Browsing by Subject "Plasmonics"
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Item Open Access A1GaN UV photodetectors : from micro to nano(Bilkent University, 2011) Bütün, SerkanThe absorption edge of AlGaN based alloys can be tuned from deep UV to near UV by changing the composition. This enables the use of the material in various technological applications such as military, environmental monitoring and biological imaging. In this thesis, we proposed and demonstrated various UV photodetectors for different purposes. The multi-band photodetectors have the unique ability to sense the UV spectrum in different portions at the same time. We demonstrated monolithically integrated dual and four-band photodetectors with multi layer structures grown on sapphire. This was achieved through epitaxial growth of multi AlGaN layers with decreasing Al content. We suggested two different device architectures. First one has separate filter and active layers, whereas the second one has all active layers which are used as filter layers as well. The full width at half maximum (FWHM) values for the dual band photodetector was 11 and 22 nm with more than three orders of magnitude inter-band rejection ratio. The self-filtering four band photodetector has FWHMs of 18, 17, 22 and 9 nm from longer to shorter bands. Whereas photodetector with separate filter layers has FWHMs of 8, 12, 11 and 8 nm, from longer to shorter bands. The overall inter-band rejection ration was increased from about one to two of magnitude after incorporating the passive filter layers. The plasmonic enhancement of photonic devices has attracted much attention for the past decade. However, there is not much research that has been conducted in UV region. In the second part of this thesis, we fabricated nanostructures on GaN based photodetectors and improved the responsivity of the device. We have fabricated Al nano-particles on sapphire with e-beam lithography. We characterized their response via spectral extinction measurements. We integrated these particles with GaN photodetectors and had enhancement of %50 at the plasmonic resonance of the nano-particles. Secondly, we have fabricated sub-wavelength photodetectors on GaN coupled with linear gratings. We had 8 fold enhancement in the responsivity at the plasmonic resonance frequency of the grating at normal incidence. Numerical simulations revealed that both surface plasmons and the unbound leaky surface waves played a role in the enhancement. We, finally, conducted basic research on the current transport mechanisms in Schottky barriers of AlGaN based materials. Experiments revealed that the tunneling current plays a major role in current transport. In addition incorporation, of a thin insulator between metalsemiconductor interface reduces the undesired surface states thereby improving the device performance.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 Chemical funneling of colloidal gold nanoparticles on printed arrays of end-grafted polymers for plasmonic applications(American Chemical Society, 2020-06) Pekdemir, S.; Torun, İ.; Şakir, M.; Ruzi, M.; Rogers, J. A.; Önses, M. SerdarSpatially defined assembly of colloidal metallic nanoparticles is necessary for fabrication of plasmonic devices. In this study, we demonstrate high-resolution additive jet printing of end-functional polymers to serve as templates for directed self-assembly of nanoparticles into architectures with substantial plasmonic activity. The intriguing aspect of this work is the ability to form patterns of end-grafted poly(ethylene glycol) through printing on a hydrophobic layer that consists of fluoroalkylsilanes. The simultaneous dewetting of the underlying hydrophobic layer together with grafting of the printed polymer during thermal annealing enables fabrication of spatially defined binding sites for assembly of nanoparticles. The employment of electrohydrodynamic jet printing and aqueous inks together with reduction of the feature size during thermal annealing are critically important in achieving high chemical contrast patterns as small as ∼250 nm. Gold nanospheres of varying diameters selectively bind and assemble into nanostructures with reduced interparticle distances on the hydrophilic patterns of poly(ethylene glycol) surrounded with a hydrophobic background. The resulting plasmonic arrays exhibit intense and pattern-specific signals in surface-enhanced Raman scattering (SERS) spectroscopy. The localized seed-mediated growth of metallic nanostructures over the patterned gold nanospheres presents further routes for expanding the composition of the plasmonic arrays. A representative application in SERS-based surface encoding is demonstrated through large-area patterning of plasmonic structures and multiplex deposition of taggant molecules, all enabled by printing.Item Open Access Chiral Structures: Manipulation of Asymmetric Transmission in Planar Chiral Nanostructures by Anisotropic Loss (Advanced Optical Materials 7/2013)(2013) Li, Z.; Gokkavas, M.; Özbay, EkmelAsymmetric transmissions of circularly polarized optical waves can be achieved when the waves are incident normal to planar chiral structures, provided that the structures are anisotropic and lossy. In order to clarify how the factor of loss affects the asymmetric transmission, Z. Li, M. Gokkavas, and E. Ozbay studied a typical planar chiral structure by using an optical lumped element model. On page 482, they found that the anisotropy of loss, instead of the whole loss, plays a crucial role for achieving asymmetric transmission. