Tunable Microlens Research Articles

Advances in the development of tunable lenses in Mexico

Advances in the development of tunable lenses have produced more compact, simple and lightweight optical systems. There are different types of tunable lenses; among the simplest we can find liquid and solid lenses, which only require a mechanical mount to change the shape of the lenses. We present the analysis and results that were obtained recently from these two types of lenses which have produced an improvement in the quality of images formation as well as some of their future applications. Full Text: PDF References N. Nguyen, "Micro-optofluidic Lenses: A review", Biomicrofluidics 4, 031501-1 (2010). CrossRef U. Levy, R. Shamai, "Tunable optofluidic devices", Microfluidics and Nanofluidics 4, 97 (2008). CrossRef G. R. Xiong et al. "Liquid microlens with tunable focal length and light transmission", Appl. Phys. Lett. 92, 241119 (2008). CrossRef A. Weber and H. Zappe, IEEE/ASME JMEMS 17, 1218 (2008). A. Santiago-Alvarado et al. "Simulating the functioning of variable focus length liquid-filled lenses using the finite element method (FEM)", Optik, 124, 1003 (2013). CrossRef B. Malouin et al. "Electromagnetic liquid pistons for capillarity-based pumping", Lab. Chip 11, 393 (2011). CrossRef W. Wang and J. Fang, "Design, fabrication and testing of a micromachined integrated tunable microlens", J. Micromech. Microeng 16, 1221 (2006). CrossRef C. López, C. Lee and A. Hirsa, "Electrochemically activated adaptive liquid lens", Appl. Phys. Lett. 87, 134102 (2005). CrossRef N. Nam-Trung, "Micro-optofluidic Lenses: A review", BIOMICROFLUIDICS 4,031501 (2010). CrossRef I. Jung et al. "Dynamically tunable hemispherical electronic eye camera system with adjustable zoom capability", Proc. Natl. Acad. Sci. 108, 1788 (2011). CrossRef A. Cruz-Félix, A. Santiago-Alvarado, IJESIT 3, 563 (2014). R. Kingslake, Lens Design Fundamental, (New York, Academic Press 1978). A. Santiago Alvarado et al. Jour. of Phys: Conf. Series 274 (2011). A. Santiago Alvarado et al. "Measurement of aberrations of a solid elastic lens using a point-diffraction interferometer", Opt. Eng. 49, 123401 (2010). CrossRef

Simulation Study on Polarization-Independent Microlens Arrays Utilizing Blue Phase Liquid Crystals with Spatially-Distributed Kerr Constants

Polarization independent liquid crystal (LC) microlens arrays based on controlling the spatial distribution of the Kerr constants of blue phase LC are simulated. Each sub-lens with a parabolic distribution of Kerr constants results in a parabolic phase profile when a homogeneous electric field is applied. We evaluate the phase distribution under different applied voltages, and the focusing properties of the microlens arrays are simulated. We also calculate polarization dependency of the microlenses arrays at oblique incidence of light. The impact of this study is to provide polarizer-free, electrically tunable focusing microlens arrays with simple electrode design based on the Kerr effect.

Tunable microlens actuated via a thermoelectrically driven liquid heat engine

We have developed a thermally actuated liquid microlens. An embedded thermoelectric element is used to actuate the liquid based heat engine. A closed-loop system is harnessed to drive and stabilize the temperature of the heat engine. Direct contact between the thermoelectric device and the water results in greatly improved, sub-second thermal rise time (0.8 s). The water based heat engine reacts to the variation in the temperature via expansion and contraction. In turn, the shape of a pinned water-oil meniscus at a lens aperture is deformed in response to the net volume change in the water, creating a tunable microlens. A method to fabricate microfluidic devices with relatively high thickness (250–750 μm) and large length-to-depth aspect ratio (280:1) was developed and used in the process. After fabrication and thermal calibration, optical characteristic of the microlens was assessed. Back focal length of the microlens was shown to vary continuously from −19.6 mm to −6.5 mm as the temperature increased from 5 °C to 35 °C. A thin film air was further introduced to insulate the heat engine from the substrate to protect the microlens area from the temperature fluctuation of the heat engine, thus preventing the change of the refractive indices and thermally induced aberrations. Wavefront aberration measurement was conducted. Surface profile of the microlens was mapped and found to have a conical shape. Both 3-dimensional and 1-dimensional thermal models for the device structure were developed and thermal simulation of the device was performed.

