This review presents a systematic perspective on the development of micro-optofluidic lenses. The progress on the development of micro-optofluidic lenses are illustrated by example from recent literature. The advantage of micro-optofluidic lenses over solid lens systems is their tunability without the use of large actuators such as servo motors. Depending on the relative orientation of light path and the substrate surface, micro-optofluidic lenses can be categorized as in-plane or out-of-plane lenses. However, this review will focus on the tunability of the lenses and categorizes them according to the concept of tunability. Micro-optofluidic lenses can be either tuned by the liquid in use or by the shape of the lens. Micro-optofluidic lenses with tunable shape are categorized according to the actuation schemes. Typical parameters of micro-optofluidic lenses reported recently are compared and discussed. Finally, perspectives are given for future works in this field.

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Micro-optofluidic Lenses: A review

The recent rapid development in microlens technology has provided many opportunities for miniaturized optical systems, and has found a wide range of applications. Of these microlenses, tunable-focus microlenses are of special interest as their focal lengths can be tuned using micro-scale actuators integrated with the lens structure. Realization of such tunable microlens generally relies on the microelectromechanical system (MEMS) technologies. Here, we review the recent progress in tunable liquid microlenses. The underlying physics relevant to these microlenses are first discussed, followed by description of three main categories of tunable microlenses involving MEMS techniques, mechanically driven, electrically driven and those integrated within microfluidic systems.

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Liquid Tunable Microlenses based on MEMS techniques.

We demonstrate a tunable in-plane optofluidic microlens with a 9× light intensity enhancement at the focal point. The microlens is formed by a combination of a tunable divergent air–liquid interface and a static polydimethylsiloxane lens, and is fabricated using standard soft lithography procedures. When liquid flows through a straight channel with a side opening (air reservoir) on the sidewall, the sealed air in the side opening bends into the liquid, forming an air–liquid interface. The curvature of this air–liquid interface can be conveniently and predictably controlled by adjusting the flow rate of the liquid stream in the straight channel. This change in the interface curvature generates a tunable divergence in the incident light beam, in turn tuning the overall focal length of the microlens. The tunability and performance of the lens are experimentally examined, and the experimental data match well with the results from a ray-tracing simulation. Our method features simple fabrication, easy operation, continuous and rapid tuning, and a large tunable range, making it an attractive option for use in lab-on-a-chip devices, particularly in microscopic imaging, cell sorting, and optical trapping/manipulating of microparticles.

Tunable optofluidic microlens through active pressure control of an air–liquid interface

This paper presents a novel optofluidic Michelson interferometer based on droplet microfluidics used to create a droplet grating. The droplet grating is formed by a stream of plugs in the microchannel with constant refractive index variation. It has a real-time tunability in the grating period through varying the flow rates of the liquids and index variation via different combinations of liquids. The optofluidic Michelson interferometer is highly sensitive and is suitable for the measurement of biomedical and biochemical buffer solutions. The experimental results show that it has a sensitivity of 66.7 nm per refractive index unit (RIU) and a detection range of 0.086 RIU.

https://nocweba.ntu.edu.sg/laq_mems/journal%20papers/2010/A%20reconfigurable%20optofluidic%20Michelson%20interferometer%20using%20tunable%20droplet%20grating.pdf

A reconfigurable optofluidic Michelson interferometer using tunable droplet grating.

Novel optical techniques for sensitive and reproducible fluorescence single cell analysis utilizing setups without single photon counting units are attractive for enhanced low-cost cell parameter screening. In this contribution we present the first microfluidic planar device to form an optofluidic adjustable convex lens with three-dimensional light focusing ability to improve optical sensor systems.

3D fluidic lens shaping—A multiconvex hydrodynamically adjustable optofluidic microlens

This letter reports the tunable liquid microlens using three laminar flows injected into an expansion chamber. Different lens shapes and curvatures can be achieved and tuned through the control of three flow rates. The expansion chamber is designed to improve the fluidic stability and maintain the ideal lens shape for precise microscale optical measurement. The optical aberration is also eliminated by minimizing the diffusive broadening at the interfaces. The collimation and focusing capabilities of three liquid microlenses are demonstrated. The tunable liquid microlens is promising as a tool to realize different optical components that can be integrated onto a microchip.

T. H. Cheng

Papers

Different curvatures of tunable liquid microlens via the control of laminar flow rate