Abstract
In this manuscript, a method of generating an ultra-long optical needle (depth-to-width ratio 37.5:1) is proposed and demonstrated by focusing an azimuthally polarized beam. In theory, the action mechanism between the incident beam and the amplitude modulation element, the spiral phase modulation element, the focusing lens were studied based on the Richards and Wolf's theory. The relationship between the intensity distribution of the optical needle and the structure parameter of the element were obtained, thus leading to the complete design model and design standard. In experiment, the annular amplitude modulation element and spiral phase modulation element were fabricated by lithography. The optical needle was obtained based on a custom-designed optical setup in our laboratory. The optical system consists of an annular aperture (3.9-mm inner diameter, 80-μm annular width), a spiral phase plate (topological charge of 1), and an objective lens with numerical aperture of 0.95. Finally, an optical needle with a subwavelength size (0.416λ) and an ultra-long depth of focus (15.6λ) was obtained, showing an excellent agreement with our theoretical model.
Highlights
Optical needles with sub-wavelength spot and ultra-long depth-of-focus (DOF) are attractive for various applications, including lithography [1], optical data storage [2] and super-resolution imaging [3]
We prove that the azimuthally polarized (AP) beam has a significant advantage over the radially polarized (RP) beam in generating optical needles by comparing the intensity distributions of focusing spots
The required annular aperture and spiral phase plate (SPP) were fabricated by lithography
Summary
Optical needles with sub-wavelength spot and ultra-long depth-of-focus (DOF) are attractive for various applications, including lithography [1], optical data storage [2] and super-resolution imaging [3]. An optical needle with a full width at half maximum (FWHM) of 0.43λ and longitudinal full width at half maximum (LFWHM) of 4λ was generated by focusing with an objective lens of NA = 0.95 Such a breakthrough promoted the study of super-resolution nano optical needles induced by RP beams. Different methods of modulating the RP beam to generate optical needles have been continuously reported [13]–[18] In these studies, Huang et al [13] showed that adjusting the area outside the normalized radius, R = 0.5293 of the light field, is more conducive to extending the DOF. Similar to RP beam, the AP beam has perfect rotationally symmetric distribution, symmetric electric field distribution and special polarization distribution [28]–[33], making it possible to generate ultra-long optical needles with a sub-wavelength spot. There is a lack of systematic and theoretical analysis of this technology, whereas the mechanism and theoretical model need to be verified by experiments
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