SiO 2 -based variable microfluidic lenses fabricated by femtosecond laser lithography-assisted micromachining

Abstract

SiO₂-based variable microfluidic lenses were fabricated by femtosecond laser lithography-assisted micromachining (FLAM). Optofluidic devices have attracted much interest because the adaptive nature of liquids in microfluidics enables unique optical performance that is not achievable within all solid state devices. SiO₂-based microfluidic devices are, particularly, attractive due to high transparency, physical and chemical stabilities. However, it is generally rather difficult to form the microstructures in microchannels because photolithography process is limited to planar substrates. In our study, we fabricated SiO₂-based variable microfluidic lenses, which had micro-Fresnel lenses inside the channels, by using FLAM, which was a combined process of nonlinear lithography and plasma etching. The resist patterns of the Fresnel lenses were directly written inside chemically amplified negative-tone photoresist on SiO₂-based microchannels of 250 μm width and 6 μm depth using femtosecond laser-induced nonlinear optical absorption. Following that, the patterns were transferred to the bottom of the channels by using CHF₃ and O₂ mixed plasma. SiO₂-based Fresnel lenses with smooth surface were formed on the bottoms. When the channel was filled with the air, the focal spot was observed 2020 μm from the lens surface. By injecting silicone oil into the channel, the incident light was switched to the dispersed.