Tunable Focusing Research Articles

Terahertz switchable vortex beam generator based on vanadium dioxide reflective metasurface

Vortex beams exhibit significant potential for applications in diverse fields, such as optical communications, particle manipulation, and quantum information, due to the orbital angular momentum (OAM) they carry. While substantial progress has been made in the generation of vortex beams and the control of their wavefronts, further optimization is necessary to enhance the variability of OAM modes and the modulation of focusing. This study presents three tunable vortex beam generators based on vanadium dioxide metasurfaces. By leveraging the insulator-metal phase transition of vanadium dioxide (VO2), these metasurfaces can convert incident left-circularly polarized (LCP) light into a deflected vortex beam with adjustable OAM modes, a switchable focused vortex beam, or a focused vortex beam with tunable OAM modes in the terahertz band. Specifically, by varying the angle of the incident light, a vortex beam carrying variable topological charges can be deflected over a range from -34° to 90°. Vortex beams with a topological charge of 1 enable reversible switching between near-focus (0.85 mm) and far-focus (3.2 mm), demonstrating highly effective tunable focusing capabilities. Additionally, focused vortex generators with switchable topological charges are proposed. Focused vortex beams with a topological charge of -1 are generated in the insulating state, while a topological charge of -2 is observed in the metallic state. These vortex beam generators allow for different phase gradient distributions in the phase transition states, enabling diverse OAM and variable focuses. This research has potential applications in areas such as dynamic imaging and optical communications.

Continuously tunable ultrasonic focusing by Moiré metalenses

Tunable ultrasonic focusing holds great significance in both medicine and engineering. Recent advancements in metalenses have introduced approaches for tunable acoustic focusing, but their complex configurations and limited tuning range remain challenges. Here, acoustic Moiré metalenses (AMMs) are proposed to achieve continuously tunable ultrasonic focusing in water. Two cascading metasurfaces that can function as Moiré diffractive elements make up the AMM. By mutually rotating the metasurface, the focal point of the AMM can be continuously tuned in a large range. The focal length can be adjusted continuously from ∼14.3λ0to ∼50λ0for the axial focusing. We further show that the well-designed AMM can achieve the continuously tunable lateral focusing, with the deflection angle of the focal point being tunable between approximately -40°,40°. Both simulation and experimental results confirm the excellent tunable focusing performances of the AMMs. The proposed AMMs with continuously tunable focusing capability may have potential applications in ultrasonic imaging and ultrasound treatment.

Formation of subwavelength tunable flat-top focus with double foci characteristic by tightly focused cylindrical vector beams

We report a numerical investigation on the generation of a subwavelength flat-top focus by tuning the polarization angle of the incident beam under tight focusing conditions. Our approach uses a binary phase mask containing two regions of opposite phases, with a variable diameter of the inner region. Properly tuning the diameter of the inner region of the mask, the flat-top profile of variable sizes (above and below the subwavelength region) is generated in a controlled manner with the various incident polarization angles of the beam and with the different numerical aperture (NA) value of the lens. Our study reveals that a particular cut-off value of polarization angle, above which the subwavelength flat-top is generated, corresponds to a specific NA that increases with the increase in the NA value. In addition, the emergence of the dual foci structure is observed in the longitudinal direction with increasing NA of the lens. The flat-top's polarization degrees-of-freedom and dual foci properties open up promising routes for potential applications in fluorescence polarization microscopy and multiplane particle trapping.

Tunable focus and multiple plasmonic bottles from the superposition of half Pearcey plasmons

Tunable focus and multiple plasmonic bottles from the superposition of half Pearcey plasmons

Focusing of mid-infrared polaritons through patterned graphene on van der Waals crystals

