Theses supervised by Prof. Dr. Atilla Aydınlı
18 theses · İhsan Doğramacı Bilkent University, Bursa Uludağ Üni̇versi̇ty
Tümleşik optik uygulamaları için kutupsal olmayan ısıl optik kipleyiciler
Abstract POLARIZATION INDEPENDENT THERMO-OPTIC MODULATORS FOR INTEGRATED OPTICS Aşkın Kocabaş M. S. in Physics Supervisor: Prof. Atilla Aydınlı September 2003 In this work, we studied MMI and Y-junction based Mach-Zehnder modulators on both silicon-on-insulator and polymer based on thermo-optic effect. Both Pw values and frequency response of the devices were measured and found to be consistent with those observed in the literature as well as results obtained from finite element simulations. We feel that our BCB based Y-junction modulators have the highest reported 3 -dB cut-off frequency to date. We also observe that all of our devices are polarization independent, an important feature for applications in optical communications. Keywords: Thermo-optic modulators, Thermo-optic effect, Finite element method, Multi-mode interference, Y-junction modulator, Poly mer modulator.
Pleksitonik kristaller ve moiré kovuklarında ışık madde etkileşimi
Surface plasmon polaritons (SPPs) are quanta of electromagnetic excitations at the interface between metal and dielectric media. SPPs with an evanescent tail in the perpendicular direction, thus their properties are sensitive to variations in the optical properties of the dielectrics film. If SPPs are created near excitonic media, coupling between excitons and SPs can be achieved. In this thesis, interaction dynamics of SPP-exciton coupling is investigated. In weak coupling case, properties of SPPs and excitons are perturbed as the enhancement of the optical absorption in excitonic matrices. In the strong coupling, coupled pairs (plexcitons) causes Rabi splitting in SPP dispersion curves. By patterning the metal-dielectric interface with sine profile grating, it is possible to form a band gap on the dispersion curve, width of which can be tuned by the groove depth and SPP-Exciton coupling can be engineered. Using this, a new type of crystal, plexcitonic crystal, is proposed and demonstrated that exhibit directional dependent coupling on square and triangular lattices. Superposing an additional grating on the initial one but with a slight difference in pitch, results in Moiré cavities, in which, slow plasmon modes can be confined. We show that we can directly image these modes using dark field microscopy. Further, the slow cavity mode in contact with an excitonic source, where SPPs are coupled with near field coupling, results in amplified light signal. Various Moiré cavities are shown to exhibit plasmonic lasing when slow plasmon modes in Ag coated cavities are excited inside a suitable gain medium.
SiOx:Ge ince filmlerde sürekli dalga lazeri ışınlaması ile germanyum nanokristal oluşumu
Germanium and silicon are the materials which have effective absorption in the visible and near infrared region of electromagnetic spectrum; therefore they are preferred for optoelectronic device and solar cell applications. Si and Ge are the material of choice when it comes to solar cell applications due to their being low cost, widely available and inert. They have indirect bandgap and the absorption coefficient of indirect bandgap materials is lower than direct ones. It is known that decreasing dimensions of materials to nanometric sizes cause transition from indirect bandgap to direct bandgap behavior along with increasing band gap. Therefore decreasing their dimensions both a shift of the band gap toward the blue as well as an increase in absorption can be achieved. In this work, thin films of SiOx:Ge were fabricated with different germanium concentrations and annealed with CW Ar+ laser operating at 488 nm that resulted in formation of Ge nanocrystals in the SiOx matrix. Composition analysis of as grown samples were done by Rutherford Backscattering Spectroscopy, optical properties were determined by ellipsometry. Nanocrystal formation within laser irradiated samples was confirmed by Raman spectroscopy. Data were also collected about crystal formation by scanning surface texture with stylus surface profilometer. As a result of all the analysis, it was shown that crystal formation depends on germanium concentration in the SiOx matrix and laser irradiation power density.
