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Optical metasurfaces have great potential to overcome the functional limitations of conventional optical devices. In addition to polarization- or wavelength-multiplexed metasurfaces, angle-multiplexed metasurfaces can provide new degrees of freedom, enabling previously unrealized complex functionality in diverse applications such as LiDAR, augmented reality glasses, and imaging. However, there have been fundamental trade-offs in transmission efficiency and angular sensitivity for practically important paraxial rays. In this paper, we overcome this limitation by breaking mirror symmetries of single-layer metasurface structures. Based on an effective medium theory, we intuitively explain which material parameters affect the sensitivity and efficiency and prove that high sensitivity and high efficiency can be achieved simultaneously by breaking the mirror symmetry. Based on this, we propose optimized metasurfaces for two applications: an angle-multiplexed beam-steering device with up to 93% relative efficiency and an angle-multiplexed metalens array that can break the fundamental resolution–density trade-off of microlens arrays with high efficiency. The proposed angle-selective designs could pave the way for the development of new classes of compact optical devices with novel functions.

Recent advances in metasurfaces and optical nanostructures have enabled complex control of incident light with optically thin devices. However, it has thus far been unclear whether it is possible to achieve complete linear control of coherent light transmission, i.e., independent control of polarization, amplitude, and phase for both input polarization states, with just a single, thin nanostructure array. Here we prove that it is possible and propose a universal metasurface, a bilayer array of high-index elliptic cylinders, that possesses a complete degree of optical freedom with fully designable chirality and anisotropy. We mathematically show the completeness of achievable light control with corresponding Jones matrices, experimentally demonstrate new types of three-dimensional holographic schemes that were formerly impossible, and present a systematic way of realizing any input-state-sensitive vector linear optical device. Our results unlock previously inaccessible degrees of freedom in light transmission control., Comment: Main text: 30 pages, 5 figures. Supplementary discussion: 12 pages, 4 figures

Metamaterials can possess extraordinary properties not readily available in nature. While most of the early metamaterials had narrow frequency bandwidth of operation, many recent works have focused on how to implement exotic properties and functions over broad bandwidth for practical applications. Here, we provide two definitions of broadband operation in terms of effective material properties and device functionality, suitable for describing materials and devices, respectively, and overview existing broadband metamaterial designs in such two categories. Broadband metamaterials with nearly constant effective material properties are discussed in the materials part, and broadband absorbers, lens, and hologram devices based on metamaterials and metasurfaces are discussed in the devices part.

Lithium metal is considered one of the most promising anode materials for realizing high volumetric and gravimetric energy density, owing to the high specific capacity (∼3860 mAh g−1) and the low electrochemical potential of lithium (−3.04 V vs. the standard hydrogen electrode). However, undesirable dendritic lithium growth and corresponding instability of the solid electrolyte interphase prevent safe and long-term use of lithium metal anodes. This paper presents a simple electrolyte approach to enhance the performance of lithium metal batteries by tuning the dielectric constant of the liquid electrolyte. Electrolyte formulations are designed by changing the concentration of ethylene carbonate to have various dielectric constants. This study confirms that high ethylene carbonate content in a liquid electrolyte enhances the cycling performance of lithium metal batteries because the electric field intensity applied to the electrolyte is reduced in relation to the polarization of the electrolyte and thus allows smooth lithium plating and formation of a stable solid electrolyte interphase. We believe that this approach provides an important concept for electrolyte system design suitable to lithium metal batteries.

In optical nanostructures used as artificial resonance-based color filters, there is unfortunate universal trade-off between spectral sharpness and angular tolerance as well as maximum extinction. We rigorously derive the maximum performance bounds of wavelength-rejection filters realized by single-layer plasmonic metasurfaces with a dominant resonance and weak near-field coupling, and propose a multi-layer approach to overcome these single-layer limits and trade-offs. We also present a realistic example that has a narrow full-width-at-half-maximum bandwidth of 24 nm with 10 dB extinction at 532 nm with good angular tolerance up to 60°. The performance of the proposed metasurface is close to the general theoretical bound.

Microscopic structuring can change the effective properties of a material by several orders of magnitude. An example of this is animal bone, which has an effective elastic modulus that is more than 1,000 times larger than that of the constituent proteins. Here, we propose a broadband-enhancement principle of photonic nonlinearity that has a similar mathematical origin as the bone example. The proposed staggered array metamaterials violate the standard Miller’s rule in nonlinear optics and can enhance the third-order nonlinearity by more than a thousand to a billion times, depending on target operation frequencies. This metamaterial principle also enables manipulation of the individual components of the linear and nonlinear susceptibility tensors. Our biomimetic approach overcomes the fundamental speed-efficiency trade-off in current resonant enhancement schemes, making faster and more efficient all-optical devices possible for 1.55 μm wavelength. The principle is also applicable to ionic diffusion, heat conduction, or other transport problems. Biological materials such as bone show enhanced mechanical properties due to their specific structures, which can inspire new biomimetic materials. Here, a broadband metamaterial exhibiting giant optical nonlinearity is proposed, who’s properties share a mathematical origin with mechanically enhanced biomaterials.

