Ultrathin van der Waals Metalenses

WSe2 Light-Emitting Device Coupled to an h-BN Waveguide

Optical information processing using photonic integrated circuits is a key goal in the field of nanophotonics. Extensive research efforts have led to remarkable progress in integrating active and passive device functionalities within one single photonic circuit. Still, to date, one of the central components, i.e., light sources, remain a challenge to be integrated. Here, we focus on a photonic platform that is solely based on two-dimensional materials to enable the integration of electrically contacted optoelectronic devices inside the light-confining dielectric of photonic structures. We combine light-emitting devices, based on exciton recombination in transition metal dichalcogenides, with hexagonal boron nitride photonic waveguides in a single van der Waals heterostructure. Waveguide-coupled light emission is achieved by sandwiching the light-emitting device between two hexagonal boron nitride slabs and patterning the complete van der Waals stack into a photonic structure. Our demonstration of on-chip light generation and waveguiding is a key component for future integrated van der Waals optoelectronics.

All-2D material photonic devices

Two-dimensional (2D) materials are extensively used in almost all scientific research areas, from fundamental research to applications. Initially, 2D materials were integrated with conventional non-2D materials having well-established manufacturing methods. Recently, the concept of constructing photonic devices exclusively from 2D materials has emerged. Various devices developed to date have been demonstrated based on monolithic or hetero 2D materials. In this review, photonic devices that solely consist of 2D materials are introduced, including photonic waveguides, lenses, and optical cavities. Exploring photonic devices that are made entirely of 2D materials could open interesting prospects as they enable the thinnest devices possible because of their extraordinarily high refractive index. In addition, unique characteristics of 2D materials, such as high optical anisotropy and spin orbit coupling, might provide intriguing applications.

Quantum Emitters in Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) has emerged as a fascinating platform to explore quantum emitters and their applications. Beyond being a wide-bandgap material, it is also a van der Waals crystal, enabling direct exfoliation of atomically thin layers─a combination which offers unique advantages over bulk, 3D crystals. In this Mini Review we discuss the unique properties of hBN quantum emitters and highlight progress toward their future implementation in practical devices. We focus on engineering and integration of the emitters with scalable photonic resonators. We also highlight recently discovered spin defects in hBN and discuss their potential utility for quantum sensing. All in all, hBN has become a front runner in explorations of solid-state quantum science with promising future prospects.

Atomic-Void van der Waals Channel Waveguides

Layered van der Waals materials allow creating unique atomic-void channels with subnanometer dimensions. Coupling light into these channels may further advance sensing, quantum information, and single molecule chemistries. Here, we examine theoretically limits of light guiding in atomic-void channels and show that van der Waals materials exhibiting strong resonances, excitonic and polaritonic, are ideally suited for deeply subwavelength light guiding. We predict that excitonic transition metal dichalcogenides can squeeze >70% of optical power in just <λ/100 thick channel in the visible and near-infrared. We also show that polariton resonances of hexagonal boron nitride allow deeply subwavelength (<λ/500) guiding in the mid-infrared. We further reveal effects of natural material anisotropy and discuss the influence of losses. Such van der Waals channel waveguides while offering extreme optical confinement exhibit significantly lower loss compared to plasmonic counterparts, thus paving the way to low-loss and deeply subwavelength optics.

Optical Constants of Several Multilayer Transition Metal Dichalcogenides Measured by Spectroscopic Ellipsometry in the 300-1700 nm Range: High Index, Anisotropy, and Hyperbolicity