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Item Open Access Colloidal photoluminescent refractive index nanosensor using plasmonic effects(De Gruyter, 2018) Guzatov, D. V.; Gaponenko, S. V.; Demir, Hilmi VolkanFluorescence enhancement by metal nanostructures which is sensitive to refractive index n of an ambient medium is suggested as an operation principle of a novel refractive index sensor for liquids. Calculations are made for spherical and spheroidal Ag particles, and potential feasibility of sensitivity of the order of Δn=10-4 is demonstrated. Sensors of this type can be made fully colloidal with metal bodies deposited on a substrate or comprising a metal layer covering colloidal assembly of dielectric particles to serve as a test strip as well as placed on a fiber tip end to get local probing of refractive index in the tip-enhanced refractometry mode. Colloidal core-shell semiconductor nanocrystals may become the best candidates for this type of sensors whereas molecular probes may be affected by chemical properties of tested liquids.Item Open Access Comparison of back and top gating schemes with tunable graphene fractal metasurfaces(American Chemical Society, 2016) Aygar, A. M.; Balci, O.; Cakmakyapan, S.; Kocabas, C.; Caglayan, H.; Özbay, EkmelIn this work, fractal metasurfaces that consist of periodic gold squares on graphene are used to increase light-graphene interaction. We show by simulations and experiments that higher level fractal structures result in higher spectral tunability of resonance wavelength. This is explained by higher field localization for higher level fractal structures. Furthermore, spectral tunability of fractal metasurfaces integrated with graphene is investigated comparing two different schemes for electrostatic gating. Experiment results show that a top-gated device yields more spectral tunability (8% of resonance wavelength) while requiring much smaller gate voltages compared to the back-gated device. © 2016 American Chemical Society.Item Open Access Correction to: Active tuning from narrowband to broadband absorbers using a sub-wavelength VO2 embedded layer(Springer, 2021-02-04) Osgouei, Ataollah Kalantari; Hajian, Hodjat; Khalichi, Bahram; Serebryannikov, Andriy E.; Ghobadi, Amir; Özbay, EkmelMetamaterial perfect absorbers (MPAs) with dynamic thermal tuning features are able to control the absorption performance of the resonances, providing diverse applications spanning from optical switches and filters to modulators. In this paper, we propose an MPA with diverse functionalities enabled by vanadium dioxide (VO2) embedded in a metal-dielectric plasmonic structure. For the initial design purpose, a silicon (Si) nanograting on a silver (Ag) mirror is proposed to have multiple resonant responses in the near infrared (NIR) region. Then, the insertion of a thin VO2 layer at the right position enables the design to act as an on/off switch and resonance tuner. In the insulator phase of VO2, in which the permittivity data of VO2 is similar to that of Si, a double strong resonant behavior is achieved within the NIR region. By increasing the temperature, the state of VO2 transforms from insulator to metallic so that the absorption bands turn into three distinct resonant peaks with close spectral positions. Upon this transformation, a new resonance emerges and the existing resonance features experience blue/red shifts in the spectral domain. The superposition of these peaks makes the overall absorption bandwidth broad. Although Si has a small thermo-optic coefficient, owing to strong light confinement in the ultrasmall gaps, a substantial tuning can be achieved within the Si nanogratings. Therefore, the proposed hybrid design can provide multi-resonance tunable features to cover a broad range and can be a promising strategy for the design of linearly thermal-tunable and broadband MPAs. Owing to the proposed double tuning feature, the resonance wavelengths exhibits great sensitivity to temperature, covering a broad wavelength range. Overall, the proposed design strategy demonstrates diverse functionalities enabled by the integration of a thin VO2 layer with plasmonic absorbers.Item Open Access Design, fabrication and characterization of surface plasmon resonance based MEMS displacement sensors(Bilkent University, 