Theses supervised by Prof. Dr. Ali Serpengüzel
15 theses · Koç University
Silisyum mikroyuvarlar ile ayarlama ve izge gözlemlemesi
Remarkable amount of research is being devoted to the merger of microelectronics and microphotonics on a single silicon chip. Additionally, substantial amount of research has demonstrated a variety of optical functionalities for microspheres. Possessing high quality factor morphology dependent resonances, microspheres have been investigated in resonant cavity enhanced optoelectronic applications. Silicon microspheres, combining the advantage of both the spherical shape and the optoelectronic properties of silicon are ideal candidates for integrated photonics. In this work, silicon microspheres have been excited in the S-band using optical fiber half couplers. The transmission spectra and the elastic scattering spectra from a silicon microsphere have been obtained. Potential difference is applied to the silicon microsphere using two gold electrodes placed on the reciprocal sides of the silicon microsphere. Optical modulation has been observed in both the transmitted and the 90o elastic scattered spectra for the transverse electric and transverse magnetic polarizations. The applied voltage at increasing duty cycles provided promising results about the spectral signature of the morphology dependent resonances.
Silisyum mikroyuvarlardaki elastik saçılmada yüksek kalite faktörlü optik çınlamalar
The whispering gallery modes resonators have received increased attention as novel structures, which are suitable for various applications in linear, nonlinear and quantum optics. Small mode volumes and high optical quality factors are unique features of these resonators. These properties make possible high concentration of optical fields and relatively long photon confinement times. The spherical cavities confine light by total internal reflection at the inner surface boundary. In this study, elastic light scattering intensity calculations at 90o and 0o for the transverse electric and transverse magnetic polarized light were performed at 1200 nm for a 50 micrometer radius and 3.5 refractive index silicon microsphere. The mode spacing between morphology dependent resonances was found to be 1.76 nm. The linewidth of the morphology dependent resonances was observed to be on the order of 1E-02, which leads to a quality factor on the order of 1E+05. Experimentally, high quality factor optical resonances are obtained in the elastic scattering spectra from a silicon microsphere with a radius of 500 micrometer and refractive index of 3.48. The mode spacing between morphology dependent resonances was found to be 0.27 nm. The linewidths of the resonances were found to be of the order 1E-04 nm, which leads to Q factors of 1E+06. As a result, silicon microspheres could be used as a resonant cavity enhanced optoelectronic devices for near infrared wavelength region as well as other application in linear, nonlinear and quantum optics.
Cam mikroyuvarların amorf ve kristal sıvılar içinde optik çınlamaları
Optical microcavities have found many applications in linear optics, nonlinear optics, quantum optics, and biophotonics as laser light sources as well as optical switching, sensing, and wavelength division multiplexing devices. Microspheres are high quality factor optical microcavities that localize light by total internal reflection. Electromagnetic boundary conditions have to be satisfied for the light localization. Optical resonances occur, when an integer multiple of the wavelength is equal to the optical pathway of the cavity.In this work, the polarization dependence of the optical resonances in an optical fiber- microsphere system is investigated experimentally. The change of the elastic light scattering spectra of the microspheres is studied for microspheres placed in various fluids. It is observed that, the shape of resonances and the intensity of the background are strongly dependent on the surrounding fluid medium. Blue and red shifts of the optical resonances are observed due to the changes of the external fluid media. This optical geometry heralds new device designs and applications for microsphere based optoelectronics and microfluidics.
Sıvı içindeki katı mikroyuvarlardan elastik ışık saçılması ve kiplenmesi
Optical microcavities have been extensively studied for the last few decades for theirpotential applications in lightwave communications and biochemical sensing. Among thedifferent microcavity shapes and configurations studied so far, the microsphere is simplestresonator type due to the self-forming, ideal, and perfect nature of the sphere. For a sphere,the solutions to the electromagnetic fields subject to the boundary conditions, lead to thehigh quality factor optical resonances, which are strongly dependent on the morphology,i.e., size, shape and refractive index, of the spherical resonator.In this work, solid dielectric and semiconductor microspheres are used asmicroresonators. The optical resonances of the microspheres are excited with tunable diodelasers through optical fiber half couplers. The observed transmission and the elasticscattering spectra reveal high quality factor optical resonances. The spheres are placed invarious liquids, such as liquid crystals, organic liquids and aqueous solutions for theinvestigation of these optical resonances.Nematic liquid crystals are highly birefringent molecules, which are vastly used indisplay technologies. Applying an external electric field to the nematic liquid crystalinduces changes in the refractive index of the medium external to the microsphere. Hencethe spectral position of the microspheres? optical resonances are tuned and used for signalmodulation. This state-of-the-art microfluidic system promises novel optical signalprocessing applications in liquid crystal display and optofluidic technologies.