Transition metal dichalcogenides (TMDs) attract significant attention due to their remarkable optical and excitonic properties. It was understood already in the 1960s and recently rediscovered that many TMDs possess a high refractive index and optical anisotropy, which make them attractive for nanophotonic applications. However, accurate analysis and predictions of nanooptical phenomena require knowledge of dielectric constants along both in- and out-of-plane directions and over a broad spectral range, information that is often inaccessible or incomplete. Here, we present an experimental study of optical constants from several exfoliated TMD multilayers obtained using spectroscopic ellipsometry in the broad range of 300-1700 nm. The specific materials studied include semiconducting WS₂, WSe₂, MoS₂, MoSe₂, and MoTe₂, as well as in-plane anisotropic ReS₂ and WTe₂ and metallic TaS₂, TaSe₂, and NbSe₂. The extracted parameters demonstrate a high index (n up to ∼4.84 for MoTe₂), significant anisotropy (n _∥ - n _⊥ ≈ 1.54 for MoTe₂), and low absorption in the near-infrared region. Moreover, metallic TMDs show potential for combined plasmonic-dielectric behavior and hyperbolicity, as their plasma frequency occurs at around ∼1000-1300 nm depending on the material. The knowledge of optical constants of these materials opens new experimental and computational possibilities for further development of all-TMD nanophotonics.

Coherently switching the focusing characteristics of all-dielectric metalenses

Flat, gradient index, metasurface optics - in particular all-dielectric metalenses - have emerged and evolved over recent years as compact, lightweight alternative to their conventional bulk glass/crystal counterparts. Here we show that the focal properties of all-dielectric metalenses can be switched via coherent control, which is to say by changing the local electromagnetic field in the metalens plane rather than any physical or geometric property of the nanostructure or surrounding medium. The selective excitation of predominantly electric or magnetic resonant modes in the constituent cells of the metalens provides for switching, by design, of its phase profile enabling binary switching of focal length for a given lens type and, uniquely, switching between different (spherical and axicon) lens types.

On-chip photonics and optoelectronics with a van der Waals material dielectric platform

During the last few decades, photonic integrated circuits have increased dramatically, facilitating many high-performance applications, such as on-chip sensing, data processing, and inter-chip communications. The currently dominating material platforms (i.e., silicon, silicon nitride, lithium niobate, and indium phosphide), which have exhibited great application successes, however, suffer from their own disadvantages, such as the indirect bandgap of silicon for efficient light emission, and the compatibility challenges of indium phosphide with the silicon industry. Here, we report a new dielectric platform using nanostructured bulk van der Waals materials. On-chip light propagation, emission, and detection are demonstrated by taking advantage of different van der Waals materials. Low-loss passive waveguides with MoS2 and on-chip light sources and photodetectors with InSe have been realised. Our proof-of-concept demonstration of passive and active on-chip photonic components endorses van der Waals materials for offering a new dielectric platform with a large material-selection degree of freedom and unique properties toward close-to-atomic scale manufacture of on-chip photonic and optoelectronic devices.

Multifunctional and Transformative Metaphotonics with Emerging Materials

Future technologies underpinning multifunctional physical and chemical systems and compact biological sensors will rely on densely packed transformative and tunable circuitry employing nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of resonant metaphotonics may provide a versatile practical platform for nanoscale science by employing resonances in high-index dielectric nanoparticles and metasurfaces. Here, we discuss the recently emerged field of metaphotonics and describe its connection to material science and chemistry. For tunabilty, metaphotonics employs a variety of the recently highlighted materials such as polymers, perovskites, transition metal dichalcogenides, and phase change materials. This allows to achieve diverse functionalities of metasystems and metasurfaces for efficient spatial and temporal control of light by employing multipolar resonances and the physics of bound states in the continuum. We anticipate expanding applications of these concepts in nanolasers, tunable metadevices, metachemistry, as well as a design of a new generation of chemical and biological ultracompact sensing devices.

Enhanced light-matter interaction in two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS₂, WS₂, MoSe₂, and WSe₂, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research.

Helicity-dependent continuous varifocal metalens based on bilayer dielectric metasurfaces

Metasurfaces offer a unique platform to realize flat lenses, reducing the size and complexity of imaging systems and thus enabling new imaging modalities. In this paper, we designed a bilayer helicity-dependent continuous varifocal dielectric metalens in the near-infrared range. The first layer consists of silicon nanopillars and functions as a half-wave plate, providing the helicity-dependent metasurface by combining propagation phase and geometric phase. The second layer consists of phase-change material Sb₂S₃ nanopillars and provides tunable propagation phases. Upon excitation with the circularly polarized waves possessing different helicities, the metalens can generate helicity-dependent longitudinal focal spots. Under the excitation of linear polarized light, the helicity-dependent dual foci are generated. The focal lengths in this metalens can be continuously tuned by the crystallization fraction of Sb₂S₃. The zoom range is achieved from 32.5 µm to 37.2 µm for right circularly polarized waves and from 50.5 µm to 60.9 µm for left circularly polarized waves. The simulated focusing efficiencies are above 75% and 87% for the circularly and linearly polarized waves, respectively. The proposed metalens has potential applications in miniaturized devices, including compact optical communication systems, imaging, and medical devices.