2009) Güner, HasanStrong dependence of surface plasmon resonance (SPR) on coupling parameters offers new varieties of sensing mechanisms in nano and micro-scale engineering fields. In this study, design, fabrication and characterization of MEMS displacement sensors that utilize angular dependence of grating coupled SPR condition are explored. Several surface plasmon polariton (SPP) excitation mechanisms are reported in the academic literature. One of them which is quite adaptable to microelectromechanical systems is grating coupling scheme. In this scheme, thin metallized grating structures are particularly designed depending on the desired wavelength and the angle of incidence of the SPP excitation light. Geometric parameters like periodicity, surface profile, depth and duty cycle of the grooves and material parameters like dispersion and thickness of the top metal layer have to be chosen with care in order to reach sharp SPR curves in the reflected intensity spectra with respect to either wavelength or angle of incidence. As the first step, geometric and material parameters of SPR gratings are numerically optimized using rigorous coupled-wave analysis (RCWA). Angular quality factors on the order of tens are shown to be achievable. Various lithographic techniques (nanoimprint, electron beam and optical lithography) are used to nanofabricate those certainly defined gratings. It is observed that p-polarized reflected intensity measurements using spectroscopic ellipsometry are in quite good agreement with those numerically calculated. Spectroscopic scan measurements are also provided to show the polarization dependence of SPP excitation. All effort to obtain high angular Q-factor grating structures is aimed at enhancing the sensitivity of angular displacement detection scheme. In this scheme, angular position of the grating structure in the polarization plane is detected through the reflected intensity response of the photodetector. Dependence of sensitivity on excitation light source wavefront parameters and photodetector noise are analyzed. MEMS displacement sensor designs relying on the principle of angular displacement detection scheme are developed. Simply, SPR grating structures are transferred on conventional micromembranes. Two types of such particular designed micromembranes are introduced: corrugated microcantilevers (singly clamped) and corrugated microbridges (doubly clamped). They are fabricated through well-known surface micromachining processes in addition to SPR grating nanofabrication procedures. Mechanical resonance frequencies, flexural mode shapes and effective spring constants are analytically, numerically and experimentally obtained. In addition, a MEMS accelerometer design with plasmonic readout with nano-G noise floor is presented. An experimental configuration for micromechanical displacement sensing is investigated. According to the results of this work, novel arrayed sensors combining the sensitivities of plasmon resonance and micromembrane type sensors may provide unprecedented performance.Item Open Access Disordered and densely packed ITO nanorods as an excellent lithography-free optical solar reflector metasurface(American Chemical Society, 2019) Yıldırım, Deniz Umut; Ghobadi, Amir; Soydan, Mahmut Can; Ateşal, Okan; Toprak, Ahmet; Çalışkan, Mehmet Deniz; Özbay, EkmelPrecise control and stabilization of the operating temperature environment of spacecraft and satellites during their life cycle is of paramount importance to increase device reliabilities and reduce the thermomechanical constraints. Optical solar reflectors are the physical interface between the spacecraft and space, and they are broadband mirrors for the solar spectrum, while having strong thermal emission in the mid-infrared part of the electromagnetic spectrum. Strong light–matter interactions in metamaterials and metasurfaces offer significant advantages compared to the conventional methods in performance, weight, launch, and assembly costs. However, the fabrication complexity of these metastructures due to necessitating lithography hinders their upscaling, reproducibility, large-area compatibility, and mass production. In this regard, we propose a facile, lithography-free fabrication route, exploiting oblique deposition to design a metasurface based on disordered and densely packed Indium Tin Oxide (ITO) nanorod forests. The excellent light trapping capability of the nanorod forests, randomness in the geometrical dimensions of these nanorods, combined with the lossy plasmonic nature of ITO in the thermal-infrared range led to strong coupling of thermal radiation to broad plasmonic resonances and, consequently, an experimental emissivity of 0.968, in a very wide range from 2.5 to 25 μm. In the solar spectrum, the low-loss dielectric characteristic of ITO resulted in an experimental solar absorptivity as small as 0.168. Our proposed design with high throughput, robustness, low