Tunable Lens Research Articles

Module for zooming in extended depth of focus in digital holographic microscopy

Microscopes with continuous magnification and a quick zoom reaction time are much needed for imaging of biological specimen. Here, we are proposing a module consisting of two electrically tunable lenses (ETL) to facilitate the digital holographic microscopy with a continuous variable zoom/magnification within the extended depth of focus of the microscope objective. The proposed module provides a large working distance for the movement of the sample as well as the elimination of break-off while switching objectives. The major benefit is that the image plane remains stationary throughout the zooming range within the extended focal depth. The range of both ETL currents for the continuous zoom operation is discussed. Post experimental calculations, over a 3.5 mm expanded focal depth, the zoom precision is determined to be 12 µm with a response time of ∼7 ms. The zoom ratio has been found to be in the range -28.75X to -122.5X experimentally. In the expanded depth of focus, the zoom ratios at different focus planes are presented with simulation and experimental results. Moreover, the quantitative information of micro samples at deep depths with high zoom ratios without sacrificing resolution over a field of view of 1.77° is provided. The results are provided for human red blood cells and a standard USAF chart present at different depths to demonstrate the feasibility of utilizing the proposed configuration for zooming into extended focal depths of objective for optically transparent samples.

Large-scale phase retrieval from coded diffraction patterns with electrically tunable lens

General optical detection devices rely on converting photons to electrons (current), and do not allow for direct recording of the phase due to the high oscillation frequency. As so, the missing phase can only be recovered from the intensity measurements. The emerging non-convex phase retrieval algorithm, represented by the Wirtinger Flow (WF) algorithm requires multiple-shot coded diffraction patterns (CDPs) for accurate recovery. To achieve the real-time acquisition for multiple CDPs, this paper proposes a 4f imaging system based on an electrically tunable lens (ETL), which can be used for real-time acquisition of multiple-shot CDPs, and can take the advantages of highspeed, high-resolution, extended depth-of-field, high-sensitivity and low-cost imaging. In this paper, the performance of 4f-ETL based imaging system for phase retrieval with multiple CDPs is compared under different iteration times, different object size, different numbers of masks and different noise levels. Numerical experiments demonstrate the effectiveness and superiority of our proposed ETL-based imaging system, and ETL allows variable-distance focusing of imaging and display systems without mechanical structures, which reduces the mechanical complexity and power consumption, improves acquisition speed.

Measurement of specular surfaces using electrically tunable lens in digital holography

In the field of industrial metrology, the 3D nondestructive imaging of reflective metallic surfaces is a delicate task. In this work, we propose a novel application of the electrically tunable lens (ETL) in digital holography for imaging specularly reflecting objects. The precise surface profile of the microscopic step height at different axial depths is obtained by tuning the liquid lens at different currents. Initially, the ETL’s focal length is set by tuning its control current to image the specular reflection observed from the surface of the reflecting sample. The current of the ETL is tuned accordingly as the sample is moved to different z-positions. In order to demonstrate the efficacy of the proposed setup, the object is kept at multiple axial distances within the depth of field of the ETL. The step height measurements are carried out and a measurement uncertainty of 0.083 µm is calculated for step height measurements at different axial positions ranging from 2 cm–21 cm. The axial range of the setup is validated by keeping two specularly reflecting samples in the field of view of the ETL. The experimental results demonstrate the ETL’s efficiency in a digital holographic system for accurately imaging specularly reflecting objects present at multiple axial depths. The setup is useful for precise step height measurements and for obtaining surface profiles of microstructures.

Spectroscopy of individual Brownian nanoparticles in real-time using holographic localization.

Individual nanoparticle spectroscopic characterization is fundamental, but challenging in liquids. While confocal selectivity is necessary to isolate a particle in a crowd, Brownian motion constantly offsets the particle from the light collection volume. Here, we present a system able to acquire holograms and reconstruct them to precisely determine the 3D position of a particle in real time. These coordinates drive an adaptive system comprising two galvanometric mirrors (x,y, transverse directions) and a tunable lens (z, longitudinal) which redirect light scattered from the corresponding region of space towards the confocal entrance of a spectrometer, thus allowing long spectral investigations on individual, freely-moving particles. A study of the movements and spectra of individual 100 nm Au nanoparticles undergoing two types of aggregations illustrates the possibilities of the method.