Abstract Manipulating the propagation of mid-infrared (mid-IR) light is crucial for optical imaging, biosensing, photocatalysis, and guiding photonic circuits. Artificially engineered metamaterials were introduced to comprehensively control optical waves. However, fabrication challenges and optical losses have impeded the progress. Fortunately, two-dimensional van der Waals (vdW) materials are alternatives because of their inherent optical properties, such as hyperbolic behavior, high confinement, low loss, and atomic-scale thickness. In this research, we conducted theoretical and numerical investigations on the α-phase molybdenum trioxide, a biaxial vdW material, with patterned graphene to assess the potential of the tunable focusing of mid-IR light. Our proposed method directly alters the path of excited light to focus mid-IR light by negative refraction. Further, the patterned graphene in our design offers enhanced focusing characteristics, featuring a significantly reduced waist diameter with 1/92 of the free-space wavelength, an enhanced beam quality without pronounced field ripples, and a fivefold increase in field intensity. Moreover, our approach significantly preserves the waist diameter of the focused beam while facilitating directional steering. Thus, the focused beam can propagate in a canalized manner toward the desired direction. These advancements lay the foundation for promising applications in planar photonics.

Quantitative phase imaging with a compact meta-microscope

Quantitative phase imaging (QPI) based on the transport-of-intensity equation (TIE) is a powerful technique in label-free microscopy. The image stack required for a successful TIE-QPI is traditionally obtained by translating the object or image plane, and the optical elements used in the conventional TIE-QPI systems are usually bulky and cumbersome. Stable and compact TIE-QPI methods capable of non-motion optical zooming can significantly facilitate applications that demand portability. Here, we propose a non-motion TIE-QPI method based on a dispersive metalens. The dispersive nature of the metalens is utilized to provide a spectral focal tuning. With fixed object and image planes, seven through-focus intensity images are captured by changing the illumination wavelength. The QPI performance is validated by retrieving the surface phase profiles of a microlens array and a phase resolution target, showing a high phase detection accuracy (deviation less than 0.03 wavelength). Subsequently, we established a compact meta-microscope by integrating the metalens with a commercially available CMOS image sensor, which shows good performance in microscopic imaging of unstained bio-samples. Our approach, based on the large-dispersive metalens, facilitates a compact and robust QPI system for optical metrology and label-free microscopy.

Electrically tunable virtual image Luneburg lens using graphene.

Virtual image lenses play essential roles in various optical devices and applications, including vision correction, photography, and scientific instruments. Here, we introduce an approach for creating virtual image Luneburg lenses (LL) on graphene. Remarkably, the graphene plasmonic lens (GPL) exhibits electrically tunable virtual focusing capabilities. The design principle of the tunability is based on the nonlinear relationship between surface plasmon polariton (SPP) wave mode index and chemical potential of graphene. By controlling the gate voltage of graphene, we can achieve continuous tuning of virtual focus. A ray-tracing technique is employed to determine the required gate voltages for various virtual focal lengths. The proposed GPL facilitates adjustable virtual focusing, promising advancements in highly adaptive and transformative nanophotonic devices.

Effect of water salinity on the focal tuning of an injected liquid lens used to modify the z-scan method

A water-injected liquid lens is fabricated to tune its focal length using the change in water salinity. It is found that when the salinity of water is changed from zero to 34.25%, the focal length can be changed by about 12.6 mm from 73.7 mm to 86.3 mm. A focal length resolution of approximately 0.75 × 10−2 mm and high temporal stability over a long period have been achieved for the lens foci. This lens is then used to modify the z-scan technique where the lens and the sample both remain fixed without displacement. The performance of the fabricated lens is evaluated by nonlinear refractive index measurement of a sample containing 10.82-pH-synthesized Silver nanoparticles suspended in water with 15 mM of concentration. For verification of the results, a nonlinear refractive index of (−10.6 ± 1.0) × 10−7 cm2 W−1 is firstly measured for the sample using a classical z-scan benefiting from a conventional focal-fixed lens. Interestingly, we found out that when the fabricated lens is replaced in the modified z-scan, the nonlinear refractive index of about (−8.1 ± 0.2) × 10−7 cm2 W−1 can be measured, indicating a similarity in the order and small difference in the coefficient compared to the classical z-scan. This outcome highlights the potential capability and simplicity of the fabricated lens in the modification of the classical z-scan technique.

Investigation into fabrication and optical characteristics of tunable optofluidic microlenses using two-photon polymerization.