Yüksek güçlü lazer diyotlar için safsızlık atomu olmadan kuantum kuyularını birbirine karıstırma
The demand for ever higher powers and efficiencies from semiconductor lasers, continues. State-of-the-art high power lasers require not only sophisticated de- signs but also complex fabrication technologies to push the boundaries. A major obstacle to ever higher powers is catastrophic optical mirror damage that occurs at the mirrors of the cavity. Among several approaches to increase the threshold for damage, local manipulation of the band gap near the mirrors stands out, as it eliminates reabsorption. The structure of modern lasers employing quantum wells surrounded by large band gap and low index claddings gives the opportunity in intermix the quantum well and increase the effective band gap close to cavity edges during fabrication. The research presented in this thesis reports the results of Impurity-Free Vacancy Disordering (IFVD) of GaAs quantum wells in high power laser diode structures that leads to blue shifting of the effective band gap. In contrast with previous work, this study concentrates on actual large optical cavity (LOC) high power laser diode structures where the waveguide and cladding layers are thick. Using selective area QWI can be extremely beneficial in terms of enhancing catastrophic optical mirror damage (COMD) threshold, spatial mode instability, propagation losses and overheating which are the main limitations to fabricate HPLDs. In the course of the fabrication of HPLDs, the last and most problematic step is to manage QWI. IFVD was realized by capping the crys- tal surface with a sputtered dielectric layer of SiO2 to enhance intermixing and thermally evaporated SrF2 to prevent intermixing for selected parts of the laser cavity. Disordering the layers takes place by diffusion of Ga atoms from GaAs QW into sputtered SiO2 layer during rapid thermal annealing (RTA), leaving Ga vacancies in QW. It allows the Ga vacancy defects free to move AlxGa1−xAs lay- ers providing interstitial Al atoms to move into QW. The results were monitored using low temperature photoluminescence spectroscopy to determine the shift in the photoluminescence peak. Relative composition in the layers that make up the laser structure was measured with X-ray photoelectron spectroscopy in con- junction with depth profiling. A blue shift of 65 nm (154 meV) was achieved, in parallel with both Ga and Al diffusion in the laser structure.
Silisyum tabanlı dielektrikler: Tümleşik optikte kullanıma yönelik büyütme ve inceleme
In recent years, growing attention has been paid to silicon based dielectrics, suchas silicon oxynitrides, silicon nitrides, and semiconductor doped silicon oxides, allcombined under the name silica on silicon technology. This attention has beenmotivated mainly due to their excellent optical properties such as well controlledrefractive index and high transparency over a wide range of wavelength.In accordance with the main goal of this study that relied on the utilization ofsilicon based dielectrics and their optimization for applications in integrated op-tics, an emphasis was given to optimize the compositional and optical propertiesof these materials. A detailed quantitative compositional analysis using Fouriertransform infrared spectroscopy resulted in identiï¬cation of the germanosilicatedielectrics as the most promising candidates for use in integrated optics. Theï¬rst reported systematic study of propagation losses for diï¬erent-index planarwaveguides by using prism coupling method was correlated with the composi-tional analysis. This study had an important outcome for planar waveguidesfabricated with germanosilicate core layers resulting in the lowest propagationloss values reported so far for as deposited CVD-grown ï¬lms at λ=1.55 µm, elim-inating the need for costly and cumbersome annealing process.An improvement of the prism coupling technique led to a new approach forelasto-optic characterization of thin polymer ï¬lms. This completely new methodallows one to determine the optical anisotropy and out-of-plane mechanical prop-erties and to correlate both in order to obtain the elasto-optical properties of thinpolymer ï¬lms, for the ï¬rst time.Of interest as potential electro-optic material, we have concentrated on ther-mally poled germanosilicate ï¬lms deposited on fused-silica substrates by PECVD.ivvAs a result of an optimization study, we demonstrated a record peak nonlinearcoeï¬cient of â¼1.6 pm/V, approximately twice as strong as the highest reliablevalue reported in a thermally poled fused silica glass.Finally, we have demonstrated several applications of this technology in theï¬eld of integrated optics. Since optical waveguides constitute the building blocksof many integrated optical devices, we had ï¬rst concentrated on design and opti-mization of waveguides employing germanosilicates as the core layers. The ï¬nalstep of our work concentrated on design and implementation of microring res-onator devices based on germanosilicate layers.Keywords: Integrated optics, Silicon dielectrics, Silicon oxide, Silicon oxynitride,Germanium, Germanosilicate, PECVD, FTIR, Waveguide, Optical absorption,Prism coupling, Elastic modulus, Elasto-optic coeï¬cient, Birefringence, Nonlin-earity, Electro-optic coeï¬cient, Thermal poling, Ring resonator.