Stretchable Transparent Polyelectrolyte Elastomers for All‐Solid Tunable Lenses of Excellent Stability Based on Electro–Mechano–Optical Coupling

AbstractLenses are ubiquitously used for imaging. Compared with conventional lenses containing rigid translating components driven by mechanical motors, nonmechanical tunable lenses have been drawing attention owing to their advantages in compactness, lightweight, low power consumption, and fast response. Herein, inspired by the accommodation mechanism of the human eye, an all‐solid electromechanically tunable lens driven by a dielectric elastomer actuator is presented. A soft, stretchable, transparent, and ionically conductive polyelectrolyte elastomer, poly(3‐acrylamidopropyl)trimethylammonium chloride, is synthesized and used as the electrodes, one of which is plano‐convex and the other planar. A mold‐free procedure is elaborated to eliminate the contamination by molding and for facile fabrication. Subject to voltage, the resulting tunable lens achieves a relative change of focal length of ≈46.4%, superior to that of the human eye. The electro–mechano–optical coupling of the lens is modeled and the theoretical predictions agree well with the experimental results. Moreover, the tunable lens responds fast, operates stably in the ambient and desiccated environment, maintains performances over 1000 cycles, and exhibits a shelf‐life longer than 12 weeks. The polyelectrolyte elastomer‐based all‐solid tunable lens promises a potential solution for lightweight, compact, and durable imaging systems.

Focus stacking in non-contact dermoscopy

Dermoscopy is the main tool for early detection of skin cancer. Non-contact dermoscopes often suffer from a small depth of field leading to images of skin topographies with regions that are not in focus. We aim to provide an easy-to-implement focus stacking-based approach to ensure all-in-focus images from a non-contact dermoscope. Further, we aim to extract additional information about the skin topography from the image stacks. The focus stacking procedure itself is implemented in a non-contact dermoscope with an electrically adjustable focus realized by using a tunable liquid lens. We show that all-in-focus imaging is possible for non-contact dermoscopy and deliver a method to extract topographical information for dermatologists from the acquired image stacks. Our finding indicate that the approach can be valuable for non-contact dermoscopic examination as well as for the early detection of skin diseases such as cancer as it possible to derive hyperfocus images and information on the skin topography. With this, we were able to develop a software for the acquisition of the raw image data and its processing into a high resolution hyperresolution dermoscopic image. In the next steps, we plan to apply the approach in the clinical environment for skin cancer diagnostics or imaging of inflammatory skin diseases.

Evaluation of a liquid membrane-based tunable lens and a solid-state LIDAR camera feedback system for presbyopia.

Presbyopia is an age-related loss of accommodation ability of the eye which affects individuals in their late 40s or early 50s. Presbyopia reduces the ability of a person to focus on closer objects at will. In this study, we assessed electronically tunable lenses for their aberration properties as well as for their use as correction lenses. The tunable lenses were evaluated in healthy subjects with cycloplegia by measuring visual acuity and contrast sensitivity for their use in presbyopia correction. Furthermore, we have developed and demonstrated the feasibility of a feedback mechanism for the operation of tunable lenses using a portable solid-state LIDAR camera with a processing time of 40 ± 5 ms.

Tunable two-element diffractive lenses with optimal bandwidth utilization.

Previous work has shown that a set of two diffractive optical elements arranged in series can form a diffractive lens with variable optical power that can be tuned by relative rotation of its two sub-elements about their common central axis. However, previous designs of these diffractive optical elements did not take advantage of the full spatial resolution required for the fabrication process because the corresponding sub-elements consisted of both, regions with very high phase gradients (requiring full resolution) and other extended regions with nearly vanishing phase gradients, where the available resolution is "wasted". Here, an advanced design is proposed that exploits the full spatial bandwidth of the production system. This is done by increasing the polar (angular) phase gradient of each sub-element such that it approaches the usually much larger radial phase gradient. A pair of these new sub-elements then composes a diffractive lens that has the same tuning range of its optical power than a standard tunable diffractive lens, but advantageously achieved within a much smaller relative rotation range. This has advantages in systems where high speed tuning of optical power is required, and in systems where the rotation angle is limited.

26.4: Force‐induced Tunable Lens for Dark‐zone Compensation in Stretchable Display

Lens array is an important component for optimizing display performances. In this paper, we further explore the lens array structure for future stretchable displays. This work proposed the force‐induced tunable lens with an adjustable diopter triggered by applied strain. Furthermore, we introduced an optical compensation method to solve the dark zone problem with this tunable lens. The optical simulation result shows the feasibility of this method used in a display array.