Tunable unidirectional nonlinear emission from transition-metal-dichalcogenide metasurfaces

Nonlinear light sources are central to a myriad of applications, driving a quest for their miniaturisation down to the nanoscale. In this quest, nonlinear metasurfaces hold a great promise, as they enhance nonlinear effects through their resonant photonic environment and high refractive index, such as in high-index dielectric metasurfaces. However, despite the sub-diffractive operation of dielectric metasurfaces at the fundamental wave, this condition is not fulfilled for the nonlinearly generated harmonic waves, thereby all nonlinear metasurfaces to date emit multiple diffractive beams. Here, we demonstrate the enhanced single-beam second- and third-harmonic generation in a metasurface of crystalline transition-metal-dichalcogenide material, offering the highest refractive index. We show that the interplay between the resonances of the metasurface allows for tuning of the unidirectional second-harmonic radiation in forward or backward direction, not possible in any bulk nonlinear crystal. Our results open new opportunities for metasurface-based nonlinear light-sources, including nonlinear mirrors and entangled-photon generation.

Metalenses: from design principles to functional applications

Lens is a basic optical element that is widely used in daily life, such as in cameras, glasses, and microscopes. Conventional lenses are designed based on the classical refractive optics, which results in inevitable imaging aberrations, such as chromatic aberration, spherical aberration and coma. To solve these problems, conventional imaging systems impose multiple curved lenses with different thicknesses and materials to eliminate these aberrations. As a unique photonic technology, metasurfaces can accurately manipulate the wavefront of light to produce fascinating and peculiar optical phenomena, which has stimulated researchers' extensive interests in the field of planar optics. Starting from the introduction of phase modulation methods, this review summarizes the design principles and characteristics of metalenses. Although the imaging quality of existing metalenses is not necessarily better than that of conventional lenses, the multi-dimensional and multi-degree-of-freedom control of metasurfaces provides metalenses with novel functions that are extremely challenging or impossible to achieve with conventional lenses.

Ultra-sensitive nanometric flat laser prints for binocular stereoscopic image

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with tantalizing layer-dependent electronic and optical properties have emerged as a paradigm for integrated flat opto-electronic devices, but their widespread applications are hampered by challenges in deterministic fabrication with demanded shapes and thicknesses, as well as light field manipulation in such atomic-thick layers with negligible thicknesses compared to the wavelength. Here we demonstrate ultra-sensitive light field manipulation in full visible ranges based on MoS₂ laser prints exfoliated with nanometric precisions. The nontrivial interfacial phase shifts stemming from the unique dispersion of MoS₂ layers integrated on the metallic substrate empower an ultra-sensitive resonance manipulation up to 13.95 nm per MoS₂ layer across the entire visible bands, which is up to one-order-of-magnitude larger than their counterparts. The interlayer van der Waals interactions and the anisotropic thermal conductivity of layered MoS₂ films endow a laser exfoliation method for on-demand patterning MoS₂ with atomic thickness precision and subwavelength feature sizes. With this, nanometric flat color prints and further amplitude-modulated diffractive components for binocular stereoscopic images can be realized in a facile and lithography-free fashion. Our results with demonstrated practicality unlock the potentials of, and pave the way for, widespread applications of emerging 2D flat optics.

The authors demonstrate fidelity colour prints and binocular stereoscopic images in multilayer MoS₂ integrated on an Au substrate, showing nanometric layer sensitivity in the Fabry-Perot resonance changed by a facile laser recipe.