Optofluidic systems, integrating microfluidic and micro-optical technologies, have emerged as transformative tools for various applications, from molecular detection to flow cytometry. However, existing optofluidic microlenses often rely on external forces for tunability, hindering seamless integration into systems. This work presents an approach using two-photon polymerization (TPP) to fabricate inherently tunable microlens arrays, eliminating the need for supplementary equipment. The optofluidic design incorporates a three-layered structure enabling dynamic manipulation of refractive indices within microchannels, leading to tunable focusing characteristics. It is shown that the TPP fabricated optofluidic microlenses exhibit inherent tunable focal lengths, numerical apertures, and spot sizes without reliance on external forces. This work signifies some advancements in optofluidic technology, offering precise and tunable microlenses with potential applications in adaptive imaging and variable focal length microscopy.

Programming quadric metasurfaces via infinitesimal origami maps of monohedral hexagonal tessellations: Part I

The control of the shape of complex metasurfaces is a challenging task often addressed in the literature. This work presents a class of tessellated plates able to deform into surfaces of preprogrammed shape upon activation by any flexural load and that can be controlled by a single actuator. Quadric metasurfaces are obtained from infinitesimal origami maps of monohedral hexagonal tessellations of the plane, that is pavings in which all tiles are congruent to each other. Monohedral tessellated portions can be joined together to obtain more complex shapes, which can be locally synclastic or anticlastic and can have a certain roughness. We broaden previous work by providing a complete characterization of all the three known types of monohedral tessellations composed by irregular hexagons. The proposed two-dimensional structures may have applications in prosthetics, tissue engineering, wearable devices, energy harvesting devices, tunable focus mirrors and adaptive facades. The study is divided in two parts. In Part I, after introducing the discrete kinematics of tessellated plates, it is proved analytically that essentially each type of monohedral hexagonal tessellation possesses only one deformation mode. Afterwards, several numerical examples are provided to demonstrate the variety of achievable surface shapes. In Part II, first the metasurfaces corresponding to assigned tile geometries are given a continuum description, which establishes that the continuous interpolant is always a quadric. Then, experimental results on fabrication, assembly and surface accuracy are reported.

Multilateral Sound Manipulation Using Magnetically Tunable Apertures

In recent years, planar aperture‐based acoustic devices have been investigated due to their thin form factor, high effective transmission, and sound modulation capability. Moreover, reconfigurable and tunable devices are being continuously researched to enable multiple degrees of freedom in real time. Herein, an aperture‐based acoustic device is presented, which enables tunable multilateral ultrasonic operation. Sound waves are manipulated in multiple directions by continuously tuning aperture width or length, using a magnetically controlled rod. The position, orientation, and geometrical properties of the rod (i.e., length, topographical curvature) are exploited to create a varied interface for the impinging sound. Through experiments and simulations, acoustic wave steering in different orthogonal planes and acoustic switching using a single tunable device unit are demonstrated. Furthermore, different four‐unit device configurations are implemented for tunable beam‐formation and tunable acoustic focusing applications by utilizing the acoustic anisotropy of the system. The tunable dynamic acoustic device is scalable to audible frequencies and enables tilted operation. This straightforward and accessible proof‐of‐concept opens a paradigm for exploring multifunctional aperture‐based designs with greater degrees of freedom, bringing us a step closer toward practical applications such as acoustic device integration with walls, window panels, and other commercial products for tunable sound transmission.

Metasurface-based varifocal Alvarez lens at microwave frequencies.

Lenses with a tunable focus are highly desirable but remain a challenge. Here, we demonstrate a microwave varifocal meta-lens based on the Alvarez lens principle, consisting of two mechanically movable tri-layer metasurface phase plates with reversed cubic spatial profiles. The manufactured multilayer Alvarez meta-lens enables microwave beam collimation/focusing at frequencies centered at 7.5 GHz, and shows one octave focal length tunability when transversely translating the phase plates by 8 cm. The measurements reveal a gain enhancement up to 15 dB, 3-dB beam width down to 3.5∘, and relatively broad 3-dB bandwidth of 3 GHz. These advantageous characteristics, along with its simplicity, compactness, and lightweightness, make the demonstrated flat Alvarez meta-lens suitable for deployment in many microwave systems.

