Beyond the Rozanov Bound on Electromagnetic Absorption via Periodic Temporal Modulations

Incorporating time-varying elements into electromagnetic systems has shown to be a powerful approach to challenge well-established performance limits, for example bounds on absorption and impedance matching. So far, the majority of these studies have concentrated on time-switched systems, where the material undergoes instantaneous modulation in time while the input field is entirely contained within it. This approach, however, necessitates accurate timing of the switching event and limits how thin the system can ultimately be due to the spatial width of the impinging pulse. To address these challenges, here we investigate the periodic temporal modulation of highly lossy materials, focusing on their relatively unexplored parametric absorption aspects. Our results reveal that, by appropriately selecting the modulation parameters, the absorption performance of a periodically modulated absorber can be greatly improved compared to its time-invariant counterpart, and can even exceed the theoretical bound for conventional electromagnetic absorbers, namely, the "Rozanov bound". Our findings thus demonstrate the potential of periodic temporal modulations to enable significant improvements in absorber performance while circumventing the limitations imposed by precise timing and material thickness in time-switched schemes, opening up new opportunities for the design and optimization of advanced electromagnetic absorber systems for various applications.Comment: 12 pages, 4 figure

Polarization-independent broadband bidirectional optical cloaking using a new type of inverse scattering approach

(c) 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.Since the advent of transformation optics a decade ago [1], the ability to achieve optical cloaking has become a matter of practical realization. However, so far extreme material requirements and large device areas have significantly posed an obstacle to realize compact cloaking schemes that are fully functional. Here, by taking a different approach and by following our recently developed general theorem to control the scattering behaviour of an arbitrary object on a specific demand [2], we show that nearly perfect bidirectional optical cloaking effect can be generated for any type of object with a given shape and size. Contrary to previous approaches, we reveal that such a method is always able to produce local refractive indices larger than one and that neither gain nor lossy materials are required. Furthermore, by means of numerical calculations, we demonstrate a highly tunable broad operational bandwidth of 550 nm (covering 650-1200 nm interval) and an angular aperture of 36° for both directions and polarizations. With these unprecedented features, we expect that the present work will hold a great potential to enable a new class of optical cloaking structures that will find applications particularly in communication systems, defence industry and in other related fields.Peer ReviewedPostprint (author's final draft

Slow light enabled wavelength demultiplexing

Photonic crystal waveguides supporting band gap guided modes hold great potential to tailor the group velocity of propagating light. We propose and explore different wavelength demultiplexer design approaches that utilize slow light concept. By altering the dielectric filling factors of each waveguide segment, one can show that different frequencies can be separated and extracted at different locations along the cascaded waveguide. Furthermore, to eliminate the inherent reflection loss of such a design, a composite structure involving a tapered waveguide with a side-coupled resonator is also presented. Such a structure features not only a forward propagating wave but also a backward propagating wave acting as a feedback mechanism for the drop channels. We show that by careful design of the waveguide and the resonator, the destructive and instructive interference of these waves can effectively eliminate the reflection loss and increase the coupling efficiency, respectively. Numerical and experimental verification of the proposed structures show that the targeted frequencies can be coupled out with low cross-talks and moderate quality factors, while maintaining a compact size. © 2016 IEEE.Peer ReviewedPostprint (published version

Enhancement of infrared absorption and sensing in two- and three-dimensional photonic media

