN that includes subwavelength apertures surrounded by nano-structured metal gratings was compared to a conventional design without gratings. Results indicated an 8 fold enhancement in the photocurrent at the resonant wavelength.Item Open Access Electrochemically swithable plasmonic surfaces(2014) Karayalçın, Nihat SerkanIn this study, we design and produce grating coupled surface plasmon surfaces which are switched by electrochemistry. Grating structures are fabricated using digital versatile discs (DVDs) which are commercially available. According to atomic force microscopy (AFM) results, we categorize the different grating structures in two groups, namely shallow and deep gratings. Plasmonic properties of the surfaces are investigated using numerical simulations. Gold and silver are used as plasmon supporting metallic layers on gratings. Refractive index sensitivity of the plasmon resonances are studied using deionized water, air and glycerol solutions as the dielectric media and results are compared with simulations. Experimental results are coherent with the simulations in terms of reflection spectra. Electrochemical switching of plasmonic properties may have applications in tunable and switchable filters, as well as enhanced colorimetric sensing. We deposit ultrathin films of copper on plasmonic surfaces and investigate reversible changes in the plasmonic properties. Copper sulfate is selected as the electrolyte. Cyclic voltammetry is performed on plasmonic surfaces while monitoring optical reflectance. Copper is observed to deposit in the form of nanoislands on silver and gold films rather than uniform thin films. The effect of copper deposition on the plasmonic properties of the grating structure is simulated by Lumerical software and is seen to be two fold. For small effective thickness of copper nanoislands, the plasmon resonance condition shifts, whereas for thicker copper deposition plasmonic resonances are eliminated. Finally, copper's oxidation and reduction reactions are controlled by changing applied voltage thus shifting the resonance wavelength. Resonances are switched reversibly multiple times not only for different molarities but also for different grating sructures and plasmon supporting metallic layers . In summary, we demonstrate that plasmonic properties of nanostructured metallic surfaces can be controlled by electrochemistry. Switchable resonance surfaces can be used as dynamic filters or may enhanced contrast in plasmon resonance imaging applications.Item Open Access Novel volumetric plasmonic resonator architectures for enhanced absorption in thin-film organic solar cells(2010) Sefünç, Mustafa AkınThere has been a growing interest in decreasing the cost and/or increasing the efficiency of clean renewable energy resources including those of photovoltaic approaches for conversion of sunlight into electricity. Today, although photovoltaics is considered a potential candidate in diversification of energy sources, the cost of photovoltaic systems remains yet to be reduced by several factors to compete with fossil fuel based energy production. To this end, new generation solar cells are designed to feature very thin layers of active (absorbing) materials in the order of tens of nanometers. Though this approach may possibly decrease the cost of solar cells, these ultra-thin absorbing layers suffer from undesirably low optical absorption of incident photons. Recently revolutionary efforts on increasing light trapping using nanopatterned metal layers in the active photovoltaic material via surface plasmon excitations have been demonstrated, which attracted interest of the academic community as well as the industry. In these prior studies, plasmonic structures, placed either on the top or at the bottom of absorbing layers, have been investigated to enhance the absorption in the active material. However, all these previous efforts were based only on using a single layer of plasmonic structures. In this thesis, different than the previous reports of our group and the others, we focus on a new design concept of volumetric plasmonic resonators that relies on the idea of incorporating two (or more) layers of coupled plasmonic structures embedded in the organic solar cells. For proof-of-concept demonstration, here we embody one silver grating on the top of the absorbing layer and another at the bottom of the active layer to couple them with each other such that the resulting field localization is further increased and extended within the volume of the active material. In addition to individual plasmonic resonances of these metallic structures, this allows us to take the advantage of the vertical interaction in the volumetric resonator. Our computational results show that this architecture exhibits a substantial absorption enhancement performance particularly under the transverse-magnetic polarized illumination, while the optical absorption is maintained at a similar level as the top grating alone under the transverseelectric polarized illumination. As a result, the optical absorption in the active layer is enhanced up to ~67%, surpassing the improvement limit of individual gratings, when the total film thickness is kept fixed. This volumetric interaction contributes to further enhancement of optical absorption in the active layer, beyond the limited photon absorption in non-metallic (bare) organic solar cell.Item Open Access Plasmonics from metal nanoparticles for solar cell applications(2013) Günendi, Mehmet CanIn today’s economy, need for development in energy is essential. Solar energy is safe, and at the same time is one of the cleanest, cheapest choices of energy alternative to fossil fuels. In this perspective, using the sun light effectively is in fundamental