The Two-Dimensional Electrides XONa (X=Mg, Ca) as Novel Natural Hyperbolic Materials

In two-dimensional electrides, anionic electrons are spatially confined in interlayer regions with high density, comparable to metals, and they are highly mobile, just as free electrons, resembling hyperbolic metamaterials with metal-dielectric multilayered structures. In this work, two-dimensional electride materials MgONa and CaONa are proposed as good natural hyperbolic materials. By using the first-principles calculations based on density functional theory (DFT), the electronic structures, stabilities, and optical properties of two-dimensional electride materials XONa (X=Mg, Ca) are investigated. Our results show that they are stable in 1-monolayer (1-ML) structures as well as in bulk states. They exhibit hyperbolic dispersions from visible to near infrared spectral range with high qualities up to about 700, which is two orders-of-magnitude larger than the preceding bulk hyperbolic materials. Numerical results reveal that they exhibit negative refraction with low losses. Their high-quality hyperbolic responses over a wide spectral range pave the way of broad photonic applications as natural hyperbolic materials.

π-phase modulated monolayer supercritical lens

The emerging monolayer transition metal dichalcogenides have provided an unprecedented material platform for miniaturized opto-electronic devices with integrated functionalities. Although excitonic light–matter interactions associated with their direct bandgaps have received tremendous research efforts, wavefront engineering is less appreciated due to the suppressed phase accumulation effects resulting from the vanishingly small thicknesses. By introducing loss-assisted singular phase behaviour near the critical coupling point, we demonstrate that integration of monolayer MoS₂ on a planar ZnO/Si substrate, approaching the physical thickness limit of the material, enables a π phase jump. Moreover, highly dispersive extinctions of MoS₂ further empowers broadband phase regulation and enables binary phase-modulated supercritical lenses manifesting constant sub-diffraction-limited focal spots of 0.7 Airy units (AU) from the blue to yellow wavelength range. Our demonstrations downscaling optical elements to atomic thicknesses open new routes for ultra-compact opto-electronic systems harnessing two-dimensional semiconductor platforms with integrated functionalities.

Here the authors report binary phase supercritical lenses by patterning monolayer TMD materials. Through placement of atomic thin 2D TMD with sufficient absorption, a spot of critical coupling is created to facilitate a π phase jump and subdiffraction focusing over bandwidth of 150 nm in visible range.

Recent Progress on Ultrathin Metalenses for Flat Optics

As technology advances, electrical devices such as smartphones have become more and more compact, leading to a demand for the continuous miniaturization of optical components. Metalenses, ultrathin flat optical elements composed of metasurfaces consisting of arrays of subwavelength optical antennas, provide a method of meeting those requirements. Moreover, metalenses have many other distinctive advantages including aberration correction, active tunability, and semi-transparency, compared to their conventional refractive and diffractive counterparts. Therefore, over the last decade, great effort has been focused on developing metalenses to investigate and broaden the capabilities of metalenses for integration into future applications. Here, we discuss recent progress on metalenses including their basic design principles and notable characteristics such as aberration correction, tunability, and multifunctionality.

Atomically Thin Noble Metal Dichalcogenides for Phase-Regulated Meta-optics

Owing to its good air stability and high refractive index, two-dimensional (2D) noble metal dichalcogenide shows intriguing potential for versatile flat optics applications. However, light field manipulation at the atomic scale is conventionally considered unattainable because the small thickness and intrinsic losses of 2D materials completely suppress both resonances and phase accumulation effects. Here, we demonstrate that losses of structured atomically thick PtSe₂ films integrated on top of a uniform substrate can be utilized to create the spots of critical coupling, enabling singular phase behaviors with a remarkable π phase jump. This finding enables the experimental demonstration of atomically thick binary meta-optics that allows an angle-robust and high unit thickness diffraction efficiency of 0.96%/nm in visible frequencies (given its thickness of merely 4.3 nm). Our results unlock the potential of a new class of 2D flat optics for light field manipulation at an atomic thickness.

