Tunable Microlens Research Articles

Highly flexible MTF measurement system for tunable micro lenses

We present an efficient, low-cost modulation transfer function (MTF) measurement approach, optimized for characterization of tunable micro-lenses; the MTF may easily be measured at a variety of different focal lengths. The approach uses a conventional optical microscope with an optimized approach for lens illumination and the measurement results have been correlated with a commercial MTF measurement system. Measurements on fixed-focus and tunable micro-lenses were performed; for the latter, resolution for lenses with back focal length of 11 mm was 55 lines/mm, decreasing to 40 lines/mm for a back focal length of 4 mm. In general, it was seen that performance was better for lenses with longer focal lengths.

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.

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

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.

Large-Stroke Dielectric Elastomer Actuators With Ion-Implanted Electrodes

In this paper, we present miniaturized polydimethylsiloxane (PDMS)-based diaphragm dielectric elastomer actuators capable of out-of-plane displacement up to 25% of their diameter. This very large percentage displacement is made possible by the use of compliant electrodes fabricated by low-energy gold ion implantation. This technique forms nanometer-scale metallic clusters up to 50 nm below the PDMS surface, creating an electrode that can sustain up to 175% strain while remaining conductive yet having only a minimal impact on the elastomer's mechanical properties. We present a vastly improved chip-scale process flow for fabricating suspended-membrane actuators with low-resistance contacts to implanted electrodes on both sides of the membrane. This process leads to a factor of two increase in breakdown voltage and to <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> time constant shorter than mechanical time constants. For circular diaphragm actuator of 1.5-3-mm diameter, voltage-controlled static out-of-plane deflections of up to 25% of their diameter is observed, which is a factor of four higher than our previous published results. Dynamic characterization shows a mechanically limited behavior, with a resonance frequency near 1 kHz and a quality factor of 7.5 in air. Lifetime tests have shown no degradation after more than 4 million cycles at 1.5 kV. Conductive stretchable electrodes photolithographically defined on PDMS were demonstrated as a key step to further miniaturization, enabling large arrays of independent diaphragm actuators on a chip, for instance for tunable microlens arrays or arrays of micropumps and microvalves.

Voltage-programmable liquid optical interface

Recently, there has been intense interest in photonic devices based on microfluidics, including displays1,2 and refractive tunable microlenses and optical beamsteerers3,4,5 that work using the principle of electrowetting6,7. Here, we report a novel approach to optical devices in which static wrinkles are produced at the surface of a thin film of oil as a result of dielectrophoretic forces8,9,10. We have demonstrated this voltage-programmable surface wrinkling effect in periodic devices with pitch lengths of between 20 and 240 µm and with response times of less than 40 µs. By a careful choice of oils, it is possible to optimize either for high-amplitude sinusoidal wrinkles at micrometre-scale pitches or more complex non-sinusoidal profiles with higher Fourier components at longer pitches. This opens up the possibility of developing rapidly responsive voltage-programmable, polarization-insensitive transmission and reflection diffraction devices and arbitrary surface profile optical devices. Voltage-programmable liquid surface profiles with large amplitudes resulting from dielectrophoresis are demonstrated. The oil interface formed can be ultrastable and static, or rapidly switchable, as shown in the case of a modulated diffraction grating. The scheme provides the possibility for responsive and polarization-insensitive transmission and reflection devices, and for optical interfaces with arbitrary surface profiles.

A negative–positive tunable liquid-crystal microlens array by printing

A tunable microlens array by printing was demonstrated. An UV-curable adhesive, NOA65, was printed and cured to form a lens array profile on an ITO glass. Then this microlens array ITO glass was assembled with a normal ITO glass to form a cell, which was later filled with a liquid crystal. The focal length of the lens array can be tuned by an electric field, which changes the index difference between liquid crystals and NOA65 due to the reorientation of the LC molecules. In our experiment, the focal length varied from -2.29 cm to 3.12 cm when the applied voltage was increased from 0 V to 13.26 V.

