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.
Using the above technique, microfluidic channels can be easily fabricated as shown in Fig. 1 (a). However, the fabricated channels typically are strongly tapered due to the limited etching selectivity, and their cross-sectional shapes are highly elliptical, as shown in Figs. 1 (c) and (d). Further, the surface roughness of sidewall of the microchannel can reach a level of ~500 nm, which is too high for many fluidic applications. To solve these problems, we perform an additional glass drawing step after the etching of the sample. Namely, the glass sample is heated with an oxyhydrogen flame until it is softened. Then we slightly draw the glass sample in the direction parallel to the channel. After the drawing process, a homogeneous channel with perfectly circular cross sectional shapes throughout itself is obtained, as shown in Figs. 1(b), 1(e) and 1(f). In addition, an inner surface roughness of ~0.2 nm has been achieved. These improvements are achieved owing to the surface tension in the molten glass during the glass drawing. The glass drawing technique also allows for creating centimeter-long microfluidic channels in complex fluidic networks with a diameter of only ~10 mm, as shown in Figs. 1(g)-(j).
With the similar technique, we also fabricate micro-optical lenses with nearly diffraction-limited focusing performance on fused silica substrates. To create the curved surface as illustrated in Fig. 2(a), we first scan the sample with the tightly focused femtosecond laser beam and then carry out the wet etching as mentioned above. Afterwards, an additional oxyhydrogen (OH) flame polishing is used to smooth the curved surface. The fabricated microlens is shown in Fig. 2(b). In comparison to the simulation result (Fig. 2(d)) for a microlens with the same geometry, the measured focal spot produced by the fabricated microlens has a comparable size, as shown in Fig. 2(c). Recently, the microlens has been used in two-photon fluorescence imaging. The two-photon images of a plant leaf tissue acquired with the micro-optical lens and with a 5× objective lens are compared in Figs. 2(e) and 2(f), showing little difference in their imaging performances. We envisage that femtosecond laser micromachining will open up a broad spectrum of opportunities for microfluidic, optical, and optofluidic applications.