A reconfigurable acoustic coding metasurface for tunable and broadband sound focusing

The targeted concentration of acoustic waves has significant implications for industrial nondestructive testing, ultrasound diagnosis, and medical treatment. Most conventional sound-focusing metasurfaces suffer from an untunable focus, narrow bandwidth, and fixed geometric configurations, which severely constrain their practical utility. In this paper, we propose a reconfigurable acoustic coding metasurface composed of two coding units with high transmittance and transmitted phases of 0 and π for realizing tunable and broadband sound focusing. Through the straightforward manipulation of each unit structure and alterations in the coding sequences, precise control of the focus position across the entire working plane is attainable, enabling both tunable axial-axis and off-axis sound-focusing effects. Moreover, the sound-focusing performance of the proposed metasurface is excellent within a broad frequency range from 3000 to 5500 Hz. The experimental results are consistent with theoretical expectations and numerical simulations. This work lays a practical foundation for the design of acoustic devices for tunable and broadband sound focusing.

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.

Tunable focus poly(vinyl chloride) gel microlens Array fabricated by shaping in patterned electric field

AbstractIn this work, we demonstrate a poly(vinyl chloride) (PVC)‐gel‐based microlens array (MLA) with tunable focus. The MLA is fabricated using a PVC solution and a glass substrate with a ring‐array‐patterned electrode. By coating the solution on the substrate and applying a direct current (DC) voltage to the electrode, a PVC gel membrane with a convex surface profile on the inner region of the electrode (anode) is formed with the solvent evaporation. After the complete evaporation of the solvent and removing the voltage, the solidified PVC gel membrane exhibits an MLA character. When an inverse DC voltage is applied to the electrode, the PVC gel accumulated on the inner region will shift toward the outer electrode connected to the anode, leading the curvature of the convex surface to decrease and the focal length to increase accordingly. The variable surface profile with voltage is precisely measured by a laser confocal microscope and the result shows that the focal length of the MLA can be tuned within a large range.

Controllable Deformation Modulated Multi‐Functionality for Phase‐Gradient Metamaterials

AbstractTunable metamaterials offer versatile approaches to achieve the dynamic manipulation of electromagnetic waves with various dimensionality. Recently, mechanical approaches have been extensively employed to construct reconfigurable metamaterials with simple composition and superior mechanical characteristics. Mechanical modulation methods in phase‐gradient metamaterials are even more worthy of being investigated to facilitate valuable applications. Here, an in‐plane rotational deformation principle is proposed to assist the structural design of reconfigurable phase‐gradient metamaterials by integrating split ring resonators (SRRs) on the hyperelastic kirigami substrate. The orientations and split angles of SRRs are explored to manipulate the amplitudes and phases of cross‐polarized transmittance under x‐polarized incidence, respectively. The tensile strains dramatically modulate the amplitudes yet not the gradient phase. As proof‐of‐concept, a reconfigurable metalens is elaborately designed to realize tunable focus location and number by regulating the transmittance amplitudes of meta‐atoms via stretching the kirigami substrate. Furthermore, a holographic metamaterial is developed to perform dynamic hologram on the imaging plane under the strain 0% and 40%. The proposed mechanical deformation principle offers a promising platform for dynamic manipulation of the wavefront and may promote the conformal and flexible designs in certain cases and enlarge the potential applications of meta‐devices in imaging and information encryption.

Stimulated-responsive refractive-diffractive biological hydrogel micro-optical element enabling achromatism via femtosecond laser lithography.

