Browsing by Subject "Plasmons (Physics)"
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Item Open Access Ald grown zno as an alternative material for plasmonic and uncooled infrared imaging applications(Bilkent University, 2014) Kesim, Yunus EmrePlasmonics is touted as a milestone in optoelectronics as this technology can form a bridge between electronics and photonics, enabling the integration of electronics and photonic circuits at the nanoscale. Noble metals such as gold and silver have been extensively used for plasmonic applications due to their ability to support plasmons, yet they suffer from high intrinsic optical losses. Recently, there is an increased effort in the search for alternative plasmonic materials including Si, Ge, III-Nitrides and transparent conductive oxides. The main appeal of these materials, most of them semiconductors, is their lower optical losses, especially in the infrared (IR) regime, compared to noble metals owing to their lower number of free electrons. Other advantages can be listed as low-cost and control on plasma frequency thanks to the tunable electron concentration, i.e. effective doping level. This work focuses on atomic layer deposition (ALD) grown ZnO as a candidate material for plasmonic applications. Optical constants of ZnO are investigated along with figures of merit pertaining to plasmonic waveguides. It is shown that ZnO can alleviate the trade-off between propagation length and mode confinement width owing to tunable dielectric properties. In order to demonstrate plasmonic resonances, a grating structure is simulated using finite-difference-time-domain (FDTD) method and an ultra-wide-band (4-15 µm) infrared absorber is computationally demonstrated. Finally, an all ZnO microbolometer is proposed, where ALD grown ZnO is employed as both the thermistor and the absorber of the microbolometer which is an uncooled infrared imaging unit that relies on the resistance change of the active material (thermistor) as it heats up due to the absorption of incident electromagnetic radiation. The material complexity and process steps of microbolometers could be reduced if the thermistor layer and the absorber layer were consolidated in a single layer. Computational analysis of a basic microbolometer structure using FDTD method is conducted in order to calculate the absorptivity in the long-wave infrared (LWIR) region (8-12 µm). In addition, thermal simulations of the microbolometer structure are conducted using finite element method, and time constant and noise-equivalent-temperature-difference (NETD) values are extracted.Item Open Access Applications of plasmon enhanced emission and absorption(Bilkent University, 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 Cascading and modifying nonradiative energy transfer mechanisms in strong coupling region of plasmons and excitons in semiconductor quantum dots(Bilkent University, 2010) Akın, OnurNonradiative energy transfer finds important applications in nanophotonics and nanobiotechnology including nanoscale optical waveguiding and biological nanosensors. Various fluorophores can take part in such energy transfer interactions in close proximity of each other. Their emission kinetics can be strongly modified and controlled as a result. For example, colloidal semiconductor quantum dots, also known as nanocrystals, have widely been shown to serve as donors and acceptors among themselves or with other fluorescent species to transfer excitation energy nonradiatively. In their close proximity, emission characteristics of such fluorophores can also be altered when coupled with plasmonic structures, e.g., metal nanoparticles. One favored result of these plasmon-exciton interactions is the emission enhancement. In principle it is possible to plasmon-couple acceptor-donor pairs of nonradiative energy transfer to modify their transfer rate. Such plasmon-mediated energy transfer has been demonstrated, where both acceptor-donor pairs are plasmoncoupled. In these cases, however, the resulting plasmon-exciton interactions are not controlled to take place either at the donor site or the acceptor site but at both of the sites. Therefore, it has previously not been possible to identify the coupled interactions. In this thesis, we propose and demonstrate cascaded plasmonic - nonradiative energy transfer interactions that are controlled by selectively plasmon-coupling either only the donor quantum dots or only the acceptor quantum dots. For that, we designed a novel self-assembly architecture of our hybrid layered systems of semiconductor nanocrystals and metal nanoparticles in a bottom-up fashion through precise spatial and spectral control. This scheme uniquely allowed for the ability to spatially control plasmonexciton interactions to take place either at the “start” site (donors) or “finish” site (acceptors) of the energy transfer. This control was achieved by placing the plasmonic layer in the right proximity of the donors (for strong donor-exciton plasmon-coupling) while sufficiently being far away from the acceptors (for weak acceptor-exciton plasmon-coupling), or vice versa. Here we comparatively studied and analyzed consequent