Exciton-Enabled Meta-Optics in Two-Dimensional Transition Metal Dichalcogenides

Optical wavefront engineering has been rapidly developing in fundamentals from phase accumulation in the optical path to the electromagnetic resonances of confined nanomodes in optical metasurfaces. However, the amplitude modulation of light has limited approaches that usually originate from the ohmic loss and absorptive dissipation of materials. Here, an atomically thin photon-sieve platform made of MoS₂ multilayers is demonstrated for high-quality optical nanodevices, assisted fundamentally by strong excitonic resonances at the band-nesting region of MoS₂. The atomic thin MoS₂ significantly facilitates high transmission of the sieved photons and high-fidelity nanofabrication. A proof-of-concept two-dimensional (2D) nanosieve hologram exhibits 10-fold enhanced efficiency compared with its non-2D counterparts. Furthermore, a supercritical 2D lens with its focal spot breaking diffraction limit is developed to exhibit experimentally far-field label-free aberrationless imaging with a resolution of ∼0.44λ at λ = 450 nm in air. This transition-metal-dichalcogenide (TMDC) photonic platform opens new opportunities toward future 2D meta-optics and nanophotonics.

Negative Refraction in the Visible and Strong Plasmonic Resonances in Photonic Structures of the Electride Material Mg2 N

Natural hyperbolic materials have recently attracted great attention due to their capability of supporting spatial mode frequency much higher than artificial metamaterials and the advantage that they do not require nanofabrication processes. For practical applications, however, hyperbolic bulk materials with lower optical losses in shorter wavelength range should be developed. This work presents the electronic structure and dielectric response of an electride Mg₂ N, revealing that this material exhibits hyperbolic responses with low optical loss in the visible and plasmonic responses with high-quality in the near-infrared range. Negative refraction in the red spectral range has been analytically and numerically demonstrated. In particular, nanoantenna structures of Mg₂ N generate strong plasmonic resonances in the near-infrared and the intensity enhancement in the gap region is one order of magnitude higher compared with silver nanoantenna due to its much higher quality factor, which can find potential applications for nanoplasmonic purposes such as single molecule detections by surface-enhanced hyper-Raman spectroscopy and nonlinear wavelength generations at the nanoscale.

Electro-optically Tunable Multifunctional Metasurfaces

Shaping the flow of light at the nanoscale has been a grand challenge for nanophotonics over decades. It is now widely recognized that metasurfaces represent a chip-scale nanophotonics array technology capable of comprehensively controlling the wavefront of light via appropriately configuring subwavelength antenna elements. Here, we demonstrate a reconfigurable metasurface that is multifunctional, i.e., notionally capable of providing diverse optical functions in the telecommunication wavelength regime, using a single compact, lightweight, electronically controlled array with no moving parts. By electro-optical control of the phase of the scattered light from each identical individual metasurface element in an array, we demonstrate a single prototype multifunctional programmable metasurface that is capable of both dynamic beam steering and reconfigurable light focusing. Reconfigurable multifunctional metasurfaces with arrays of tunable optical antennas thus can perform arbitrary optical functions by programmable array-level control of scattered light phase, amplitude, and polarization, similar to dynamic and programmable memories in electronics.

Remarkably high-Q resonant nanostructures based on atomically thin two-dimensional materials

Planar optical resonant structures with high quality (Q) factors play a crucial role in modern photonic technologies. In this paper, a type of remarkably high-Q resonant nanostructure based on atomically thin two-dimensional (2D) materials is proposed. It is shown theoretically and numerically that with the excitation of leaky modes in the proposed structures, guided mode resonant (GMR) gratings, can achieve resonances with extremely narrow linewidths down to 0.0005 nm and high Q-factors up to millions in the telecom range. The thickness of 2D materials and thus the high-Q resonances can be precisely controlled by changing the layer number of 2D materials, providing a versatile platform for strong light-matter interactions. As an example, dramatic nonlinear reflectance can be realized around the resonance at a power level of a few kW cm-2 with the Kerr effect. This new type of 2D material resonant nanostructure can be employed for a variety of applications ranging from lasers, filters and polarizers to nonlinear optical devices.

