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

Responsive Liquid Crystal Network Microstructures with Customized Shapes and Predetermined Morphing for Adaptive Soft Micro-Optics

Stimuli-responsive materials have garnered substantial interest in recent years, particularly liquid crystal networks (LCNs) with sophisticatedly designed structures and morphing capabilities. Extensive efforts have been devoted to LCN structural designs spanning from two-dimensional (2D) to three-dimensional (3D) configurations and their intricate morphing behaviors through designed alignment. However, achieving microscale structures and large-area preparation necessitates the development of novel techniques capable of facilely fabricating LCN microstructures with precise control over both overall shape and alignment, enabling a 3D-to-3D shape change. Herein, a simple and cost-effective in-cell soft lithography (ICSL) technique is proposed to create LCN microstructures with customized shapes and predesigned morphing. The ICSL technique involves two sequential steps: fabricating the desired microstructure as the template by using the photopolymerization-induced phase separation (PIPS) method and reproducing the LCN microstructures through templating. Meanwhile, surface anchoring is employed to design and achieve molecular alignment, accommodating different deformation modes. With the proposed ICSL technique, cylindrical and spherical microlens arrays (CMLAs and SMLAs) have been successfully fabricated with stimulus-driven polarization-dependent focusing effects. This technique offers distinct advantages including high customizability, large-area production, and cost-effectiveness, which pave a new avenue for extensive applications in different fields, exemplified by adaptive soft micro-optics and photonics.

Reconfigurable Microlens Array Enables Tunable Imaging Based on Shape Memory Polymers

Microlens arrays (MLAs) with a tunable imaging ability are core components of advanced micro-optical systems. Nevertheless, tunable MLAs generally suffer from high power consumption, an undeformable rigid body, large and complex systems, or limited focal length tunability. The combination of reconfigurable smart materials with MLAs may lead to distinct advantages including programmable deformation, remote manipulation, and multimodal tunability. However, unlike photopolymers that permit flexible structuring, the fabrication of tunable MLAs and compound eyes (CEs) based on transparent smart materials is still rare. In this work, we report reconfigurable MLAs that enable tunable imaging based on shape memory polymers (SMPs). The smart MLAs with closely packed 200 × 200 microlenses (40.0 μm in size) are fabricated via a combined technology that involves wet etching-assisted femtosecond laser direct writing of MLA templates on quartz, soft lithography for MLA duplication using SMPs, and the mechanical heat setting for programmable reconfiguration. By stretching or squeezing the shape memory MLAs at the transition temperature (80 °C), the size, profiles, and spatial distributions of the microlenses can be programmed. When the MLA is stretched from 0 to 120% (area ratio), the focal length is increased from 116 to 283 μm. As a proof of concept, reconfigurable MLAs and a 3D CE with a tunable field of view (FOV, 160-0°) have been demonstrated in which the thermally triggered shape memory deformation has been employed for tunable imaging. The reconfigurable MLAs and CEs with a tunable focal length and adjustable FOV may hold great promise for developing smart micro-optical systems.

Switchable liquid crystal lenticular microlens arrays based on photopolymerization-induced phase separation for 2D/3D autostereoscopic displays

Conventionally, the fabrication of liquid crystal lenticular microlens arrays (LCLMLAs) is complicated and costly. Here, we demonstrate a one-step fabrication technique for LCLMLAs, which is prepared through the photopolymerization-induced phase separation in the LC/polymer composite. The LCLMLAs possess both polarization-dependent and electrically tunable focusing properties. Furthermore, we construct a 14-view 2D/3D switchable autostereoscopic display prototype based on a 2D LCD panel and the prepared LCLMLA, which has a viewing angle of 14° and a crosstalk of 46.2% at the optimal viewing zone. The proposed LCLMLAs have the merits of simple fabrication, large-scale production, and low cost.

Technologies for depth scanning in miniature optical imaging systems [Invited]

Biomedical optical imaging has found numerous clinical and research applications. For achieving 3D imaging, depth scanning presents the most significant challenge, particularly in miniature imaging devices. This paper reviews the state-of-art technologies for depth scanning in miniature optical imaging systems, which include two general approaches: 1) physically shifting part of or the entire imaging device to allow imaging at different depths and 2) optically changing the focus of the imaging optics. We mainly focus on the second group of methods, introducing a wide variety of tunable microlenses, covering the underlying physics, actuation mechanisms, and imaging performance. Representative applications in clinical and neuroscience research are briefly presented. Major challenges and future perspectives of depth/focus scanning technologies for biomedical optical imaging are also discussed.

