Varifocal Lenses Research Articles

Stretchable Dual-Axis Terahertz Bifocal Metalens with Flexibly Polarization-Dependent Focal Position and Direction.

Varifocal lenses are essential components in any optical system, while traditional lenses suffer from bulky volume, fixed focal position, and limited working spectra. As well-arranged subwavelength structures, metalenses overcome the abovementioned obstacles and exhibit merits of ultrathin thickness, flexible focal length, and multifocus. The electromagnetic responses of metasurfaces, including metalens, rely on the phase distributions of phase-shifting elements. The steerable focal direction is investigated to obtain the combinations of focusing and anomalous refraction phase distribution. To fully explore the flexibility of focal length and direction, seven designs of double layers of terahertz (THz) bifocal metalenses are proposed and investigated in this study. They exhibit dependent and independent relationships of tunable focal length and direction with flexible tuning mechanisms. Along with polarization multiplexing, two different focuses can be obtained when the incident waves are x-linear and y-linear polarization states, respectively. The simulation results agreed well with the theoretical predictions. These designs provide a new method to modulate the focal position precisely with promising applications in wireless communication, imaging, and on-chip optical integration systems.

Frequency characteristics of an ultrasonic varifocal liquid crystal lens.

Compound lens systems with mechanical actuators are used to focus objects at near to far distances. The focal length of ultrasound varifocal liquid crystal (LC) lenses can be controlled by modulating the refractive index spatial distribution of the medium through the acoustic radiation force, resulting in thin and fast-response varifocal lenses. The frequency characteristics of such a lens are evaluated in this paper, and several axisymmetric resonant vibration modes over 20kHz are observed. The effective lens aperture decreased with the wavelength of the resonant flexural vibration generated on the lens, meaning that this parameter can be controlled with the driving frequency.

Talbot-Lau devices: a reappraisal

The Talbot effect and the Lau effect have been usefully applied in optical interferometry, and for designing novel X-ray devices, as well as for implementing useful instruments for matter waves. In temporal optics, the above phenomena play a significant role for reconstructing modulated, optical short pulses that travel along a dispersive medium. We note that the Talbot-Lau devices can be spatial frequency tuned if one employs varifocal lenses as a nonmechanical technique. Thus, we identify a pertinent link between the Talbot-Lau sensors and the development of artificial muscle materials, for generating tunable lenses. Our discussion unifies seemly unrelated topics, for providing a global scope on the applications of the Talbot-Lau effect.

3D Stretchable Devices: Laser‐Patterned Electronic and Photonic Structures

AbstractRealizing three‐dimensional stretchable structures of functional materials with a minimum footprint on Silicone polymer is highly desirable in soft robotics, stretchable electronics, and photonics. However, material processing on a stretchable substrate requires a sophisticated deposition system with integrated substrate cooling facilities, delamination of materials from the stretchable substrate due to stretching‐releasing cycles, and coating the functional materials. Here, a methodology to address these challenges using in situ graphitization within silicone polymer, referring to transforming the material into graphite‐like structures using three‐dimensional laser printing is reported. In this case, the graphitization process occurs due to the interaction of the material with a spatially controllable, tightly focused femtosecond laser beam in the confined region within the polymer. Three‐dimensional printed embedded, stretchable electrodes and varifocal lenses of thickness 1/20th compared to the epidermis layer thickness of human skin, which can contribute to achieving compact, highly sensitive wearable sensing and imaging systems are demonstrated and characterized. This process will open a new door for forming non‐metallic stretchable three‐dimensional conductors and photonics with minimum exposure to atmospheric conditions and a pathway to interface with thin films to develop low‐dimensional devices. These graphitized three‐dimensional structures can make them integral to intelligent skins, e‐textiles, and implantable devices.

Low-loss tunable beam collimator and expander assembly with no moving parts using anengineered diffuser and varifocal lenses.

