Emre Bor

In this study, we present the numerical design and experimental demonstration of an all-dielectric low refractive index polarization-insensitive multilevel diffractive lens (MDL) at microwave frequencies. The proposed MDL structure is composed of concentric rings (zones) having different widths and heights. Here, the heights and widths of each dielectric concentric rings of lens structure are optimized by using the differential evolution (DE) algorithm to obtain the desired polarization-insensitive focusing performance. The DE method is incorporated with the three-dimensional finite-difference time-domain method to design an MDL structure and evaluate its wave focusing ability. The design frequency is fixed to 10 GHz and, at the design frequency, the DE method is applied to achieve light focusing with the full-width at half-maximum (FWHM) values of 0.654λ and 0.731λ for transverse-magnetic (TM) and transverse-electric (TE) polarizations, respectively, where λ is the wavelength of incident light in free space. Moreover, focusing efficiencies and numerical apertures are calculated as 60.3% and 0.853 at the design frequency, respectively, for both polarizations. Besides, experimental verifications of the numerical results are carried out in microwave regime where the MDL design is fabricated by 3D printing technology by using a polylactic acid material. In the microwave experiments, MDL focuses the TM and TE polarized waves at the focal distances of 71.82 mm and 69.3 mm with the FWHM values of 0.701λ and 0.887λ, respectively. We believe that the proposed design approach can be further expanded to design low refractive index lenses for visible and near-infrared wavelengths.

In this Letter, we numerically and experimentally demonstrate the carpet cloaking effect by a rectangular lattice two-dimensional photonic crystal (PC) exhibiting a semi-Dirac cone (SDC) dispersion phenomenon. The proposed SDC PC with an anisotropic zero refractive index medium operates as an optical carpet cloak for a perfect electric conductor surface bump. The experimental verification of the cloak is realized at microwave frequencies at around 12.1 GHz via dielectric rods. A good agreement between experimental measurements and numerical calculations is observed. Finally, features such as rendering larger objects invisible are possible with the proposed idea.

In this paper, we present the numerical and experimental demonstration of a wavelength demultiplexer (WDM) based on the photonic crystal (PC), in which the waveguide has a tapered width. Owing to the tapered waveguide, propagating light can be slowed down and be trapped by a local mode gap effect at certain distances from the entrance of the waveguide. The corresponding effect leads to the localization of four different wavelengths at different points inside the waveguide. The drop-channels are introduced at these specified locations to separate selected wavelengths. Here, we utilized an optimization algorithm to enhance the coupling efficiencies of the introduced drop-channels. The presented WDM PC separates the wavelengths of 22.29, 21.63, 20.80, and 19.87 mm (13.46, 13.87, 14.42, and 15.10 GHz, respectively) into different drop-channels with coupling efficiencies at around 80%. Experimental verifications of the numerically presented results are realized at the microwave frequency regime where the coupling efficiencies of each drop-channel are measured as around 75%. The designed WDM PC structure is all dielectric, compact, and efficient, and it exhibits low cross talk between drop-channels. Experimental measurements show a rainbow-trapping phenomenon and verify the simulation results of wavelength demultiplexing design with the margin of error between 0.8% and 1% frequency shifts in peak transmission values.

In the presented study, we proposed a design approach based on optimization algorithm to obtain mode order conversion in photonic crystal (PC) structures and experimentally verify the numerical result of the corresponding approach. The finite-difference time-domain method is integrated with an evolutionary algorithm to develop the introduced design method. Here, we designed two different mode order converter PCs with ultra-compact sizes that transform a fundamental mode to a higher order mode. The first designed structure realizes a mode order conversion in free-space host medium, and the second structure performs the same operation in a PC waveguide with optimized region. Also, asymmetric light transmission effect in PC waveguide structure is investigated. Additionally, we performed microwave experiments to confirm the numerical results of mode order conversion. In conclusion, we demonstrated a novel design approach to obtain mode order conversion, which can be feasible to design diverse photonic structures in the future applications of nanophotonics.

In this study, we propose the metaheuristic design of a polarization-insensitive diffractive lens. The designed lens structure consists of concentric dielectric rings with optimally modulated widths and heights. Here, the evolutionary optimization approach is conducted to find the optimum values of concentric rings’ widths and heights to obtain polarization-insensitive subwavelength focusing effect of the incident light. Three-dimensional finite-difference time-domain (3D FDTD) method is incorporated with the optimization algorithm to evaluate the focusing ability of instantly designed lens structures throughout the optimization process. The presented lens structure is designed at operating frequency of 10 GHz, and has a diameter of 18.45 cm with thickness of 2.4 cm. The detailed analysis of the polarization-insensitive focusing effect of designed diffractive flat lens is numerically investigated at microwave frequency regime considering the scenario of experimental verification to be conducted at the same spectral region. Possible experimental verifications are discussed along with the 3D-printing of the designed lens structure. As a further step, the presented design concept may be applied to design metasurfaces operating at visible wavelengths.

