Abstract
The movable super-diffraction optical needle (MSON) is a tightly focused beam like a “needle”, which can realize vector scanning on the focusing plane. Not only does it have a long focal depth, but its resolution also exceeds the diffraction limit. The modulation and control technology required for generating MSON by oblique incidence is explored in this manuscript for the purpose of processing high-aspect-ratio, sub-wavelength structures. As the optical needle generated by traditional methods is static and sensitive to variation of the angle information of the incident beam, here we introduce a confocal scanning system by using a two-dimensional galvanometer system, a scan lens, and a tube lens to control the oblique incidence angle. The effects of the oblique incidence angle on the resolution, depth of focus, uniformity, and side lobes of the MSON were analyzed. Further, the voltage-controlled liquid crystal located between the scan lens and the 2D galvanometer system can be used to compensate for the additional phase difference caused by oblique incidence. The aspect ratio is defined as the ratio of depth of focus to resolution. By modulating and controlling the light field, the MSON with high aspect ratio (7.36), sub-diffractive beam size (0.42λ), and long depth of focus (3.09λ) has been obtained with homogeneous intensity, and suppressed side lobes. High speed, high axial positioning tolerance, and high-resolution laser processing can also be achieved, which removes the restrictions presented by traditional laser processing technology, for which high resolution and long depth of focus cannot be achieved simultaneously.
Highlights
High-aspect-ratio, sub-wavelength structures have a wide range of applications in major projects such as metasurfaces, X-ray diffraction spectroscopy devices, space particle capture, and deep-grain film imaging devices for space-to-ground observation [1,2]
High axial positioning tolerance, and high-resolution laser processing can be achieved, which removes the restrictions presented by traditional laser processing technology, for which high resolution and long depth of focus cannot be achieved simultaneously
By modulating and controlling the light field, we obtained a movable super-diffraction optical needle (MSON) with sub-diffractive beam size (0.42λ), long depth of focus (DOF) (3.09λ), high aspect ratio (7.36), uniform intensity, and suppressed side lobes, which can resolve the scientific problem that DOF and resolution cannot be improved at the same time
Summary
High-aspect-ratio, sub-wavelength structures have a wide range of applications in major projects such as metasurfaces, X-ray diffraction spectroscopy devices, space particle capture, and deep-grain film imaging devices for space-to-ground observation [1,2]. To manufacture a high-aspect-ratio, sub-wavelength structure, the resolution and depth of focus (DOF) in laser processing technologies need to be improved simultaneously. To improve the DOF and resolution of the optical needle, many scientists have designed filters with different structures to modulate the incident beam. According to the principle of multi-focus superposition, an amplitude filter based on Euler transformation, which was applied to modulate the amplitude and phase of the incident radial polarized Bessel–Gaussian beam, was proposed to generate the longitudinally polarized long focal depth structure [15]. By modulating and controlling the light field, we obtained a MSON with sub-diffractive beam size (0.42λ), long DOF (3.09λ), high aspect ratio (7.36), uniform intensity, and suppressed side lobes, which can resolve the scientific problem that DOF and resolution cannot be improved at the same time. Compared with the traditional method of generating static optical needles with normal incidence, a combination of vector scanning and mechanical scanning can be used in this oblique incidence method to increase processing speed
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