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.

Liquid crystal microlens array with positive and negative focal lengths based on a patterned electrode

A liquid crystal microlens array (LCMLA) with positive and negative focal lengths based on a ring-array patterned electrodes is demonstrated. By carefully designing patterned electrodes with a circular electrode array area and outer ring electrode region area, the switching of the positive and negative lens effect can be easily achieved in a single cell. A positive lens effect appeared when the voltage was applied to the outer ring electrode region and the top substrate. The focal length changed from infinity to 1 mm as the voltage varied from 0 to 3V_rms. A negative lens effect occurred when the voltage was applied to the circular electrode array and the top substrate. The focal length varied from infinity to -1mm when the voltage changed from 0 to 2V_rms. The imaging properties of the LCMLA at different voltages are evaluated. Our LCMLA, with simple structure, low driving voltage, and good stability, has potential applications in optical communication, imaging processing, and displays.

High-throughput and controllable manufacturing of liquid crystal polymer planar microlens array for compact fingerprint imaging

The microlens array (MLA) with a small geometric footprint and unique performances, is the key enabler to push the development of photonic devices toward miniaturization, multi-function and large-scale integration. However, the realization of 100% fill-factor (FF) MLAs with high controllability and its mass manufacturing without complex steps has always been a difficult issue. Here, we propose an efficient, highly flexible and low-cost manufacturing approach for MLAs with a high FF via snapshot polarization patterning. The digitalized linear polarization pattern was distributed across the photo-alignment layer with both high efficiency and accuracy, enabling large-area liquid crystal MLA with parameter controllability from element to element. The MLA manufacturing process does not involve developing, etching and deposition steps and is suitable for industry up-scaling. We further proposed a novel compact compound-eye imaging system for biometrics with the obtained MLAs. The 100% FF MLA enables high light utilization efficiency and low background crosstalk, yielding compact biometrics indentation with high recognition accuracy. The realization of such planar optics would lead to a plethora of different miniaturized multiaperture imaging systems in the future.

Real-time measurement of the liquid-crystal optic-axis angle and effective refractive index distribution based on a common-path interferometer

A common-path interferometer for the real-time measurement of the liquid-crystal (LC) optic-axis angle and effective refractive index distribution is proposed. This method involves adding a polarizer and polarization camera to a general optical microscope. This requires only single-exposure imaging without changing any optical elements, and greatly simplifies the measurement process and system. In addition, the measurement results are unaffected by light-source power fluctuations or a non-uniform spatial distribution. Therefore, this method is suitable for measuring the LC optic-axis angle and effective refractive index of electrically controlled LC devices. Finally, the feasibility and validity of the proposed method are verified by simulation and experimentation.

Tunable liquid crystal multifocal microlens array

A novel liquid crystal microlens array with tunable multifocal capability, high optical power and fill-factor is proposed and experimentally demonstrated. A specific hole pattern design produces a multifocal array with only one voltage control. Three operations modes are possible, “Off”, “Tunable Multifocal” and “Unifocal”. The design is patterned in both substrates. Then, the substrates are arranged in symmetrical configuration. The result is a high optical power in comparison with typical hole patterned structures. Besides, it is proposed a hexagonal pattern that produces a high fill factor, specially indicated for some applications as Integral Imaging. The array has several useful characteristics for this type of application: tunability for the loss of resolution; multifocal for extended DOF; high fill factor for increase the number of views; and low power consumption for integration in portable devices. Moreover, the optical characteristics of the proposed device could bring new applications in other fields.

Enhancement of the depth-of-field of integral imaging microscope by using switchable bifocal liquid-crystalline polymer micro lens array

An integral imaging microscopy (IIM) system with improved depth-of-field (DoF) using a custom-designed bifocal polarization-dependent liquid-crystalline polymer micro lens array (LCP-MLA) is proposed. The implemented MLA has improved electro-optical properties such as a small focal ratio, high fill factor, low driving voltage, and fast switching speed, utilizing a well-aligned reactive mesogen on the imprinted reverse shape of the lens and a polarization switching layer. A bifocal MLA switches its focal length according to the polarization angle and acquires different DoF information of the specimen. After two elemental image arrays are captured, the depth-slices are reconstructed and combined to provide a widened DoF. The fabricated bifocal MLA consists of two identical polarization-dependent LCP-MLAs with 1.6 mm and f/16 focal ratio. Our experimental results confirmed that the proposed system improves the DoF of IIM without the need for mechanical manipulation.

