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

Abstract

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.

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

“…Similar multiple beam interference mechanism has been also observed in Ref. 59, where subwavelength focusing of light is achieved by applying a sophisticated optimization algorithm to the design of an alldielectric PC lens structure. As a result, the combination of the above-mentioned physical phenomena, enhancement in terms of focusing as well as steering of output beam can be achieved by using RDPCs.…”

“…The obtained spot sizes indicate that a RDPC structure can outperform the widely studied ordered GRIN PC medium, [51][52][53] where the lack of any disorder may prevent a sub-diffraction limited focus. Although we note that a well-optimized semi-disordered PC medium can focus light down to one fifth of its wavelength, 59 the fabrication of such precisely placed dielectric rods at the nano-or micro-scale still remains as an open task. On the other hand, the concept we propose may alleviate such fabrication challenges, due to the relaxed requirement of definite rod positions.…”

Machine Learning Inspired Design of Complex-Shaped GaN Subwavelength Grating Reflectors

“…Outof-bounds constraints were handled by resetting the out-of-bounds parameter(s) to the boundary [35], [36]. We note at this point that the differential evolution (DE) algorithm approach can be extended in principle to the design of other photonic structures such as thermophotovoltaic (TPV) gratings, light-focusing structures [37] and waveguides. Whenever there is a non-closed-form or non-linear mapping between input parameters and desired output characteristics, as is often the case in photonics design, there is potentially an opportunity to apply the DE algorithm.…”

Asymmetric light transmission effect based on an evolutionary optimized semi-Dirac cone dispersion photonic structure

“…It should also be noted that the idea of using an optimization approach to design a photonic structure attracts great attention [40,41]. To date, a variety of optimization algorithms has been proposed as a design approach to form unique optical devices based on PCs such as lenses [42], mode order converters [43], cavities [44], large photonic band-gap structures [45,46], optical couplers [47], and obtaining of a semi-Dirac-point [32].…”