Browsing by Subject "Electromagnetic waves."

Now showing 1 - 5 of 5

Open Access
Beaming and localization of electromagnetic waves in periodic structures
(2010) Çağlayan, Hümeyra
We want to manipulate light for several applications: microscopy, data storage, leds, lasers, modulators, sensor and solarcells to make our life healthier, easier or more comfortable. However, especially in small scales manipulating light have many difficulties. We could not focus or localize light into subwavelength dimensions easily, which is the key solution to beat today’s devices both in performance and cost. Achievements in three key research fields may provide the answer to these problems. These emerging research fields are metamaterials, photonic crystals and surface plasmons. In this thesis, we investigated beaming and localization of electromagnetic waves in periodic structures such as: subwavelength metallic gratings, photonic crystals and metamaterials. We studied off-axis beaming from both a metallic subwavelength aperture and photonic crystal waveguide at microwave regime. The output surfaces are designed asymmetrically to change the beaming angle. Furthermore, we studied frequency dependent beam steering with a photonic crystal with a surface defect layer made of dimmers. The dispersion diagram reveals that the dimer-layer supports a surface mode with negative slope. Thus, a photonic crystal based surface wave structure that acts as a frequency dependent leaky wave antenna was presented. Additionally, we investigated metamaterial based cavity systems. Since the unit cells of metamaterials are much smaller than the operation wavelength, we observed subwavelength localization within these metamaterial cavity structures. Moreover, we introduced coupled-cavity structures and presented the transmission spectrum of metamaterial based coupled-cavity structures. Finally, we demonstrated an ultrafast bioassay preparation method that overcomes the today’s bioassay limitations using a combination of low power microwave heating and split ring resonator structures.
Open Access
Design and fabrication of resonant nanoantennas on chalcogenide glasses for nonlinear photonic applications
(2013) Duman, Hüseyin
Optical nanoantennas are the metallic nanostructures which confine electromagnetic waves into sub-wavelength volumes at resonant conditions. They are used for various applications including biological and chemical sensing, single molecule spectroscopy, manipulation and generation of light. Combining extremely large electromagnetic field enhancement in plasmonic resonant nanoantenna with high optical nonlinearity of chalcogenide glass leads to a low-threshold broadband light generation scheme in sub-wavelength chip-scale structures. New frequency generation with ultra-low pumping power in plasmonic nanostructures allows compact on-chip light sources which can find applications in single molecule spectroscopy, optical signal processing and broadband lasers. We propose plasmonic nanoantenna chalcogenide glass systems for initiating nonlinear phenomena at low threshold. Size and shape of antennas are optimized according to linear refractive index of substrate and surrounding media for this purpose by finite difference time domain (FDTD) simulations. Resonant behaviour of antennas at their near-field and nonlinear response of optically highly nonlinear chalcogenide glasses are investigated. On resonance, strong field accumulation at the interface of the gold stripe and highly nonlinear As2Se3 glass triggers a start of the spectral broadening of incident beam accompanied by third harmonic generation at an ultra-low threshold power level of 3 W/µm2 . Moreover, we fabricate the designed structures by electron beam lithography, wet chemical techniques and optimize each fabrication step of processes by several experiments. Fabrication steps are explained and SEM images of related steps are presented.
Open Access
Experimental demonstration of transmission enhancement through subwavelength apertures at microwave frequencies
(2012) Ateş, Damla
Metamaterials are artificial materials with novel electromagnetic characteristics. They are used in many applications including imaging, super lenses, cloaking, transmission enhancement, beaming and recently in nano applications. One of the major building blocks is the split ring resonators (SRR). We can construct metamaterials by using a single or an array of the SRRs. In this thesis, enhanced transmission through subwavelength apertures, which is one of the applications of metamaterials, is obtained by using various split ring resonators configurations. We demonstrated transmission enhancement with Connected Split Ring Resonators (CSRRs), Omega-like Split Ring Resonators and Stack-like Split Ring Resonators through circular and rectangular subwavelength apertures experimentally and numerically at the microwave frequencies. We report the highest experimental transmission enhancement results in the literature so far. Besides high factors, we also obtained multi-peak resonant characteristics with Stack-like SRR designs. Furthermore, we analyzed these various SRR samples numerically in order to understand the resonance behavior. We also discuss the effects of shorting the loops, omitting the components of the SRRs and aperture geometry to the resonance frequency. Finally, we applied Tight Binding methods to analyze the multi-peak characteristics of the Stack-like SRR design.