Low aberration and fast switching microlenses based on a novel liquid crystal mixture

In this work, we present a novel kind of LC mixture (5005) for photonic applications, with emphasis on a LC microlens array. This mixture is a nematic composition of three different families of rod like liquid crystals. The key is that frequency dependence of parallel component of electric permittivity is different for each component, resulting in a strongly dependent on frequency dielectric anisotropy. The unique properties of this LC mixture are demonstrated to work in a frequency modulated LC microlens array. A hole patterned structure is used. Thanks to the special characteristics of this mixture, the microlenses are reconfigurable by low voltage signals with variable frequency. This is a first demonstration of a LC lens with tunable focal length by frequency in an analog way. The result of this type of control are microlenses with low aberrations and fast switching (the frequency switching is around 10 times faster than amplitude modulation). The tunability with frequency and the fast switching, makes this liquid crystal of special interest not only for microlenses but for all kind of optical phase modulators.

Arrayed optical switches based on integrated liquid-crystal microlens arrays driven and adjusted electrically

Based on our fundamental work on liquid-crystal microlens arrays (LCMAs) driven and adjusted electrically, a new kind of arrayed optical switch (AOS) constructed by a key LCMA with a special dual-mode function of converging and diverging incident beams according to electrical signals applied over the LCMA is proposed. The LCMA leading to the AOS is constructed by a microcavity with a couple of paralleled electrodes. The top electrodes of the LCMA are fabricated by depositing a layer of indium-tin-oxide (ITO) film and a layer of aluminum film, respectively. The aluminum film is continuously patterned into a circular microhole array, and the ITO film only acts as a planar conductor. Both functioning films are effectively separated by the SiO₂ wafer. Another SiO₂ wafer is also coated by an ITO film as a planar conductor. The measurements show that the developed AOS can effectively switch on or off beams propagating in arrayed fibers by applying proper voltage signals to them. Compared with other conventional AOSs, the developed AOS demonstrates several merits, including greater integration level, lower cost, and suitability to high-power propagating beams.

Embedded Ag mesh electrodes for polymer dispersed liquid crystal devices on flexible substrate

An embedded Ag mesh transparent conductive electrode (TCE) on flexible substrate, which is suitable for polymer dispersed liquid crystal (PDLC) device, is demonstrated. With the combination of soft ultra-violet nanoimprinting lithography and scrape technique, it offers parallel processing with high resolution (10000dpi), as well as remarkable simplicity and fully controllable flexibility to tailor the transmittance and sheet resistance. While being able to achieve maximum transmittance 60% in the on state and the minimum 0.1% in the off state, the PDLC smart window displays low sheet resistance (5.58 Ω/sq.) under low driven voltage (30V) safe for human. The main advantage of adoption of PDLC as an optically scattering element lies in the fact that there needs no mechanical part for in situ tunability. An enhancement factor of 50 of the diffraction intensity is observed experimentally. The embedded Ag mesh TCE for PDLC device has an environmentally-friendly additive manufacturing process inherently. Compared to existing solutions, the fabricated sample shows superior performance in both optoelectronic and mechanic characteristics. We envision that the embedded Ag mesh TCE will enable economically widen application of PDLC devices on flexible substrate.

Liquid Crystal Microlenses for Autostereoscopic Displays

Three-dimensional vision has acquired great importance in the audiovisual industry in the past ten years. Despite this, the first generation of autostereoscopic displays failed to generate enough consumer excitement. Some reasons are little 3D content and performance issues. For this reason, an exponential increase in three-dimensional vision research has occurred in the last few years. In this review, a study of the historical impact of the most important technologies has been performed. This study is carried out in terms of research manuscripts per year. The results reveal that research on spatial multiplexing technique is increasing considerably and today is the most studied. For this reason, the state of the art of this technique is presented. The use of microlenses seems to be the most successful method to obtain autostereoscopic vision. When they are fabricated with liquid crystal materials, extended capabilities are produced. Among the numerous techniques for manufacturing liquid crystal microlenses, this review covers the most viable designs for its use in autostereoscopic displays. For this reason, some of the most important topologies and their relation with autostereoscopic displays are presented. Finally, the challenges in some recent applications, such as portable devices, and the future of three-dimensional displays based on liquid crystal microlenses are outlined.

