B. B. Oner

Efficient mode converter design using asymmetric graded index photonic structures

Asymmetric graded-index (GRIN) inhomogeneous medium is proposed to enable even-to-odd mode conversion. An exponential refractive index profile is implemented to induce an asymmetric refractive index distribution. A two-dimensional photonic crystal structure composed of constant dielectric radii with rectangular unit cells is deployed. Efficient mode transformation in a compact structure may promote the manipulation of light for the creation of other types of higher-order modes in asymmetric GRIN structures.

High-efficiency beam bending using graded photonic crystals.

We explore beam-bending properties of graded index (GRIN) waveguide with hyperbolic secant profile. The transmission efficiency and bandwidth features are extracted for GRIN photonic crystal (PC) media composed of dielectric rods. Light guiding performance of the GRIN PC medium is analyzed for 90° and 180° waveguide bends. The finite-difference time-domain method is deployed to investigate the performance of the designed GRIN waveguides. By the help of proposed photonic configuration, bending of light is achieved with a high efficiency within a broad bandwidth, which promotes the use of GRIN PC structures for efficient light-bending purposes.

Modified Maxwell fish-eye approach for efficient coupler design by graded photonic crystals.

We present a novel design of two dimensional graded index medium that provides coupling of light with high coupling efficiency between two planar dielectric waveguides of different widths (15.46 μm vs. 2.21 μm). Poor light coupling performance of butt-coupler can be mitigated by implementing tapered coupler at the expense of long coupler section. In order to reduce coupling losses, a new coupling device approach based on graded index (GRIN) concept is proposed. The refractive index distribution is in the form of modified version of the Maxwell fish-eye lens. The inhomogeneous refractive index distribution is approximated by photonic crystals (PCs) such that the positions of each PC rods are appropriately arranged. Strong electric field focusing ability of the designed GRIN PC medium provides relatively high coupling efficiency that is around 90%. Spectral region corresponding to coupling efficiency over 75% has a bandwidth of Δω = 18.56% (284 nm). Finally, we discuss the durability of the proposed coupler against the lateral displacement and angular misalignment of output waveguides.

Mode transformation using graded photonic crystals with axial asymmetry

We propose a mode conversion method that enables transformation of the propagating mode from fundamental to higher-order modes by utilizing asymmetric graded index (A-GRIN) structures. Refractive index variations of two different asymmetric gradient profiles, i.e., exponential and Luneburg lens profiles, have been approximated by two-dimensional photonic crystals (PCs). The basic structure is composed of constant radii with different lattice sizes. The designed GRIN mode converters provide relatively high transmission efficiency in the spectral region of interest and achieve the transformation in compact configuration. Numerical approaches utilizing the finite-difference time-domain and plane wave expansion methods are used to analyze the mode conversion phenomenon of proposed GRIN PC media. Analytical formulation based on ray theory is outlined to explore both ray trajectories and the physical concept of a wavefront retardation mechanism.

Large bandwidth mode order converter by differential waveguides

In this article, we propose a large bandwidth mode-order converter design by dielectric waveguides with equal lengths but different cross-sectional areas. The efficient conversion between even and odd modes is verified by inducing required phase difference between the equal length waveguides of different widths. Y-junctions are composed of both tapered mode splitter and combiner to connect mono-mode waveguide to multi-mode waveguide. The converted mode profiles at the output port show that the device operates successfully at designed wavelengths with wide bandwidth. This study provides a novel technique to implement compact mode order converters and direction selective/sensitive photonic structures.

Dual polarized broadband and all dielectric partial cloaking using stacked graded index structures

We propose broadband one-dimensional optical cloaking design based on isotropic and purely dielectric non-absorbent materials. The photonic structures are formed by utilizing graded index (GRIN) concept in stacked form. All simulations are performed by finite-difference time-domain and plane wave basis frequency domain numerical methods. Indications in ray optics are also presented for the cloaking device. The refractive index distribution of the design is also obtained via effective medium theory. The cloaking devices can reroute wavelengths of light in one dimension. The rerouted light is avoided to reach the interior region of the stacked GRIN structure. Unidirectional GRIN cloaking structure demonstrates low-loss and large bandwidth characteristics. It is shown that the structure operates in dual polarization mode. Performed numerical analyses reveal the capability of cloaking devices to hide arbitrary shaped large objects from the incident light.

