Zeki Hayran

Numerical and experimental demonstration of a wavelength demultiplexer design by point-defect cavity coupled to a tapered photonic crystal waveguide

We propose and experimentally demonstrate a demultiplexer with point-defect resonators and a reflection feedback mechanism in a photonic crystal waveguide (PCW). A tapered PCW has been chosen as the necessary reflector, which enhances the drop efficiency. Due to the variation of the single-mode waveguide width of the tapered PCW, spatial alteration of the effective refractive index can be achieved. This phenomenon is used to reflect back the forward propagating wave which is then coupled again to the drop channels via the resonators. High transmission efficiency to the dropout channels is numerically predicted by calculations, either in two- and three-dimensional models, and analytically described by a coupled-mode theory. Moreover, an experimental realization in the microwave regime provides confirmation that the targeted wavelengths can be properly transmitted at the drop channels with low crosstalk and relatively high efficiencies.

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.

Manipulation of photonic nanojet using liquid crystals for elliptical and circular core-shell variations

We present theoretical studies on a tunable photonic nanojet (PNJ) created by adapting a shell and liquid crystal (LC) core architecture. The shell is made of indium tin oxide and the core is infiltrated with nematic LCs. The application of an external static electric field to the LCs modifies their refractive index, and this allows tuning the PNJ effect in the proposed system. In addition to nonresonant excitation, resonant PNJ excitation is also obtained from a hybrid structure. Both nonresonant and resonant internal field excitations of circular and elliptical PNJ configurations are examined by using a high-resolution finite-difference time-domain method. The calculated results indicate that the proposed PNJ configurations with tunable refractive indices lead to significant changes in some parameters such as decay length, focal distance, full width at half maximum and electric field intensity. Such PNJ designs can be employed in high-resolution optical sensors, optical trapping, and high-density data storage.

Focusing of light beyond the diffraction limit by randomly distributed graded index photonic medium

Sub-wavelength focusing of light holds great potential in various applications of science and engineering, including nanolithography, optical microscopy, optical measurements, and data storage. In the present paper, we propose a new concept to obtain sub-wavelength focusing of light by using structures composed of all-dielectric materials. The approach utilizes the design of an inhomogeneous refractive index profile with random distributions of individual elements occupying the unit cells of two-dimensional photonic crystals (PCs). Light focusing phenomenon is both systematically and quantitatively analyzed at different selected frequencies and we show that the randomly generated graded index (GRIN)-like photonic medium provides light focusing in air with a spot size below λ/3, where λ is the wavelength of light. The numerically obtained minimum spot size is equal to 0.260λ. Gaussian probability function is used to implement numerous random designs to investigate the optical characteristics of the photoni...

Target detection and ranging with the 2.4 GHz MIT Coffee Can radar

For the purpose of increasing the undergraduate student interest in and knowledge of radar, a 2.4 GHz FMCW MIT Coffee Can radar was built by 2nd and 3rd year undergraduates in the TOBB ETÜ Radar Systems Laboratory, and used for target detection and ranging. The radar was constructed both on a protoboard and on a printed circuit board (PCB). SPICE simulations were conducted to aid in debugging and demonstrate the proper functioning of the transmitter and receiver. The potential to discriminate slow-moving targets was assessed by measuring the micro-Doppler of walking and running people, low-speed vehicles, and bicyclists. The project was successful in increasing student interest in radar and signal processing. In the future, it is aimed to further develop the system at an RF circuit design level into a pulse Doppler radar.

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

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 noninvasive near-field probing applications, where conventional transformation optics based cloaks fail to provide the interaction of the wave with the object.

PT-vector fields

Non-Hermitian potentials, as known since a decade, can favor unidirectionality of the flows in one and two-dimensional systems. Inspired by such counterintuitive property of non-Hermitian potential, we propose a novel concept of PT-vector fields to manipulate the field flows in two- (or higher) dimensional systems. The idea is based on designing complex potentials favoring arbitrary vector fields of directionality 𝑝⃗(𝑟⃗) with desired shapes and topologies. To achieve this, we derive a new mathematical tool referred as local Hilbert transform. We study interesting cases of such vector fields in the form of sink, vortex, and circular channel, constructed from different background patterns using local Hilbert transform. This new concept provides a precise control over the dynamics of the probe fields, which may have potential applications in technological systems.

All-Dielectric Self-Cloaked Structures

A general procedure to design objects that are intrinsically invisible (without the necessity of an external cloak) has not been demonstrated so far. Here we propose a flexible method to design such self-cloaked objects by uncoupling the scattered waves from the incident radiation via judiciously manipulating the scattering potential of the object. We show that such a procedure is able to yield optical invisibility for any arbitrarily shaped object within any specified frequency bandwidth by simply employing isotropic nonmagnetic dielectric materials, without the usage of loss or gain material. The validity of the design principle has been verified by direct experimental observations of the spatial electric field profiles and scattering patterns at the microwave regime. Our self-cloaking strategy may have profound implications especially in noninvasive probing, cloaked sensor applications, and scattering-free non-Hermitian optics based systems.

Light localization and filtering | in three dimensional photonic structures

Three dimensional photonic structures specifically, woodpile photonic crystals, have great potential for manipulating light propagation such as localization and filtering. Efficient harvesting of the energy of the incident photons require spatially localized waves interacting strongly with the absorbing material. Meanwhile, one can also utilize similar concept in order to implement filtering of light via defining drop channels that are linked with the main waveguide. One unique property of the woodpile photonic crystals is the complete band gap feature that may reduce the out-of-plane losses. Besides, band gap width and edges can be tuned by chirping three-dimensional woodpile photonic crystals. In the present work, we show light localization via “rainbow trapping” concept and propose a drop-out mechanism based on the enhanced interaction between a defect waveguide and defect micro-cavities. Frequency resolved light detection/absorption and filtering capabilities are important in photodetector applications, optical communication, and solar energy.

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

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.