Study of focusing parameters of wavelength-scale binary phase Fresnel zone plate

Abstract

Binary Fresnel zone plates (FZP) are among the most commonly used focusing elements of 2D-planar optical circuits in micro- and nano-photonics. When the diameter and focal distance of a FZP are reduced to the wavelength dimensions, the parameters of the focal area experience strong influence by FZP constructive design (material, thickness, depth of zone relief). By means of the numerical simulations, the near-field diffraction of monochromatic optical wave on a wavelength-scale binary phase plate is investigated. We found a range of optimal depths of zone plate grooves etching as well as substrate thicknesses providing the best focusing of the incident circularly polarized optical wave in terms of maximum field intensity and minimum size of the focal spot. A certain improvement of these focus parameters can be achieved by filling the zone grooves with a dielectric having a specific refractive index contrast against the FZP substrate. Additionally, the concept of a super-focus binary phase plate with a solid immersion layer (SIL) in the form of a truncated cone made of the ZP substrate material is proposed. Similar to conventional SIL device, this flat SIL-FZP can focus a circularly polarized optical radiation into a subdiffraction spot with a full-width of the order λ/2n (n is FZP refraction index).

Optical Phenomena in Mesoscale Dielectric Particles

“…The similar effect of resonance super-resolution imaging is considered in [96]. It is interesting to note that 0.27 × 0.6λ focal spot for the linear polarization of the illuminating wave can be obtained in an off-resonance frequency mode for a specific configuration of a zone plate [97] with a low level of the side lobes, but at a maximum field intensity, which is 1000 times lower. This is shown in Figure 7.…”

Fresnel zone plate metalens using silicon subwavelength gratings

“…This can be compensated for by introducing an extra relative phase shift of π for one set of zones to create constructive interference at the focal point. This concept guides the design of BPFZPs which impose the phase profile of alternating zones of 0 and π relative phase to perform high efficiency focusing 11,21 .…”

Silicon Fresnel Zone Plate Metalens with Subwavelength Gratings

“…As a result, the full aperture contributes constructively at the focal point. This concept guides the design of binary phase Fresnel zone plates, which imposes the phase profile of alternating zones with 0 and π relative phase difference to perform high-efficiency focusing [24,26,37]. Conventional metalenses focus light by converting incoming planar wavefronts into spherical waves that converge at a point.…”

“…Without access to the full phase range from 0 to 2π, the transmitted wavefronts would not be perfectly spherical, and the lens would suffer from spherical aberrations, which diminishes the quality of the focus [2]. A possible solution to this is to limit the size of the lens according to the available phase coverage [24]. However, this places a constraint on the numerical aperture that can be achieved.…”

“…Dielectric focusing metalenses based on a binary phase Fresnel zone plate (BPFZP) are capable of breaking the diffraction limit [24,25]. They have also been demonstrated with impressive achromatic focusing abilities.…”