Fast-Response Liquid Crystal Microlens

Electrically tunable liquid crystal microlenses have attracted strong research attention due to their advantages of tunable focusing, voltage actuation, low power consumption, simple fabrication, compact structure, and good stability. They are expected to be essential optical devices with widespread applications. However, the slow response time of nematic liquid crystal (LC) microlenses has been a significant technical barrier to practical applications and commercialization. LC/polymer composites, consisting of LC and monomer, are an important extension of pure LC systems, which offer more flexibility and much richer functionality than LC alone. Due to the anchoring effect of a polymer network, microlenses, based on LC/polymer composites, have relatively fast response time in comparison with pure nematic LC microlenses. In addition, polymer-stabilized blue phase liquid crystal (PS-BPLC) based on Kerr effect is emerging as a promising candidate for new photonics application. The major attractions of PS-BPLC are submillisecond response time and no need for surface alignment layer. In this paper, we review two types of fast-response microlenses based on LC/polymer composites: polymer dispersed/stabilized nematic LC and polymer-stabilized blue phase LC. Their basic operating principles are introduced and recent progress is reviewed by examples from recent literature. Finally, the major challenges and future perspectives are discussed.

Wafer-scale fabricated thermo-pneumatically tunable microlenses

We have developed a self-contained, liquid tunable microlens based on polyacrylate membranes integrated with compact on-chip thermo-pneumatic actuation fabricated using full-wafer processing. Silicone oil is used as the optical liquid, which is pushed or pulled into the lens cavity via an extended microfluidic channel structure without any pumps, valves or other mechanical means. The heat load generated by the thermal actuator is physically isolated from the lens chamber. The back focal length may be tuned from infinity to 4 mm with a maximum power consumption of 300 mW. The principal application is fine tuning of the back focal length, for which tuning time constants as small as 100 ms are suitable.

Polarization-independent and fast tunable microlens array based on blue phase liquid crystals

This work investigates a polarization-independent and fast response microlens array. This array is composed of a concave polymer microlens array and blue phase liquid crystals (BPLCs). The microlens array can be either positive or negative, depending on the birefringence of the BPLCs. The experimental results show that the microlens array is fast switched between positive and negative focal lengths via controlling the electric fields, and the response time is a few hundred microseconds. Additionally, the focusing efficiency is independent of the polarization of the incident light.

Design and Fabrication of a Tunable Two-fluid Micro-lens Device with a Large Deflection Polymer Actuator

A large deflection polymer bending actuator was employed in the design of an integrated micro-lens with electrically adjustable focal length. This micro-lens consisted of micro-fluidic chambers bonded together and sealed by an elastomeric lens membrane to which the polymer actuator has been attached. For focal length optimization, the chambers could be filled with fluids of different refractive indices. The multilayered actuator of radial shape and the variable micro-lens chip design were optimized using 2D FEM simulations. Fabrication steps included bonding of polymer actuator stack with the elastomeric lens membrane on a micro-machined silicon wafer. For a tunable micro-lens with 3mm aperture, a tuning range in optical power between 0 and more than 15 dpt was achieved.

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.

Tunable liquid crystal microlenses with crater polymer prepared by droplet evaporation

A simple and low-cost technique is proposed to construct a tunable liquid crystal (LC) microlens with a crater polymer structure, which is prepared using micro-drop technology and 2-step UV polymerization. The dimensions and the geometric profile of the restructured polymer surface significantly depend on the volume of the micro droplet, and the UV irradiation dose. In this work, the focal length of the LC microlens is controlled electrically from infinity to 7.8 cm. Such a microlens has prospective applications in optical communications, image processing, and switchable 2D/3D displays.