modifications of quantum dot emission kinetics in response to both cases of plasmon-coupling to only the donors and to only the acceptors through steady-state and time-resolved photoluminescence measurements, along with their lifetime and rate calculations. Such cascaded energy transfer interactions in the strong exciton-plasmon coupling region hold great promise for innovative near-field photonic devices and biological tags. system.Item Open Access Chiral metamaterial and high-contrast grating based polarization selective devices(Bilkent University, 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(Bilkent University, 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(Bilkent University, 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(Bilkent University, 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 Enhanced confined microwave transmission by single subwavelength apertures(Bilkent University, 2005) Çağlayan, HümeyraGrating-coupling phenomena between surface plasmons and electromagnetic waves are studied in the microwave spectrum using metallic circular apertures surrounded by an array of grooves. The measurements are performed in the microwave spectrum of 10-18 GHz, corresponding to a wavelength region of 16.7-30 mm. The metallic samples have a subwavelength hole with a diameter of 8 mm and have concentric grooves with a periodicity of 16 mm. We first present the experimental and theoretical results of enhanced microwave transmission though a subwavelength circular aperture with concentric periodic grooves around the surface plasmon resonance frequency. This is followed by transmission studies through circular annular apertures with and without concentric periodic grooves around the aperture. We demonstrate a 145-fold enhancement factor could be obtained with a subwavelength circular annular aperture surrounded by concentric periodic grooves. Moreover, we study the diffraction of electromagnetic waves from subwavelength metallic circular annular apertures in the microwave spectrum. The theoretical and experimental demonstration of the near- and far-field EM distributions for subwavelength circular apertures and circular annular apertures surrounded by concentric periodic grooves is reported. We present the angular transmission distributions from circular apertures and circular annular apertures surrounded by concentric periodic grooves. At the surface mode resonance frequency the transmitted electromagnetic waves from the subwavelength circular annular aperture surrounded by concentric periodic grooves have a strong angular confinement with an angular divergence of ±3°. This represents a fourfold reduction when compared to the angular divergence of the beam transmitted from a subwavelength aperture. These results show, that not only high transmission but also a confined beam is achieved at the surface plasmon resonance frequency using a circular annular aperture with grooves .Item Open Access Fabrication, characterization and simulation of plasmonic cavities(Bilkent University, 2010) Karabıyık, MustafaSurface plasmon polaritons (SPPs) originate from the collective oscillations of conduction electrons coupled with photons propagating at metal-dielectric interfaces. A uniform metallic gratings change the dispersion (energy-momentum relation) of a flat metal surfaces due to the interaction of SPPs with the periodic structure. By breaking the symmetry of the periodic plasmonic structure, SPP cavities can be achieved and SPPs can be localized inside the cavity regions. The aim of this thesis is to understand the physics of phase shifted grating based plasmonic cavities. To this end, we fabricated uniform gratings and phase shifted gratings using electron beam lithography, and optically characterized these SPP structures with polarization dependent reflection spectroscopy. We verified experimental results with numerical simulations SPP propagation and localization on the grating structures. Dispersion curves of SPPs have been calculated by solving Maxwell’s wave equations using finite difference time domain method (FDTD) with appropriate boundary conditions in agreement with experimentally obtained data. We studied the dispersion curve as a function of grating profile modulation where we vary the ridge height and width of the ridges. We find that the plasmonic band gap width increases as the ridge height of the ridges in the grating increases. Optimum duty cycle of grating to observe plasmonic band gap is determined to be half of the grating period. Amount of the phase shift added to the periodicity of the uniform grating defines the energy of the cavity state, which is periodically related to the phase shift. A plasmonic cavity with a quality factor 80 has been achieved. The propagation mechanism of SPPs on coupled cavities is plasmon hopping from a given cavity to the next one.Item Open Access Grating based plasmonic cavities(Bilkent University, 2009) Şenlik, Servet SeçkinSurface plasmon polaritons are dipole carrying electromagnetic excitations occur- ing at metal-dielectric interfaces. Metallic periodic structures exhibit modi¯ed transmission and re°ection spectra owing to the