Optically isotropic, electrically tunable liquid crystal droplet arrays formed by photopolymerization-induced phase separation

Phase separation has been an interesting and important topic in liquid crystal (LC)-polymer composites. We investigated the photopolymerization-induced phase separation in an LC-polymer composite through a maskless lithographic system based on an amplitude-modulated spatial light modulator. By optimizing both exposure conditions and materials, we achieved a two-dimensional (2D) liquid crystal droplet array (LCDA) in the LC-polymer composite. Further investigations revealed that such 2D LCDAs, working as microlens arrays, demonstrated polarization-independent, electrically tunable focusing properties under a certain voltage. With advantages in cost-effectiveness, fast fabrication, and polarization-independent, electrically tunable focusing, such phase-separated microlens arrays in the LC-polymer composite could find many potential optical applications.

Liquid crystal microlens arrays recorded by polarization holography

We report the characterization of diffractive microlens arrays (MAs) using a polarization holographic approach assisted by a spatial light modulator (SLM), in a nematic liquid crystal (NLC) cell. The MAs were recorded in the photoaligning substrates of the cell and then replicated in the NLC bulk, through the surface interactions. The transparency of the NLC on a wide range of wavelengths and the ability to tune its optical birefringence, through an external voltage, allowed us to create MAs with high efficiency. We have presented the results obtained for diverse MAs configurations, composed by spherical and cylindrical microlenses and characterized by different focal lengths. The efficiency reaches a value of 90%, at a wavelength of 633 nm.

An adaptive liquid lens with a reciprocating movement in a cylindrical hole

We demonstrate a liquid droplet which can do a reciprocating movement in a cylindrical hole. The droplet in the hole exhibits a lens character. By applying a voltage, the border of the droplet is stretched to expand by the generated dielectric force. Due to the fixed volume, the dome of the droplet in the hole has to move toward the substrate without changing its surface profile. Therefore, the focal length of the droplet remains unchanged although the focal point is shifted. Once the voltage is removed, the droplet can return to its original state. The droplet with such a movement functions as an adaptive lens. Our lens can provide a high resolution (~114 lp/mm) whether or not it is actuated. The dynamic response time is reasonably fast. Integrating with a solid lens, the compound lens can provide a variable focal length.

Polarization-insensitive liquid crystal microlens array with dual focal modes

We demonstrate a liquid crystal (LC) microlens array (MLA) fabricated by LCs possessing negative dielectric anisotropy, in conjunction with a cell with a three-electrode structure. The presented LC MLA is polarization-insensitive and can be operated in both concave and convex modes. The shortest focal length of the LC MLA is -2.54 and 2.22 mm in concave and convex mode, respectively. Disclination lines that are usually observed in conventional hole-patterned LC lens can also be avoided because of the vertical alignment treatment of LCs.

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).

Speed, optical power, and off-axis imaging improvement of refractive liquid crystal lenses

Two design approaches (multicell and addition of phase resets in single cell) are introduced to optimize the performances of tunable refractive liquid crystal lenses, including improvements on the switching speed, optical power, and the off-axis, wide-angle imaging performance. Key parameters and advantages for each method are discussed, and their effects on the performance are demonstrated in detail with numerical calculations.

Liquid crystal-based square lens array with tunable focal length

We demonstrate a liquid crystal (LC)-based square lens array with two focusing modes according to the polarization state of the input light. The homogeneously aligned LC layer is placed on an array of static square lenses fabricated using a photo-curable polymer whose refractive index is matched with the refractive index of the LC. For the input beam polarized parallel to the easy axis of the LC, the focal length is varied with the applied voltage from a few meters to 21 mm which corresponds to the focal length of the static lens. For the perpendicularly polarized input beam, the focal length is independent of the applied voltage and remains constant. The two focusing effects with high optical performance over fully activated areas are useful for polarization-dependent imaging systems and three-dimensional displays in projection and integral imaging.

Fabrication of large curvature microlens array using confined laser swelling method

This Letter proposes a confined laser swelling method to fabricate large curvature microlens arrays. Unlike the polymers in conventional free laser swelling, the swelling polymer, which is methyl red-doped polymethyl methacrylate here, is confined between walls formed by a substrate and a flexible cover layer. Because swelling occurs in an enclosed space, decomposed segments remain in the matrix, resulting in a large hump at the side of the flexible cover layer. The results show that these humps are tens of times higher than those acquired by conventional methods and this method has potential for high efficiency large curvature microlens fabrication.

Near-diffraction-limited and low-haze electro-optical tunable liquid crystal lens with floating electrodes

A near-diffraction-limited, low-haze and tunable liquid crystal (LC) lens is presented. Building on an understanding of the key factors that have limited the performance of lenses based on liquid crystals, we show a simple design whose optical quality is similar to a high quality glass lens. It uses 'floating' electrodes to provide a smooth, controllable applied potential profile across the aperture to manage the phase profile.

