Chenhui Li

Electrowetting-driven variable-focus microlens on flexible surfaces

We demonstrate a flexible, electrowetting-driven, variable-focus liquid microlens. The microlens is fabricated using a soft polymer polydimethylsiloxane. The lens can be smoothly wrapped onto a curved surface. A low-temperature fabrication process was developed to reduce the stress on and to avoid any damage to the polymer. The focal length of the microlens varies between -15.0 mm to +28.0 mm, depending on the applied voltage. The resolving power of the microlens is 25.39 line pairs per mm using a 1951 United States Air Force resolution chart. The typical response time of the lens is around 50 ms.

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.

Synthesis of a photoresponsive liquid-crystalline polymer containing azobenzene.

The synthesis of an oriented liquid-crystalline photoresponsive polymer, prepared by polymerization of mono- and di-acrylates, both of which contain azobenzene chromophores, is reported. The prepared free-standing polymer film shows strong reversible photoinduced deformation upon exposure to unpolarized UV light at 366 nm, as a result of an optically induced isomeric change of the azobenzene moieties in the polymer network. The synthesis process is relatively simple and more efficient compared to conventional ones, and can be used to synthesize other liquid-crystalline photoresponsive polymers. The use of this photoresponsive polymer film as an optical high-pass/low-pass switch under UV or natural light irradiation for a laser beam is demonstrated. This photoresponsive polymer may have applications in robotic systems, artificial muscles, and actuators in microelectromechanical systems (MEMS) and labs on chips.

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.

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.

Fabrication and Characterization of Flexible Electrowetting on Dielectrics (EWOD) Microlens

We present a flexible variable-focus converging microlens actuated by electrowetting on dielectric (EWOD). The microlens is made of two immiscible liquids and a soft polymer, polydimethylsiloxane (PDMS). Parylene intermediate layer is used to produce robust flexible electrode on PDMS. A low-temperature PDMS-compatible fabrication process has been developed to reduce the stress on the lens structure. The lens has been demonstrated to be able to conform to curved surfaces smoothly. The focal length of the microlens is 29–38 mm on a flat surface, and 31–41 mm on a curved surface, varying with the voltage applied. The resolving power of the microlens is 25.39 line pairs per mm by a 1951 United States Air Force (USAF) resolution chart and the lens aberrations are measured by a Shack-Hartmann wavefront sensor. The focal length behavior on a curved surface is discussed and for the current lens demonstrated the focal length is slightly longer on the curved surface as a result of the effect of the curved PDMS substrate.

Hydrogel-Based Tunable-Focus Liquid Microlens Array With Fast Response Time

We present a hydrogel-driven focus-tunable liquid microlens array on a curvilinear surface with much faster response time than previously reported. Water-oil interfaces pinned at polymer apertures serve as microlenses. Thermoresponsive poly(N-isopropylacrylamide) hydrogel incorporating glycidyl-methacrylate-functionalized graphene oxide is employed to provide faster actuation for focal length tuning. Thermoelectric modules based on Peltier effect were implemented to enhance the heat transfer to and dissipation from the hydrogel actuators. The average response time improves to 5 s, and the focal length ranges from 7 to 120 mm. Simulations were performed to characterize the thermal behavior of the microlens array during actuation. The microlens array is also demonstrated with an ability to be remotely controlled by infrared light.

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.

Light Actuation of Graphene-Oxide Incorporated Liquid Crystalline Elastomer Nanocomposites

The authors demonstrate high-performance photo-actuation of nematic liquid crystal elastomer (LCE) nanocomposites incorporating graphene oxide (GO). The nematic LCE serves as the matrix with reversible thermomechanical response. The incorporated GO absorbs photons and converts the photonic energy to heat, thus actuating the LCE nanocomposite. Both infrared and visible lights of wide spectrum (white light) or various wavelength ranges irradiations are able to effectively actuate the LCE nanocomposites, thus proves that they can fully utilize the photo energy of a light source for their mechanical actuation. Attributed to the well dispersity of GO in LCE matrix, sensitive (in seconds) and reversible photo-induced strain of LCE nanocomposites with consistent shape-changing ratio and significantly enhanced mechanical properties are observed. The contraction of the LCE nanocomposite films under light irradiation is about one third of the original length. The effective load-actuation capability is elevated about 50%.

Three-Dimensional Surface Profile Measurement of Microlenses Using the Shack–Hartmann Wavefront Sensor

We present a 3-D surface profiling method for microlenses that utilizes a Shack-Hartmann wavefront sensor. This method applies to both solid microlenses and liquid-liquid interfaces in liquid microlenses. The wavefront at the aperture stop of a microlens is measured by a Shack-Hartmann wavefront sensor and is then used to calculate the 3-D surface profile of the microlens. Three types of microlenses-a photoresist microlens, a hydrogel-driven tunable liquid lens, and an electrowetting-driven tunable liquid lens-were fabricated and measured. The variable-focus liquid lenses were tested within a wide focal length range. The obtained surface profiles were fitted to spherical and conical surface models to study their geometrical properties. The photoresist microlens was found to be approximately spherical. For the hydrogel-driven microlens, the profile was smooth and nearly spherical at the center but became steep and linear at the aperture edges. The electrowetting-driven liquid lens was also fitted better with the conical model, and its conic constant was determined. The obtained surface profiles were used to estimate the optical properties of microlenses in an optical analysis software package. The comparison between the simulation and experiment results indicated that the accuracy of the estimation is rough and the error could be due to the wavefront measurement and surface fitting approximation.