Kadir Üstün

Study of different spectral regions and delay bandwidth relation in slow light photonic crystal waveguides

We investigate slow light propagation in monomode photonic crystal waveguides with different spectral features such as constant group index, high bandwidth and low group velocity dispersion. The form of the waveguide mode alters dramatically and spans three different spectral intervals by tuning the size of the boundary holes. Namely, slope of the band gap guided mode changes sign from negative to positive toward the Brillouin zone edge. In between there is a transition region where modes have nearly zero slopes. Maximum group index occurs at these turning points at the expense of high dispersion and narrow bandwidth. The apparent trade-off relationship between group index and bandwidth is revealed systematically. We show that as the radius of the innermost hole is increased above a certain value, the former one decreases and the latter one increases both exponentially but with a different ratio. The product of average group index and bandwidth is defined as a figure of merit which reaches up to a value of approximately 0.30 after a detailed parametric search. The findings of the frequency domain analysis obtained by plane wave expansion method are confirmed via finite-difference time-domain study.

Ultra slow light achievement in photonic crystals by merging coupled cavities with waveguides

We explore slow light behavior of a specially designed optical waveguide by carrying out structural dispersion using numerical techniques. The structure proposed is composed of square-lattice photonic crystal waveguide integrated with side-coupled cavities. We report three orders of magnitude reduction in group velocity at around υ(g) ≅ 0.0008c with strongly suppressed group velocity dispersion. The analysis is performed by using both plane-wave expansion and finite-difference time-domain methods. For the first time, we succeeded to show such a low group velocity in photonic structures. Slow light pulse propagation accompanied by light tunneling between each cavity is observed. These achievements show the feasibility of photonic devices to generate extremely large group index which in turn will eventually pave the way to new frontiers in nonlinear optics, optical buffers and low threshold lasers.

Slow light structure with enhanced delay–bandwidth product

In this study, we propose a special type of slow light photonic crystal (PC) waveguide structure to achieve slow light with an improved delay and bandwidth product (DBP). The waveguide is based on a triangular lattice PC with a line defect imposed by changing the radii and locations of the holes lying along the waveguide centerline. By altering the locations of these central holes, group indices ranging approximately from 25 to 40 are obtained over frequency intervals, attaining a nearly constant group index. It is also observed that the group index spectrum has an S-like shape under certain circumstances. The manipulations of structural parameters easily allow attaining higher or lower group indices. For special configurations, normalized DBPs can be enhanced up to a value of 0.554. According to the best of the authors’ knowledge, this value is the highest value achieved with PC waveguide structures, and this value is achieved without using any special optimization methods such as topology optimization. Group velocity dispersion values of various configurations are minimized to enable proper optical pulse propagation.

Highly efficient and broadband light transmission in 90° nanophotonic wire waveguide bends

Nanophotonic wire silicon waveguides are indispensable components of integrated photonic circuits. Because of the inherent nature of these waveguides, such as narrow width and high-index contrast, corners with large bending radii are inevitable for efficient light transmission with small loss values, which, in turn, impedes the miniaturization of photonic components. To alleviate huge bending losses of a right angle waveguide, we designed a structure incorporating a two-dimensional (2D) photonic crystal, along with careful engineering of the individual cell at the corner. The low transmission efficiency of around 55% can be increased to 99% by implementing 2D analysis. The implementation of the computationally heavy three-dimensional finite-difference time domain method, on the other hand, produces power transmission efficiencies of approximately 52% and 92% for a regular wire bend and optimized structure, respectively. The method asserts compact size and guarantees broadband operation, which, in turn, may assist the implementation of optical interconnects to distribute effectively optical clock signals through the chip.

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.

Slow light based on optical surface modes of two-dimensional photonic crystals

We investigate slow light properties of optical surface modes sustaining at the interface of two-dimensional photonic crystals and uniform medium (air). The manipulation of the structural parameters at the surface governs the modal field distribution of the surface state. The spectral and temporal behaviors of the slow mode are numerically explored by utilizing both plane-wave expansion and finite-difference time-domain methods. We show that the group index and bandwidth can be tuned within a wide range. The distortion free optical pulse propagation is supported by the presence of low group velocity dispersion behavior of the slow light surface mode photonic structure.