Silis ve silisyum mikroçınlaçlardan esnek ışık saçılımı
Optical microresonators are gaining an important role in a variety of applications such as sensing, novel light source, and optical communication applications. Lightwave circuits are already using these optical microresonators extensively.Silicon being the main material for the current electronic integrated circuits can also be used for producing future photonic integrated circuits or photonic optoelectronic circuits. These photonic integrated circuits can easily upgrade their electronic counterparts, using the well developed CMOS process technology.This work covers simulation and analysis of the elastic light scattering from various silica silicon microresonators such as microslabs, microcylinders, microspheres, and microspheroids. Absorption, extinction, and scattering efficiencies as well as elastic scattering intensities are calculated and analyzed for silica and silicon microresonators. The resulting morphology dependent resonances, which are apparent in the elastic scattering spectra, can have novel applications in silicon photonics.
Yuvarlak silisyum mikroçınlaçlarda elastik ışık saçılması ve optoelektronik yanıtı
We review the microsphere optical resonators with their whispering gallery modes (WGM) and the coupling of light to the microspheres by optical fiber half couplers (OFHC). A theoretical background for the optical resonances in spherical particles is given for plane wave excitation using the Lorenz-Mie Theory, and Gaussian beam excitation using the Generalized Lorenz-Mie Theory (GLMT). Numerical simulations for plane wave excitation using LMT and Gaussian beam excitation using GLMT are obtained to explain the experimental light scattering results. A brief description of the experimental light scattering setup is given together with the properties of the silicon microsphere. The elastic light scattering and the observation of the WGMs on the silicon microspheres are presented for 0o transmission and 90o elastic scattering. The mode structure is analyzed with the mode spacing and quality factor measurements, which corresponds well with the theoretical expectations. The analysis of the WGM spectrum continues with the characterization of the WGM clusters. These clusters can either be explained by the angular distribution of the WGMs, azimuthal mode splitting due to ellipticity or the mode splitting induced by a particle. The observed asymmetrical lineshapes in the spectra are also explained. The presented measurements show a Fano type lineshape for the 90o elastic light scattering. This line shape is explained by the phase relation of the WGMs with the background created by the surface imperfections and the glare spots. The Fano lineshape observed in the measurements can be used for fast switching and filtering applications. A biosensing scheme for DNA particles is also presented, where a particle adsorbed on the microsphere creates an observable shift in the WGM spectrum. All in all, the optical resonances, namely WGMs are characterized towards possible applications in sizing, sensing and filtering applications. The silicon microsphere is a promising candidate for various novel photonic applications.