importance. One of the problems, because of the indirect band gap of the material Si, is small energy conversion ratios of various solar cell structures and limited absorption of red light. Because of the material properties, Si cells cannot absorb red light, which contributes great amount of the sun light. One of the recent developed techniques to use red light is using metal nanoparticles (MNP) embedded in a semiconductor medium as sub-wavelength antennas or MNP scatterers, hence increasing the effective path length of light in the cell. Absorption and scattering are mostly in plasmon resonances. Shifting the plasmon resonance peaks is possible by changing various parameters of the system like the size of the MNPs. In this work, Finite-Difference Time-Domain (FDTD) method is used to analyze various systems worked. Mainly the MEEP package, developed at MIT, is used to simulate systems and other codes, related to analytical work, have also used to compare results. The plasmon resonances of various sizes of Ag MNPs embedded in different mediums at different positions are analyzed. Critical parameters like particle size, shape, dielectric medium, film thickness are discussed for improved solar cell applications.Item Open Access Subwavelength surface plasmon interferometer for high-throughput sensing(2012) Yavaş, ÖzlemSmall detection volume, increased analysis speed and reduced cost are the main driving forces for miniaturized lab-on-a-chip systems. Subwavelength holes on opaque metal films provide a unique configuration for miniaturized sensors. Transmitted light through these tiny holes is governed by the electronic resonance on the surface of the metal film. Excitation of surface plasmon-polaritons (SPPs) on the metal-dielectric interface characterizes the resonance condition. The sensitive dependence of the plasmon resonance condition on the dielectric constant of the medium is used for label free sensing applications. In this thesis, we demonstrate a refractive index sensor based on a subwavelength plasmon interferometer using monochromatic light. Very high contrast fringe pattern is generated by the plasmon interferometer that consists of a sub-wavelength slitgroove pair with a small angle between them. The small angle between the groove and the slit provides spatially varying slit-groove distance which generates a highcontrast interference pattern. By interrogating the relative position of interference fringes, one can determine the refractive index of the dielectric medium on the metal surface. The presented plasmon interferometer provides a practical yet sensitive refractive index measurement scheme with very small detection volume.Item Open Access Three dimensional nanoplasmonic surfaces : modeling, fabrication and characterization(2013) Güngör, KıvançToday designing functional nanoplasmonic structures specific to a variety of applications attracts great interest from various fields ranging from optoelectronics to life sciences. There are numerous ways of making nanoplasmonic structures. Among them, nanopatterning of a thin-film metal layer is one of the most common approaches, which allows for finely controlled fabrication of a plasmonic unit and their repeating layout in the plane of the starting metal film. Although there are many examples of such nanopatterned plasmonic structures reported to date, they are typically designed and implemented on a planar surface. In these architectures, plasmonic layout commonly covers significantly less than 100% of the substrate surface and can provide field localization most strongly around the sharp corners and small gaps between the patterns. In the case of using a periodic layout, which is commonly employed for experimental realization (although periodicity is not necessary), the plasmonic array inherently yields a duty cycle substantially less than unity (usually close to 0.5). As a result, the surface coverage of nanopatterned plasmonic structures on a planar surface has intrinsically been limited and the field enhancement across their nanoplasmonic layout has been possible mostly in the plane and slightly above it. To address these limitations, this thesis proposed and demonstrated three-dimensional (3D) nanoplasmonic arrayed structures designed and implemented on a non-planar surface that allows for strong field enhancement in the out-of-plane direction and enables a very large surface coverage of the substrate close to unity. The thesis work included both numerical modeling and experimental characterizations. As a proof-of-concept demonstration, we fabricated non-planar arrays of checkerboard nanostructures, each with two-fold rotational symmetry, laid out in a volumetric fashion as two interlocked square lattice arrays at two different levels, facilitating strong field localization vertically between these two complementary planes. The resulting nanofabricated samples exhibited a maximum surface coverage of 100% in plan view. With full electromagnetic solution of such 3D nanoplasmonic surfaces, we showed that the out-of-plane field localization is 7.2-fold stronger than the inplane localization, in comparison to their two-dimensional (2D) components alone. These numerical results agree well with the experimental observations including far-field optical transmission and reflection measurements. The absorption spectroscopy further revealed that the resulting spectrum of the 3D checkerboard features a unique signature arising from the out-of-plane localization, which does not exist in the case of the 2D counterparts. These results indicate that 3D nanoplasmonics of such non-planar surfaces provides us with the ability to generate and better utilize the plasmonic volume, possibly useful for increased plasmonic coupling and interactions.