In this paper, we present a novel design for a tunable beam collimator. A variable collimator assists in achieving an adaptive size of an output collimated beam. Alternatively, it can also provide an adjustable output beam divergence angle for a noncollimated beam output. Tunable collimators are highly desirable for various applications in testing, engineering, and measurements. Such devices are also useful in providing tunable illumination of samples or targets in microscopes and emulating different target distances for characterizing the performance of camera systems in laboratory settings. The proposed collimator has two distinct advantages: it is light-efficient compared with pinhole-based collimator designs, and it delivers a large range of output beam sizes without involving the mechanical motion of bulk components. These attributes are achieved via the use of an engineered diffuser (in the place of a pinhole) and a pair of large aperture tunable focus lenses, which deliver a tunable magnification to the output collimated beam. In laboratory experiments, we achieve an optical transmission efficiency of 90% for the proposed tunable collimator.

Development of the personalized manufacturing system framework for freeform vari-focal lenses and its implementation and application perspectives

Development of the personalized manufacturing system framework for freeform vari-focal lenses and its implementation and application perspectives

Development of the personalised manufacturing system framework for freeform vari-focal lenses and its implementation and application perspectives

Development of the personalised manufacturing system framework for freeform vari-focal lenses and its implementation and application perspectives

Millimeter-scale focal length tuning with MEMS-integrated meta-optics employing high-throughput fabrication

Miniature varifocal lenses are crucial for many applications requiring compact optical systems. Here, utilizing electro-mechanically actuated 0.5-mm aperture infrared Alvarez meta-optics, we demonstrate 3.1 mm (200 diopters) focal length tuning with an actuation voltage below 40 V. This constitutes the largest focal length tuning in any low-power electro-mechanically actuated meta-optic, enabled by the high energy density in comb-drive actuators producing large displacements at relatively low voltage. The demonstrated device is produced by a novel nanofabrication process that accommodates meta-optics with a larger aperture and has improved alignment between meta-optics via flip-chip bonding. The whole fabrication process is CMOS compatible and amenable to high-throughput manufacturing.

Monitoring and Predicting the Surface Generation and Surface Roughness in Ultraprecision Machining: A Critical Review

The aim of manufacturing can be described as achieving the predefined high quality product in a short delivery time and at a competitive cost. However, it is unfortunately quite challenging and often difficult to ensure that certain quality characteristics of the products are met following the contemporary manufacturing paradigm, such as surface roughness, surface texture, and topographical requirements. Ultraprecision machining (UPM) requirements are quite common and essential for products and components with optical finishing, including larger and highly accurate mirrors, infrared optics, laser devices, varifocal lenses, and other freeform optics that can satisfy the technical specifications of precision optical components and devices without further post-polishing. Ultraprecision machining can provide high precision, complex components and devices with a nanometric level of surface finishing. Nevertheless, the process requires an in-depth and comprehensive understanding of the machining system, such as diamond turning with various input parameters, tool features that are able to alter the machining efficiency, the machine working environment and conditions, and even workpiece and tooling materials. The non-linear and complex nature of the UPM process poses a major challenge for the prediction of surface generation and finishing. Recent advances in Industry 4.0 and machine learning are providing an effective means for the optimization of process parameters, particularly through in-process monitoring and prediction while avoiding the conventional trial-and-error approach. This paper attempts to provide a comprehensive and critical review on state-of-the-art in-surfaces monitoring and prediction in UPM processes, as well as a discussion and exploration on the future research in the field through Artificial Intelligence (AI) and digital solutions for harnessing the practical UPM issues in the process, particularly in real-time. In the paper, the implementation and application perspectives are also presented, particularly focusing on future industrial-scale applications with the aid of advanced in-process monitoring and prediction models, algorithms, and digital-enabling technologies.

Design of thermal imaging continuous-zoom optical system

The report tells about the design of a middle wave infrared continuous-zoom imaging optical system. To capture the image in an «eye-type» infrared optical system a multi-element sensor is used. To increase the contrast of the thermal image the sensor is equipped with a cold aperture. The use of an optical system based on the continuous-zoom lens makes it possible to solve the problems of object detection and identification more effectively but requires taking into account some features of varifocal lenses. The optical system being analyzed in the report includes front zoom objective system and secondary imaging system. The front zoom optical system is composed of four groups and provides the focal length that is continuously changed with a large zoom ratio (15–20X) under the condition of a constant total length of the system. The secondary imaging system is designed for zoom system exit stop and sensor cold stop conjugation. The design technique represents analytical expressions and equations to obtain optical parameters of the front and secondary system components and the motion curves for movable groups.