Plazmon bant aralığı kovukları
Surface 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.
Düşük boyutlu yapıların elektrik ve optik uygulamaları
Low dimensional structures such as quantum dots have been particularly attrac-tive because of their fundamental physical properties and their potential appli-cations in various devices in integrated optics and microelectronics. This thesispresents optical and electrical applications of low dimensional structures. For thispurpose we have studied silicon and germanium nanocrystals for flash memoryapplications and InAs quantum dots for optical modulators.As a quantum dot, nanocrystals can be used as storage media for carriers in°ash memories. Performance of a nanocrystal memory device can be expressed interms of write/erase speed, carrier retention time and cycling durability. Chargeand discharge dynamics of PECVD grown nanocrystals were studied. Electronand hole charge and discharge currents were observed to differ signifıcantly andstrongly depend on annealing conditions chosen for the formation of nanocrystals.Our experimental results revealed that, discharge currents were dominated by theinterface layer acting as a quantum well for holes and route for direct tunnelingfor electrons.On the other hand, possibility of obtaining quantum dots with enhancedelectro-optic and/or electro-absorption coeffcients makes them attractive for usein light modulation. Therefore, waveguides of multilayer InAs quantum dotswere studied. Electro-optic measurements were conducted at 1.5 micron and clearFabry-Perot resonances were obtained. The voltage dependent Fabry-Perot mea-surements revealed that 6 V was suffcient for full on/off modulation. Electro-absorption measurements were conducted at both 1.3 and 1.5 micron. Since thestructure lases at 1285 nm, high absorption values at 1309 nm were obtained.The absorption spectrum of the samples was also studied under applied elec-tric field. Absorption spectra of all samples shift to lower photon energies with increasing electric field.
Optoelektronik aygıtlar için germanyum bileşikleri
Silicon has been the backbone of the mainstream electronics of the last fifty years. It is however, used in conjunction with other matierals, mainly with its oxides and nitrides. Germanium, on the other hand, is also a group IV element and has been used in the early stages of transistor and detector development. In addition to Si/Ge heterojunctions, bandgap engineering through SiGe alloys has also been used in photodetectors. Recent progress in light emitting devices utilizing Si nanocrystals suggest the use of Ge1-xNx layers as barriers due to its suitable band offsets [1]. Experiments have shown that Ge1-xNx is also a promising material for applications in photodiodes, amplifiers, optic fibers, protective coatings, etc [2]. Both Si and Ge are, however indirect bandgap semiconductors, lacking efficient light emission. On the other hand, strong light emission observed in Si nanocrystals has made the study of semiconductor nanocrystals an expanding field of interest due to potential applications in novel optoelectronic devices [1]. These nanocrystals exhibit strong luminescence and nonlinear optical properties that usually do not ppear in the bulk materials [3-4]. SiGe nanocrystals attract attention due to the possibility of a tunable band gap with composition.In this study, formation of Ge1-xNx thin films and SiGe nanocrystals by plasma enhanced chemical vapor deposition (PECVD) reactor has been studied. We present the growth conditions and experimental characterization of the resulting thin films and nanocrystals. We used ellipsometry, Raman Spectrometry, Fourier Infrared Spectrometry (FTIR) and X-ray photoelectron Spectroscopy (XPS). For SiGe nanocrystals, 4 peaks in the Raman Spectra were observed around 295 cm-1, 400 cm-1, 485 cm-1 and 521 cm-1. These peaks are assigned to the Ge-Ge, Si-Ge, local Si-Si and crystalline Si-Si vibrational modes, respectively [5]. For the Ge1-xNx thin films FTIR spectrum showed the existence of the Ge-N bonds and its band offsets determined by XPS confirm its suitability for optoelectronic devices.
