All-Glass 3D Optofluidic Microchip with Built-in Tunable Microlens Fabricated by Femtosecond Laser-Assisted Etching

Development of tunable microlenses by taking advantage of the physical adaptability of fluids is one of the challenges of optofluidic techniques, since it offers many applications in biochips, consumer electronics, and medical engineering. Current optofluidic tuning methods using electrowetting or pneumatic pressure typically suffer from high complexity involving external electromechanical actuating devices and limited tuning performance. In this paper, a novel and simple tuning method is proposed that changes the liquid refractive index in an optofluidic channel while leaving the shape of the microlens unchanged. To create an optofluidic microlens with high robustness and optical performance, built-in microlenses are fabricated inside 3D glass microfluidic channels by optimized single-operation wet etching assisted by a femtosecond laser. Tuning of focusing properties is demonstrated by filling the channel with media having different indices. Continuous tuning over a wide range (more than threefold tunability for both focal length and focal spot size) is also achieved by pumping sucrose solutions with different concentrations into the microchip channels. Reversible tuning is experimentally verified, indicating intriguing properties of the all-glass optofluidic microchip. Both the proposed tuning method and the all-glass architecture with built-in microlens offer great potential toward numerous applications, including microfluidic adaptive imaging and biomedical sensing.