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The ability to manipulate light and liquids on integrated optofluidics chips has spurred a myriad of important developments in biology, medicine, chemistry and display technologies. Here we show how the convergence of optofluidics and metasurface optics can lead to conceptually new platforms for the dynamic control of light fields. We first demonstrate metasurface building blocks that display an extreme sensitivity in their scattering properties to their dielectric environment. These blocks are then used to create metasurface-based flat optics inside microfluidic channels where liquids with different refractive indices can be directed to manipulate their optical behaviour. We demonstrate the intensity and spectral tuning of metasurface colour pixels as well as on-demand optical elements. We finally demonstrate automated control in an integrated meta-optofluidic platform to open up new display functions. Combined with large-scale microfluidic integration, our dynamic-metasurface flat-optics platform could open up the possibility of dynamic display, imaging, holography and sensing applications.

Scientists are exploring new material designs to make smaller and denser pixel displays

Resonant optical nanomaterials with a high refractive index, such as silicon, have become key elements for controlling free-space light. Here, we show that silicon nanoparticles can manipulate highly confined guided waves in the form of surface plasmon polaritons (SPPs) on a subwavelength scale. Using electron energy-loss spectroscopy in a transmission electron microscope, we demonstrate that SPPs in ultrathin metal films can be efficiently launched due to the strong coupling between the Mie resonances of the nanoparticle and the SPP modes. We find that the SPP excitation wavelength can be tuned across the entire near-infrared by varying the particle size. For insight into the coupling mechanism, we also measure the electron-beam-induced response of the Mie resonances in isolated silicon nanostructures in a broad size range. Finally, we show that the silicon nanoparticles act as scatterers of the SPPs supported by the film. Our results may pave the way for using high-refractive-index dielectric nanoantennas as compact elements for manipulating highly confined SPPs.

For 20 years, nanoscale 3D printing has been based on two-photon absorption, requiring expensive pulsed lasers. Now, via a two-step absorption process, such printing has been demonstrated using a low-cost, low-power continuous-wave laser diode, showing the potential for dramatic cost reductions in 3D nanoprinting.

Layered semiconducting transition metal dichalcogenides (TMDs) are promising materials for high-specific-power photovoltaics due to their excellent optoelectronic properties. However, in practice, contacts to TMDs have poor charge carrier selectivity, while imperfect surfaces cause recombination, leading to a low open-circuit voltage (VOC) and therefore limited power conversion efficiency (PCE) in TMD photovoltaics. Here, we simultaneously address these fundamental issues with a simple MoOx (x ≈ 3) surface charge-transfer doping and passivation method, applying it to multilayer tungsten disulfide (WS2) Schottky-junction solar cells with initially near-zero VOC. Doping and passivation turn these into lateral p-n junction photovoltaic cells with a record VOC of 681 mV under AM 1.5G illumination, the highest among all p-n junction TMD solar cells with a practical design. The enhanced VOC also leads to record PCE in ultrathin (2 photovoltaics. This easily scalable doping and passivation scheme is expected to enable further advances in TMD electronics and optoelectronics.

Controlling the temporal coherence of thermal radiation plays an important role in energy harvesting and thermal management. Conventional approaches for thermal radiation control rely on resonances in bulk thermal emitters, which usually have a large footprint and display little tunability. In this work, we explore the possibility to control coherent thermal emission with atomically thin emitters. We show that, despite the atomic thickness, the peak emissivity of these emitters could be as high as the bulk emitters. We identify several linear scaling laws for the radiative properties of atomic-scale systems, providing a pathway for controlling thermal radiation with atomic precision.

The development of flat optics has taken the world by storm. The initial mission was to try and replace conventional optical elements by thinner, lightweight equivalents. However, while developing this technology and learning about its strengths and limitations, researchers have identified a myriad of exciting new opportunities. It is therefore a great moment to explore where flat optics can really make a difference and what materials and building blocks are needed to make further progress. Building on its strengths, flat optics is bound to impact computational imaging, active wavefront manipulation, ultrafast spatiotemporal control of light, quantum communications, thermal emission management, novel display technologies, and sensing. In parallel with the development of flat optics, we have witnessed an incredible progress in the large-area synthesis and physical understanding of atomically thin, two-dimensional (2D) quantum materials. Given that these materials bring a wealth of unique physical properties and feature the same dimensionality as planar optical elements, they appear to have exactly what it takes to develop the next generation of high-performance flat optics.