3.1: Force‐induced Tunable Lens for Dark‐zone Compensation in Stretchable Display

3.1: Force‐induced Tunable Lens for Dark‐zone Compensation in Stretchable Display

54.3: Invited Paper: Surface Control of LC Alignment for Creation of Liquid Crystal Lenses Arrays

New photosensitive alignment materials, capable to obtain pretilt angle from 90 to 0 degree in single technological process of UV exposure, are developed. The novel technological method for fabrication LC lens and LC lenses arrays has been developed based on new pretilt angle gradient alignment materials. Laboratory samples of voltage tunable LC lens and LC lenses arrays are obtained and characterized. The developed low cost fabrication technology of tunable micro‐lenses arrays based on photosensitive alignment materials has low complexity and is perspective for light field technology applications.

Closed-loop experimental optimization of tunable lenses.

Tunable lenses (TLs) are optical devices that can change their optical power in response to an electrical signal. In many applications, they are often pushed to or beyond their temporal limits. Fast periodic and/or abrupt variations of the optical power induce undesired distortions in their transient response and produce a decrease in their performance. A low-cost focimetry system, along with a custom closed-loop iterative optimization algorithm, was developed to (1) characterize a TL's response at high speed and (2) optimize their performance in realistic TL working conditions. A significant lens performance improvement was found in about 23 iterations with a decrease in the area under the error curve and an improved effective time. Applying the closed-loop optimization algorithm in a depth scanning experiment enhanced the image quality. Quantitatively, the image quality was evaluated using the structural similarity index metric that improves in individual frames, on average, from 0.345 to 0.895.

Active control of terahertz amplitude and phase based on graphene metasurface

Metasurfaces with actively tunable features are highly demanded for advanced applications in terahertz (THz) systems. However, realizing dual-tunability is still challenging. In this paper, we demonstrate a dual function modulator based on graphene metasurface. It can actively control the amplitude and phase of THz wave. The unit cell of the device is made up of two T-shaped resonators and a graphene patch between two resonators. By electrically tuning the Fermi level of the graphene patch, the maximum amplitude modulation depth at the central frequency of 0.55 THz can be up to 92%. Meanwhile, the dynamic phase modulation at 0.3 THz can change from −20° to 70°, and the maximum phase difference reaches to 90°. When the incident angle is up to 60°, the amplitude modulation depth is still high than 80%, and the phase difference is more than 70°. Therefore, the device has good performance for incident angles. Our work provides deeper insight into multifunctional integrated THz devices, which will enable a variety of practical applications, such as THz imaging, tunable lens and phase array antenna . • A dual function modulator based graphene metasurface was proposed. • The amplitude and phase of terahertz wave can be actively controlled by changing the Fermi level of graphene. • The modulation depth is high to 92%, and the phase modulation is 90°. • The maximum transmission is 0.7 when the Fermi level is 1 eV.

Electric field induced buckling of inversion walls in lens-shape liquid crystal droplets

Lens-shaped liquid crystal samples have the potential for their use in tunable multifocal optical lenses. In previous observations (Phys. Rev. Research, 2, 023261 (2020) and J. Mol. Liq., 334, 116085 (2021)) plano-convex spherical lens-shaped liquid crystal sessile droplets were studied either in magnetic or AC electric fields, and the formation of an inversion wall (a one-dimensional soliton) moving toward the periphery was observed and explained.Here we present experimental observations of the structure of plano-convex lens-shaped nematic liquid crystal droplets subjected to simultaneous competing magnetic and electric fields. It is found that above a threshold electric field the initially straight defect wall buckles to form a zig-zag shape. This phenomenon is compared to those observed in sandwich cells with uniform thickness and the differences in their theoretical description are discussed.

Electrically tunable lens with a non-monotonic wavefront control capability.

We describe an electrically tunable liquid crystal lens that can produce a rich variety of wavefronts, including sombrero-type (non-monotonic) phase modulation, enabling the focusing of light into a ring-shaped intensity distribution. The lens can also generate axicons or standard spherical lenses with a bipolar response (providing both positive and negative optical powers). The design of the lens requires only a single-step lithography process, dramatically simplifying its manufacturing. We describe various driving modes of this lens and present the first experimental results and discuss its possible applications in miniature cameras and microscopes. We think that this device can revolutionize the optical design in many areas of photonics.

Nonlinear Two-Photon Absorption in the Near-Infrared Band for Lead Bromide Perovskite Films Using an F-Scan Nonlinear Spectrometer.