Işığın geniş bir tayfsal aralıkta verimli bir şekilde sezilmesi, görüntülemeden haberleşmeye kadar çok geniş bir yelpazede kilit rol oynamaktadır. İyi bir sezim gerçekleştirilebilmesi, ortamdaki ışığın verimli bir şekilde soğurulmasına bağlıdır. Kızılötesi gibi çok zengin uygulamalar barındıran frekanslarda tipik soğurucuların (örneğin grafen) soğurma verimlilikleri düşüktür. Dahası, bu tip soğurucuların frekans seçicilikleri zayıftır. Diğer bir deyişle, soğurucu malzeme sezilen ışığın şiddetini algılayabiliyor iken, ışığın tayfsal içeriğini tespit edememektedir. Bu durum frekansa duyarlı ve yüksek soğuruma sahip pratik kızılötesi detektörlerin ihtiyacını doğurmaktadır. Diğer taraftan yakın-alan görüntülemede kullanılan optik detektörlerin, kırınım kısıtından ötürü ölçümü çok yakın bir mesafede gerçekleştirmeleri gerekmektedir. Ancak ölçümün çok yakın bir mesafede gerçekleşmesi, ölçüm alınan bölgedeki elektromanyetik dalgayı bozulmaya uğratabilmektedir. Sonuçta elde edilen ölçümde detay kaybı meydana gelebilmektedir. Bu tez çalışmasında, ışığın fotonik yapılarda yerelleştirilmesi ve verimli bir şekilde sezilmesi konusu incelenmiştir. Tezin ilk bölümünde ışığın yerelleştirilmesi amacıyla özgün fotonik mimariler geliştirilmiştir. Geliştirilen fotonik yapılarda ışığın yerelleştirilmesi yavaş ışık konsepti veya Hermitian-olmayan ortamlar sayesinde gerçekleştirilmiştir. Kullanılan yavaş ışık konsepti sayesinde ortamdaki ışığın Fourier bileşenlerine ayrılması ve tuzaklanması sağlanmıştır. Bu sayede yüksek verimlilikte frekansa duyarlı soğurum elde edilebildiği gösterilmiştir. Bir diğer fotonik aygıtta ise, yerelleşen dalganın yavaş ışık konsepti sayesinde frekans seçici kusurlar tarafından yüksek iletim verimliliği ve kalite faktörü ile sezilebildiği gösterilmiştir. Ayrıca Hermitian-olmayan ortamlarda dengeli bir şekilde yerleştirilmiş kazanç ve kayıp bölgelerinin dalga yerelleşmesine neden olduğu gösterilmiştir. Tezin ikinci bölümünde, sensör görünmezliği üzerine özgün yöntemler sunulmuştur. Önerilen yöntemler ile herhangi bir şekle sahip nesnelerin (örneğin sensor uçlarının) görünmezliği sağlanmıştır. Görünmezlik, nesnelerin saçılım potansiyellerinin doğrudan manipüle edilmesiyle elde edilmiştir. Bu sayede saçılımlar, gelen dalga vektöründen çözülmüş olduğundan nesne etkin bir şekilde görünmez kılınmıştır. Kuramsal olarak elde edilen görünmez nesneler, mikrodalga frekanslarında deneysel olarak gerçeklenmiştir. Bu bağlamda, kararlı hal elektrik alan dağılımları ve açısal saçılım örgüleri ölçümleri yapılarak, önerilen yöntemler deneysel olarak doğrulanmıştır. Önerilen yöntemler, sensör görünmezliğinden anten kaportalarına kadar geniş bir yelpazede uygulama alanı bulabilir.The detection of light over a broad spectrum plays a key role in a wide variety of fields ranging from imaging to communication. The ability to detect light efficiently depends on the level of absorption of the available light. At the infrared regime, where potential optical applications are abundant, the intrinsic absorption efficiency of typical absorbers (e.g. graphene) are usually low. Moreover, such absorbers have a low frequency selectivity. In other words, while the absorber is able to detect the amplitude of light, it can not sense the spectral content of it. Hence, there is a need for highly efficient frequency-selective infrared detectors. On the other hand, near-field optical detectors are required to make measurements very near the object due to the diffraction limit. However, due to the closeness of the object and the detector, the field along the object can get distorted, resulting in detrimental effects of image quality. In this thesis, the phenomenon of light trapping and sensing in engineered photonic structures has been inverstigated. In the first part of this thesis, novel photonic architectures have been proposed to achieve light trapping. The trapping of light in such structures has been achieved via the concept of slow light and non-Hermitian optics. Owing to the slow light based design principle, it has been shown that light can be trapped and be separated into its Fourier components. In result, highly efficient multicolor infrared absorption has been achieved. In another propsed photonic device, it has been shown that owing to the slow light phenomena, the trapped light can be selectively filtered with high transmission efficiency and quality factor via frequency sensitive cavities. Furthermore, it has been shown that in non-Hermitian optical media having balanced gain and loss regions, that the incoming light can be localized at predefined locations. In the second part of this thesis, various novel invisibility methods have been propsed to achieve sensor invisibility. By means of the proposed methods, the invisibility of arbitrary shaped objects (e.g. tip of a sensor) have been achieved. The proposed invisibility technique is based on the judicious tailoring of the scattering potential of a given object. Owing to such a tailoring, it has been shown that certain scattered waves can be uncoupled from the incident radiation, leading to invisibility effects. The theoretically analyzed cloaks have been experimentally realized at the microwave regime. In this regard, by measuring the steady-state electric field profiles and angular scattering patterns, the proposed methods have been experimentally verified. The proposed methods may find useful applications especially in cloaked sensor and antenna radome applications