Long distance optical transport of ultracold atoms: A compact setup using a Moiré lens.

We present a compact and robust setup to optically transport ultracold atoms over long distances. Using a focus-tunable moiré lens that has recently appeared in the market, we demonstrate transport of up to a distance of 465mm. A transfer efficiency of 70% is achieved with a negligible temperature change at 11 μK. With its high thermal stability and low astigmatism, the moiré lens is superior to fluid-based varifocal lenses. It is much more compact and stable than a lens mounted on a linear translation stage, allowing for simplified experimental setups.

Electrically Actuated Varifocal Lens Based on Liquid-Crystal-Embedded Dielectric Metasurfaces.

Compact varifocal lenses are essential to various imaging and vision technologies. However, existing varifocal elements typically rely on mechanically actuated systems with limited tuning speeds and scalability. Here, an ultrathin electrically controlled varifocal lens based on a liquid crystal (LC) encapsulated dielectric metasurface is demonstrated. Enabled by the field-dependent LC anisotropy, applying a voltage bias across the LC cell modifies the local phase response of the silicon meta-atoms, in turn modifying the metalens focal length. In a numerical implementation, a voltage-actuated metalens with continuous zoom and up to 20% total focal shift is demonstrated. The LC-based metalens concept is experimentally verified through the design and fabrication of a bifocal metalens that facilitates high-contrast switching between two discrete focal lengths upon application of a 9.8 Vpp voltage bias. Owing to their ultrathin thickness and adaptable design, LC-driven dielectric metasurfaces open new opportunities for compact varifocal lensing in a diversity of modern imaging applications.

Design and characteristics of a Maxwell force-driven liquid lens.

Varifocal lenses (especially large-aperture lenses), which are formed by two immiscible liquids based on electrowetting and dielectrophoretic effects, are usually modulated by an external high-voltage power source, with respect to the volume of the liquid. Hence, a Maxwell force-driven liquid lens with large aperture and low threshold voltage is proposed. With the polarization effect, the accumulated negative charges on the surface of the polyvinyl chloride/dibutyl adipate gel near the anode results in the generation of Maxwell force and deformation with cosine wave. The effect of surface roughness on wettability is linear with the cosine of the contact angle, leading to a sharp reduction in the threshold voltage when the volume of liquid is increased. When the volume of the droplet increases to 80 μl, the threshold voltage is about 10 V. Hence, the aperture of polarization effect-driven liquid lenses can potentially reach the centimeter level. Moreover, when Maxwell force increases, the lens ranges from concave to convex lens, which holds great promise in rich application such as those in light-sheet microscopes and virtual reality systems.

Anomalous bi-directional scanning electro-optic KTN devices with UV-assisted electron and hole injections.

In this Letter, we reported anomalous electro-optic potassium tantalate niobate (KTN) devices, in which both electrons and holes were injected into the KTN crystal via ultraviolet (UV) illumination-assisted charge injection. This could not only significantly enhance the performance of electro-optic devices (e.g., a 270% increase in the deflection angle in terms of the KTN deflector) but also enable the new bi-directional scanning capability. The results in this work would be very useful for a variety of devices and applications, such as electro-optic based vari-focal lenses.

Two-conjugate zoom system: the zero-throw advantage.

We present the Gaussian design of a two-conjugate zoom system, which does not require any mechanical compensation. The device works in two stages. First, with fixed optical power, a lens images the pupil aperture, forming a pair of conjugate planes. Then, we invert the conjugate planes for setting the two-conjugate condition. At the second stage, two varifocal lenses generate a tunable magnified virtual image, at the fixed object plane. The varifocal lenses have fixed interlens separation, and they work with zero-throw. We specify the optical powers of the composing elements and the equivalent optical power as functions of the variable magnification.

