Super-Oscillatory Metalens at Terahertz for Enhanced Focusing with Reduced Side Lobes

Abstract

In this paper, we design and numerically demonstrate an ultra-thin super-oscillatory metalens with a resolution below the diffraction limit. The zones of the lens are implemented using metasurface concepts with hexagonal unit cells. This way, the transparency and, hence, efficiency is optimized, compared to the conventional transparent–opaque zoning approach that introduces, inevitably, a high reflection in the opaque regions. Furthermore, a novel two-step optimization technique, based on evolutionary algorithms, is developed to reduce the side lobes and boost the intensity at the focus. After the design process, we demonstrate that the metalens is able to generate a focal spot of 0.46λ0 (1.4 times below the resolution limit) at the design focal length of 10λ0 with reduced side lobes (the side lobe level being approximately −11 dB). The metalens is optimized at 0.327 THz, and has been validated with numerical simulations.

Highly Efficient Focusing of Terahertz Waves with an Ultrathin Superoscillatory Metalens: Experimental Demonstration

“…In this context, SOLs arise as an interesting and powerful alternative, as they can focus light into a subwavelength hotspot located further than 10 times the wavelength in free space. [ 37–42 ]…”

“…The lens analyzed in this study is based on the design theory and methods presented in a previous article published by some of the authors of this work, [ 38 ] and has been conceived to overcome the challenge of balancing the trade‐off between the different parameters involved. Thus, here we demonstrate experimentally a metaSOL designed and operating in the lower frequency band of the THz spectrum (≈300 GHz) to 1) enhance the transmission efficiency above 70%, improving the 50% transmission efficiency achieved with the typical transparent and opaque ring masks configuration [ 43–46 ] and, all the more, the 5–10% transmission efficiency of other SOL design methods such as hole arrays [ 47 ] and superlenses [ 48 ] ; 2) enhance the focusing efficiency, also known as the yield parameter, defined as the ratio of the energy located in the hotspot to the full focal plane (see Supporting Information for a more detailed explanation of this parameter).…”

Terahertz smart dynamic and active functional electromagnetic metasurfaces and their applications

“…It demonstrates the metasurface effectively split the normal and incident x-and y-polarized components at 0.32-0.42 THz in different planes. There are also various reports on functions including the metasurface lens [16,[18][19][20][21][22][23][24][25][26], vortex beam generators [27][28][29][30][31][32][33][34][35][36] and polarization beam splitters [37][38][39][40][41][42][43]. The phase of metallic structures on the gradient metasurface maintains a certain special relationship, which provides powerful electromagnetic wave manipulation capabilities.…”

“…One important early example is the work of Chen et al [65] from Los Alamos National Lab, who combined a metasurface with a hybrid semiconductor to realize a tunable artificial microstructure that can modulate the amplitude and phase of THz waves. Subsequently, different mechanisms, materials and geometric constructs were proposed and implemented to realize different functional devices in the THz band [17,24,[66][67][68][69][70][71][72]. Thus, in the past decade, an increasing number of outstanding achievements have been reported.…”

Photonic nanojets with mesoscale high-index dielectric particles

“…The necessity to improve the performance of imaging systems for microscopy applications resolving the subwavelength features of an object has pushed forward the scientific community in order to overcome this diffraction limit. Different methods and techniques have been proposed such as solid immersion lenses [2][3][4] , diffractive optics 5 , microspherical dielectrics [6][7][8] , superoscillatory devices [9][10][11] and metamaterials [12][13][14][15] . In this realm, photonic nanojets (PNJs) were first proposed and demonstrated several years ago at optical frequencies by using micrometer scaled dielectric elements with cylindrical (2D) and also spherical (3D) geometries [16][17][18][19][20][21] .…”