An F-scan nonlinear spectrometer is used to measure the two-photon absorption coefficient for CH3NH3PbBr3 perovskite films from 690 to 995 nm. This spectrometer uses an electrically focused tunable lens and a tunable femtosecond-pulse laser (Mai Tai-HP) with a resolution of 5 nm. Two-band models and saturation irradiance corrections are used to fit the experimental data. The nonlinear absorption in this wavelength range is of the order of cm/MW. We found that the best agreement between the experimental data, the reported values in the literature, and the theoretical model is obtained for an extended two-band model with irradiance saturation correction.

Design and Fabrication of a Tunable Optofluidic Microlens Driven by an Encircled Thermo-Pneumatic Actuator.

This paper presents the design, simulation, fabrication, assembly, and testing of a miniature thermo-pneumatic optofluidic lens. The device comprises two separate zones for air heating and fluid pressing on a flexible membrane. A buried three-dimensional spiral microchannel connects the two zones without pumps or valves. The three-dimensional microfluidic structure is realized using a high-resolution three-dimensional printing technique. Multi-physics finite element simulations are introduced to assess the optimized air chamber design and the low-temperature gradient of the optical liquid. The tunable lens can be operated using a direct-current power supply. The temperature change with time is measured using an infrared thermal imager. The focal length ranges from 5 to 23 mm under a maximum voltage of 6 V. Because of the small size and robust actuation scheme, the device can potentially be integrated into miniature micro-optics devices for the fine-tuning of focal lengths.

A bifocal compound liquid lens with continuous zoom based on selective wettability.

In this paper, a tunable zoom bifocal liquid lens based on selective wettability is proposed. This lens consists of internal and external immiscible coaxial droplets surrounded by immiscible ambient liquid. Since curvatures and refractive indexes of the internal and external droplets are different, the system forms a long focus and a short focus, respectively. By applying different voltages, the curvatures of the internal and external droplets change exerting continuous movement of a long/short focal point in a certain range. To verify the feasibility and practicability of this concept, a prototype of the bifocal compound lens is fabricated in experiment, and the modulation ability of its long/short focal length is detected. The short focal length of our proposed lens varies from 15.46 mm to 17.47 mm, while the relative long focal length ranges from 96.25 mm to 70.31 mm driven by 200 V.

Liquid crystal lens array with positive and negative focal lengths.

A positive-negative tunable liquid crystal lens array is proposed by electrode design. The electrode structure consists of two main units, one of them is used to generate parabolic voltage profile and the other one distributes the voltage homogeneously across the lens aperture. The proposal features the advantages of high-quality performance, simple fabrication process (a single lithographic step), compact design, low voltages and simple driving method. In addition, the lens array can be driven as a square lens array or a rotatable cylindrical lens array. The voltage difference between the electrodes on the inner face of two substrates is controlled within the range that the phase of liquid crystal layer responds linearly to voltage difference, then the phase of the lens array maintains parabolic profile in the whole focus range. In experiments, a lens array with 30 µm liquid crystal layer is fabricated using the designed electrode. The size of the array area is 11 × 11 mm, and the side length of an individual square lens is 1.0 mm. The results show that the phase profile matches with the parabolic profile during focus tuning, and good focusing effect of the positive lens is observed. As a result, a liquid crystal lens array with high-quality performance is experimentally demonstrated, and the experimental results are consistent with the theoretical analyses.

Mid‐Infrared Dynamic Wavefront Transformer Based on a Two‐Degrees‐of‐Freedom Control System

AbstractVast tunable optical components are realized based on dynamic reconfigurations of the incident wavefronts, such as beam steering and tunable lens. However, the dominant paradigm of current wavefront reconfiguration technologies relies on complex control systems with degrees of freedom much larger than output wavefronts, e.g., beam steering based on spatial light modulator or phased array antennas. Here, a new paradigm for dynamic reconfiguration of arbitrary output wavefronts using control systems with the same degrees of freedom is proposed. As an example, a wavefront transformer is demonstrated using an in‐plane two‐degree‐of‐freedom (2DOF) mechanical actuation system of metasurface doublet for semi‐omnidirectional beam steering, which measures a 6‐kHz modulation speed and a 131.2° × 131.2° field of view. This paradigm can be applied to metasurface transformers for dynamic wavefront reconfiguration with any control system for vast applications, such as tunable lens, beam steering, and dynamic beam profiler, just to name a few.