Herein, we report a novel biological hydrogel-based achromatic refractive-diffractive micro-optical element with single-material apochromatism. Benefiting from the stimulated responsive property of the hydrogel, pH modulation yielded swelling and affected the refractive index of the element, enabling multi-wavelength focusing performance tuning and chromatic aberration adjustment. Using femtosecond laser lithography, we fabricated a separate hydrogel microlens and Fresnel zone plate and measured the tunable focusing performance while varying pH; the results were consistent with our simulation results. Furthermore, we designed and fabricated a hydrogel-based achromatic refractive-diffractive micro-optical element and demonstrated achromatism with respect to three wavelengths using only one material consisting of a microlens and a Fresnel zone plate. We characterized the optical focusing properties and observed smaller chromatic aberration. The potential applications of such hybrid microoptical elements include biomedical imaging and optical biology sensing.

Kirigami‐Inspired Planar Deformable Metamaterials for Multiple Dynamic Electromagnetic Manipulations

AbstractThe Kirigami technique has recently inspired the design of reconfigurable electromagnetic metamaterials that can be easily realized by embedding a patterned cutting array into a supportive substrate. However, these existing designs mainly focus on the 2D‐to‐3D transformable morphology that induces the modulation of the spectral responses. This work proposes a kirigami‐based planar reconfiguration mechanism and extends advanced applications in the dynamic near‐field imaging and tunable double‐foci metalens. These extraordinary tunabilities originate from three meta‐atoms that can be mutually transformed into the morphologies with non‐chirality and opposite chirality under the tensile strain. By utilizing the transmittance characteristics of reconfigurable meta‐atoms, the metamaterial is constructed to achieve three‐foldable near‐field displays and a longitudinal double‐foci metalens empowered with tunable focusing length and intensity ratio. Two anisotropic chiral meta‐atoms in deformed states that possess overwhelming transmittance difference of circularly cross‐polarized components to magnify the focusing intensity ratio of two foci for the proposed metalens are further proposed. The exhibited observations provide typical examples of a wide spectrum of applications of the kirigami‐based reconfiguration mechanism, and it may pave the avenue to spark manifold functional devices for applicable electromagnetic manipulations such as multifunctional hologram display, dynamic beam shaping, and steering, as well as the conformal design of meta‐devices.

Multifunctional metalens generation via bilayer phase-change metasurfaces at near-infrared wavelength

Multifunctional metalens with tunable focus and intensity is powerfully attractive for compact planar optics. However, the limited degree of freedom in the single-layer metasurface design makes it difficult to satisfy more complicated functionalities, which restricts the application scenarios of metalens. In this paper, a multifunctional metalens with three operating modes is designed at near-infrared wavelength by combining the phase-change material antimony selenide (Sb2Se3) with the bilayer Pancharatnam-Berry (P-B) phase. By reversibly transforming the upper and lower Sb2Se3 nanofins between the crystalline and amorphous states, the metalens can achieve arbitrary switching among dual-focus F1 and F2, single-focus F1, and single-focus F3. Moreover, when the upper Sb2Se3 nanofin is remained in the amorphous state, a bifocal metalens with tunable relative intensity is achieved by controlling the crystallization state of the lower Sb2Se3 nanofin, and the total focusing efficiency of the bifocal metalens is above 86%. Similarly, when the upper Sb2Se3 nanofin is in the crystalline state, a single-focus metalens with adjustable intensity is realized via adjusting the crystallization fraction of the lower Sb2Se3 nanofin. Regardless of whether the metalens operates in bifocal or single focal mode, the simulated focusing efficiency is higher than 53% and the full-width at half-maximum (FWHM) of each focus approaches its diffraction-limited value. The proposed multifunctional metalens can enable wide potential applications in biomedical imaging, optical communication systems, multifunctional devices, and so on.

Acoustic metalens with switchable and sharp focusing

Sharp and tunable focusing functionality is highly desired in various acoustic application fields. Here, we propose a simply structured metalens for water-borne sounds with a switchable focusing functionality. Each meta-atom in the lens is composed of two elliptical iron cylinders, and is smartly designed so that it can redirect a normally incident plane wave toward the same focal spot. A switchable focusing functionality between a transmissive lens and a reflective one can be achieved by simply rotating the elliptical cylinders. Furthermore, a sharp focusing effect is obtained with a high-intensity concentration ratio along both transverse and longitudinal directions.