Silikon mikrokürelerde fısıldayan galeri kiplerinin optik uyarım ile kontrolu
Geçtiğimiz yüzyılda silikon maddesi, yarı-iletken endüstrisi için temel bir malzeme haline gelmiş ve fotonik alanında da CMOS teknolojisi ile birlikte kullanılacağı ön görülmüştür. Geçtiğimiz yıllarda ise fısıldayan galeri kipleri (WGM) optik çınlaçları, optik filtreler, modülatörler, yükselticiler ve faz yöneticileri gibi birçok farklı fotonik uygulamasında yer bulmuştur. Buna göre, WGM çınlaçları CMOS teknolojisi ile birlikte kullanılarak tümleşik optik cihazların üretiminde yer alabilir. Bu tezde, 1427 nm dalga boyunda çalışan 500 µm yarı çaplı silikon WGM çınlaçlarının ürettiği WGM çınlamaları, 405 nm dalga boyunda çalışan bir pompa lazeri kullanılarak kırmızıya doğru kaydırılmıştır. Elde edilen WGM kırmızıya kaymaları, silikon kürelerdeki pompa lazerinin etkisi ile ortaya çıkan termo-optik etkiye (TOE) dayanmaktadır. Silikon küresel çınlaçların karakterize edilmesi amacıyla bir ölçüm düzeneği kurulmuş ve kip aralığı ∆λ = 0.23 nm ve kalite faktörü Q ≈ 60000 olan WGM kipleri gözlemlenmiştir. Bu adımdan sonra pompa-ölçüm deney düzeneğine geçilmiştir. Bu düzenekte, 405 nm dalga boyunda çalışan odaklanmamış sürekli dalga (CW) pompa uyarımı kullanılarak silikon küre WGM izgesinde 13.5 pm kırmızıya kayma ve odaklanmış pompa uyarımı ile 24.5 pm kırmızıya kayma gözlemlenmiştir. Ek olarak, pompa lazeri 8 Hz frekansında % 80 çevrim ile uyarım yaptığında WGM kiplerinde 40 pm'ye kadar WGM kayması da elde edilmiştir. Sonuçlara göre, silikon kürelerde WGM optik çınlamalarının yine optik yoluyla kontrol edilebileceği ortaya konulmuştur. Deney düzeneği ve analizlere göre silikon mikro kürelerin ultra hızlı optik anahtarlama alanında uygulama bulacağı öngörülmektedir.
X-band uygulamaları tasarım ve geniş bant meandering anten imalatı
Novel meandering antenna designs have been proposed for broadband communication applications in X-band spectral region. Broad band antennas are becoming more popular as a result of growing demand for bandwidth, for example, video conferencing; direct sequence spread spectrum (DSSS) communication system and frequency hopping. Meanders introduce longer current paths and hence add multiple staggered resonating modes. This design technique can be used to achieve multiband designs using resonating modes far from each other. However, if the design is engineered to let these resonant modes come closer to each other, they will complement the effect of closely spaced resonances, forming staggered resonating behavior, and broadband antennas are achieved. Current in the adjacent arms of the meandering antennas cancel out each other due to symmetry of the design, and radiation efficiency is compromised as a result. Antisymmetric meandering arms are presented in this work, to achieve higher radiation efficiency. A relationship between physical dimensions of the meandering loops, that is, height, thickness, mutual coupling and material properties are presented and resonant frequencies are mathematically modeled. The physical dimensions are selected to obtain set of resonant frequencies, which are closely spaced in the frequency spectrum. The model is simulated using finite element method (FEM) numerical modeling technique. A good agreement between mathematically calculated resonant response and simulated results is obtained for different designs geometries. Six designs are modeled and fabricated on a single sided FR4 substrate of dielectric constant 4.4 and thickness 1.6 mm. In order to evaluate the performance for suitable comparison, each antenna is identically implemented using a 50-Ohm coplanar waveguide (CPW) as feeding probe. Antennas are independently assessed using a Network Analyzer. Both S11 and S12, VSWR, efficiency, gain and bandwidth (BW) parameters are measured. The overall performance of the antennas has been found good.
Silisyum mikro-yuvarlarda optik anahtarlama
Silicon is a cheap and naturally abundant material and has contributed to the electronics revolution, which in turn has led to the foundation of separate fields known as silicon electronics and silicon photonics. In silicon photonics optical resonances are utilized, which are useful for various applications, i.e., optical components such as Fabry-Pérot resonators. However, with the passage of time focus has shifted to three dimensional optical resonators with microspheres being the prime example. Nowadays, optical resonances excited in microspheres are called whispering gallery modes (WGMs) and they can be beneficial for various photonics applications. In this work, a silicon microsphere, with a 1 mm diameter and a refractive index of 3.48, is used to excite the WGMs of the silicon microsphere by evanescent coupling with a silica optical fiber half coupler (OFHC). A near infrared CW laser with the central wavelength of 1472 nm is used for the excitation of the WGMs. Additionally, these WGMs are tuned with the help of a pulsed 405 nm Fabry-Pérot laser, which acts as pump laser to modify the refractive index of the microsphere. As the pump energy changes, an average red shift of 0.37 nm in the WGMs spectrum of the silicon microsphere is observed for 50% duty cycle of the pump laser, which leads to a refractive index change of ΔN = 8.75x10‑4 and a temperature change of ΔT = 4.70 K. This behavior can be explained by the thermo-optic effect (TOE) in silicon. All-optical switching in time domain is also carried out for the WGMs of the silicon sphere, as the pump modulates the refractive index of silicon, the resonant peaks are observed to be shifting in time domain. Overall, TOE tuning of the resonant spectra is shown to be beneficial for the all optical control of silicon resonators. This technique can be useful for applications in silicon photonics.