An entirely soft varifocal lens based on an electro-hydraulic actuator

Varifocal lenses have been promptly developed but show many shortcomings in the area of artificial pupils, such as rigid frames, prestress and large structures. This paper presents an entirely soft varifocal lens to adapt to the increasingly ingenious soft robotic structures. Such a varifocal lens is mainly composed of a PDMS frame, a liquid dielectric and a pair of flexible electrodes. The stable focal length of the lens varies from 533.5 to 49.4 mm by applying DC voltages from 3 to 12 kV. Surprisingly, a novel ‘jump’ phenomenon is found in the varifocal lens at the moment of power-on. Then, the varifocal effects are studied under alternating voltages with a square wave, a triangle wave or a sine wave. The varifocal range remains basically stable after repeated cycles under alternating voltages. As the frequency increases, the range of the focal length change also gradually decreases and becomes more stable. As a result, the stable focal length of the lens is able to vary as wide as from 690.3 to 24.5 mm by applying square waveform voltages from 1 to 8 kV at a frequency of 10 Hz. The proposed lens is simple, thin and flexible, which is expected to develop the artificial eyes of soft robots.

Minimized two- and four-step varifocal lens based on silicon photonic integrated nanoapertures.

Integration of optical waveguide and subwavelength structure may help address the problems of large footprint, low robustness, and small operation bandwidth, those of that are typically inborn in traditional integrated optical devices. Here, a design method of an ultra-compact small footprint lens is proposed. Combing particle swarm optimization (PSO) algorithm with spatial multiplexing technology, we successfully integrated two- and four-step varifocal lenses on SOIs chips with small footprint of 35×35 µm2, non-mechanically leading to 2.5× and 3.4× zoom capacity, respectively. The proposed designed method may shed a new light on compact on-chip display devices and offer an alternative approach to design integrated optical communication with high information storage capacity.

Hopkins procedure for tunable magnification: surgical spectacles

We analyze the use of two varifocal lenses, with fixed interlens separation, for achieving tunable magnification at a specific throw. Our discussion extends the Hopkins procedure circumscribed to the determination of fixed optical powers in a multilens system. We illustrate our results by presenting the Gaussian optics design of surgical spectacles, which have tunable magnification while generating virtual images with zero throw. We also report novel formulas describing this type of two-lens zoom system, which works without any mechanical compensation.

Reprogrammable multifocal THz metalens based on metal–insulator transition of VO2-assisted digital metasurface

Metalenses, which consist of subwavelength metasurfaces, have attracted attention due to their more condensed dimension in comparison with conventional lenses. The limitations of traditional varifocal lenses are apparent, such as the large size and absence of tunability. In this paper, we propose a reprogrammable digital metasurface hybridized with vanadium dioxide (VO2) for wavefront manipulation at terahertz (THz) frequencies. The proposed meta-atom is integrated with four similar VO2 patches to manipulate the electromagnetic response. Based on this configuration, the meta-atoms can reflect as a “1” bit at room temperature. Also, when the heat is increased, VO2 would be in a fully metallic state; therefore, meta-atoms can act as a “0” bit. Furthermore, by adjusting the meta-atoms digital distribution on a metalens surface, the suggested device can focus the incident wave at any arbitrary position by selecting an appropriate design. Numerical simulations demonstrate that the designed VO2-assisted digital metasurface can construct one and multi-focal spot in reflection mode as expected. Moreover, obtained simulation results depict an excellent agreement with theoretical results. The proposed varifocal mirror may play a pivotal role in designing ultrathin reconfigurable THz devices with applications in high-speed parallel imaging, efficient optical coding, and confocal microscopy.

Experimental demonstration of a continuous varifocal metalens with large zoom range and high imaging resolution

Varifocal lenses find significant applications in telescopy, photography, and microscopy. Conventionally, a varifocal lens is implemented by changing the axial distance between multiple conventional bulky refractive elements. Recent progress in metasurfaces offers an alternative based on ultrathin and lightweight metadevices, but suffering from a limited zoom range (typically no more than 2×) or numerical aperture (typically no more than 0.3). Here, we experimentally demonstrate a continuous varifocal metalens in the microwave band, which can be continuously zoomed by changing the mutual angle between two combined geometric metasurfaces with the same design. The results reveal that a 3.5× zoom range is realized when the mutual angle increases from 20° to 90°, with changes of the focal length from 295 mm to 85 mm and numerical aperture from 0.56 to 0.92. Furthermore, the diffraction-limited focusing and high imaging resolution are experimentally demonstrated.