In this study, we propose the design of photonic crystals with linearly tapered waveguide such that waveguide width is gradually varied. Four drop-channels are included to implement wavelength demultiplexing applications. Here, a tapering of the waveguide has been chosen to localize propagating light within the desired positions. Afterwards, the corresponding drop-channels have been opened to confine the light with the targeted wavelengths. The designed structure consists of cylindrical alumina (Al 2 O 3 ) dielectric rods and operates at microwave frequencies between 13.2 GHz and 15.3 GHz. An optimization algorithm is applied to enhance transmission efficiencies of the drop-channels and to minimize possible cross-talks between the channels by optimally modulating the position of the dielectric rods inside the channels which form cavity-coupling regions. The optimization algorithm is incorporated with the finite-difference time-domain method to evaluate the transmission efficiencies of the drop-channels for the instant designed structure. The physical mechanism of wavelength demultiplexing is related to slowing down and trapping of light in the tapered waveguide and its coupling to drop-channels for selected microwave wavelengths at different positions. Moreover, an experimental verification of the numerical analyses is demonstrated in the microwave regime and corresponding results will be shared in the conference.

In this paper, we propose a graded index (GRIN) medium to obtain subwavelength focusing of light in high and low normalized frequency regimes by using all-dielectric materials. Distributed profile of hyperbolic secant GRIN structure is approximated by using two-dimensional (2D) photonics crystals (PCs). The focusing of light is quantitatively and systematically analyzed at low frequencies. It is clearly shown that the efficiency of focusing phenomenon changes as in terms of full-width at half-maximum (FHWM) and focal point distances according to structure length and other important parameters. Light focusing at subwavelength dimension has the capability of achieving unique contributions to lasing, optical imaging and microscopy, nanolithography, data storage and optical precision measurements. Moreover, an experimental verification of the numerical analyses is demonstrated in the microwave regime and corresponding results will be shared in the conference.

In this study, we propose the design of a one-dimensional photonic crystal rectangular hole based nanobeam cavity structure for refractive index sensing. The designed sensor structure consists of periodic air slots on a thin substrate which is made of a dielectric thermoplastic material. Here, three-dimensional finite-difference time-domain method is used to evaluate resonator and refractive index sensing characteristic of the designed sensor structure. The light localization in the cavity region is numerically investigated for various refractive index values at microwave wavelengths. The sensitivity of 5.739 mm/RIU is calculated for a liquid analyte with refractive index value at around 1.33. The possible experimental validation of numerical results can be demonstrated at microwave wavelengths by 3D-printing the designed sensor structure.

In this study, we present the design of ultra-compact and highly efficient photonic integrated devices by applying a meta-heuristic approach. Here, we integrated the three-dimensional finite-difference time-domain method into an evolutionary optimization algorithm to specifically design waveguide crossing, optical attenuator and reflector. The proposed devices have ultra-compact footprints of 2×2 μm ² with slab thickness of 220 nm and consist of 100×100 nm ² silicon or air cells on a SiO 2 substrate. We demonstrate an ultra-compact waveguide cross on silicon photonic platform with transmission efficiency greater than 80% and with a negligible crosstalk. Also designed attenuator and reflector devices are providing 3 dB signal reduction and over 85% reflectivity, respectively. All of the devices are excited by a fundamental transverse-electric mode guided in a silicon waveguide with a width of 500 nm. Throughout the optimization process, fabrication constraints are taken into account to enable the realization of the designed devices in applications. The introduced design method can be further expanded to form either diverse photonic integrated devices or even plasmonic devices.

In this study, we propose and experimentally demonstrate spatial filtering of light by the photonic wavy structures designed to be operated in a Bragg regime. The proposed configuration is designed, optimized and analyzed by 3D finite-difference time-domain (FDTD) method. Both incidence angle variation and wavelength dependency are examined in detail. The multilayered photonic structure with preselected structural parameters is fabricated by physical vapour deposition on the microstructured substrate. The numerical results as well as experimental measurements of the angle/wavelength transmission of the proposed structure indicate the evidence of the angular filtering based on the selective diffraction of light.