Open Access
Resonant optical nanoantennas and applications
(2010) Kılınç, Murat Celal
Being one of the fundamentals of electrical engineering, an antenna is a metallic shape structured to transmit or receive electromagnetic waves. Thanks to the recent advances in nano fabrication and nano imaging, metallic structures can be defined with sizes smaller to that of visible light, wavelengths of several nanometers. This opens up the possibility of the engineering of antennas working at optical wavelengths. Nanoantennas could be thought of optical wavelength equivalent of common antennas. Today physics, chemistry, material science and biology use optical nanoantennas to control light waves. Optical nanoantennas are tailored for many technological applications that include generation, manipulation and detection of light. The near field enhancement of resonant optical nanoantennas at specific wavelengths is their most promising advantage that attracts technological applications. In this thesis, we study the resonance characteristics of optical nanoantennas and investigate the governing factors by numerical calculations. We also evaluate radiated electric field from the resonating nanoantenna. Finally, we employ the resonant near field enhancement in optical nonlinear generation. The fabrication of nanoantennas with FIB milling is also explored.
Open Access
Waveguiding of electromagnetic waves and investigation of negative phase velocity in photonic crystals and metamaterials
(2012) Çolak, İlyas Evrim
Electromagnetic wave propagation is characterized in periodic dielectric and metallic structures: Photonic Crystals (PCs) and Metamaterials, respectively. The applications of these structures are demonstrated in the Microwave regime. In the first application, Graded Index (GRIN) PC is used to focus the incoming waves into a small spot. Speaking in terms of PC period a, for an incident beam with Full Width Half Maximum of 9.20a the power of the focusing behavior is quantified by looking at the spot size conversion ratio, which is around 3.9. PCs can act as an efficient input coupler for the PC Waveguide (PCW). The GRIN PC has been experimentally shown to yield a coupling efficiency of 5 dB over the single PCW at 18 GHz. This method can be applied to provide a solution for the input coupling losses between PC structures and other lightwave circuits. PCs can also be used to achieve dual-bandpass and bandstop spatial filtering by proper adjustments of the lattice parameters and the frequency range. For the plane-wave excitation, a wideband spatial filtering is shown to exist due to the specific Fabry-Perot type resonances, which are nearly independent on the angle of incidence. The effect of the finite angular distribution of the Gaussian-beam excitation is also demonstrated. The spatial filtering in the incidence and observation angle domains has been discussed both numerically and experimentally for the non-plane-wave excitations under the light of calculated iso-frequency contours. In addition to bandstop characteristics, the dispersion relation of the PCs can be modified with the proper arrangement, namely by employment of the dimer layer. This surface layer supports the surface waves and serves like a waveguide for the electromagnetic waves. At higher frequencies above the lightline, surface waves radiate into air in the form of backward leaky waves and frequency dependent steering is reported from 0 º to 70º for the outgoing beam. The leaky wave behavior and backward radiation is similar to that is seen in Left-Handed (LH) Metamaterials. Metallic fishnet layers are used to demonstrate negative refractive index (NRI) in conjunction with the left-handed behavior in this class of metamaterial. A wedge structure formed by fishnet layers is used to measure the NRI which was also verified by the retrieval analysis. The limits of homogenization are discussed. The dependence of the LH properties on the fishnet parameters is investigated parametrically. For example, the NRI changes from -1.8 to -1.3 as the interseperation distance of the layers varies from as=λ/10.5 (2mm) to as=λ/4.2 (4mm) at magnetic resonance frequency around 14.3 GHz (ωm). It is also shown that the fishnet layers behave as an LC resonator as well as a TEM waveguide and a 1D transmission line at ωm.