Liquid Microlenses Research Articles

Low voltage electrowetting microlens of two immiscible liquids

Liquid microlenses are widely used due to their unique advantages such as no mechanical moving, tunable focal length, and relevance in optical systems for imaging, concentrating, and biosensing. In this work, we demonstrate development of liquid lenses of two immiscible liquids based on electrowetting on dielectric (EWOD) material. A droplet of electrolytic KCl buffer solution acts a conducting liquid confined in an immiscible insulating oil can work as the liquid lens. The applied electric field seems to control the shape of the conducting droplet –oil interface curvature, altering the focal length. Accordingly, the variations as regards contact angle (CA), droplet height, focal length, etc. have been analyzed for two insulating oil media namely, silicone oil and Johnson’s mineral oil. While CA versus applied voltage trends offered distinct curves for two different kinds of oil, contact angle saturation (CAS) was seen to occur at a CA value, θv=120o.

In Situ Electrohydrodynamic Jet Printing-Based Fabrication of Tunable Microlens Arrays

Tunable microlens arrays (MLAs) with controllable focal lengths have been extensively used in optical sensors, biochips, and electronic devices. The commonly used method is electrowetting on dielectric (EWOD) that controls the contact angle of the microlens to adjust the focal length. However, the fabrication of tunable MLAs at the microscale remains a challenge because the size of MLAs is limited by the external electrodes of EWOD. In this study, a highly integrated planar annular microelectrode array was proposed to achieve an electrowetting tunable MLA. The planar microelectrode was fabricated by electrohydrodynamic jet (E-jet) printing and the liquid microlens was then deposited in situ on the microelectrode. This method could realize 36 tunable liquid microlenses with an average diameter of 24 μm in a 320 × 320 μm2 plane. The fabricated tunable MLAs with higher integration levels and smaller sizes can be beneficial for cell imaging, optofluidic systems, and microfluidic chips.

Dual-responsive shape memory polymer arrays with smart and precise multiple-wetting controllability

Smart-controlled surface wettability from superhydrophilicity to superhydrophobicity has been extensively explored, and stimulus-responsive strategies have been widely accepted as a useful method to realize reversibility. However, achieving smart and precise wetting control remains challenging because most previous studies focused on stimulating single surface chemistry or microstructures. Herein, a dual-stimulus-responsive strategy that can synergistically stimulate surface chemistry and microstructures is demonstrated on the pH-responsive molecule poly(2-(diisopropylamino)ethyl methacrylate (PDPAEMA)-modified temperature-triggered shape memory polymer (SMP) arrays. The responsive PDPAEMA and SMP can provide the surface with tunable surface chemistry and microstructures, respectively. Thus, the wetting of the surface between various states can be reversibly and precisely controlled from superhydrophilicity to superhydrophobicity with contact angle (CA) differences of less than 15° under the cooperative effect between the adjustable surface microstructure and chemistry. The surface is further utilized as a platform to create gradient wettings based on its excellent controllability. Therefore, this work presents a strategy for surface wetting control by combining tunable surface microstructures and chemistry. The prepared samples with a special wetting controllability can be applied to numerous fields, including adaptive liquid microlenses, accurate drug release, and selective catalysis. This work also proposes novel expectations in designing smart functional surfaces.

Bi-phase emulsion droplets as dynamic fluid optical systems

Micro-scale optical components play a critical role in many applications, in particular when these components are capable of dynamically responding to different stimuli with a controlled variation of their optical behavior. Here, we discuss the potential of micro-scale bi-phase emulsion droplets as a material platform for dynamic fluid optical components. Such droplets act as liquid compound micro-lenses with dynamically tunable focal lengths. They can be reconfigured to focus or scatter light and form images. In addition, we discuss how these droplets can be used to create iridescent structural color with large angular spectral separation. Experimental demonstrations of the emulsion droplet optics are complemented by theoretical analysis and wave-optical modelling. Finally, we provide evidence of the droplets utility as fluidic optical elements in potential application scenarios.

Reconfigurable and responsive droplet-based compound micro-lenses

Micro-scale optical components play a crucial role in imaging and display technology, biosensing, beam shaping, optical switching, wavefront-analysis, and device miniaturization. Herein, we demonstrate liquid compound micro-lenses with dynamically tunable focal lengths. We employ bi-phase emulsion droplets fabricated from immiscible hydrocarbon and fluorocarbon liquids to form responsive micro-lenses that can be reconfigured to focus or scatter light, form real or virtual images, and display variable focal lengths. Experimental demonstrations of dynamic refractive control are complemented by theoretical analysis and wave-optical modelling. Additionally, we provide evidence of the micro-lenses’ functionality for two potential applications—integral micro-scale imaging devices and light field display technology—thereby demonstrating both the fundamental characteristics and the promising opportunities for fluid-based dynamic refractive micro-scale compound lenses.