interaction of propagating SPPs with the periodicity. These periodic surfaces are used to demonstrate localiza- tion of propagating SPPs. Thin metallic ¯lms surrounded by Bragg re°ectors, selective loading of biharmonic metallic surfaces and Moire patterns are used to demonstrate plasmonic cavity formation. The quality factor, Q, a characteristic value that indicates rate of energy loss relative to the stored energy in the cavity is a crucial parameter for classifying these cavities. It was proposed that the Q factor should strongly depend on the surface geometry. However, there was not a sytematic study on the Q factor of these cavity structures. In this work, we report on a comparative study of grating based plasmonic band gap cavities. Numerically, we calculate the quality factors of the cavities based on three types of grating surfaces; uniform, biharmonic and Moirµe surfaces. Experimentally, we demonstrate the existence of plasmonic cavities based on uniform gratings. E®ective index perturbation and cavity geometries are obtained by additional dielectric loading. Furthermore, we fabricate 2D plasmonic structures, observe plasmonic band gaps in the symetry axis and propose cavity geometries for this structure.Item Open Access Novel light-sensitive nanocrystal skins(Bilkent University, 2013) Akhavan, ShahabLight sensing devices traditionally made from crystalline or amorphous silicon, operating at the visible and near-infrared wavelengths, have led to a multibillion-dollar annual market. However, silicon faces various limitations including weak detection at long wavelengths (insufficient beyond 1.1 µm) with a cut-off at short wavelengths (in the ultraviolet) and small-area applications. On the other hand, solution-processed semiconductor nanocrystals (NCs), also known as colloidal quantum dots, offer large-area light sensing platforms with strong absorption cross-section. In this thesis we propose and demonstrate a new class of large-area, semi-transparent, light-sensitive nanocrystal skin (LS-NS) devices intended for large-surface applications including smart transparent windows and light-sensitive glass facades of smart buildings. These LS-NS platforms, which are fabricated over areas up to many tens of cm2 using spraycoating and several cm-squares using dip-coating, are operated on the basis of photogenerated potential buildup, as opposed to conventional charge collection. The close interaction of the monolayer NCs of the LS-NS with the top interfacing metal contact results in highly sensitive photodetection in the absence of external bias, while the bottom side is isolated using a high dielectric spacing layer. In operation, electron-hole pairs created in the NCs of the LS-NS are disassociated and separated at the NC monolayer - metal interface due to the difference in the workfunctions. As a result, the proposed LS-NS platforms perform as highly sensitive photosensors, despite using a single NC monolayer, which makes the device semi-transparent and reduces the noise generation Furthermore, because of the band gap tunability, it is possible to construct cascaded NC layers with a designed band gap gradient where the NC diameters monotonically change. Here we present the first account of exciton funneling in an active device, which leads to significant performance improvement in the device. We show highly photosensitive NC skins employing the exciton funneling across the multiple layers of NC film. To further enhance the device photosensitivity performance, we demonstrate embedding plasmonic nanoparticles into the light-sensitive skins of the NCs. In addition, we exhibit the LS-NS device sensitivity enhancement utilizing the device architecture of semi-transparent tandem skins, the addition of TiO2 layer for increased charge carrier dissociation, and the phenomenon of multiexciton generation in infrared NCs. With fully sealed NC monolayers, LS-NS is found to be highly stable under ambient conditions, promising for low-cost large-area UV/visible sensing in windows and facades of smart buildings. We believe the findings presented in this thesis have significant implications for the future design of photosensing platforms and for moving toward next generation large-surface light-sensing platforms.Item Open Access Novel plasmonic devices for nano-photonics applications(Bilkent University, 2013) Şahin, LeventPlasmonics have attracted a great deal of interest because of their potential to design novel photonics devices which have unique optical properties. This dissertation focuses on novel plasmonic device designs for photonics applications. Electromagnetic properties of metamaterials are characterized and the resonance mechanism of Split Ring Resonator (SRR) structure is investigated. Furthermore, novel SRR-based metamaterial structures are studied. We demonstrated the significant plasmonic enhancement in the transmission characteristics through a sub-wavelength aperture by utilizing SRR resonances. Electrical tuning of plasmonic resonance with varying gate bias by using graphene is observed. Also, electrical properties of graphene is investigated. Fabrication of electrically gated graphene based