The article comments on a recently proposed innovative process that uses direct laser writing to achieve vivid, fine-tunable color at centimeter scale by leveraging the fabrication speed and the spatial resolution of pixelated F-P cavity structures.

The explosion in the amount of information that is being processed is prompting the need for new computing systems beyond existing electronic computers. Photonic computing is emerging as an attractive alternative due to performing calculations at the speed of light, the change for massive parallelism, and also extremely low energy consumption. We review the physical implementation of basic optical calculations, such as differentiation and integration, using metamaterials, and introduce the realization of all-optical artificial neural networks. We start with concise introductions of the mathematical principles behind such optical computation methods and present the advantages, current problems that need to be overcome, and the potential future directions in the field. We expect that our review will be useful for both novice and experienced researchers in the field of all-optical computing platforms using metamaterials.

We theoretically and experimentally demonstrate an RGB achromatic metalens that operates concurrently at three visible wavelengths (λ=450, 532, and 700 nm) with a high numerical aperture of 0.87. The RGB metalens is designed by simple integration of metalens components with the spatial interleaving method. The simulated spatial interleaving metalens shows RGB achromatic operation with focusing efficiencies of 25.2%, 58.7%, and 66.4% at the wavelengths of 450, 532, and 700 nm, respectively. A 450 μm diameter metalens operating at three designated wavelengths is fabricated with low-loss hydrogenated amorphous silicon. The fabricated metalens has the measured focusing efficiencies of 5.9%, 11.3%, and 13.6% at λ=450, 532, and 700 nm, respectively. The Strehl ratios of 0.89, 0.88, and 0.82 are obtained at given wavelengths, which show a capability of diffraction-limited operation.

Metasurfaces have attracted great attention due to their ability to manipulate the phase, amplitude, and polarization of light in a compact form. Tunable metasurfaces have been investigated recently through the integration with mechanically moving components and electrically tunable elements. Two interesting applications, in particular, are to vary the focal point of metalenses and to switch between holographic images. We present the recent progress on tunable metasurfaces focused on metalenses and metaholograms, including the basic working principles, advantages, and disadvantages of each working mechanism. We classify the tunable stimuli based on the light source and electrical bias, as well as others such as thermal and mechanical modulation. We conclude by summarizing the recent progress of metalenses and metaholograms, and providing our perspectives for the further development of tunable metasurfaces.

Chiro-optical effects offer a wide range of potential applications in nanophotonics, such as advanced imaging and molecular sensing and separation. Flat single-layer metasurfaces composed of subwavelength meta-atoms have gained significant attention due to their exceptional characteristics in light–matter interactions. Although metasurface-based devices have manipulated electromagnetic waves, the compact on-chip realization of giant chiro-optical effects remains a challenge at optical frequencies. In this work, we experimentally and numerically demonstrate an all-dielectric metasurface to realize large chiro-optical effects in the visible regime. Notably, the proposed strategy of utilizing achiral nanofins instead of conventional chiral structures provides an extra degree of design freedom. The mutual coupling between carefully engineered nanofins produces constructive and destructive interference, leading to the asymmetric transmission of 70% and average circular dichroism exceeding 60%. We investigate the underlying mechanism behind the chiro-optical effects using the theory of multipolar decomposition. The proposed design mechanism maximizes the chiro-optical response through a single-layer metasurface with potential applications in high-efficiency integrated ultrathin polarization rotators and shapers, chiral polarizers for optical displays, chiral beam splitters, and chiral sensors.

Structural coloration techniques have improved display science due to their high durability in terms of resistance to bleaching and abrasion, and low energy consumption. Here, we propose and demonstrate an all-solid-state, large-area, lithography-free color filter that can switch structural color based on a doped semiconductor. Particularly, an indium-gallium-zinc-oxide (IGZO) thin film is used as a passive index-changing layer. The refractive index of the IGZO layer is tuned by controlling the charge carrier concentration; a hydrogen plasma treatment is used to control the conductivity of the IGZO layer. In this paper, we verify the color modulation using finite difference time domain simulations and experiments. The IGZO-based color filter technology proposed in this study will pave the way for charge-controlled tunable color filters displaying a wide gamut of colors on demand.