Liquid microlenses and waveguides from bulk nematic birefringent profiles

We demonstrate polarization-selective microlensing and waveguiding of laser beams by birefringent profiles in bulk nematic fluids using numerical modelling. Specifically, we show that radial escaped nematic director profiles with negative birefringence focus and guide light with radial polarization, whereas the opposite - azimuthal - polarization passes through unaffected. A converging lens is realized in a nematic with negative birefringence, and a diverging lens in a positive birefringence material. Tuning of such single-liquid lenses by an external low-frequency electric field and by adjusting the profile and intensity of the beam itself is demonstrated, combining external control with intrinsic self-adaptive focusing. Escaped radial profiles of birefringence are shown to act as single-liquid waveguides with a single distinct eigenmode and low attenuation. Finally, this work is an approach towards creating liquid photonic elements for all-soft matter photonics.

Transparent h- BN/polyacrylamide nanocomposite hydrogels with enhanced mechanical properties

Abstract In this study, a facile way has been proposed to prepare transparent, tough and flexible polyacrylamide (PAM) hydrogels which is composed of a dually crosslinked single network by chemical crosslinking of N , N ′-methylenebisacrylamide (BIS) and physical crosslinking of hydrophilic hexagonal boron nitride ( h- BN) nanosheets. The resulting h- BN/PAM nanocomposite hydrogels are highly transparent, and exhibit significantly enhanced mechanical properties compared to the dark (GO)/PAM nanocomposite hydrogels or chemical crosslinking PAM hydrogels. Thus it opens up new opportunities for developing next-generation transparent, tough and flexible hydrogels that hold great promise in such important applications as light responsive soft robot and liquid microlenses.

Numerical simulation of astigmatic liquid lenses tuned by a stripe electrode

We propose a new design for tuning the astigmatism of liquid micro-lenses using electric field and hydrostatic pressure as control parameters. We explore the feasibility and operating range of the lens with a self-consistent numerical calculation of the electric field distribution and the shape of the two-phase interface. Equilibrium shapes, including surface profiles parallel and perpendicular to a stripe electrode, are extracted to determine the astigmatism. The wavefronts are decomposed into Zernike polynomials under zero defocus conditions using a commercial ray-tracing software. We observe that the global curvature of the lens is primarily controlled by the hydrostatic pressure, while asphericity and astigmatism are controlled by the electric field. For optimized electrode geometries and simultaneous control of pressure and electric fields the astigmatism can be tuned from Z6 = 0…0.38 μm with minor changes in the focal length.

Design and fabrication of an electrohydrodynamically actuated microlens with areal density modulated electrodes

In this paper, we introduce an electrode design for electrohydrodynamically actuated liquid microlenses. The effective electrode areal density radially increases which results in centering of the liquid tunable microlens with a planar device structure. A model was developed to demonstrate the centering mechanism of the liquid microlens. 3D electrostatic simulation was conducted and validity of the idea was examined. A simple fabrication process was developed that uses a surface modified SU-8 as the insulator. The focal length of the microlens was measured to vary from 10.1 mm to 5.8 mm as the voltage varied from zero to 100 V.

Optofluidic lens with tunable focal length and asphericity.

Adaptive micro-lenses enable the design of very compact optical systems with tunable imaging properties. Conventional adaptive micro-lenses suffer from substantial spherical aberration that compromises the optical performance of the system. Here, we introduce a novel concept of liquid micro-lenses with superior imaging performance that allows for simultaneous and independent tuning of both focal length and asphericity. This is achieved by varying both hydrostatic pressures and electric fields to control the shape of the refracting interface between an electrically conductive lens fluid and a non-conductive ambient fluid. Continuous variation from spherical interfaces at zero electric field to hyperbolic ones with variable ellipticity for finite fields gives access to lenses with positive, zero, and negative spherical aberration (while the focal length can be tuned via the hydrostatic pressure).

Large scale arrays of tunable microlenses

We demonstrate a simple and robust method to produce large 2-dimensional and quasi-3-dimensional arrays of tunable liquid microlenses using a time varying external electric field as the only control parameter. With increasing frequency, the shape of the individual lensing elements (~40 μm in diameter) evolves from an oblate (lentil shaped) to a prolate (egg shaped) spheroid, thereby making the focal length a tunable quantity. Moreover, such microlenses can be spatially localized in desired configurations by patterning the electrode. This system has the advantage that it provides a large dynamic range of shape deformation (with a response time of ~30 ms for the whole range of deformation), which is useful in designing adaptive optics.

Optical Detection and Sizing of Single Nanoparticles Using Continuous Wetting Films

The physical interaction between nanoscale objects and liquid interfaces can create unique optical properties, enhancing the signatures of the objects with subwavelength features. Here we show that the evaporation on a wetting substrate of a polymer solution containing submicrometer or nanoscale particles creates liquid microlenses that arise from the local deformations of the continuous wetting film. These microlenses have properties similar to axicon lenses that are known to create beams with a long depth of focus. This enhanced depth of focus allows detection of single nanoparticles using a low-magnification microscope objective lens, achieving a relatively wide field-of-view, while also lifting the constraints on precise focusing onto the object plane. Hence, by creating these liquid axicon lenses through spatial deformations of a continuous thin wetting film, we transfer the challenge of imaging individual nanoparticles to detecting the light focused by these lenses. As a proof of concept, we demonstrate the detection and sizing of single nanoparticles (100 and 200 nm), CpGV granuloviruses, as well as Staphylococcus epidermidis bacteria over a wide field-of-view of 5.10 × 3.75 mm(2) using a 5× objective lens with a numerical aperture of 0.15. In addition to conventional lens-based microscopy, this continuous wetting-film-based approach is also applicable to lens-free computational on-chip imaging, which can be used to detect single nanoparticles over a large field-of-view of >20-30 mm(2). These results could be especially useful for high-throughput field analysis of nanoscale objects using compact and cost-effective microscope designs.