plasmonic structures are realized. In addition, we utilized metamaterials to design novel photonic devices and we experimentally studied and numerically verified the novel propagation characteristics of graphene-based photonic devices and 3D nanostructures. The proposed structures are designed, simulated, fabricated and measured. The simulations and experimental results are in good agreement and shows significant enhancement of transmission characteristics of plasmonic devices. The dimensions of the structures that are designed in our work is less than 10 times smaller than the incident wavelength (r/λ~0.1) which is a desired property for enhanced light confinement of sensors. Also, the gate tuning of SRR's plasmonic resonance is the first demonstration in the contemporary literature according to our knowledge.Item Open Access Novel volumetric plasmonic resonator architectures for enhanced absorption in thin-film organic solar cells(Bilkent University, 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 Plasmonic band gap cavities(Bilkent University, 2008) Kocabaş, AşkınSurface plasmon polaritons (SPP’s) are trapped electromagnetic waves coupled to free electrons in metals that propagate at the metal-dielectric interfaces. Due to their surface confinement and potential in sub-wavelength optics, SPP’s have been extensively studied for sensing and nanophotonic applications. Dielectric structures and metallic surfaces, both periodically modulated, can form photonic band gaps. Creating a defect cavity region in the periodicity of dielectrics allows specific optical modes to localize inside a cavity region. However, despite the demonstration of numerous plasmonic surfaces and unlike its dielectric counterparts, low index modulation in metallic surfaces limits the formation of plasmonic defect cavity structures. This thesis describes new approaches for plasmonic confinement in a cavity through the use of selective loading of grating structures as well as through the use of Moiré surfaces. In our first approach, we demonstrate that a high dielectric superstructure can perturb the optical properties of propagating SPPs dramatically and enable the formation of a plasmonic band gap cavity. Formation of the cavity is confirmed by the observation of a cavity mode in the band gap both in the infrared and the visible wavelengths. In addition to the confinement of SPP’s in the vertical direction, such a cavity localizes the SPP’s in their propagation direction. Additionally, we have demonstrated that such biharmonic grating structures can be used to enhance Raman scattering and photoluminescence (PL). Using biharmonic grating structure 105 times enhancement in Raman signal and 30 times enhancement in PL were measured. Furthermore, we show that metallic Moiré surfaces can also serve as a basis for plasmonic cavities with relatively high quality factors. We have demonstrated localization and slow propagation of surface plasmons on metallic Moiré surfaces. Phase shift at the node of the Moiré surface localizes the propagating surface plasmons in a cavity and adjacent nodes form weakly coupled plasmonic cavities. We demonstrate group velocities around v = 0.44c at the center of the coupled cavity band and almost zero group velocity at the band edges can be achieved. Furthermore, sinusoidally modified amplitude about the node suppresses the radiation losses and reveals a relatively high quality factor for plasmonic cavities.Item Open Access Plasmonic nanoparticles by laser dewetting of thin metallic films(Bilkent University, 2013) Sarıtaş, SevalIn this work, formation of metal nanoparticles via laser induced dewetting and their plasmonic properties have been investigated. The effects of metal film, substrate type, laser power density and dwell time on dewetting phenomenon were analyzed. Silver and gold thin films were fabricated with thermal evaporation on various substrates. Next, they were characterized by the ellipsometry, UV-VIS spectroscopy and atomic force microscopy (AFM) as the characteristic of the thin film affects dewetting. Samples were then processed by a cw argon laser. Varying the dwell time and power density, Ag and Au nanoparticles with different morphology were obtained. At the final stages of dewetting, nanoparticles attained spherical shapes. Particle size distribution and length scale analysis were performed using the images obtained from scanning electron microscope (SEM). Using these results, relations between the average particle size and film thickness, as well as the relation between length scale and film thickness were obtained to verify the occurrence of dewetting. Substrate and film type were observed to affect the particle morphology and particle size. Moreover, plasmonic resonance effect of Ag and Au nanoparticles were observed via the optical absorbance measurements. Multilayered metallic nanoparticles and embedded nanoparticles were fabricated and were found to display plasmonic properties.Item Open Access Plasmonically enhanced silicon infrared Schottky detector(Bilkent University, 2011) Polat, Kazım GürkanItem Open Access Plasmonics from metal nanoparticles for solar cell applications(Bilkent