cost, and high performance, therefore, shows great promise not only for space missions, but also for promoting environmentally friendly passive radiative cooling for our planet and thermal imaging in the field of security labeling.Item Open Access Disordered and densely packed ITO nanorods as an excellent lithography-free optical solar reflector metasurface for the radiative cooling of spacecraft(SPIE, 2019) Yıldırım, Deniz Umut; Ghobadi, Amir; Soydan, Mahmut Can; Ateşal, Okan; Toprak, Ahmet; Çalışkan, Mehmet Deniz; Özbay, EkmelOptical Solar Reflectors (OSRs) form the physical interface between the spacecraft and space and they are essential for the stabilization and uniform distribution of temperature throughout the spacecraft. OSRs need to possess a spectrally selective response of broadband and perfect electromagnetic wave absorption in the thermal-infrared spectral range, while strongly reflecting the solar energy input. In this work, we experimentally show that disordered and densely packed ITO nanorod forests can be used as an excellent top-layer metasurface in a metal-insulator-oxide cavity configuration, and a thermal-emissivity of 0.97 is experimentally realized in the spectral range from 2.5 to 25 μm. The low-loss dielectric response of ITO in the solar spectrum, from 300 nm to 2.5 μm range limited the solar absorptivity to an experimental value of 0.167. These make our proposed design highly promising for its application in space missions due to combining high throughput, robustness, low cost with ultra-high performance.Item Open Access Eco-friendly fabrication of plasmonically active substrates based on end-grafted poly(ethylene glycol) layers(American Chemical Society, 2019) Karabel Öcal, S.; Pekdemir, S.; Serhatlıoğlu, Murat; İpekçi, H. H.; Şahmetlioğlu, E.; Narin, İ.; Duman, F.; Elbüken, Çağlar; Demirel, G.; Önses, Mustafa SerdarWe report completely sustainable processes and materials for inexpensive and scalable fabrication of plasmonically active solid substrates, which are critical for emerging applications in sensing, catalysis, and metasurfaces. Our approach involves grafting of poly(ethylene glycol) (PEG) onto silicon oxide terminated solid substrates using all-water based processing leading to an ultrathin (12 nm) and smooth (roughness of ∼1 nm) functional layer. The resulting surfaces facilitate robust and effective immobilization of gold nanoparticles (NPs) with a density that is superior to the organic solvent based processing. This new process achieves size dependent assembly of the citrate-stabilized gold NPs resulting in high plasmonic activity in surface-enhanced Raman scattering (SERS). The use of leaf extracts derived from Quercus pubescens as a reducing and stabilizing agent allowed for green synthesis of gold NPs with an average diameter of 25.6 ± 11.1 nm. The assembly of the green synthesized gold NPs on all-water processed PEG grafted layers enabled a fully sustainable route for fabrication of plasmonically active solid substrates. The resulting substrates exhibited high SERS response over the entire (∼1 cm2) substrate surface with an analytical enhancement factor of 9.48 × 104 for the probe molecule rhodamine 6G under 532 nm laser excitation. A microfluidic device was also constructed on the fabricated platform for SERS mediated simultaneous detection of two nonsteroidal anti-inflammatory drugs, dexketoprofen and ibuprofen, which are widely used in human medicine and present as contaminants in wastewater. The biocompatibility of PEG together with all-water based processing overcome the need for waste management and ventilation of the working place enabling cost and energy efficient, environmentally benign fabrication of plasmonic devices.Item Open Access Effect of gold nanoparticles size on light scattering for thin film amorphous-silicon solar cells(Elsevier Ltd, 2014-05) Islam, K.; Alnuaimi, A.; Battal, E.; Okyay, Ali Kemal; Nayfeh, A.In this work, the effect of gold (Au) nanoparticles on the performance of a-Si:H solar cells is investigated experimentally. The solar cell stack is grown on a highly doped p-type Si wafer and consists of 20nm heavily doped p-type a-Si, 500nm undoped a-Si, 20nm heavily doped n-type a-Si and finally 80nm Indium Tin Oxide (ITO) on the top. Au nanoparticles of 10, 20, 50, 80, 100, 200 and 400nm are spin coated on top of the ITO before metallization. The plasmonic effect of the Au nanoparticles allows for additional scattering at the surface thus reducing the overall reflectivity. The larger the nanoparticle size the more scattering is obtained and the median reflectivity drops from about 23% to 18%. The results show an increase in the short-circuit current density (Jsc) and efficiency with increasing nanoparticle size. The Jsc increases from 9.34 to 10.1mA/cm2. In addition, the efficiency increases from 4.28% to 5.01%. © 2014 Elsevier Ltd.