Femtosecond Laser Microfabrication of Optofluidic Lab-on-a-chip with Selective Chemical Etching

The three-dimensional (3-D) writing capability of a high repetition rate (1 MHz) fiber-amplified femtosecond laser with a wavelength of 522 nm was harnessed together with wet-chemical etching for laser-patterning of 3-D optofluidic microsystems in fused silica glass, by the method of Femtosecond Laser Irradiation followed by Chemical Etching (FLICE). Selective chemical etching of laser irradiated glass with dilute hydrofluoric acid (HF) enabled micro-fabrication of high aspect-ratio embedded micro-channels and fine-period 3-D glass meshes in a 3-D inverted woodpile (IWP) arrangement that permitted high density lab-on-a-chip (LOC) integration of flow channels, reservoirs, glass chromatography columns, and optical circuit devices. Optical waveguides, reservoirs, micro-channels, and IWP structures were first laser patterned and followed by selective wet etching controlled by the polarization orientation of the writing laser. With the laser polarization perpendicular to the scanning direction, the volume nanogratings were aligned perpendicular to glass surfaces to facilitate HF etching and thus created designer shaped micro-channels with the smoothest sidewall surfaces measured at present and terminated with open reservoirs. An array of vertical access holes spaced periodically apart facilitated etching of continuous and highly uniform buried channels of unrestricted length in the glass to interconnect flow channels and reservoirs. Alternatively, laser polarization parallel to the scan direction provided low-loss optical waveguides with nanograting walls resisting the acid etching, providing a convenient one-step laser scanning process of optofluidic microsystems prior to wet etching. For the first time, dual-channel capillary electrophoresis was demonstrated by simultaneous fluorescent detection of separating dyes in a 3-D microsystem having over- and under-passing crossed channels in fused silica. In addition, an on-chip particle counting device based on capillary force to drive analytes through an embedded micro-channel into a calibrated reservoir for particle counting was designed and demonstrated. Further, a new type of glass mesh structure is presented where a 3-D IWP micro-channel array with diamond-like symmetry was integrated inside a micro-channel for capillary electrophoretic chromatography. The FLICE technique thus enables rapid prototyping of fully integrated 3-D optofluidic systems in bulk fused silica glasses for numerous applications, and these provide the groundwork and open new 3-D design approaches for advanced microsystems in the future.