Tunable liquid microlens arrays in electrode-less configuration and their accurate characterization by interference microscopy

A special class of tunable liquid microlenses is presented here. The microlenses are generated by an electrowetting effect under an electrode-less configuration and they exhibit two different regimes that are named here as separated lens regime (SLR) and wave-like lens regime (WLR). The lens effect is induced by the pyroelectricity of polar dielectric crystals, as was proved in principle in a previous work by the same authors (S. Grilli et al., Opt. Express 16, 8084, 2008). Compared to that work, the improvements to the experimental set-up and procedure allow to reveal the two lens regimes which exhibit different optical properties. A digital holography technique is used to reconstruct the transmitted wavefront during focusing and a focal length variation in the millimetre range is observed. The tunability of such microlenses could be of great interest to the field of micro-optics thanks to the possibility to achieve focus tuning without moving parts and thus favouring the miniaturization of the optical systems.

Tunable Liquid Gradient Refractive Index (L-GRIN) lens with two degrees of freedom

We report a tunable optofluidic microlens configuration named the Liquid Gradient Refractive Index (L-GRIN) lens for focusing light within a microfluidic device. The focusing of light was achieved through the gradient refractive index (GRIN) within the liquid medium, rather than via curved refractive lens surfaces. The diffusion of solute (CaCl(2)) between side-by-side co-injected microfluidic laminar flows was utilized to establish a hyperbolic secant (HS) refractive index profile to focus light. Tailoring the refractive index profile by adjusting the flow conditions enables not only tuning of the focal distance (translation mode), but also shifting of the output light direction (swing mode), a second degree of freedom that to our knowledge has yet to be accomplished for in-plane tunable microlenses. Advantages of the L-GRIN lens also include a low fluid consumption rate, competitive focusing performance, and high compatibility with existing microfluidic devices. This work provides a new strategy for developing integrative tunable microlenses for a variety of lab-on-a-chip applications.

Electrowetting for Tunable Microoptics

The design, fabrication, and optical properties of tunable liquid microoptical components based on electrowetting are presented. Two optical microsystems are discussed: a single liquid microlens with tunable focal length, centered and stabilized in a micromachined positioning structure, and laterally repositionable tunable liquid microlens arrays. The latter employs electrowetting for lateral movement as well as focal length tuning of the optical elements. Through the use of novel microstructuring techniques, a high positioning accuracy may be achieved with low electrode count.

Numerical simulation for meniscus shape and optical performance of a MEMS-based liquid micro-lens

It is very difficult to fabricate tunable optical systems having an aperture below 1000 micrometers with the conventional means on macroscopic scale. Krogmann et al. (J. Opt. A 8, S330-S336, 2006) presented a MEMS-based tunable liquid micro-lens system with an aperture of 300 micrometers. The system exhibited a tuning range of back focal length between 2.3mm and infinity by using the electrowetting effect to change the contact angle of the meniscus shape on silicon with a voltage of 0-45 V. However, spherical aberration was found in their lens system. In the present study, a numerical simulation is performed for this same physical configuration by solving the Young-Laplace equation on the interface of the lens liquid and the surrounding liquid. The resulting meniscus shape produces a back focal length that agrees with the experimental observation excellently. To eliminate the spherical aberration, an electric field is applied on the lens. The electric field alters the Young-Laplace equation and thus changes the meniscus shape and the lens quality. The numerical result shows that the spherical aberration of the lens can be essentially eliminated when a proper electric field is applied.

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 Pneumatic Microoptics

A new class of tunable and actuated microoptical devices is presented: pneumatic microoptics. Using microelectromechanical system fabrication technology extended by the use of polydimethylsiloxane (PDMS) membranes, tunable microlenses, and lens arrays, actuated micromirrors with large tilt angles and tip-tilt piston mirrors have been designed and fabricated. Actuation is by pressure: Gas- or liquid-filled microfluidic cavities are employed to distend the microfabricated PDMS structures which then act as a lens surface or as an actuator for a micromirror. Thermopneumatic actuation is also employed for completely integrated tunable optical systems in which all actuator and optical components are fabricated on-chip. The technology is particularly promising for microsystem applications in which significant movement is required but high voltages or external fields are impractical. [2007-0301].

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

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.

A liquid crystalline polymer microlens array with tunable focal intensity by the polarization control of a liquid crystal layer

We propose a focal intensity tunable microlens array by using a birefringent liquid crystalline polymer for lensing action. Due to the difference of effective refractive indices, it acts as a positive or negative microlens with respect to the polarization state. As we control the incident polarization by adding a liquid crystal layer, the focal intensity can be tuned by an applied voltage. Twisted nematic and bistable ferroelectric liquid crystal modes were applied to demonstrate the possibility of various driving features such as a continuously tunable focal intensity or fast switching with memory effect.