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 Electroluminescence efficiency enhancement in quantum dot light-emitting diodes by embedding a silver nanoisland layer(Wiley-VCH Verlag, 2015) Yang, X.; Hernandez-Martinez, P. L.; Dang C.; Mutlugün, E.; Zhang, K.; Demir, Hilmi Volkan; Sun X. W.A colloidal quantum dot light-emitting diode (QLED) is reported with substantially enhanced electroluminescence by embedding a thin layer of Ag nanoislands into hole transport layer. The maximum external quantum efficiency (EQE) of 7.1% achieved in the present work is the highest efficiency value reported for green-emitting QLEDs with a similar structure, which corresponds to 46% enhancement compared with the reference device. The relevant mechanisms enabling the EQE enhancement are associated with the near-field enhancement via an effective coupling between excitons of the quantum dot emitters and localized surface plasmons around Ag nano-islands, which are found to lead to good agreement between the simulation results and the experimental data, providing us with a useful insight important for plasmonic QLEDs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Item Open Access Fabrication of flexible, cost-effective, and scalable silver substrates for efficient surface enhanced Raman spectroscopy based trace detection(Elsevier BV, 2021-06-20) Khan, G. A.; Demirtaş, Ö.; Aytekin, Ö.; Demir, Ahmet Kemal; Bek, A.; Bhatti, A. S.; Ahmed, W.The fabrication and optimization of cost-effective, eco-friendly, uniform and flexible SERS platforms by facile synthesis routes has recently attracted great attention for trace detection of various analytes. Herein, we report the fabrication of interconnected Ag nanostructures on the unmodified filter paper-based flexible substrates by a facile recipe, which involves evaporation of Ag precursor solution on the filter paper followed by its reduction with a strong reducing agent, NaBH4. The fabrication process is time-efficient, reproducible, and has the potential of being scaled up. The presence of inter-connected nanostructures results in high concentration of uniformly distributed hotspots on the substrate, which in turn provide excellent and reproducible SERS sensitivity. The finite element simulation results showed that fabricated nanostructures are much more effective for field enhancement as compared to the agglomerated spherical nanoparticles. Crystal violet (CV) concentration of 10−8 M was easily detected with these substrates both via solution drying and swabbing. Similarly, AgFPS have proven effective for swab-based detection of urea nitrate (UN), a well-known constituent of homemade explosives. The C-N and NO3- symmetric stretching modes of UN are detectable down to 1 µM and 100 µM concentrations, respectively. Moreover, these substrates showed SERS signal uniformity both from different spots of a single substrate, and substrates from different batches, with spot-to-spot relative standard deviation (RSD) of 15% and sample-to-sample RSD of 15–19%. This makes them excellent for reliable and quick quantitative detection. The simple synthesis strategy, flexibility, cost-effectiveness, and quantitative detection makes these substrates ideal for SERS based trace detection, especially for on-site analysis, and various sensing applications.Item Open Access Forces and torques on the nanoscale: from measurement to applications(SPIE, 2012) Volpe,GiovanniThe possibility of measuring microscopic forces down to the femtonewton range has opened new possibilities in fields such as biophysics and nanophotonics. I will review some of the techniques most often employed, namely the photonic force microscope (PFM) and the total internal reflection microscope (TIRM), which are able to measure tiny forces acting on optically trapped particles. I will then discuss several applications of such nanoscopic forces: from plasmonic optical manipulation, to self-propelled microswimmers, to self-organization in large ensembles of particles.Item Open Access Fourier transform plasmon resonance spectrometer(Bilkent University, 2017-01) Aibek uulu, DoolosNanophotonics is an emerging field of research aiming to control interaction of light with matter in nanometer scale. Electrons localized on metallic nanoparticles generate localized plasmon oscillations with interesting optical properties that can be used for various sensing applications. Spectroscopic sensing using plasmonic particles could provide more detailed information, however it requires bulky spectrometers which limits its applications. In this thesis, a nanometer scale Fourier Transform Plasmon Resonance (FTPR) spectrometer is presented. FTPR spectrometer consists of a nanometer slit-grove or slit-ridge plasmon interferometer with varying optical path. The inherent coherence of the surface plasmons propagating through the sub-wavelength holes yield high contrast spatial interference pattern. FTPR spectrometer converts this spatial interference pattern in to spectroscopic