Xuefeng Zeng

Liquid tunable microlenses based on MEMS techniques

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.

Tunable liquid microlens actuated by infrared light-responsive hydrogel

We report on liquid-based tunable microlenses actuated by infrared (IR) light. Multiple micropost structures, made of IR light-responsive hydrogel with entrapped gold nanoparticles, are photopatterned around a lens aperture. The volumetric change in the hydrogel, controlled by IR light, regulates the curvature of a liquid-liquid interface forming the microlens at the aperture and its focal length. The focal length of the microlens can be tuned from −17.4mmto+8.0±0.4mm in seconds under IR irradiation. This microlens can be integrated into optical systems, for instance, for fiber endoscopy.

Tunable-focus microlens arrays on curved surfaces

We present a microlens array consisting of multiple liquid-based tunable-focus microlenses omnidirectionally fabricated on a hemisphere, resulting in large field of view. Polymer bridge structure is formed between microlenses to reduce the stress and deformation in each lens structure. Each microlens in the array is formed via a water-oil interface at its lens aperture. Photopatterned thermo-responsive hydrogel actuators are used to regulate the curvature of the water-oil interface, thus tuning the focal length, ranging from millimeters to infinity.

Tunable microlens arrays actuated by various thermo-responsive hydrogel structures

We report on liquid-based tunable-focus microlens arrays made of a flexible polydimethylsiloxane (PDMS) polymer. Each microlens in the array is formed through an immiscible liquid–liquid interfacial meniscus. Here deionized water and silicone oil were used. The liquids were constrained in the PDMS structures fabricated through liquid-phase photopolymerization for molding and soft lithography. The microlenses were actuated by thermo-responsive N-isopropylacrylamide (NIPAAm) hydrogel microstructures and could be tuned individually by changing the local temperature. The NIPAAm hydrogels expanded and contracted, absorbing and releasing water, at different temperatures. Thus the pressure across the water–oil interface in the microlenses varied responding to the temperature, tuning their corresponding focal lengths. The microlens diameter was 2.4 mm. The typical microlens focal length was measured to be from 8 to 60 mm depending on the temperature. The microlens response time actuated by different structures and components of the NIPAAm hydrogels were compared. The normalized light intensities of the microlens focused spots were measured, matching well with a Zemax simulation, to study the microlens spherical aberrations. The NIPAAm hydrogel durability was also measured.

Polydimethylsiloxane Microlens Arrays Fabricated Through Liquid-Phase Photopolymerization and Molding

We report on polydimethylsiloxane (PDMS) microlens arrays fabricated through liquid-phase photopolymerization and molding. The gist of this fabrication process is to form liquid menisci of variable radii of curvature at an array of apertures through pneumatic control, followed by photopolymerization under ultraviolet radiance. The resultant polymerized structures are then transferred to PDMS utilizing two molding steps. By adjusting the pneumatic pressure during the process, a single aperture array can be used to fabricate PDMS microlens arrays with variant focal lengths. The liquid menisci are formed by liquid-air interfaces that are pinned at the top edges of the apertures along hydrophobic-hydrophilic boundaries generated through surface chemical treatments. The microlens arrays are optically characterized. Variant focal lengths from 2.35 to 5.54 mm and f-numbers from 1.27 to 5.88, dependent on the diameter of apertures and the applied pressure to form the liquid menisci, are achieved with this relatively simple process and match well with the physical model. Owing to the formation from the liquid-air interfaces, the surface roughness of microlenses is measured to be around 25 nm.