Chemically Tunable Lensing of Stimuli‐Responsive Hydrogel Microdomes

Chemically tunable hydrogel microlenses have been developed from acrylamide hydrogels based on covalently bound hinge-motion binding proteins, like calmodulin, and low affinity ligands, i.e. phenothiazine. Initially the protein binds to phenothiazine forming chemical crosslinks within the hydrogel, which are released in the presence of the higher affinity ligand chlorpromazine resulting in the swelling of the microlenses.

Reconfigurable liquid micro-lenses with high positioning accuracy

A new technique to improve the positioning accuracy of liquid droplets in electrowetting-on-dielectric applications is presented. The approach is based on a structured dielectric layer providing defined pinning barriers for the droplet, and allows the reduction of the number of implemented electrodes while preserving the positioning accuracy of the liquid droplet. By use of voltage pulses, the droplet is shifted from one defined position to the next. A tunable and repositionable liquid micro-lens system based on this approach is demonstrated.

Tunable and movable liquid microlens in situ fabricated within microfluidic channels

The authors report on a tunable and movable liquid microlens in situ fabricated through fluid manipulation within microfluidics. Taking advantage of surface tensions at the microscale, the microlens is formed by a liquid droplet interfacing air. Through pneumatic control of the droplet, the microlens can be tuned in focal length and moved within the microfluidic channel on demand, thus being highly reconfigurable. A focal length tuning range from 1.5to8.9mm is demonstrated. The in-plane optical axis of the microlens provides the flexibility in designing micro-optics within microfluidics, as demonstrated by realizing a planar fluorescence detection device.

Materials, effects and components for tunable micro‐optics

AbstractAn overview of recent activities in the area of tunable micro‐optical components is given. These include polymer‐based deformable mirrors for adaptive optics, tunable microlenses and arrays using fluids and membranes, pneumatically actuated scanning micromirrors and tunable Bragg filters and mirrors using swelling polymers. For each device, the structure, essential fabrication technology and optical characteristics, as well as a discussion of application areas are presented. Copyright © 2007 Institute of Electrical Engineers of Japan© 2007 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.

Hydrodynamically tunable optofluidic cylindrical microlens

In this work, we report the design, fabrication, and characterization of a tunable optofluidic microlens that focuses light within a microfluidic device. The microlens is generated by the interface of two co-injected miscible fluids of different refractive indices, a 5 M CaCl(2) solution (n(D) = 1.445) and deionized (DI) water (n(D) = 1.335). When the liquids flow through a 90-degree curve in a microchannel, a centrifugal effect causes the fluidic interface to be distorted and the CaCl(2) solution bows outwards into the DI water portion. The bowed fluidic interface, coupled with the refractive index contrast between the two fluids, yields a reliable cylindrical microlens. The optical characteristics of the microlens are governed by the shape of the fluidic interface, which can be altered by simply changing the flow rate. Higher flow rates generate a microlens with larger curvature and hence shorter focal length. The changing of microlens profile is studied using both computational fluid dynamics (CFD) and confocal microscopy. The focusing effect is experimentally characterized through intensity measurements and image analysis of the focused light beam, and the experimental data are further confirmed by the results from a ray-tracing optical simulation. Our investigation reveals a simple, robust, and effective mechanism for integrating optofluidic tunable microlenses in lab-on-a-chip systems.

Tunable microfluidic optical devices with an integrated microlens array

Interest in dynamically tuning light has attracted great attention to the fabrication of tunable microlens arrays. Here we discuss the fabrication and characterization of a simple, robust, yet tunable microfluidic optical device with an integrated microlens array. The microfluidic device with a desired channel structure was micromachined on a polycarbonate plate with a resolution of up to 100 µm, followed by thermal bonding two plates above their glass transition temperature. The microlens arrays were replica molded on a glass slide, which was then attached to the polycarbonate plates. By simply actuating the liquids with variable refractive index into the fluidic channel to immerse the lens arrays without moving or deformation of microlenses, a large change of focal length of more than ten times (f = 0.74–8.53) was achieved. When a dye-containing liquid was pumped into the microfluidic channel to cover the lenses, the light transmission through the lenses was reduced from about 95% to 55% when the dye concentration was increased to 10 w/v%. The knowledge we gain from these studies will provide important insights to construct new, adaptive, micro-scale optical devices with multiple functionalities.