Abstract
The nanoresolution of geometric phase elements for visible wavelengths calls for a flexible technology with high throughout and free from vacuum. In this article, we propose a high-efficiency and simple manufacturing method for the fabrication of geometric phase elements with femtosecond–laser direct writing (FsLDW) and thermal annealing by combining the advantages of high-efficiency processing and thermal smoothing effect. By using a femtosecond laser at a wavelength of 343 nm and a circular polarization, free-form nanogratings with a period of 300 nm and 170-nm-wide grooves were obtained in 50 s by laser direct ablation at a speed of 5 mm/s in a non-vacuum environment. After fine-tuning through a hot-annealing process, the surface morphology of the geometric phase element was clearly improved. With this technology, we fabricated blazed gratings, metasurface lens, vortex Q-plates and “M” holograms and confirmed the design performance by analyzing their phases at the wavelength of 808 nm. The efficiency and capabilities of our proposed method can pave the possible way to fabricate geometric phase elements with essentially low loss, high-temperature resistance, high phase gradients and novel polarization functionality for potentially wide applications.
Highlights
Micro-optical elements have drawn great attention for their advantages of miniaturization and easy integration [1,2,3,4,5,6]
By tailoring the profiles and the corresponding optical path differences—named dynamic phase—various refractive/diffractive optical elements have been realized for micro-optics, microelectronics and biomedical sciences [5,6,7]
femtosecond–laser direct writing (FsLDW), nanogratings with different orientations were constructed on the gold film (Figure 2a)
Summary
Micro-optical elements have drawn great attention for their advantages of miniaturization and easy integration [1,2,3,4,5,6]. By tailoring the profiles and the corresponding optical path differences—named dynamic phase—various refractive/diffractive optical elements have been realized for micro-optics, microelectronics and biomedical sciences [5,6,7]. Pancharatnam–Berry (PB) phase elements—or geometric phase elements—have emerged as an option [8,9,10,11,12]. They generate continuous phase variation by tuning the direction of the nanogratings, but without changing their thickness [10,11,12]. Erez Hasman et al initially proposed a polarization-dependent 10.6-μm lens based on geometric phase by using computer-designed phase distribution and optical lithography for fabricating diffraction gratings with different directions [15]. Chen et al fabricated the designed a three-foci metasurface lens on a 40-nm-gold-film-coated glass substrate; electron-beam lithography (EBL) and a lift-off process were used [9]
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