Wideband long wave infrared metamaterial absorbers based on silicon nitride

In this paper, we present silicon nitride metamaterial absorber designs that accomplish large bandwidth and high absorption in the long wave infrared (LWIR) region. These designs are based on the metal-insulator-metal topology, insulator (silicon nitride), and the top metal (aluminum) layers are optimized to obtain high absorptance values in large bandwidths, for three different silicon nitride based absorber structures. The absorption spectrum of the final design reaches absorptance values above 90% in the wavelength interval between 8.07 μm and 11.97 μm, and above 80% in the wavelength interval between 7.9 μm and 14 μm, in the case of normal incidence. The difficulty in the design process of such absorbers stems from the highly dispersive behavior of silicon nitride in the LWIR region. On the other hand, silicon nitride is a widely used material in microbolometers, and accomplishing wide band absorption in silicon nitride is crucial in this regard. Therefore, this study will pave the way for more effici...

Compact coupling of light from conventional photonic wire to slow light waveguides

In this study, efficient input and output power coupling schemes for transition regions at the interface of conventional and slow light waveguides are investigated. By optimizing the tapered nano-tip of the input and output slab waveguides that support a group index of 3.58, we achieved 97% coupling efficiency to a square-lattice based slow light photonic crystal waveguide with a group index of 1200. The complementary slow waveguide structure based on triangular-lattice is also designed to support same order of magnitude slow light mode and targeted to alleviate the severity of the coupling loss. An acceptable efficiency value is recorded for the second type of slow waveguide mode. For the sake of targeting only input and output coupling losses, we made an assumption that other loss mechanisms are absent in the structure. The successful demonstration of effective and compact slow light couplers will assist the deployment of slow light devices in important applications, such as nonlinear optics, optical bu...

Broadband LWIR and MWIR metamaterial absorbers with a simple design topology: almost perfect absorption and super-octave band operation in MWIR band

Infrared absorbers are essential structures in the design of thermal emitters and thermal infrared imagers. In this study, we propose simple topologies of wideband metamaterial absorbers operating in the long-wave infrared or in the mid-wave infrared (MWIR) wavelengths of the electromagnetic spectrum where the atmosphere shows transparent behavior. Suggested metamaterial absorbers are mostly thin structures that consist of three functional layers from top to bottom: a periodically patterned metal layer, a planar dielectric layer, and a ground metal layer. The pattern of the top metal layer is four-fold symmetric to guarantee the polarization insensitivity of the absorber under normal incidence of light. In addition, a geometrically simple metamaterial pattern is preferred to facilitate the process of lithography. As titanium is known to be a high-loss metal, it is deliberately used at the top layer of the absorber to increase the overall absorption bandwidth. Highly satisfactory absorber results, such as almost perfect absorption and super-octave band operation are demonstrated, especially in the MWIR region. As oxidation of the top titanium layer may cause performance degradation in long-term use, a design modification is also suggested where a very thin protective coating layer is applied over the titanium metasurface.

Ultra-broadband long-wavelength infrared metamaterial absorber based on a double-layer metasurface structure

In this paper, we report a metamaterial absorber design that achieves a broad absorption band encompassing the whole long-wavelength infrared (LWIR) region. The structure consists of two parallel metasurfaces buried in an amorphous silicon dielectric layer, where the minimum size for all possible planar details does not go below 1 μm, making the use of standard optical lithography possible for fabrication. The dielectric layer of the structure is placed over a metallic ground plane that inhibits the transmission of incident waves. A substrate underneath the ground plane may also be needed for the purpose of mechanical support. This structure achieves a minimum absorptivity of about 90% in almost the full LWIR band in the case of normal incidence in a polarization-insensitive manner due to the four-fold symmetry of the structural geometry. The absorber also shows reduced sensitivity to off-normal incidence angles, satisfying a minimum absorption level of approximately 80% up to the incidence angle of 45 deg. This broadband metamaterial absorber design is anticipated to find applications in thermal emitters/coolers and in thermal infrared sensors.