Lateral tunable liquid microlenses for enhanced fluorescence emission in microfluidic channels

Microlenses are important components of optofluidics systems and their lab-on-a-chip applications. We report on liquid microlenses that can be in situ formed in microchannels in a single batch via pneumatic control. These microlenses have optical axes that are parallel to the substrate, and their focal length can be pneumatically tuned separately and independently. In addition, the microlenses can also be pneumatically removed individually and reformed on demand. The parameters affecting the profiles and optical properties of the microlenses, such as the pressure difference and gravity, were investigated by simulation. We then demonstrated the enhancement of fluorescence emission in a microfuidic channel using our microlenses to focus attenuated excitation laser beams into regions of interest in the channel. With the microlenses, the region with visible fluorescence response was enlarged by up to 13 times and the intensity of fluorescence emission was enhanced by up to 38 times, compared to the cases without the lenses.

High speed adaptive liquid microlens array

Liquid microlenses are attractive for adaptive optics because they offer the potential for both high speed actuation and parallelization into large arrays. Yet, in conventional designs, resonances of the liquid and the complexity of driving mechanisms and/or the device architecture have hampered a successful integration of both aspects. Here we present an array of up to 100 microlenses with synchronous modulation of the focal length at frequencies beyond 1 kHz using electrowetting. Our novel concept combines pinned contact lines at the edge of each microlens with an electrowetting controlled regulation of the pressure that actuates all microlenses in parallel. This design enables the development of various shapes of microlenses. The design presented here has potential applications in rapid parallel optical switches, artificial compound eye and three dimensional imaging.

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.

Dynamic Behavior of Liquid Microlenses Actuated Using Dielectric Force

This paper presents a study of the dynamic responses of dielectric liquid microlenses via simulation and experiment. The dielectric force applied to the liquid microlenses was generated on the interface of two nonconductive isodensity liquids under ac biases. The dynamic responses of liquid microlenses of 0.5-10 mm in diameter were experimentally characterized; responses such as the dynamic contact angle and the response time with respect to viscosity, voltage, ac frequency, and droplet size were measured. The experimental results indicate that the viscosity and the droplet size are proportional to the response time. The applied ac frequency in the range of 1-50 kHz was inversely proportional to the response time. The contact angles in equilibrium were found to be inversely proportional to the ac frequency and the droplet size but nearly invariant to the viscosity.

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.

Exploring the capabilities of Digital Holography as tool for testing optical microstructures

A demonstration of the capabilities of Digital Holography (DH) in microscope configuration is presented for inspecting and qualifying optical microstructures. Different structures with dimensions ranging from hundreds to few tens of microns are investigated, analyzed and characterized by DH showing that the technique is suitable either for liquid as well as polymeric microlenses. Thanks to the numerical reconstruction of the complex wavefields, we show that DH is able to retrieve not only the morphology of each single structure element but also to furnish accurate information on all the changes (spontaneous or induced) occurring during the inspection time, such as the curvature variation of liquid microlenses or the wavefront distortion.

An optofluidic volume refractometer using Fabry–Pérot resonator with tunable liquid microlenses

This letter reports the development of an optofluidic Fabry-Pérot (FP) resonator, which consists of a microcavity and a pair of liquid microlenses. The microcavity forms part of the microchannel to facilitate sample injection. The liquid microlenses are used for efficient light coupling from the optical fiber to the microcavity. The liquid microlens collimates the diverging light from the optical fiber into the FP cavity, which provides real-time tuning to obtain the highest possible finesse up to 18.79. In volume refractive index measurement, a sensitivity of 960 nm per refractive index unit (RIU) and a detection range of 0.043 RIU are achieved.

Diamond turning and soft lithography processes for liquid tunable lenses

Making use of the capability of high precision diamond turning in producing 3-dimensional free form optical surfaces with excellent surface finish, molds for various types of liquid tunable microlenses are fabricated. Subsequently, a rapid prototyping process known as soft lithography is applied to the fabricated mold to replicate multiple lens structures. This method provides an efficient and reliable way of generating rotationally symmetric free form optical surfaces that are otherwise difficult to produce with conventional methods such as lithography and etching methods. Using atomic force microscopy, white light interferometry and a mechanical profiler, it is verified that the surface quality and dimensional accuracy of the replicas are preserved. We demonstrate the practical usefulness of the proposed fabrication methods by developing and experimentally testing three different liquid tunable lenses, namely (1) a diffractive/refractive hybrid lens that reduces chromatic aberration within the visible spectrum, (2) a double focusing lens and (3) an aspherical lens that minimizes spherical aberration.