Metasurface-generated complex 3-dimensional optical fields for interference lithography

Fast submicrometer-scale 3D printing techniques are of interest for various applications ranging from photonics and electronics to tissue engineering. Interference lithography is a versatile 3D printing method with the ability to generate complicated nanoscale structures. Its application, however, has been hindered by either the complicated setups in multibeam lithography that cause sensitivity and impede scalability or the limited level of control over the fabricated structure achievable with mask-assisted processes. Here, we show that metasurface masks can generate complex volumetric intensity distributions with submicrometer scales for fast and scalable 3D printing. These results push the limits of optical devices in controlling the light intensity distribution and significantly increase the realm of possibilities for 3D printing.

Fast, large-scale, and robust 3-dimensional (3D) fabrication techniques for patterning a variety of structures with submicrometer resolution are important in many areas of science and technology such as photonics, electronics, and mechanics with a wide range of applications from tissue engineering to nanoarchitected materials. From several promising 3D manufacturing techniques for realizing different classes of structures suitable for various applications, interference lithography with diffractive masks stands out for its potential to fabricate complex structures at fast speeds. However, the interference lithography masks demonstrated generally suffer from limitations in terms of the patterns that can be generated. To overcome some of these limitations, here we propose the metasurface-mask–assisted 3D nanofabrication which provides great freedom in patterning various periodic structures. To showcase the versatility of this platform, we design metasurface masks that generate exotic periodic lattices like gyroid, rotated cubic, and diamond structures. As a proof of concept, we experimentally demonstrate a diffractive element that can generate the diamond lattice.

Aerosol-Assisted Fine-Tuning of Optoelectrical Properties of SWCNT Films

We propose a novel, scalable, and simple method for aerosol doping of single-walled carbon nanotube (SWCNT) films. This method is based on aerosolization of a dopant solution (HAuCl₄ in ethanol) and time-controlled deposition of uniform aerosol particles on the nanotube film surface. The approach developed allows fine-tuning of the SWCNT work function in the range of 4.45 (for pristine nanotubes) to 5.46 eV, controllably varying the sheet resistance of the films from 79 to 3.2 Ω/□ for the SWCNT films with 50% transmittance (at 550 nm). This opens a new avenue for traditional and flexible optoelectronics, both to replace existing indium-tin oxide electrodes and to develop novel applications of the highly conductive transparent films.

Design and analysis of thin optical lens composed of low-index subwavelength structures

We design a polarization-insensitive subwavelength optical lens capable of focusing plane waves of visible wavelength using traditional optical materials such as glass. Using analytical effective medium theory and finite difference time domain (FDTD) method, the phase of transmission of arrayed subwavelength inclusions is studied with respect to their size and shape. It is shown that the phase relations can be accurately predicted using an analytical method, simplifying the design process. A guideline is established for selecting a set of subwavelength inclusions so that complete phase coverage can be achieved. Analytical calculation of focal length using diffraction equations (Fresnel or Rayleigh-Sommerfeld) along with effective medium approximation is done and compared with FDTD and experimental results, showing high accuracy. Large-scale optical lenses with subwavelength thickness are designed and their performance analyzed using an analytical approach. The optical lenses show high focusing efficiency at small numerical apertures, insensitivity to polarization angle, and robustness to uncertainty in their structural parameters. This work shows that using an analytical method, thin optical lenses with subwavelength structure can be designed and studied.

Mechanically tunable focusing metamirror in the visible

A compact, flat lens with dynamically tunable focal length will be an essential component in advanced reconfigurable optical systems. One approach to realize a flat tunable lens is by utilizing metasurfaces, which are two-dimensional nanostructures capable of tailoring the wavefront of incident light. When a metasurface with a hyperboloidal phase profile, i.e., a metalens, is fabricated on a substrate that can be actuated, its focal length can be adjusted dynamically. Here, we design and realize the first reflection type, tunable metalens (i.e., metamirror) operating in the visible regime (670 nm). It is shown that the focal length can be continuously adjusted by up to 45% with a 0% to 20% lateral stretching of the substrate, while maintaining diffraction-limited focusing and high focusing efficiency. Our design as a flat optics element has potential in widespread applications, such as wearable mixed reality systems, biomedical instruments and integrated optics devices.