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

Optofluidics, taking advantage of the synergy of microfluidics and micro­ optics, facilitates its wide applicability by providing a platform for generating and manipulating light on a microchip. Development of tunable optical systems based on the physical adaptability of fluids has long been one of the most important tasks in optofluidics research.[1–4] As a fundamental element of optofluidics, the microlens affords a wide range of appli­ cations in microfluidic engineering, such as cell counting,[5] optical trapping,[6] and fluidic lasing and sensing.[7] Develop­ ment of a tunable microlens has recently attracted considerable attention due to its unique potential applications.[8–13] A macroscale analogue is the zoom lens in a commercial camera, in which the focal length of the lens group can be changed by displacing the lens components along the axis parallel to the lens arrangement to realize autofocusing functionality for taking photos at different depths of field. By benefiting from compact size, low light energy consumption, and high integration level, achieving tun­ ability of microscale lenses would be advantageous for distinct implementations, such as adaptive plenoptic cameras, tissue endoscopes, and advanced solar concentrators. Although the challenges of manageable control of the microlens shape and position remain unaccomplished, a cer­ tain degree of tunability has been achieved over the past dec­ ades.[14,15] Electrowetting has emerged as an attractive way to control the shape of a liquid lens on a hydrophobic solid sur­ face by electrostatically tuning the interfacial tension before solidification.[16,17] However, the liquid used for the lens is limited to conductive and photosensitive materials, and a large optical aperture is extremely difficult to achieve. Further­ more, the process is accompanied by liquid evaporation.[18] Meanwhile, liquid­filled microlenses placed inside a flexible polymeric chamber or between thin elastic membranes can vary their optical characteristics due to shape changes actu­ ated by an external force, for example, periphery movement,[19] fluidic pressure,[20] or electromagnetic actuation,[21] providing a wide range of tunability. However, external actuation devices such as mechanical pumps are required, and their operation is 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.