Laser Micro-Patterning by Means of Optical Fibers with Micro-grinded Lens End Faces


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Introduction

Nano-patterning of polymer surface has found a wide range of applications including biotechnology, where holes in the size of hundreds – nm can influence the growth and differentiation of relatively large living cells [1]. Instead of the e-beam patterning technique used in the Ref. 1, we ap-plied direct writing by laser-light. In our previous papers [2,3], we have demonstrated that this technique allows the production of sub-wavelength structures in the optical near field of a fiber tip. In conventional scanning near-field op-tical microscopy (SNOM), the light is confined to a sub-wavelength region in the near-field of a drawn metal coated optical fiber tip [4].


Experimental

The experiments were performed with 250 μm thick step index quartz multimode fibers with a numerical aper-ture NA of 0.22 (laser Components, type 2524002). The fibers were first chemically etched to a conical shape and then grinded to a hemispherical shaped lens with curvature radii of 1 - 3 μm.
The setup for direct writing and lensed fiber grinding is based on a home-made AFM/SNOM and described in de-tails in [7]. The main changes that transform scanning sys-tem of the SNOM to a micro-grinding station are the soft-ware algorithms for tip trajectory control.


Results and Discussion

In a first process step, the fibers were chemically etched to conical tips in 40% HF aqueous solution like in ref. [21]. Then we produced the lensed shape of fiber tips by grind-ing inside the conical indentation in the polishing surface. The feature 4a shows an 3D AFM image of an indentation recorded with our setup in the AFM operation mode with a freshly etched tip. The trace with arrows shows schemati-cally the trajectory of the relative movement of the tip in-side the indentation during the grinding procedure. The trajectory is a circle in the horizontal plane with the center at the indentation axis. The radius and XYZ circle centre coordinates are adaptively adjusted during the grinding process to provide constant contact of the tip to polishing surface at all trajectory points.


Conclusion
We adapted a home-made scanning probe setup for mi-cro-grinding of lensed fibers from chemically etched tips to predefined shapes with radii down to the sub-micron range. In the same setup, it is possible to perform a tip quality characterization by AFM or SNOM type measurements. We performed direct pattering of PMMA surfaces by UV laser irradiation through optical fibers with micro-grinded lens end faces. Produced features are slightly larger than features patterned with sharp conical etched fiber tips in contact mode. But the main advantage in the used of lensed fibers is the absence of the direct contact between tip and sample surface.