The femtosecond laser-induced multi-photon polymerization of a zirconium–silicon based
sol–gel photopolymer was employed for the fabrication of a series of micro-optical
elements with single and combined optical functions: convex and Fresnel lenses,
gratings, solid immersion lenses on a glass slide and on the tip of an optical fiber.
The microlenses were produced as polymer caps of varying radii from 10 to 90
µm. The matching of refractive indices between the polymer and substrate was exploited for
the creation of composite glass-resist structures which functioned as single lenses.
Using this principle, solid immersion lenses were fabricated and their performance
demonstrated. The magnification of the composite solid immersion lenses
corresponded to the calculated values. The surface roughness of the lenses was below ∼ 30 nm, acceptable for optical applications in the visible range. In addition, the integration
of micro-optical elements onto the tip of an optical fiber was demonstrated. To
increase the efficiency of the 3D laser polymerization, the lenses were formed by
scanning only the outer shell and polymerizing the interior by exposure to UV light.
Light-initiated quasi-instant solidification of a liquid polymer is attractive for its
ultra-precise spatial and temporal control of the photochemical reaction. In this paper we
present microlenses structured by femtosecond laser-induced photopolymerization.
Due to nonlinear phenomena the fabrication resolution is not restricted to the
diffraction limit for the applied laser excitation wavelength but is determined by
the intensity of a focused beam. Furthermore, pin-point structuring enables one
to produce three-dimensional structures of any form from the photopolymer.
The smallest structural elements of 200 nm lateral dimensions can be achieved
reproducibly by using high numerical aperture oil immersion focusing optics (NA = 1.4). Axial resolution (which is fundamentally a few times worse than lateral resolution due to
the distribution of light intensity in the focal region) can be controlled to a precision of a
few hundred nanometers by decreasing the scanning step. In our work we applied the
commercially available and widely used zirconium–silicon based hybrid sol–gel
photopolymer (Ormosil, SZ2080). Arrays of custom-parameter spherical microlenses for
microscopy applications have been fabricated. Their surface roughness, focal distance and
imaging quality were tested. The obtained results show potential for fast and
flexible fabrication of custom-parameter microlenses by the proposed technique.
Femtosecond laser photo-polymerization of zirconium-silicon based sol-gel photopolymer SZ2080 is used to fabricate micro-optical elements with a single and hybrid optical functions. We demonstrate photo-polymerization of the solid immersion and Fresnel lenses. Gratings can be added onto the surface of lenses. The effective refractive index of polymerized structures can be controlled via the volume fraction of polymer. We used woodpile structure with volume fraction of 0.65-0.8. Tailoring of dispersion properties of micro-optical elements by changing filling ratio of polymer are discussed. Direct write approach is used to form such structures on a cover glass and on the tip of an optical fiber. Close matching of refractive indices between the polymer and substrate in visible and near infra red spectral regions (n SZ2080 = 1.504, n glass = 1.52) is favorable for such integration. The surface roughness of laser-polymerized resits was ∼30 nm (min-max value), which is acceptable for optical applications in the visible range. For the bulk micro-optical elements the efficiency of 3D laser polymerization is increased by a factor ∼ (2 − 4) × 10 2 times (depends on the design) by the shell-formation polymerization: (i) contour scanning for definition of shell-surface, (ii) development for removal of nonfunctional resist, and (iii) UV exposure for the final volumetric polymerization of an enclosed volume.
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