Sensitized gelatin as a versatile biomaterial with tailored mechanical and optical properties

We have found that gelatin doped with tot'hema (medication used for curing anemia) and eosin becomes elastic, while retaining good optical properties. The mechanical properties of tot'hema–eosin-sensitized gelatin films (TESG) have been investigated for various concentrations of tot'hema (ranging from 5 to 30% v/v). TESG specimens were prepared according to the ASTM standards for elastic materials. Tensile strength, strain at break and Young's modulus were measured. The results show that the extensibility of TESG film increases with increasing tot'hema concentration, while the Young's modulus and stress at break exponentially decrease. Mechanical properties can be tailored to suit various biomedical applications such as blood vessels, human lens capsules and biosensors based on microlenses. Tunable (strain responsive) TESG microlenses were produced and a mechanical model of blood vessels was prepared.

An EpoClad/EpoCore-based platform for MOEMS fabrication

EpoClad and EpoCore are the photopatternable epoxies with slightly different refractive indices that were developed to create optical waveguides. In this paper, we present a simple, robust and flexible fabrication platform for micro-opto-electro-mechanical systems applications based on these materials. EpoClad and EpoCore are used for the cladding and core layers of integrated optical waveguides, respectively, as well as for the surrounding mechanical framework that includes a built-in fiber clamping system that allows both horizontal and vertical clamping of optical fibers. The transmission properties of various waveguide configurations are studied. Then, both mechanical and thermal modulators of optical transmission are investigated. A tunable in-plane hydrogel microlens setup is then shown that further demonstrates the concept of the proposed optical platform.

MEMS-based 3D confocal scanning microendoscope using MEMS scanners for both lateral and axial scan

A fiber-optic 3D confocal scanning microendoscope employing MEMS scanners for both lateral and axial scan was designed and constructed. The MEMS 3D scan engine achieved a lateral scan range of over ±26° with a 2D MEMS scanning micromirror and a depth scan of over 400μm with a 1D MEMS tunable microlens. The lateral resolution and axial resolution of this system were experimentally measured as 1.0μm and 7.0μm, respectively. 2D and 3D confocal reflectance images of micro-patterns, micro-particles, onion skins and acute rat brain tissue were obtained by this MEMS-based 3D confocal scanning microendoscope.

Thermally actuated tunable liquid microlens with sub-second response time

This letter reports a thermally actuated liquid microlens. In this design, a thermoelectric device was brought into direct contact with water to alter the water temperature and drive the lens through the thermal expansion of water. The shape of a pinned water meniscus at an aperture was deformed in response to the net volume change in the water, creating a tunable lens with a fast thermal response time of 0.8 s. Focal length of the microlens varied continuously from −82 mm to −29 mm as the temperature was increased from 20 °C to 30 °C.

Multifunctional LTCC Substrates for Thermal Actuation of Tunable Micro-Lenses Made of Aluminum Nitride Membranes

Thin membranes are widely used for tunable micro-lenses, where the membrane, usually made from polymers, defines the surface of a subjacent liquid. If the liquid is pressurized, the membrane deflects and forms a lens. In many cases, a macroscopic pump is used to generate the pressure. Here, we use a multifunctional LTCC substrate, which consists of two cavities. A micro-fluidic network allows their independent filling. For the actuation, the actuating cavity is filled with air and the optical cavity is filled with immersion oil. If the actuating cavity is heated by screen-printed resistors, the fluid expands and a pressure is transmitted to the optical cavity via a micro-fluidic channel. The optical cavity is covered with a glass chip and a membrane chip, respectively. We use 500 nm thick membranes of aluminum nitride (AlN), which have a 3 mm diameter and are fabricated using technologies of silicon based micro-electro-mechanical systems. With 6 W electrical heating power a temperature increase of 100 K within the actuating cavity is achieved. The generated pressure causes a membrane deflection in the optical cavity of about 30 μm. The thermal actuation is investigated using thermal imaging. By heating the actuating cavity, the optical cavity also heats up, but no temperature gradients along the silicon chip is found. Optical experiments show the imaging capability of the lens and confirm a tunable refractive power of 0..17 dpt.