Fiber Endoscopes Utilizing Liquid Tunable-Focus Microlenses Actuated Through Infrared Light

We report on prototype fiber endoscopes with tunable-focus liquid microlenses integrated at their distal ends and actuated through infrared (IR) light. Tunable-focus microlenses allow minimal back-and-forth movements of the scopes themselves and different depths of focus (DOFs), thus having spatial depth perception in the obtained images. The liquid microlens was formed by a water-oil meniscus pinned at a hydrophobic-hydrophilic boundary at an aperture. IR light-responsive hydrogel microstructures were formed by photopatterning thermo-responsive N-isopropylacrylamide hydrogel with entrapped IR light absorbing gold nanoparticles. The volumetric change in the hydrogel microstructures regulated the pressure difference across the water-oil interface and thus varied its focal length. The operations of the microlenses were realized through light transmitted via optical fibers. The images obtained from the microlenses were transferred via image fiber bundles. For two alignments between the hydrogel structures and the fibers, the response times of the microlenses are 65 and 20 s, respectively. Images of the simulated polyps in simulated colons were obtained. The range of focal length of a typical microlens was from 52 to 8 mm. The angle of view of an endoscope was from 77° to 128°. A microlens array could potentially be utilized to simultaneously obtain different DOFs and to increase the field of view.

Focus-Tunable Microlens Arrays Fabricated on Spherical Surfaces

We present microlens arrays consisting of multiple focus-tunable microlenses omnidirectionally fabricated on spherical surfaces, to realize large field of view. Thin flexible polymer bridges connecting adjacent microlenses are designed to reduce the wrapping stress and deformation of the microlens array. Each microlens, formed via water-oil interface, is individually tuned by a thermo-responsive hydrogel actuator. The range of the focal length of each microlens in this array varied from millimeters to infinity. A prototype of optical imaging system based on such a microlens array on a spherical surface, including a charged-coupled device camera, a fiber bundle, and a 3-D rotational stage, is demonstrated as well.

Three-dimensional surface profiling and optical characterization of liquid microlens using a Shack―Hartmann wave front sensor

We demonstrate three-dimensional (3D) surface profiling of the water-oil interface in a tunable liquid microlens using a Shack-Hartmann wave front sensor. The principles and the optical setup for achieving 3D surface measurements are presented and a hydrogel-actuated liquid lens was measured at different focal lengths. The 3D surface profiles are then used to study the optical properties of the liquid lens. Our method of 3D surface profiling could foster the improvement of liquid lens design and fabrication, including surface treatment and aberration reduction.

Fabrication of Complex Structures on Nonplanar Surfaces Through a Transfer Method

We report on a method to fabricate complex microelectromechanical systems (MEMS) structures onto a flexible membrane. Three-dimensional and high-aspect-ratio MEMS structures were formed on a silicon-on-insulator wafer by deep reactive-ion etching and then were partially buried in a flexible polymer membrane. After being released from the silicon substrate, the flexible membrane with the complex structures could deform to curvilinear shapes and then could be transferred onto curvilinear surfaces. The effect of the structure and density of the pillars on the transferring procedure is studied. Potentially, this method could be utilized to transfer any MEMS structures and devices to any nonplanar surfaces, thus possessing great potential in MEMS devices.

Fabrication of Large-Area Three-Dimensional Microstructures on Flexible Substrates by Microtransfer Printing Methods

This paper presents two robust microtransfer printing methods, namely, multiple transfer printing and peeling microprinting methods, to fabricate three-dimensional (3-D) and high-aspect-ratio microelectromechanical systems (MEMS) structures over large areas on flexible polydimethylsiloxane (PDMS) substrates. These techniques enable conformal wrapping of 3-D microstructures, initially fabricated in two-dimensional (2-D) layouts with standard fabrication technology onto a wide range of surfaces with complex and curvilinear shapes. The processes exploit the differential adhesive tendencies of the microstructures formed between a donor and a transfer substrate to accomplish an efficient release and transfer process. Experimental and theoretical studies show that the MEMS structures with a wide variety of pattern densities can be conformally transferred to bendable device substrates while keeping the structural integrity and density intact. Quantitative stress analysis on the micromechanics of such a curvilinear system suggests that the stress induced by wrapping the complete structure onto a cylinder is mostly in the flexible PDMS substrate, while the MEMS structures experience little stress.