Kırınım ağı tabanlı plazmonik kovuklar
Surface plasmon polaritons are dipole carrying electromagnetic excitations occur-ing at metal-dielectric interfaces. Metallic periodic structures exhibit modi¯edtransmission and re°ection spectra owing to the interaction of propagating SPPswith 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 todemonstrate plasmonic cavity formation. The quality factor, Q, a characteristicvalue that indicates rate of energy loss relative to the stored energy in the cavityis a crucial parameter for classifying these cavities. It was proposed that theQ factor should strongly depend on the surface geometry. However, there wasnot 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 typesof 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 additionaldielectric loading. Furthermore, we fabricate 2D plasmonic structures, observeplasmonic band gaps in the symetry axis and propose cavity geometries for thisstructure.
InSb kızılötesi detektörlerin pasifleştirilmesi
Infrared detectors have wide range applications in both military and civilian life.One of the most commonly used infrared detectors is InSb detectors. InSb detectortechnology has been developing since 1950s. Fabricating p-n diodes todetect infrared radiation is a common way of constructing InSb detectors. Dueto high free carrier concentration at room temperature, InSb detectors need tobe cooled down to operate properly and usually liquid nitrogen is preferred forcooling. However, even at 77 K, tunneling and generation-recombination andsurface leakage are not negligible and these effects result in dark current. Improvingthe photo current-to-dark current ratio is the main goal in design andfabrication of InSb photo detectors. One way of decreasing the dark current ispassivating the exposed edges of the detector to reduce surface leakage current.Passivating the edges can result in decreasing in the surface leakage by eliminatingthe surface states (dangling bonds). Dielectric thin films like SiO2 and SiNxare commonly used for passivation. In this work, different sized detectors arefabricated and characterized by measuring I-V curves and spectral response. Differentapproaches are tested for passivation and a detailed comparison betweendetectors with different treatments is presented.
Kırınım ağı yüklü tümleşik optik kaldıraçlar
Cantilever beams are the most important parts of standard scanning probe microscopy. In this work, an integrated optical approach to sense the deflection of a cantilever beam is suggested and realized. A grating coupler loaded on the upper surface of the cantilever beam couples the incident light to the chip, which is then conveyed through a taper structure to a waveguide to be detected by a photodiode. Deflections of the cantilever beam change the optical path and hence the total transmitted intensity. Finally an optical signal is produced and this signal is measured. Resonance peak of 27.2 Q factor is obtained, which could be further enhanced by proper vibration isolation and employment of vacuum environment.
Plazmonik kovukların üretimi, karakterizasyonu ve benzetimi
Surface 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.
Eksiton-plazmon çiftlenmesinin akortlanması
Exciton-plasmon coupling has recently drawn much interest. In this work, FDTD simulations of exciton-plasmon coupling in plasmonic cavity structures with corrugation patterns are investigated. Excitonic modes are obtained from a Lorentz absorber modeling of a J-aggregate organic dye. The coupling of these excitonic and plasmonic modes on Ag thin films is demonstrated. Rabi splitting due to coupling was clearly observed. Flat metallic surfaces, uniform gratings and Moiré surfaces are used in simulations as corrugation patterns. Metal film thickness and dye concentration dependence of Rabi splitting via exciton-plasmon coupling was also observed on thin flat Ag films. We show that Rabi splitting occurs even at low dye concentrations, and the magnitude of splitting increases as dye concentration increases. A new state in the band gap is observed when the total oscillator strength is increased. Large Rabi splitting is observed when plasmon damping is modulated. Exciton-plasmon coupling on uniform gratings is studied as a function of cavity size, corrugation periodicity and depth. Q factor and Rabi splitting behavior of exciton-plasmon coupling on Moiré cavities are investigated as a function of cavity size. Strong anti-crossing is observed when the excitonic absorption matches with the cavity state.Keywords: Exciton-Plasmon Coupling, Plasmonic Cavities, J-aggregates, Rabi Splitting