University, 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 Probing hot-electron effects in plasmonic surfaces using X-ray photoelectron spectroscopy(Bilkent University, 2014) Çupallari, AndiHot-electron effects in plasmonic structures have been recently investigated as potential alternative mechanisms for solar energy harvesting and photodetection. [1][2][3] Hot-electron effects provide a semiconductor free route for the conversion of photons into electrical power. Here we investigate plasmonic hot electron effects in Metal-Insulator-Metal (MIM) structures using X-ray photoelectron spectroscopy (XPS). XPS has been previously used to investigate optoelectronic effects in semiconductors and nanocomposite surfaces. [4][5][6] Here, a similar approach is used to characterize the plasmonic and hot electron effects in MIM Junctions. Monochromatic Laser excitation with 450, 532 and 650 nm wavelengths are employed to illuminate the plasmonic surfaces fabricated using thermal evaporation, atomic layer deposition and electron beam lithography. The top metal of the MIM structures act as the plasmonic antenna (metal nanodiscs and gratings/stripes) that provide wavelength selective or wide band optical absorption. Plasmonic enhancement at the interface between the top metal and the insulator enhances the absorption of light in the device and leads to excitation of a larger number of hot electrons from the metal. Hot electron effects are characterized through studying the metal-insulator-metal junction and comparing shifts of binding energy belonging to the top metal islands for dark and illuminated conditions. XPS spectrum provides important information regarding the plasmonic and hot electron effects in the interface between top metal and the dielectric. A systematic study of the dependence of the XPS spectra on excitation wavelength, light intensity, polarization, insulator thickness and nanostructure geometry is presented. Effects of using different metals and insulator materials are also studied in symmetric and asymmetric tunnel junctions. Keywords:Item Open Access Selective plasmonic control of excitons and their non-radiative energy transfer in colloidal semiconductor quantum dot solids(Bilkent University, 2009) Özel, TuncayTo date extensive research has proved that semiconductors and metals exhibit extraordinary optical properties in nano-dimensions compared to their bulk counterparts. For example, an interesting effect is observed in metal nanostructures/nanoparticles (NPs) that we form to obtain localized plasmons, with their optical response highly tuneable using the size effect. Another field of interest at the nanoscale is the investigation of light generation and harvesting using colloidal semiconductor quantum dot nanocrystals (NCs) that we synthesize in few nanometers, with their emission and absorption excitonic peaks conveniently tuneable using the size effect. In this thesis, we proposed and demonstrated the first accounts of selectively plasmonically-controlled colloidal quantum dot emitters assembled in innovative architectures, with a control achieved either through spatial selection or spectral selection. In the first set of designs, we developed for the first time plasmonic NC-composites that rely on spatially-selected plasmon-coupled CdTe NC-monolayers interspaced with respect to Au NP-monolayers in a repeating three-dimensional layer-by-layer architecture. In these bottom-up designs of hybrid nanocomposites, the photoluminescence kinetics is strongly modified and a record quantum efficiency of 30% is achieved for such CdTe NC solids. In the second set of designs, we showed the first spectrally-selected plasmon-coupling of surfaceemitting CdS NCs using optimized Ag NP deposits. This architecture allowed for the surface-state emission to be selectively enhanced while the interband emission is simultaneously suppressed in the same plasmon-coupled NCs, leading to the strongest surface-state emission from such CdS NCs reported with respect to their interband emission (with a >12-fold enhancement). Yet another important proximity phenomenon effective among quantum dot emitters is the Förster-type non-radiative resonance energy transfer (ET), in which excitonic excitation energy of the donor-NCs is non-radiatively transferred to the acceptor-NCs via dipole-dipole coupling. In the third set of our designs, we combined two fundamental proximity mechanisms of plasmon coupling and non-radiative energy transfer in the same NC solids. In plasmonic ET, we reported for the first time selectively plasmon-coupling of NC-acceptors and then that of NC-donors in the ET pair, both of which result in substantial enhancement of the acceptor emission with respect to ET with no plasmon coupling (with a maximum of 2-fold enhancement) as verified by their steadystate and time-resolved photoluminescence. This concept of spectrally/spatiallyselective plasmon coupling in quantum dots paves a new path for devices and sensors in nanophotonics.Item Open Access Subwavelength surface plasmon interferometer for high-throughput sensing(Bilkent University, 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.