Kristal ve amorf sıvılarda elastik ve elastik olmayan saçılma ile desteklenmiş yuvarlak ve düz yüzeyli silisyum ışık algılayıcıları
When light interacts with matter, absorption, scattering and fluorescence of light can take place depending on the wavelength and energy of the light together with the size of the interacting particle. In this work, we present light scattering enhancement of spherically shaped silicon photodetectors immersed in liquid environments. Our results indicate higher detection performance for spherical surface detectors as compared to conventional flat surface photodiodes. In order to characterize the photodiodes, we devised a measurement setup with a diode pumped solid state laser (DPSS) operating at 532 nm wavelength to induce light scattering in the liquid solutions. The solutions used were pure ethanol, 0.1 mM Rhodamine 640 perchlorate in ethanol, pure 5CB nematic liquid crystal, and 0.1 mM Rhodamine 640 perchlorate in 5CB, Rhodamine solution gives the ability of fluorescence in addition to the light scattering. We obtained elastic light scattering in ethanol and liquid crystal solutions and inelastic light scattering in fluorescent solutions with the laser dye. The silicon photodiodes were immersed in the solutions and characterized for their responsivity for the detection of scattering. Positional tomography for the voltage response of the photodetectors was recorded. It is seen that the flat photodetector responded as directional, whereas the sphere's responsivity pattern showed omnidirectional detection angle. Inelastic scattering, i.e. fluorescence increases the response. Elastic scattering by the LC also increases the response. Dye in LC has decreased response. Different solutions of laser dye in LC need to be studied for optimum performance. The results indicate higher performance for spherically shaped silicon photodetectors and these results can be applied to optics of highly scattering media.
X-bant aralığında çalışan 2 boyutlu altıgen fotonik kristaller
Photonic crystals have been a hot area of scientific and technological research, since their proposal in 1987. Photonic crystals are generally known as the "semiconductors" of photonics, and found applications in photonic circuits, optical imaging, and telecommunications. The scope of this research is to study microwave transmission response of 2D hexagonal photonic crystals of graphite rods. We used graphite rods to fabricate a photonic crystal structure and stacked the rods together to form a hexagonal lattice. In order to characterize the structure, we measured the transmission of the photonic crystal in the microwave X-band. No structure is observed in the transverse electric (TE) polarization transmission spectrum, as the graphite rods absorb more efficiently for that geometry. A photonic band gap with a width of 0.5 GHz is observed for transverse magnetic (TM) polarization at 10.8 GHz. As the angle of incidence is scanned, we observed the shifting of the bandgap frequency. A similar bandgap frequency was observed at 00 and 600 coinciding with the symmetry of the photonic crystal. We induced defect states in the photonic crystal, and observed a mode in the band gap, for a point defect. As Maxwell's equations are scalable, we can scale down our hexagonal photonic crystal to operate in micro to nano length scales. Our results show that, 2D graphite photonic crystals can be useful in microwave engineering.