Silisyum zengini oksitlerin lazerle işlenmesiyle silisyum nanoyapıların oluşumu
Silicon nanocrystals are well known to exhibit strong luminescence in the visible. Extension of this into a nanocrystal network would be beneficial for many applications. In the light of recent advances on exciton-plasmon interactions and photovoltaic cells, there is renewed interest in the use of nanostructures. Due to quantum confinement, silicon nanoclusters with increased band gaps, are promising for down conversion light and enhanced emission on plasmonic surfaces. Conventional techniques utilize high-temperature processing to obtain the Si-SiO2 phase separation which uses high thermal budget, not suitable for localized applications not compatible with glass substrates or thin-film stacked structures. An alternative approach capable of avoiding high temperature processing is laser irradiation of substochiometric amorphous silicon oxides.In this work, continuous-wave laser processing of Si-rich oxide thin films with varying Si content were performed in order to obtain Si nanocrystals embedded in silica. The role of composition, dwell times and power densities were investigated for Si-SiO2 phase separation.We present cw laser processing of PECVD grown and sputtered SiOx films. XPS, RBS and ERDA techniques were used for the stoichiometry analysis of different composition as grown samples and their optical properties were determined through ellipsometry analysis. Processing was performed with an Ar+ laser at 488 nm. The structural changes due to processing were investigated by Raman and photoluminescence spectroscopy.It has been shown that silicon nanocrystals formation depends both on precursor gas composition (hydrogen-diluted SiH4 and N2O or CO2 gases) and on laser power density. PECVD grown hydrogenated SiOx films were compared with sputtered films with and without hydrogen to identify the role of hydrogen for phase separation.
InAs/GaSb süperörgü fotodedektörlerde karanlik akim kontrolü
Since every object with a finite temperature accepted to emit infrared radiation, detection of this part of the electromagnetic spectrum is very important. For example the temperature of the exhaust of a vehicle or a missile makes it radiate in the midwave window or the body temperature of a person makes it glow in the long-wave window. Therefore detection in mid-wave and long-wave windows of infrared radiation spectrum is useful for both military and civil purposes. In order to satisfy the need in these field thermal and optical detectors are being developed. Optical detectors are based on the absorption of the thermally generated photons from the objects. However, due to the low energy of the infrared radiation narrow band gap materials are needed to be used. Having narrow gap seems to be useful but the energy required to make an electron to excite from the valance band to conduction band can also be supplied by the ambient temperature. This situation is the main disadvantage of optical detectors. There are too many mechanisms that can generate dark current in the device which make the optically generated current to be less measurable and reduce the device performance. In order to solve this unintentional current problem, optical detectors should be cooled down to cryogenic temperatures. Even for this low temperature regime generation-recombination, tunneling and surface leak currents are not suppressed enough. To increase the device performance these mechanisms should be suppressed. Suppression of surface leakage current which is caused by the dangling bonds on the device side-walls can be achieved with a passivation layer that eliminates the surface states. This thesis suggests the usage of two new methods for passivation of InAs/GaSb type II superlattice mid-waveinfrared detectors. Application of atomic layer deposited Al2O3 and monolayer thick octadecanethiol as passivation layers are compared with their respective unpassivated detectors on their effect on suppression of dark current. Additionally, contact resistance measurements are made in order to check that if there is a contribution from the contact structures that can complicate our device model.Keywords: InAs/GaSb, Infrared photodetector, Passivation, Al2O3, Octadecanethiol