Spatial light modulators are essential optical elements in applications that require the ability to regulate the amplitude, phase and polarization of light, such as digital holography, optical communications and biomedical imaging. With the push towards miniaturization of optical components, static metasurfaces are used as competent alternatives. These evolved to active metasurfaces in which light-wavefront manipulation can be done in a time-dependent fashion. The active metasurfaces reported so far, however, still show incomplete phase modulation (below 360°). Here we present an all-solid-state, electrically tunable and reflective metasurface array that can generate a specific phase or a continuous sweep between 0 and 360° at an estimated rate of 5.4 MHz while independently adjusting the amplitude. The metasurface features 550 individually addressable nanoresonators in a 250 × 250 μm2 area with no micromechanical elements or liquid crystals. A key feature of our design is the presence of two independent control parameters (top and bottom gate voltages) in each nanoresonator, which are used to adjust the real and imaginary parts of the reflection coefficient independently. To demonstrate this array’s use in light detection and ranging, we performed a three-dimensional depth scan of an emulated street scene that consisted of a model car and a human figure up to a distance of 4.7 m. By controlling two voltage gates separately from one another, a spatial light modulator has been made that can continuously vary the phase of 360 degrees while independently adjusting the amplitude.

Metasurface-based microdisplays Organic light-emitting diodes (OLEDs) have found wide application in high-resolution, large-area televisions and the handheld displays of smartphones and tablets. With the screen located some distance from the eye, the typical number of pixels per inch is in the region of hundreds. For near-eye microdisplays—for example, in virtual and augmented reality applications—the required pixel density runs to several thousand pixels per inch and cannot be met by present display technologies. Joo et al. developed a full-color, high-brightness OLED design based on an engineered metasurface as a tunable back-reflector. An ultrahigh density of 10,000 pixels per inch readily meets the requirements for the next-generation microdisplays that can be fabricated on glasses or contact lenses. Science , this issue p. 459

The highly engineerable scattering properties of resonant optical antennas underpin the operation of metasurface-based flat optics. Thus far, the choice of antenna has been limited to shaped metallic and high-index semiconductor nanostructures that support geometrical plasmonic or Mie resonances. Whereas these resonant elements offer strong light–matter interaction and excellent control over the scattering phase and amplitude, their electrical tunability has proven to be quite limited. Here, we demonstrate how excitonic resonances in atomically thin semiconductors can be harnessed as a different, third type of resonance to create mutable, flat optics. These strong materials-based resonances are unmatched in their tunability with various external stimuli. To illustrate the concept, we first demonstrate how excitons can enhance the focusing efficiency of a millimetre-scale, patterned WS2 zone plate lens. We also show how electrical gating can completely turn on and off the exciton resonance and thereby modulate the focusing efficiency by 33%. By harnessing the excitonic resonances of a monolayer of WS2 in the visible spectral range, large-area, actively tunable and atomically thin optical lenses can be realized.

Manipulating liquids with tunable shape and optical functionalities in real time is important for electroactive flow devices and optoelectronic devices, but remains a great challenge. Here, we demonstrate electrotunable liquid sulfur microdroplets in an electrochemical cell. We observe electrowetting and merging of sulfur droplets under different potentiostatic conditions, and successfully control these processes via selective design of sulfiphilic/sulfiphobic substrates. Moreover, we employ the electrowetting phenomena to create a microlens based on the liquid sulfur microdroplets and tune its characteristics in real time through changing the shape of the liquid microdroplets in a fast, repeatable, and controlled manner. These studies demonstrate a powerful in situ optical battery platform for unraveling the complex reaction mechanism of sulfur chemistries and for exploring the rich material properties of the liquid sulfur, which shed light on the applications of liquid sulfur droplets in devices such as microlenses, and potentially other electrotunable and optoelectronic devices., Manipulating liquids with tunable shape and optical functionalities in real time remains a great challenge. Here, the authors demonstrate electrotunable liquid sulfur microdroplets in an electrochemical cell and tune its characteristics in a fast, repeatable, and controlled manner.

We demonstrate the ultrafast conversion of statically passive dielectrics (e.g., amorphous TiO2) to transient second-order nonlinear media following the femtosecond transfer of hot electrons, enabling active control of nonlinear interactions, envisioned for terahertz functionalities.

We present the customization of non-paraxial structured light fields of 3D polarization and their application for optical manipulation of zeolite-L nanocontainers. Unlocking its full potential, we identify invisible non-paraxial properties by a self-assembled monolayer.

We report the broadband fluorescence enhancement of erbium ions embedded in a single non-resonant reverse nano-focusing waveguide. We measure a large radiative Purcell enhancement of a total emission rate enhancement of > 250 across the entire measured spectrum including the prominent telecoms C-band. Further, we observe the enhancement of single electric dipole transitions from Stark-split levels at room temperature.