information using Fast Fourier Transform (FFT) algorithm. In our design, there is no need for a bulky dispersive spectrometer or dispersive optical elements. We anticipate that high sensitivity of surface plasmons together with spectroscopic information and nanometer dimensions provides new avenues for plasmonic sensors.Item Open Access Graphene-based plasmonics(Pan Stanford Publishing, 2015) Balci S.; Polat E.O.; Kocabaş, Coşkun; Szunerits, S.; Boukherroub, R.The unique electrical, optical, and chemical properties of the two-dimensional crystal of carbon, graphene, stimulated great interest for plasmonics in reduced dimensions. Notably, controlled electrostatic doping on graphene opens new perspectives for gate-tunable active plasmonics. In this chapter, we review the fundamental aspects of plasmons on graphene and their applications ranging from surface plasmon sensors to active plasmonic devices. 11.1 Introduction: Plasmons in Reduced DimensionsGraphene is a two-dimensional (2D) crystal. Studying plasma oscillation on 2D materials sometimes causes confusions between surface plasmons and surface plasmon polaritons (SPPs). First, we would like to clarify these definitions. Plasmons are quasiparticles of quantized plasma oscillations. Polaritons are also quasiparticles, but they are formed by strong coupling of electromagnetic waves and excitations on a material. For example, (SPPs) are the coupled state of plasmons and electromagnetic waves that propagate on the surface of 3D material. Plasmons on 2D materials are usually named as surface plasmons because they live on a surface of 2D material but they are not polaritons. When we say surface plasmons of graphene we talk about bulk plasma oscillations on graphene surface. To understand the physics of plasmons, we should start with the comparison of plasma oscillations in different dimensions such as 3D bulk materials, i.e., gold and silver; 2D materials, i.e., graphene and inversion layer on silicon; 1D materials, i.e., carbon nanotubes or atomic nanowires. Depending on the dimensionality of the material, dispersion characteristics of the plasma oscillations differ. For 3D materials, the plasma oscillations are dispersionless with a plasma frequency of 23D 4= nempw , (11.1)where e is the elementary charge, m is the mass of an electron, and n is the charge density, which is the only material-dependent parameter that defines the plasma frequency. In reduced dimensions, however, plasma oscillations yield dispersion characteristics because the electric field of the plasma oscillations penetrates out of the material resulting in less restoring forces. This causes momentum-dependent plasma frequency. The early experiments on 2D plasma oscillations have been performed on liquid helium surface by Gregory Adams in 1976 [6]. The image-potential-induced surface states on liquid helium traps electrons in close proximity to the surfaces. These trapped electrons form a sheet of 2D electron gas with extremely large carrier mobilities of 107 cm2/Vs. However, the charge densities on liquid helium are limited to 108 cm-2. Later, metal-oxide-semiconductor (MOS) structures have been used to generate tunable 2D electron gas in the inversion layer on silicon surface [3]. A grating-shaped gate electrode was used to generate the inversion layer and to couple light to plasmons. With Si-MOS devices, carrier densities of 4 × 1012 cm-2 with plasma frequency of 30 cm-1 can be achieved. Very recently, graphene has been used to reveal the physics of 2D-plasmons [4, 5]. Graphene in back- gatedtransistor geometry provides a unique system to study plasma oscillations. Various techniques have been used to probe the plasmons on graphene sheet. Because of large momentum mismatch, plasmon on graphene cannot be excited by free propagating light. Therefore, a sharp atomic force microscope tip attached with an infrared spectrometer has been used to probe the tunable graphene plasmon in a back-gated transistor.Item Open Access Guided plasmon modes of a graphene-coated kerr slab(Springer New York LLC, 2016) Hajian, H.; Rukhlenko, I. D.; Leung, P. T.; Caglayan, H.; Özbay, EkmelWe study analytically propagating surface plasmon modes of a Kerr slab sandwiched between two graphene layers. We show that some of the modes that propagate forward at low field intensities start propagating with negative slope of dispersion and positive flux of energy (fast-light surface plasmons) when the field intensity becomes high. We also discover that our structure supports an additional branch of low-intensity fast-light guided modes. The possibility of dynamically switching between the forward and the fast-light plasmon modes by changing the intensity of the excitation light or the chemical potential of the graphene layers opens up wide opportunities for controlling light with light and electrical signals on the nanoscale.
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