Lazer topaklanma yoluyla ince metal filmlerden plazmonik nano parçacık
In 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.Keywords: Laser induced dewetting, thin film, plasmonics
Fotonik kristallerde yavaş ışık temelli sensör tasarımı
Işığın yayılımını denetlemek ve yönetmek optik ve fotonik alanının önemli konularından biridir. Işığın birçok özelliği yüksek hassasiyette sensör tasarımlarını mümkün kılmaktadır. Bu sensör tasarımlarında son yıllarda fotonik kristallerin kullanımı yaygınlaşmıştır. Fotonik kristaller, dielektrik sabitinin bir, iki veya üç boyutta periyodik olarak değiştiği, yarı iletkenlerdeki elektronik bant aralığına benzer olarak fotonik bant aralığına sahip olan yapılardır. Bu fotonik bant aralığı belirli dalga boylarının kristal içerisinde ilerlemesini sağlarken belirli dalga boylarını geri yansıtarak kristalin içinde ilerlemesini engellemektedir. Fotonik kristal yapılarda oluşturulan bilinçli kusurlarla fotonik bant aralığında kusur kipleri elde edilir. Bu kusur kiplerinin dalga kılavuzu içerisinde ilerlerken grup hızının azalması sonucu artan madde-alan etkileşimi sensör hassasiyetini de arttırmaktadır. Tez çalışmasında kare örgülü fotonik kristallerin tasarımı yapılmış ve bilinçli olarak yapılan kusurların fotonik kristallerin özellikleri üzerindeki değişimleri incelenmiştir. Fotonik kristalin örgü simetrisinde yapılan çeşitli değişimler sayesinde ışığın grup hızı azaltılmış ve bunun sonucunda madde-alan etkileşimi arttırılmıştır. Yapılan tasarım ile sensör olarak kullanılan dalga boyu aralığında ışığın grup hızı 0,13*c olarak hesaplanmış ve sensör hassasiyeti 212 nm/RIU olarak elde edilmiştir. Yüksek dereceden kiplerin sensör hassasiyetine etkisinin incelenmesi amacıyla yapılan ikinci tasarımda iki farklı kip incelenmiştir. İkinci tasarımda birinci kiple 166 nm/RIU ve ikinci kiple 181 nm/RIU sensör hassasiyetine ulaşılmıştır. Bu tasarımda yüksek dereceden kiplerin azalan dalgasının (evanescent field) dielektik çubuklar üzerinde yoğunlaşmasının sensör hassasiyetini arttırdığı gösterilmiştir.
Tümleşik optikte parite zaman simetrisi uygulamaları
Optik dalga kılavuzlarında farklı dereceden enine kipler ayırmak veya birleştirmek tümleşik optiğin araştırma alanlarından biridir. Enine kiplerin ayrılması için farklı geometrik yapıya sahip çeşitli pasif dalga kılavuzu birleştirici tasarımları yapılmaktadır. Son zamanlarda, PZ simetrisi kavramının optikte uygulanması, farklı tümleşik optik aygıtların tasarımına olanak sağlamaktadır. Bu kavram ilk önce kuvantum fiziğinde incelenmiştir. Bu çalışmalarda V (x) = V*(- x) koşulunu sağlayan potansiyellerin, Hermisyen sistemlerde olduğu gibi gerçek özdeğerler alabileceği gösterilmiştir. Daha sonra Schrödinger denklemi ile EM dalga denklemi arasındaki benzerlikten yararlanılarak, kompleks kırılma indisi dağılımında 𝑛(𝐫)=𝑛𝑅(𝐫)+𝑖𝑛𝐼(𝐫) , 𝑛(𝐫)=𝑛∗(−𝐫) şartının sağlanması halinde PZ simetrisinin elde edilebileceği gösterilmiştir. Öte yandan, süperörgü ve kuvantum kuyuları kullanarak kızılötesi ve terahertz bölgesini de kapsayan ışık kaynağı olarak geliştirilmekte olan kuvantum çağlayan lazerler son yıllarda önemli gelişmelere sahne olmuştur. Bu çalışmada kuvantum çağlayan lazerlerde PZ simetri uygulaması incelenmiştir. Kazanç ve kayıp parametrelerinin modülasyonu ile enine kiplerin eşleşme katsayılarını değiştirerek PZ simetrik kuantum çağlayan lazerlerde yönlü çiftleyicilerde enine kiplerin ayrıştırılması sağlanmıştır. nmıştır. Geliştirilen tasarım, sonlu fark öz kip çözücü ve sonlu fark ışın yayılımı yöntemi ile simüle edilmiştir. Önce kuantum çağlayan lazerlerde öz kip çözücü yöntemi kullanılarak enine kiplerin özellikleri elde edilmiş sonrasında enine kiplerin ayrıştırılması için uygun kazanç ve kayıp değerleri hesaplanmıştır. Daha sonra belirlenen kazanç ve kayıp ortamına sahip çiftleyici sonlu farklar ışın ilerleme yöntemi ile simüle edilerek uygun kazanç ve kayıp değerleri altında TE0 ve TE1 kiplerinin ayrılabileceği gösterilmiştir.