Efficient light focusing through tunable spiralized Fresnel zone plate

Spiral antennas for the generation of bessel beams with tunable nondiffractive range

“…In [31,32], the controllable transmission and focusing of an in-line propagating optical wave through a spiral lens with a circle-involute profile are discussed. Here, we aim to take advantage of a similar structure working at microwaves and featuring subwavelength mutual distances between filaments, whose rotation around a central joint will induce the formation of an envelope figure presenting circular density bands.…”

“…The original feature introduced here is the possibility to control the local densification by rigid simultaneous mechanical displacements of spiral filaments. More precisely, we use an interesting transformation based on rotations around each filament pivot point (the overall envelope figure can be estimated by means of analytical expressions and simple numerical codes as shown in [31]) to gain dynamic control of the NDR extent.…”

“…The solution presented here is based on an LWA made of spiral filaments with subwavelength spacings which, under proper displacements, assume a variable envelope configuration with a given density function equivalent to a concentric arrangement of solid metal rings (see figure 1). Such displacements mainly consist of simple rigid translations and rotations, as shown for optical lensing [31,32], but they can also involve elastic deformations. We shall also consider this latter case by pushing the filament rotation beyond the first-contact angle, to provide the overall structure with a sharper density pattern and so with a further perturbation of the surface current distribution or, equivalently, a higher leakage constant.…”