Modern-day sensing and imaging applications increasingly rely on accurate measurements of the primary physical quantities associated with light waves: intensity, wavelength, directionality, and polarization. These are conventionally performed with a series of bulky optical elements, but recently, it has been recognized that optical resonances in nanostructures can be engineered to achieve selective photodetection of light waves with a specific set of predetermined properties. Here, we theoretically illustrate how a thin silicon layer can be patterned into a dislocated nanowire-array that affords detection of circularly polarized light with an efficiency that reaches the theoretical limit for circular dichroism of a planar detector in a symmetric external environment. The presence of a periodic arrangement of dislocations is essential in achieving such unparalleled performance as they enable selective excitation of nonlocal, guided-mode resonances for one handedness of light. We also experimentally demonstrate compact, high-performance chiral photodetectors created from these dislocated nanowire-arrays. This work highlights the critical role defects can play in enabling new nanophotonic functions and devices.

Metasurface-based optical elements typically manipulate light waves by imparting space-variant changes in the amplitude and phase with a dense array of scattering nanostructures. The highly localized and low optical-quality-factor (Q) modes of nanostructures are beneficial for wavefront shaping as they afford quasi-local control over the electromagnetic fields. However, many emerging imaging, sensing, communication, display and nonlinear optics applications instead require flat, high-Q optical elements that provide substantial energy storage and a much higher degree of spectral control over the wavefront. Here, we demonstrate high-Q, non-local metasurfaces with atomically thin metasurface elements that offer notably enhanced light–matter interaction and fully decoupled optical functions at different wavelengths. We illustrate a possible use of such a flat optic in eye tracking for eyewear. Here, a metasurface patterned on a regular pair of eye glasses provides an unperturbed view of the world across the visible spectrum and redirects near-infrared light to a camera to allow imaging of the eye. High optical-quality-factor, non-local metasurfaces enable independent functions across different wavelength bands.

The ability to manipulate light and liquids on integrated optofluidics chips has spurred a myriad of important developments in biology, medicine, chemistry and display technologies. Here we show how the convergence of optofluidics and metasurface optics can lead to conceptually new platforms for the dynamic control of light fields. We first demonstrate metasurface building blocks that display an extreme sensitivity in their scattering properties to their dielectric environment. These blocks are then used to create metasurface-based flat optics inside microfluidic channels where liquids with different refractive indices can be directed to manipulate their optical behaviour. We demonstrate the intensity and spectral tuning of metasurface colour pixels as well as on-demand optical elements. We finally demonstrate automated control in an integrated meta-optofluidic platform to open up new display functions. Combined with large-scale microfluidic integration, our dynamic-metasurface flat-optics platform could open up the possibility of dynamic display, imaging, holography and sensing applications.

The possibility to manipulate light fields with metasurfaces can facilitate more compact and efficient devices for encoding computer-generated holograms. However, most of the reconstructed holographic images for reported metasurfaces are either monochromatic or two-dimensional, limiting their applicability. Here, we design and fabricate a silicon metasurface, whose constituent meta-molecules are composed of three different kinds of meta-atoms capable of modulating red, green, and blue light independently. A modified Gerchberg–Saxton algorithm is utilized to retrieve the wavefront for the three-dimensional (3D) hologram. The reconstructed 3D full-color images can be switched by changing the helicity of the illuminating circularly polarized light. The spin-switched 3D full-color scenes greatly increase the information capacity of the device, which may find use in 3D dynamic displays, virtual reality, and data storage applications.

Plasmonic antennas and metasurfaces can effectively control light-matter interactions, and this facilitates a deterministic design of optical materials properties, including structural color. However, these optical properties are generally fixed after synthesis and fabrication, while many modern-day optics applications require active, low-power, and nonvolatile tuning. These needs have spurred broad research activities aimed at identifying materials and resonant structures capable of achieving large, dynamic changes in optical properties, especially in the challenging visible spectral range. In this work, we demonstrate dynamic tuning of polarization-dependent gap plasmon resonators that contain the electrochromic oxide WO3. Its refractive index in the visible changes continuously from n = 2.1 to 1.9 upon electrochemical lithium insertion and removal in a solid-state device. By incorporating WO3 into a gap plasmon resonator, the resonant wavelength can be shifted continuously and reversibly by up to 58 nm with less than 2 V electrochemical bias voltage. The resonator can remain in a tuned state for tens of minutes under open circuit conditions.

