Synthesis of sub-diffraction quasi-non-diffracting beams by angular spectrum compression

Abstract

Quasi-non-diffracting beams are attractive for various applications, including optical manipulation, super-resolution microscopes, and materials processing. However, it is a great challenge to design and generate super-long quasi-non-diffracting beams with sub-diffraction and sub-wavelength size. In this paper, a method based on the idea of compressing a normalized angular spectrum is developed, which makes it possible and provides a practical tool for the design of a quasi-non-diffracting beam with super-oscillatory sub-wavelength transverse size. It also presents a clear physical picture of the formation of super-oscillatory quasi-non-diffracting beams. Based on concepts of a local grating and super-oscillation, a lens was designed and fabricated for a working wavelength of λ = 632.8 nm. The validity of the idea of normalized angular spectrum compression was confirmed by both numerical investigations and experimental studies. An optical hollow needle with a length of more than 100λ was experimentally demonstrated, in which an optical hollow needle was observed with a sub-diffraction and sub-wavelength transverse size within a non-diffracting propagation distance of 94λ. Longer non-diffracting propagation distance is expected for a lens with larger radius and shorter effective wavelength.

High‐Numerical‐Aperture Dielectric Metalens for Super‐Resolution Focusing of Oblique Incident Light

“…In the past several years, various types of superoscillatory lenses have been demonstrated, including linear‐focusing lenses, [ 18,19 ] spot‐focusing lenses, [ 20,21 ] long‐depth‐of‐focusing lenses, [ 22–24 ] and vector wave superoscillatory lenses. [ 25–30 ] So far, the smallest PSF that has ever been experimentally demonstrated has a full‐width‐at‐half‐maximum (FWHM) value of 0.33λ, [ 31 ] where λ is the working wavelength. Due to their comparative large sidelobes, those super‐resolution lenses are not suitable for direct imaging.…”

Superoscillation: from physics to optical applications

“…In a later study, surprisingly, classical binary Fresnel zone plates were used to generate subdiffraction optical needles for multiple polarizations via optimization-free design 130 . A numerical simulation showed that, compared to the optical needle reported in reference 129 , those that were created by classical Fresnel zone plates have smaller fluctuations in optical intensity along the optical axis. The experimental results showed that the transverse sizes and the axial lengths were 0.40λ–0.54λ and 90λ, 0.43λ–0.54λ and 73λ and 0.34λ–0.41λ and 80λ for the generated optical needles with circular, longitudinal and azimuthal polarizations, respectively, as shown in Fig.…”

“…The value of the wavelength λ is determined by several parameters, including the lens radius, the working wavelength λ 0 , the focal length and the optical needle propagation distance. The extension of an optical needle in water 124,129 can be explained similarly. Interestingly, utilizing the same strategy, subdiffraction diffractionless beams can be created with the same transverse size and propagation distance but lower intensity fluctuations along the optical axis using a classical Fresnel zone plate, which is entirely free from optimization and allows one to design subdiffraction diffractionless beams for multiple polarizations using very simple algebra 130 , as shown in Fig.…”

“…As shown in Fig. 9b, utilizing the concept of angular spectrum compression 129 , an SOL was designed with a single subdiffraction focal spot at wavelength λ. Then, under illumination at a shorter wavelength λ 0 (<λ), a subdiffraction diffractionless beam was generated with a length of approximately 100λ 0 in air and 200λ 0 in water.…”

“…Based on this concept, a superoscillation point-focusing lens that uses a BP mask was optimized at a wavelength of 672.8 nm. When illuminated with an azimuthally polarized wave at a shorter wavelength of 632.8 nm, the lens generated an optical hollow needle with a subdiffraction and subwavelength transverse size within the nondiffracting propagation distance of 94λ 129 . A numerical simulation also revealed that when the lens was immersed in water, the propagation distance was further extended to 180λ, while the beam remained superoscillatory with a transverse size of ~0.35λ–0.4λ.…”