Passive daytime radiative cooling (PDRC) involves spontaneously cooling a surface by reflecting sunlight and radiating heat to the cold outer space. Current PDRC designs are promising alternatives to electrical cooling but are either inefficient or have limited applicability. We present a simple, inexpensive, and scalable phase inversion–based method for fabricating hierarchically porous poly(vinylidene fluoride-co-hexafluoropropene) [P(VdF-HFP)HP] coatings with excellent PDRC capability. High, substrate-independent hemispherical solar reflectances (0.96 ± 0.03) and long-wave infrared emittances (0.97 ± 0.02) allow for subambient temperature drops of ~6°C and cooling powers of ~96 watts per square meter (W m−2) under solar intensities of 890 and 750 W m−2, respectively. The performance equals or surpasses those of state-of-the-art PDRC designs, and the technique offers a paint-like simplicity.

Metasurfaces offer a unique platform to precisely control optical wavefronts and enable the realization of flat lenses, or metalenses, which have the potential to substantially reduce the size and complexity of imaging systems and to realize new imaging modalities. However, it is a major challenge to create achromatic metalenses that produce a single focal length over a broad wavelength range because of the difficulty in simultaneously engineering phase profiles at distinct wavelengths on a single metasurface. For practical applications, there is a further challenge to create broadband achromatic metalenses that work in the transmission mode for incident light waves with any arbitrary polarization state. We developed a design methodology and created libraries of meta-units—building blocks of metasurfaces—with complex cross-sectional geometries to provide diverse phase dispersions (phase as a function of wavelength), which is crucial for creating broadband achromatic metalenses. We elucidated the fundamental limitations of achromatic metalens performance by deriving mathematical equations that govern the tradeoffs between phase dispersion and achievable lens parameters, including the lens diameter, numerical aperture (NA), and bandwidth of achromatic operation. We experimentally demonstrated several dielectric achromatic metalenses reaching the fundamental limitations. These metalenses work in the transmission mode with polarization-independent focusing efficiencies up to 50% and continuously provide a near-constant focal length over λ = 1200–1650 nm. These unprecedented properties represent a major advance compared to the state of the art and a major step toward practical implementations of metalenses.

Metasurfaces are optically thin metamaterials that promise complete control of the wavefront of light but are primarily used to control only the phase of light. Here, we present an approach, simple in concept and in practice, that uses meta-atoms with a varying degree of form birefringence and rotation angles to create high-efficiency dielectric metasurfaces that control both the optical amplitude and phase at one or two frequencies. This opens up applications in computer-generated holography, allowing faithful reproduction of both the phase and amplitude of a target holographic scene without the iterative algorithms required in phase-only holography. We demonstrate all-dielectric metasurface holograms with independent and complete control of the amplitude and phase at up to two optical frequencies simultaneously to generate two- and three-dimensional holographic objects. We show that phase-amplitude metasurfaces enable a few features not attainable in phase-only holography; these include creating artifact-free two-dimensional holographic images, encoding phase and amplitude profiles separately at the object plane, encoding intensity profiles at the metasurface and object planes separately, and controlling the surface textures of three-dimensional holographic objects.

Research on two-dimensional designer optical structures, or metasurfaces, has mainly focused on controlling the wavefronts of light propagating in free space. Here, we show that gradient metasurface structures consisting of phased arrays of plasmonic or dielectric nanoantennas can be used to control guided waves via strong optical scattering at subwavelength intervals. Based on this design principle, we experimentally demonstrate waveguide mode converters, polarization rotators and waveguide devices supporting asymmetric optical power transmission. We also demonstrate all-dielectric on-chip polarization rotators based on phased arrays of Mie resonators with negligible insertion losses. Our gradient metasurfaces can enable small-footprint, broadband and low-loss photonic integrated devices.

No abstract

No abstract

Quasi-bound states in the continuum (q-BICs) are resonant states of suitably tailored nanostructures with long optical lifetimes controlled by symmetry-breaking perturbations. While in planarized ultrathin devices the resulting Fano resonance is limited to linear polarization, we show here that chiral perturbations extend q-BIC concepts to arbitrary elliptical polarizations.Using geometric phase engineering, we realize metasurfaces with ultrasharp Fano spectral features that can shape the impinging wavefront with near-unity efficiency, while at the same time precisely filtering their spectral content.Suitably designed photonic crystal slabs (PCSs) have been recently shown to support quasi-bound states in the continuum (q-BICs), opening new opportunities to enhance and control light-matter interactions. Q-BICs are modes whose radiative lifetime is controlled by a symmetry-lowering perturbation [1]; they would be ideally non-radiating states [2]-[4] due to symmetry-protection in the absence of perturbation, despite their momentum being compatible with coupling energy to the radiation continuum. When light with a polarization state matching the eigenpolarization of the q-BIC impinges on the structure, an ultrasharp Fano response arises [5], and the resonantly scattered light maintains the same polarization. This property, combined with strong in-plane Bragg scattering in high-contrast index systems [6], enables compact optical devices concentrating light 2 in both space and time [7]- [11]. By perturbing every other unit cell in such systems, the Brillouin zone folds, enabling access to previously bound modes and providing additional design freedom to control q-BICs in real-and momentum-space [10]- [13]. Photonic crystals supporting q-BICs hence offer a highly versatile platform for biological sensing [14], planar optical modulators [15], notch filters [16] and nonlinear optics [17], [18].Recently, the selection rules for q-BICs in planar photonic crystals have been classified for both mono-atomic and multi-atomic lattices, clarifying to which (if any) free-space polarization state a q-BIC may couple due to a chosen symmetry perturbation [13]. This result implies that in planar structures that preserve symmetry across a horizontal plane, the q-BIC polarization must necessarily be linear. By adiabatically varying the polarization angle of the supported q-BICs in the lateral direction and thereby introducing a spatial variation of the geometric phase, it is possible to tailor the impinging wavefront and realize metasurface functionalities, such as anomalous reflection and refraction for circularly polarized light emanating from the metasurface [19].However, due to the planarized symmetry and the linear polarization constraint, the maximum achievable efficiency of wavefront shaping is 25% [20].Breaking this symmetry with the introduction of optical chirality may enable devices with strong circular dichroism [22]- [29]. Recent work has demonstrated metasurfaces that leverage this principle by varying the geometric phase ...

No abstract