MOEMS tunable microlens made of aluminum nitride membranes

We present tunable lenses based on aluminum nitride membranes. The achievable tuning range in the refractive power is 0 to 25 dpt with an external pressure load of ≤20 kPa . The lenses are manufactured using MOEMS technology. For 500-nm-thick membranes with a diameter of 3 mm, a spherical deflection profile is found. The system provides good long-term stability showing no creep or hysteresis. A model for the refractive power versus applied pressure is derived and validated experimentally. Based on this model, design guidelines are discussed. One essential parameter is the residual stress of the aluminum nitride layer that can be controlled during deposition.

Imprinted hydrogels for tunable hemispherical microlenses

We report on the development of an ultraviolet curable hydrogel, based on combinations of poly(ethylene glycol) dimethacrylate (PEGMA), acrylic acid (AA) and N-Isopropylacrylamide (NIPPAm) for imprint lithography processes. The hydrogel was successfully imprinted to form dynamic microlens arrays. The response rate of the microlenses by volume change to water absorption was studied optically showing tunable focalisation of the light. Important optical refractive index change was measured between the dry and wet state of the microlenses. Our work suggests the use of this newly developed printable hydrogel for various imprinted components for sensing and imaging systems.

Fabrication of Tunable Liquid Crystal Microlens Using Micro-Drop Technology

In this work, the micro-drop technology was applied to fabricate tunable LC microlens, which was composed of LC and UV curable polymers. The formed mechanism was significantly dependent on the dropped liquid volume, UV dose, and precuring time. The crater morphology of the microlens can be controlled by changing these parameters. The focal length of the proposed LC microlens can be electrically controlled from -5.6 cm to -22.4 cm.

An in-plane optofluidic microchip for focal point control

A polydimethylsiloxane (PDMS) optofluidic microfluidic chip comprising a tunable optofluidic in-plane biconvex microlens and a tunable optofluidic in-plane microprism is proposed for controlling the focal length and deviation angle of a light beam. In the proposed device, the microlens comprises an expansion chamber containing a high refractive index stream sandwiched between two low refractive index streams. Meanwhile, the microprism comprises a triangular chamber filled with two liquids, one with a higher refractive index than that of PDMS and the other a lower refractive index than that of PDMS. It is shown that the radius of curvature (and therefore the focal length) of the microlens can be adjusted by controlling the flow rate ratio of the core and cladding streams. In addition, it is shown that the deviation angle of the light ray exiting the microprism depends on the refractive indices of the two working fluids, the apex angle of the prism chamber, and the flow rate ratio of the two working fluids. In general, the results show that the biconvex microlens enables the focal length to be adjusted in the range of 2.9-7.6 mm when using benzothiazole and ethylene glycol-ethanol as the core and cladding fluids, respectively. Moreover, a deviation angle range of -6.2° to 22.3° can be achieved when using a microprism chamber with an apex angle of 90° and benzothiazole and DI water as the working fluids. The integrated optofluidic chip therefore can manipulate the focal length and deviation angle of a light beam by adjusting the relative flow rates of fluids.

Tunable Microlenses Optically Recorded in Dye-Doped Liquid Crystals

We report on optically recorded microlenses in conventional liquid crystal cells doped with the azo-dye methyl-red. The focal length can be tuned electrically with low external voltage and changed in a wide range with just a small variation of the applied dc field. No patterned electrodes, built-in polymeric lens or patterned molecular reorientation are required. The effect underlying the proposed fabrication method is connected to the surface induced nonlinear effect (SINE) observed a few years ago in dye doped liquid crystals.

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.