Focus tuning by liquid crystal lens in imaging system

A quantitative study of the focus tuning by a liquid crystal lens in an imaging system composed of a camera module and the liquid crystal lens that performs the focusing function is reported. The resolving capability of the imaging system is investigated by analyzing the image of an ISO12233 chart formed by the system. Measurements show that with the focus tuning by the liquid crystal lens, the resolving power of the system can be very close to that of the camera module.

Printing of polymer microlenses by a pyroelectrohydrodynamic dispensing approach

The investigation of a method for fabricating microlenses by a nozzle-free inkjet printing approach is reported. The new method, based on a pyroelectrohydrodynamic mechanism, is also able to dispense viscous liquids and to draw liquid phase drops directly from the reservoir. Specifically, by dispensing optical grade polymer dissolved in different solvent mixtures, microlenses were printed with a pattern defined directly through this deposition method. The reliability of the microlenses and the tunability of their focal properties were demonstrated through profilometric and inteferometric analyses.

Tunable liquid crystal microlens array using hole patterned electrode structure with ultrathin glass slab

A configuration of hole patterned electrode liquid crystal microlens array with an ultrathin glass slab was fabricated. To reduce the fringing electric field effect and avoid the occurrence of disclination lines, an ultrathin glass slab was introduced between the patterned electrode and liquid crystal layer. The glass slab thickness played an important role in effecting the optical performance of the liquid crystal microlens array. An optimum thickness of 30 μm was selected employing numerical simulation method. Using this method, we demonstrated a microlens array that greatly improved the phase profile and focus power. The dynamic focal range of the liquid crystal microlens array may extend from <1.2 mm to >8 mm and the minimum diameter of the focus spot could be as small as 15 µm.

Optically tunable chirped fiber Bragg grating

This work presents an optically tunable chirped fiber Bragg grating (CFBG). The CFBG is obtained by a side-polished fiber Bragg grating (SPFBG) whose thickness of the residual cladding layer in the polished area (D(RC)) varies with position along the length of the grating, which is coated with a photoresponsive liquid crystal (LC) overlay. The reflection spectrum of the CFBG is tuned by refractive index (RI) modulation, which comes from the phase transition of the overlaid photoresponsive LC under ultraviolet (UV) light irradiation. The broadening in the reflection spectrum and corresponding shift in the central wavelength are observed with UV light irradiation density of 0.64mW/mm. During the phase transition of the photoresponsive LC, the RI increase of the overlaid LC leads to the change of the CFBG reflection spectrum and the change is reversible and repeatable. The optically tunable CFBGs have potential use in optical DWDM system and an all-fiber telecommunication system.

Maskless multiple-beam laser lithography for large-area nanostructure/microstructure fabrication

This paper reports a maskless multiple-beam laser lithography technique for large-area nanostructure/microstructure fabrication. This lithography technique can flexibly generate arbitrary nanostructures/microstructures over a large area at a high speed. The feature size of the nanostructures/microstructures can be controlled by exposure time and moving speed of the nanostage. Functional predesigned patterns, including split-ring resonator metamaterials for terahertz waves, can be obtained. More complicated structures can be made by single- and double-exposure schemes to make hybrid nanostructures/microstructures and tune surface plasmonic resonance properties. Meanwhile, microstructures with large height to lateral dimension ratios (2.5D microstructures) fabricated on silicon substrates can be used as mold tools for soft lithography. This technology shows its unique capacity to create various nanostructures/microstructures for extensive applications.

Optically tunable microfiber-knot resonator

This paper demonstrates light-induced tuning of the optical spectrum by a microfiber-knot resonator overlaid with a photoresponsive liquid crystal (LC) mixture containing photosensitive diluents (non-mesogenic azobenzene molecules), a chiral dopant and a nematic LC. The high-quality resonator is made by drawing a single mode fiber to a micro-size diameter and causing the microfiber to self-twist into a knot. A thin layer of a photosensitive mixture was placed on the overlap (knot) area and gentle heating was used to obtain a uniform thin film which coated the fiber's surface. Upon irradiation with UV light, noticeable changes to the peak resonance wavelengths were observed which we associate with a local change in the refractive index (RI) in the fiber's tapering area. Repeatable and reversible spectral shifting (0.15 nm) of the resonance wavelength is demonstrated by irradiation with 50 mW/cm² UV light.