By: Ya Cheng, Fei He, Yang Liao, Lingling Qiao, Zhizhan Xu, Koji Sugioka and Katsumi Midorikawa
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, China
Laser Technology Laboratory, RIKEN – Advanced Science Institute, Japan
Nowadays, microfluidic systems for controlling and manipulating tiny volumes of liquids with high precision have attracted significant attention due to their capability of downsizing both chemistry and biology. In addition, it is often desirable to incorporate micro-optical structures into the microfluidic chips, which leads to not only compact chemical and biological sensors but also tunable and reconfigurable photonic devices. For both microfluidic and micro-optical applications, fused silica can be an ideal substrate material due to its excellent physical and chemical properties, such as chemical inertness, low thermal expansion coefficient, low autofluorescence, exceptional transmittance over a wide spectral range, and so on. On the other hand, fabrication of three-dimensional (3D) microstructures with fused silica, including embedded microfluidic channels and microspherical optical lenses, has long been a challenge because traditional approaches based on photolithography inherently produce planar structures. Here, we show that 3D micromachining of fused silica for both microfluidic and micro-optical applications can be achieved using femtosecond laser direct writing followed by a wet etching in hydrofluoric (HF) acid. In this process, the internal areas modified by the femtosecond laser irradiation will gain a significantly higher etch rate than those unmodified areas, so that hollow structures embedded in fused silica can be produced by preferentially removing the materials in the laser-scanned areas.