Polarization-Independent Photonic Crystal Fabry–Perot Cavity

We present the design steps for 3D polarization-independent Fabry-Perot cavity. Inclusion of isolated dielectric rods into the limited number of holes at around the central region enables the generation of cavity that sustains both transverse electric and transverse magnetic modes. Localization of light is obtained at the same frequency for both polarizations with moderately high quality factors. Polarization-independent nanobeam cavity is expected to be a platform for studying strong light matter interaction at the wavelength scales.

Polarization independent nanobeam cavity tuning using annular photonic crystals

Nanobeam cavity waveguides have drawn great attention of the researchers due to being a useful optical platform for several applications, e. g. optical switching and filtering.1 Almost all of the past studies investigated high quality (Q) factors without considering polarization independency. In the literature Zhang et al. proposed a device that enables high Q for both transverse electric (TE) and transverse magnetic (TM) modes for a specific frequency.2 In our study we demonstrate a three-dimensional study of polarization independent nanobeam cavity waveguide that consists of annular photonic crystals (PCs) showing similar optical properties for both TE and TM modes.3 Besides, a detailed analysis of the shift of the overlapped frequency is investigated with respect to height and width variation of the nanobeam structure. The designed waveguide is composed of 12 air holes and 4 annular PCs located in the Silicon (nSi=3.46). The radii of all air holes in the structure are 0.36a. The annular PCs at the interior section have inner dielectric radii of 0.18a and the outer ones have inner dielectric radii of 0.20a. Silica (nSilica=1.52) material is used as a substrate. The width and height of the waveguide may be tuned in order to obtain high Q factors at the desired frequency for both polarizations. In our analysis, we investigated the relation between widths, height and cavity frequency for both TE and TM cases. Obtained frequencies are fitted to cubic polynomials of the structural parameters width and height. Overlapped frequency curve is revealed by an equalization of the polynomials of TE and TM resonant frequencies. The findings elucidate the effect of the parameters on the overlap mechanism of resonant mode matching for both polarizations.

Broadband directional cloaking using graded index structures

We propose broadband two-dimensional cloaking designs based on isotropic and all dielectric lossless materials. The photonic structures are created by implementing artificially generated graded index (GRIN) media. The desired refractive index distribution is achieved with effective medium theory. Directional graded index cloaking structure shows unique characteristics such as low-loss, large bandwidth and scalable to other frequency regions in the electromagnetic spectrum. We numerically demonstrate feasibility of hiding arbitrary shaped large objects from the incident light. The unique features of GRIN medium tailoring the light propagation allow designing advanced configurations that enable cloaking under the oblique incidence cases.

Partial cloaking by graded index photonic crystals

Since the first proposal of the idea of optical cloaking, huge research effort has been spent to implement hiding objects. We propose a broad band all-dielectric partial (unidirectional) cloaking device that hides arbitrary shaped objects. The cloaking structure is designed utilizing graded index (GRIN) photonic crystals. Refractive index distribution of the structure is chosen as a hyperbolic secant profile. In order to generate desired index profiles, both low and high dielectric backgrounds are chosen. The main principle of the cloaking in the study is separating the beam into two main parts while propagating through the composite device. Each part of the separated beam is strongly focused at the center of the stacked GRIN devices. Then these beams diverge and converge repeatedly without deteriorating the planar input field profile. This mechanism dramatically reduces the intensity at the center of the device. Therefore, existence of an object at the cloaked region almost does not affect wave front of the exiting beam due to this special light manipulation mechanism. In this manner, an observer cannot detect the hidden object. GRIN medium is a special type of inhomogeneous environment and light propagation is greatly affected by the presence of GRIN. Any partial cloaking solution as long as being practical and broadband in nature can be preferred. In this case, material selection and easy transferring the design to other electromagnetic spectrum regions become crucial. Therefore, the proposed idea in this work collects these desirable features.