Elmas mikroyuvarın elastik ışık saçılması, fotoışıma ve Raman izge ölçümleri
Over the past decades, morphology dependent resonances, the optical resonances occurring by the interaction of light with microspheres of various materials, were vastly investigated. Spherical microcavities have diverse applications due to their high concentration of modes in small volumes at optical frequencies and high Q factors. The localization of the electromagnetic waves inside the microsphere is realized by total internal reflection. Microspheres provide the concave surface for light to circumnavigate around the microcavity, giving rise to whispering gallery modes. Because of its unique lattice structure, robust physical properties, as well as its distinctive optical properties, different forms of diamond were vastly investigated for various applications. As is well known, nitrogen vacancy (NV) centers inside the diamond lattice give rise to such applications by replacing two carbon atoms by one nitrogen atom and a vacancy, that yields a floating electron at the NV- center. In this thesis, Raman and photoluminescence spectroscopy is performed with various diamond samples including a diamond microsphere. The measurement setup is presented for both spectroscopies. NV centers exhibit 575 nm or 637 nm zero phonon lines along with their corresponding phonon side bands depending on the charge state of the point defect, NV0 and NV-, respectively. A 572 nm inelastic Raman scattering (1332.25 cm-1) is observed with 532 nm excitation laser due to the vibrational state of carbon-carbon bonds with sp3 configuration, whereas sp2 hybridization (1444.56 cm-1) yields inelastically scattered photons with 576 nm wavelength. It is possible to identify the main impurities within the diamond lattice, and confirm the carbon-carbon bonds with spectroscopic techniques. In addition, the demonstration of a diamond microsphere whispering gallery modes resonator in the standard telecom wavelengths between 1426.10 nm and 1427.42 nm in the 90° elastic light scattering for both transverse magnetic and transverse electric polarizations is realized. The highest measured whispering gallery mode Q-factor is on the order of 104, and the mode spacing 0.332 nm for both transverse polarizations. The coupling of the continuous wave tunable infrared excitation laser to the diamond microsphere is achieved by a single mode silica optical fiber half coupler. All in all, diamond is a unique material with particular optical properties. Utilizing such properties in a spherical morphology can give rise to distinctive applications. It is essential to observe the aspects of the whispering gallery modes created by the light circumnavigating a diamond microsphere such as mode spacing and Q-factor to classify the properties of a 1 mm synthetically grown spherical diamond microresonator. Such a diamond resonator can further be used as stable optical frequency comb generators or lasing microcavities by exploiting the nitrogen vacancy centers present within the lattice of the diamond.
Silisyum ve elmas mikroyuvarların fısıldayan dalga kiplerinin femtosaniye lazer ile yazılmış cam ve elmas dalga kılavuzları ve cam fiber bağlayıcı ile uyarılması
Whispering gallery modes (WGMs) are the optical resonances, where light is trapped inside a dielectric and symmetric cavity by total internal reflection (TIR) on the boundary facets. Due to the unique optical properties, such as very high quality factors, and very narrow linewidths, due to the long photon lifetime inside the cavity, and confinement of the field in very small mode volumes, resulting in high power densities, cavities of different geometries, supporting WGMs, have been extensively studied for more than four decades now. However, the efficient coupling to WGMs mostly rely on experimentally challenging schemes including prism coupling, optical fiber half coupling (OFHC) or free space coupling. This thesis consists of the excitation of silicon microsphere WGMs using an alternative coupling method: shallow waveguides fabricated by femtosecond laser inscription. The evanescent field coupling from the guided modes of the femtosecond micromachined glass and diamond waveguides has been used to excite microsphere WGMs. Furthermore, this thesis includes the excitation of a diamond sphere using an OFHC. Employment of diamond material in photonic applications is substantial, because among all materials, diamond exhibits unique optical properties such as its color centers, large band-gap, high Raman gain, as well as a unique crystal lattice, and robust physical properties. Utilizing diamond as an excitation waveguide material or the cavity material opens the way for combination of both diamond's and WGMs' unique properties for applications in electronics, quantum information processing, and sensing.