The capability of on-chip wavefront modulation has the potential to revolutionize many optical device technologies. However, the realization of power-efficient phase-gradient metasurfaces that offer full-phase modulation (0 to 2π) and high operation speeds remains elusive. We present an approach to continuously steer light that is based on creating a virtual frequency-gradient metasurface by combining a passive metasurface with an advanced frequency-comb source. Spatiotemporal redirection of light naturally occurs as optical phase-fronts reorient at a speed controlled by the frequency gradient across the virtual metasurface. An experimental realization of laser beam steering with a continuously changing steering angle is demonstrated with a single metasurface over an angle of 25° in just 8 picoseconds. This work can support integrated-on-chip solutions for spatiotemporal optical control, directly affecting emerging applications such as solid-state light detection and ranging (LIDAR), three-dimensional imaging, and augmented or virtual systems.

The reflection of light from metallic mirrors results in a near-zero electric field at their surface. This precludes strong light–matter interaction between such mirrors and two-dimensional (2D) ma...

Minimizing reflection losses is required for the efficient operation of a wide range of optical components. Antireflection coatings supporting Fabry-Pérot resonances are commonly used to solve this problem and can be applied on an industrial scale. Recent work has shown that reflections can also be reduced by placing an array of high-index nanostructures on a surface. In such coatings, antireflection is achieved by tailoring the scattering by optical Mie resonances. Here, we design and fabricate Si metasurfaces that combine both Fabry-Pérot and Mie resonances in a single metasurface to realize a multiresonant broadband antireflection response. We optically characterize the metasurfaces demonstrating 4.1% AM1.5-averaged reflectance across the visible spectrum (425-900 nm). Our metasurface design strategy is generally applicable to different materials and frequency ranges, making our approach relevant for a broad variety of applications.

Metasurfaces enable the manipulation of light’s disorder strength in a two-dimensional photonic system. Here we report on the spectral interleaving of an ordered and a disordered system within a geometric phase metasurface. The efficiency of prevalent interleaving techniques is limited by the number of functions incorporated within the metasurface. We present a shared-aperture extinction cross-section approach relying on interleaving of spectrally selective nanoantenna arrays, each having a large extinction cross-section, thus allowing to overcome this limitation. Using this approach, we realize a silicon-based spectral interleaving metasurface for spectrum-dependent disguise, holographic tagging, and imaging of a target object. The shared-aperture extinction cross-section concept opens the path for the generation of multiple, efficient, and spectrally resolved functions in a two-dimensional photonic system. The presented order–disorder interleaving approach offers new prospects for the manipulation of li...

Layered semiconducting transition metal dichalcogenides (TMDs) are promising materials for high-specific-power photovoltaics due to their excellent optoelectronic properties. However, in practice, contacts to TMDs have poor charge carrier selectivity, while imperfect surfaces cause recombination, leading to a low open-circuit voltage (

Metasurface-based optical elements typically manipulate light waves by imparting space-variant changes in the amplitude and phase with a dense array of scattering nanostructures. The highly localized and low optical-quality-factor (Q) modes of nanostructures are beneficial for wavefront shaping as they afford quasi-local control over the electromagnetic fields. However, many emerging imaging, sensing, communication, display and nonlinear optics applications instead require flat, high-Q optical elements that provide substantial energy storage and a much higher degree of spectral control over the wavefront. Here, we demonstrate high-Q, non-local metasurfaces with atomically thin metasurface elements that offer notably enhanced light-matter interaction and fully decoupled optical functions at different wavelengths. We illustrate a possible use of such a flat optic in eye tracking for eyewear. Here, a metasurface patterned on a regular pair of eye glasses provides an unperturbed view of the world across the visible spectrum and redirects near-infrared light to a camera to allow imaging of the eye.

The ability of two nearly-touching plasmonic nanoparticles to squeeze light into a nanometer gap has provided a myriad of fundamental insights into light–matter interaction. In this work, we construct a nanoelectromechanical system (NEMS) that capitalizes on the unique, singular behavior that arises at sub-nanometer particle-spacings to create an electro-optical modulator. Using in situ electron energy loss spectroscopy in a transmission electron microscope, we map the spectral and spatial changes in the plasmonic modes as they hybridize and evolve from a weak to a strong coupling regime. In the strongly-coupled regime, we observe a very large mechanical tunability (~250 meV/nm) of the bonding-dipole plasmon resonance of the dimer at ~1 nm gap spacing, right before detrimental quantum effects set in. We leverage our findings to realize a prototype NEMS light-intensity modulator operating at ~10 MHz and with a power consumption of only 4 fJ/bit., Squeezing light into a nanometer gap offers strong light–matter interaction. Here, the authors develop a nanoelectromechanical system to dynamically control the gap of a plasmonic dimer at nanometer scale, enabling the realization of a light-intensity modulator that operates at high speed and with a low power consumption.

We showcase the application of resonant flat optical elements as insertable angular filters that facilitate phase contrast imaging. With high accuracy direct angular filtering, quantitative phase contrast imaging is achieved akin to Fourier optics approaches. © 2021 The Author(s)