Emre Bor

Differential evolution algorithm based photonic structure design: numerical and experimental verification of subwavelength λ/5 focusing of light.

Photonic structure designs based on optimization algorithms provide superior properties compared to those using intuition-based approaches. In the present study, we numerically and experimentally demonstrate subwavelength focusing of light using wavelength scale absorption-free dielectric scattering objects embedded in an air background. An optimization algorithm based on differential evolution integrated into the finite-difference time-domain method was applied to determine the locations of each circular dielectric object with a constant radius and refractive index. The multiobjective cost function defined inside the algorithm ensures strong focusing of light with low intensity side lobes. The temporal and spectral responses of the designed compact photonic structure provided a beam spot size in air with a full width at half maximum value of 0.19λ, where λ is the wavelength of light. The experiments were carried out in the microwave region to verify numerical findings, and very good agreement between the two approaches was found. The subwavelength light focusing is associated with a strong interference effect due to nonuniformly arranged scatterers and an irregular index gradient. Improving the focusing capability of optical elements by surpassing the diffraction limit of light is of paramount importance in optical imaging, lithography, data storage, and strong light-matter interaction.

T-shape slotted photonic crystal based sensor with high sensitivity

In this paper, we propose a novel photonic crystal sensor based on refractive index sensing method. The proposed photonic crystal sensor design has a T-shaped sensing slot region to inject bio-material samples with different refractive index values between 1.00 and 1.35. The corresponding highest refractive index unit (RIU) sensitivity of the designed photonic crystal is numerically calculated as 1016 nm/RIU. The designed photonic crystal sensor can be especially utilized for analysing different gas contaminations and diabetic blood samples in the healthcare sector.

Photonic crystal sub-wavelength λ/5 focusing lens design using optimization method

In this paper, differential evolution (DE) optimization algorithm is applied to design sub-wavelength focusing PCs structure for the first time to the best of our knowledge. For this purpose, the transverse cross section of the input beam is spatially shaped by optimized PC structure. Strong focusing of beam with FWHM equal to λ/5 and with suppressed magnitude of side lobe intensities is achieved. In addition, microwave experiments to verify the numerical results are conducted. The numerical and experimental results given in the paper are in good agreement with each other.

Polarization independent focusing of light by gradually modulated annular photonic structure

In this manuscript, we propose the design of an inhomogeneous artificially modeled graded photonic crystal (PC) medium to control and enrich the polarization insensitive focusing ability of light by using annular type all-dielectric materials. To obtain the graded annular PC structure, the lattice spacing along the transverse direction is modulated according to a parabolic function. The inner and outer radii of the annular PCs are optimized to attain frequency response of the graded structure. Even though the frequency responses of the transmission spectrum for different radial parameters exhibit different oscillation patterns for both TE and TM polarizations, optimum radii are determined where overlapping high transmission region is observed. Light focusing characteristics at those overlapping frequency regions are systematically and quantitatively investigated. Polarization insensitive focusing is achieved for bandwidth corresponding to ~6%. The designed structure can be used in integrated nanophotonics as a compact optical element with flat surfaces operating at both polarizations with high power transmission efficiency.