Femtosaniye lazer ile yazılmış sığ optik cam ve elmas dalgakılavuzlarına bağlaştırılmış silisyum ve elmas mikroyuvarların esnek saçılması
Elastic light scattering of silicon and diamond microspheres excited by femtosecond (fs)-laser written glass and diamond waveguides is studied in order to assemble an all diamond optical system towards integrated diamond photonics applications. Transverse electrically (TE) and transverse magnetically (TM) polarized light from a wavelength tunable laser operating in the near-infrared region is coupled to a 1 mm silicon sphere by using the Gorilla and Eagle2000™ glass optical waveguides. Silicon sphere elastic scattering characteristics are gathered on both glass and diamond optical waveguides, in order to examine diamond optical waveguide characteristics. After which, an all-diamond optical system is established with diamond sphere on fs-laser written diamond shallow waveguide. The assembled diamond system integrates a Type-Ib (nitrogen impurity > 5 ppm) diamond microsphere with a fs-laser written Type-IIa (nitrogen impurity of 100 ppb) diamond waveguide. The diamond waveguide is fabricated by exploiting type II fabrication method to achieve stress induced waveguiding. TE and TM polarized light from a wavelength tunable laser operating in the near-infrared region is coupled to a 1 mm diamond sphere by using the diamond optical waveguide. By carefully engineering the microsphere's high quality factor resonances, and further exploiting the nonlinear properties of existing nitrogen-vacancy (NV) centers in diamond microspheres and/or diamond waveguides in such configurations, it is possible to realize various applications in integrated diamond photonics.
Silisyum kıvrılan dağıtılmış geribildirimli yapıların tümleşik fotonik incelemeri: Biyolojik ligandlar için örnek bir düzen
Halka çınlaçları, dağıtılmış Bragg yansıtıcıları (DBY), Mach-Zehnder girişimölçerleri (MZG) ile bunların ardışık bağlanmış sürümleri gibi tümleşik fotonik yapılar virüs, protein, biyoimleyici, deoksiribonükleik asit (DNA), ve mikroribonükleik asit (mikroRNA) gibi örneklerin biyoalgılanması yaygın olarak çalışılmış, ve başarıyla denenmiştir. Biz biyoalgılama uygulamaları için yeni bir tür yalıtkan-üzeri-silisyum (YÜS) kıvrımlı dağıtılmış geribildirim (KDG), antisimetrik KDG (AKDG), ile simetrik KDG (SKDG) yapılarını öneriyoruz. Bağdaşmak için taşıyıcı dalgakılavuzu gerektiren halka çınlaçlarının tersine, bu tektaş yapıları tasarlamak için bir enine elektrik (EE) kutuplu silisyum dalgakılavuzu kullanılmıştır. Bu tasarımların yapıtaşı kıvrımlı halka yansıtıcıdır (KHY). İzgesel kip ayrımı gösteren, KDG yapısını oluşturmak için 3 tane KHY ardışık olarak kullanılmıştır. AKDG yapısı için 4 tane içiçe geçmiş KHY kullanılmıştır. AKDG yapısı izgesinde, C = 0.09 bağlaşım katsayısı için izgesel kip ayrımı; 0.27 < C < 0.51 için elektromanyetik irkiltilmiş saydamlık (EIS)-benzeri tepeler görülmüştür. SKDG yapısı 5 tane içiçe geçmiş KHY kullanılmıştır. SKDG izgesi, C = 0.24 için kip ayrımlı Fano çınlaması; 0.78 < C < 0.94 için elektromanyetik irkiltilmiş saydamlık (EIS)-benzeri tepeler gösterir. SKDG yapısının Fano çizgibiçimli deneysel izgesi 26 dBm sönümleme oranı ve 368 dBm/nm eğim oranı gösterir. Bir çınlaç algılayıcının öz algılama sınırı λ/QS olarak gösterilebilir; burada λ boş uzay dalgaboyu, Q çınlacın nitelik katsayısı, S duyarlılıktır. KDG yapısının izgesel ayrılmış tepeleri ile, AKDG, ve SKDG yapılarının elektromanyetik irkiltilmiş saydamlık (EIS)-benzeri tepeleri Q = 50000 düzeyinde nitelik katsayısı gösterir; öyle ki 1550 nm dalgaboyundaki biyomolekül soğurması katılmadığında, λ = 1550 nm dalgaboyunda, S = 50 nm/RIU duyarlılık değerinde, silisyum fotonik yapılarımızın kuramsal öz sınırı 0.0006 [RIU] olarak bulunur. Var olan tümleşik fotonik devre yapılarına ek olarak, bizim sunduğumuz yeni tektaş yalıtkan-üzeri-silisyum (YÜS) fotonik yapılar biyoalgılama uygulamalarında ümit vermektedir.