Transparent conducting electrodes (TCEs) are essential components in various optoelectronic devices. Nanostructured metallic thin film is one of the promising candidates to complement current metal oxide films, such as ITO, where high cost rare earth elements have been a longstanding issue. Herein, we present that multiscale porous metal nanomesh thin films prepared by bimodal self-assembly of block copolymer (BCP)/homopolymer blends may offer a new opportunity for TCE. This hierarchical concurrent self-assembly consists of macrophase separation between BCP and homopolymer as well as microphase separation of BCP, and thus provides a straightforward spontaneous production of a highly porous multiscale pattern over an arbitrary large area. Employing a conventional pattern transfer process, we successfully demonstrated a multiscale highly porous metallic thin film with reasonable optical transparency, electro-conductance, and large-area uniformity, taking advantage of low loss light penetration through microscale pores and significant suppression of light reflection at the nanoporous structures. This well-defined controllable bimodal self-assembly can offer valuable opportunities for many different applications, including optoelectronics, energy harvesting, and membranes.

The mode orthogonality fundamentally influences the scattering spectra of multi-resonance systems, such as plasmonic color filters. We show that planar arrays of silver nanostructures with dual localized surface plasmon resonances and the right mode orthogonality can function as transmissive RGB color filters with peak transmittances higher than 70%, and color gamut areas larger than 90% of the sRGB space. These are the brightest and most saturated of all designs proposed thus far. We present the Pareto frontier from designs with more than 80% peak transmittance, to designs that achieve a color gamut larger than 120% of the sRGB space.

The refractive index is the fundamental property of all optical materials and dictates Snell's law, propagation speed, wavelength, diffraction, energy density, absorption and emission of light in materials. Experimentally realized broadband refractive indices remain 1,800 resulting from a mesoscopic crystal with a dielectric constant greater than three million. This gigantic enhancement effect originates from the space-filling curve concept from mathematics. The principle is inherently very broad band, the enhancement being nearly constant from zero up to the frequency of interest. This broadband giant-refractive-index medium promises not only enhanced resolution in imaging and raised fundamental absorption limits in solar energy devices, but also compact, power-efficient components for optical communication and increased performance in many other applications., The refractive index of natural materials only covers a limited range. Here, Chang et al. use the principle of space-filling curves to construct a mesoscopic crystal with a refractive index greater than 1000 at GHz frequencies. The concept is inherently broadband and scalable.

Resonance phenomenon in general allows large responses from small inputs. This effective amplification effect has been in successful use in diverse range of sensing applications. More specifically, in case of optical resonances, various micro- and nano-resonator designs have been proposed so far with a large range of performance parameters. On the one hand, micro-scale dielectric resonators can provide extreme sensitivity in terms of (change in the measured light intensity) per (unit change in the molar concentration of analytes). The downside is that the sensing volume is typically large in comparison and the system may be susceptible to environmental changes such as temperature. On the other hand, nano-scale resonators can achieve very high performance values in terms of (change in the measured light intensity) per (unit change in the mass of analytes), especially if optical nonlinearity is involved. The downside is that it requires careful design such that the resonators can be fabricated in a reproducible and cost-effective fashion. In this talk, we focus on nano-scale resonator designs and show preliminary results of enhanced detection sensitivity based on array of tiny optical resonators that can be fabricated over a large area.

The refractive index of natural transparent materials is limited to 2–3 throughout the visible wavelength range. Wider controllability of the refractive index is desired for novel optical applications such as nanoimaging and integrated photonics. We report that metamaterials consisting of period and symmetry-tunable self-assembled nanopatterns can provide a controllable refractive index medium for a broad wavelength range, including the visible region. Our approach exploits the independent control of permeability and permittivity with nanoscale objects smaller than the skin depth. The precise manipulation of the interobject distance in block copolymer nanopatterns via pattern shrinkage increased the effective refractive index up to 5.10. The effective refractive index remains above 3.0 over more than 1,000 nm wavelength bandwidth. Spatially graded and anisotropic refractive indices are also obtained with the design of transitional and rotational symmetry modification., Wider control of the refractive index is desired for new optical applications. Here the authors manipulate block copolymer self-assembled nanopatterns via shrinkage in order to control the refractive index. They achieve an index above 3 over 1,000 nm bandwidth.

Localized surface plasmon resonance of metallic nanostructures receives noticeable attention in photonics, electronics, catalysis, and so on. Core-shell nanostructures are particularly attractive due to the versatile tunability of plasmonic properties along with the independent control of core size, shell thickness, and corresponding chemical composition, but they commonly suffer from difficult synthetic procedures. We present a reliable and controllable route to a highly ordered uniform Au@Ag core-shell nanoparticle array via block copolymer lithography and subsequent seeded-shell growth. Size-tunable monodisperse Au nanodot arrays are generated by block copolymer self-assembly and are used as seed layers to grow Ag shells with variable thickness. The resultant Au@Ag core-shell nanoparticle arrays exhibit widely tunable broadband enhancement of plasmonic resonance, greatly surpassing single-element nanoparticle or homogeneous alloy nanoparticle arrays. Surface-enhanced Raman scattering of the core-shell nanoparticle arrays showed an enhancement factor greater than 270 from Au nanoparticle arrays.

We propose and prove that it is possible to decouple the electric and magnetic response of an array of conductive nanoparticles and realize a very wide range of combinations of effective permittivities and permeabilities. The principle exploits the large differences in the Thomas-Fermi screening length for longitudinal electric fields and the classical penetration depth for time-varying transverse magnetic fields. This non-resonant principle allows frequency invariance of the effective material properties with a bandwidth spanning many octaves, orders of magnitude larger than previous resonant metamaterials. An effective medium with a record-high refractive index over broadband is demonstrated as an example.

We propose a broadband enhancement scheme of optical nonlinearity with mesoscopically structured material. The one billion enhancement is larger than any previous result. The nonlinear susceptibility tensor components can be individually tuned and 18 different crystal classes can be realized.