By Ya Cheng

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 laser 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 these difficulties can be overcome by means of femtosecond laser micromachining.

Fig.1 (a-b) Flow chart of fabrication of microfluidic channels in porous glass by femtosecond laser (for details, please see text). (c) A 3D microfluidic channel fabricated by femtosecond laser which is filled with fluorescein solution. (d) Schematic illustration of a 3D micromixer. (e) Close up view of the mixing units of the fabricated micromixer. (f) Mixing of two fluorescence solutions in the micromixer (Green: Fluorescein; red: Rhodamine 6G). Scale bar in all panels: 150 um.

FEMTOSECOND FLEXIBILITY

The 3D nature of the femtosecond laser direct writing offers flexibility for constructing complex microfluidic networks in glass. The main fabrication process includes two steps: (1) direct formation of hollow microchannels in a porous glass substrate immersed in water by femtosecond laser ablation (Fig. 1(a)); and (2) postannealing of the glass sample at ~ 1150 °C by which the porous glass can be consolidated due to collapse of the nanopores (Fig. 1(b)). The consolidated glass sample can then be used to confine liquids in the fabricated 3D microfluidic channel without any leakage, as evidenced by Fig. 1(c). Because of its capability to directly form large-scale microfluidic structures embedded in glass with arbitrary 3D configurations, this technique can be used for fabricating functional microfluidic devices such as the 3D microfluidic mixer illustrated in Fig. 1(d) which is usually difficult to achieve with other fabrication technologies. Fig. 1(e) shows the details of the middle part of the fabricated micromixer which is composed of a series of mixing units of a true 3D geometry. Indeed, such 3D micromixer exhibits a high mixing efficiency as shown in Fig. 1(f), owing to the chaotic flow in the twisted microfluidic channels. In contrast, we observe that efficient mixing of the same solutions does not occur in a straight 1D microfluidic channel with the similar diameter even after passing a distance of ~1.5 mm, because of the inherent laminar nature in the microfluidic channel.

Fig.2 (a) Schematic and (b) optical micrograph of the microlens fabricated by femtosecond laser micromachining. (c) The measured focal spot and (d) the calculated point-spread function of the microlens in (b). Two-photon fluorescence images of the leaf tissue acquired using (e) the microlens and (f) an objective lens (5x magnification).

Micro-optical components can be fabricated in a slightly different manner, namely, by femtosecond laser direct writing in fused silica followed by chemical 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 this technique, we 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 model lens 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.

Fig.3 (a) Schematic illustration of fabrication for embedded microelectrodes, and optical micrographs showing (b) top view and (c) cross-sectional view of the embedded electrodes. (d) Schematic layout of an electro-optical modulator. Insets: near-field intensity distributions at the exit of the Mach-Zehnder interferometer at different direct current (DC) voltages of 0 V (upper left) and 19 V (lower right).

Further, femtosecond laser micromachining can allow for electro-optic (EO) integration in active materials such as lithium niobate (LiNbO3) crystal. For this purpose, we develop a technique for selective metallization of dielectric materials by femtosecond laser ablation followed by selective electroless plating, which permits to fabricate high-aspect-ratio microelectrodes deeply embedded in the substrates. As illustrated in Fig. 3(a), the fabrication of embedded electrodes mainly consists of four steps: (1) formation of microgrooves on the surface of substrates by femtosecond laser ablation; (2) formation of AgNO3 films on the surfaces of the substrates by dip coating in AgNO3 solution; (3) exposure of the coated substrate to ultraviolet (UV) light for reducing silver ions to silver nanoparticles, which become the seeds for the subsequent electroless copper plating; and (4) electroless copper plating at a temperature of ~45 °C. The embedded electrodes fabricated by femtosecond laser micromachining, as shown in Fig. 3(b) and (c), are integrated with a Mach-Zenhder (MZ) interferometer buried in LiNbO3 crystal, which is composed of optical waveguides written by femtosecond laser, to construct an EO modulator, as illustrated in Fig. 3(d). The voltage required to completely switch on and off a He-Ne laser beam coupled into the MZ interferometer is measured to be ~19 V, indicating an excellent EO overlap integral of ~0.95. Since microfluidic, micro-optical and microelectronic structures can be simultaneously fabricated in dielectric materials using femtosecond laser direct writing, we envisage that femtosecond laser micromachining will open up a broad spectrum of opportunities for fluidic-photonic-electronic circuit applications.