Rainbow trapping in a chirped three-dimensional photonic crystal

Light localization and intensity enhancement in a woodpile layer-by-layer photonic crystal, whose interlayer distance along the light propagation direction is gradually varied, has been theoretically predicted and experimentally demonstrated. The phenomenon is shown to be related to the progressive slowing down and stopping of the incident wave, as a result of the gradual variation of the local dispersion. The light localization is chromatically resolved, since every frequency component is stopped and reflected back at different positions along the crystal. It has been further discussed that the peculiar relation between the stopping position and the wave vector distribution can substantially increase the enhancement factor to more than two orders of magnitude. Compared to previously reported one- and two-dimensional photonic crystal configurations, the proposed scheme has the advantage of reducing the propagation losses by providing a three-dimensional photonic bandgap confinement in all directions. The slowing down and localization of waves inside photonic media can be exploited in optics and generally in wave dynamics, in many applications that require enhanced interaction of light and matter.Peer Reviewe

Polarization independent high transmission large numerical aperture laser beam focusing and deflection by dielectric Huygens' metasurfaces

In this letter, we propose all-dielectric Huygens' metasurface structures to construct high numerical aperture flat lenses and beam deflecting devices. The designed metasurface consists of two-dimensional array of all dielectric nanodisk resonators with spatially varying radii, thereby introducing judiciously designed phase shift to the propagating light. Owing to the overlap of Mie-type magnetic and electric resonances, high transmission was achieved with rigorous design analysis. The designed flat lenses have numerical aperture value of 0.85 and transmission values around 80%. It also offers easy fabrication and compatibility with available semiconductor technology. This spectrally and physically scalable, versatile design could implement efficient wavefront manipulation or beam shaping for high power laser beams, as well as various optical microscopy applications without requiring plasmonic structures that are susceptible to ohmic loss of metals and sensitive to the polarization of light. (C) 2017 Elsevier B.V. All rights reserved.H.K. gratefully acknowledges the partial support of the Turkish Academy of Sciences

Experimental demonstration of broadband perfect invisibility cloak composed of all-dielectric materials

Photonic and Phononic Properties of Engineered Nanostructures VIII (2018 ; San Francisco, United States)With the development of various recent tools to control electromagnetic wave propagation, such as transformation optics, the long-sought dream of rendering objects invisible has become a matter of practical implementation. However, the required index profile derived with such techniques leads to material properties that are not readily available in nature and, hence, various experimental simplifications and performance scarifications are inevitable. Therefore, it has been a widespread belief that perfect cloaking cannot be achieved with conventional materials. Here, we follow a different direction and provide a unique method based on scattering cancellation rather than conventional coordinate transformations, and show that perfect invisibility can be indeed achieved for any specified angular range and operational bandwidth by employing merely all-dielectric materials. The presented method is based on our recently proposed generalized Hilbert-like transform [1] that is able to eliminate the undesired scattered waves for any type of object, regardless of its shape/size, by directly tailoring the object's scattering potential. In this direction, we show that the impinging wave on an object can be perfectly restored owing to the effective cancellation of the scattered waves emanating from the object and the surrounding index profile. We demonstrate this effect by experimental analyses conducted at the gigahertz regime. The proposed method represents an important step towards the ultimate goal of cloaking arbitrarily large objects at various wavelength regimes and may have profound implications especially in non-invasive near-field probing applications, where conventional transformation optics based cloaks fail to provide the interaction of the wave with the object