A multifocus optical vortex metalens, with enhanced signal-to-noise ratio, is presented, which focuses three longitudinal vortices with distinct topological charges at different focal planes. The design largely extends the flexibility of tuning the number of vortices and their focal positions for circularly polarized light in a compact device, which provides the convenience for the nanomanipulation of optical vortices.

The dielectric metasurface hologram promises higher efficiencies due to lower absorption than its plasmonic counterpart. However, it has only been used, up to now, for controlling linear-polarization photons to form single-plane holographic images in the near-infrared region. Here, we report a transmissiontype metahologram achieving images in three colors, free from high-order diffraction and twin-image issues, with 8-level modulation of geometric phase by controlling photon spin via precisely patterned Si nanostructures with varying orientations. The resulting real and virtual holographic images with spin dependence of incident photons natively enable the spin degeneracy removal of light, leading to a metahologram-enabled spin Hall effect of light. Low-absorption dielectrics also enable us to create holograms for short-wavelength light down to 480 nm, thus spanning the three primary colors. It possesses the potential for compact color-display chips using mature semiconductor processes, and holds significant advantages over previous metaholograms operating at longer wavelengths.

This paper reports an effective method to enhance the surface plasmon resonance (SPR) on Ag films by using a thin Ni seed layer assisted deposition. Ag films with a thickness of about 50 nm were deposited by electron beam evaporation above an ultrathin Ni seed layer of approximately 2 nm on both silicon and quartz substrates. The root-mean-square (rms) surface roughness and the correlation length have been reduced from >4 nm and 28 nm for a pure Ag film to approximately 1.3 and 19 nm for Ag/Ni films, respectively. Both experimental and simulation results show that the Ag/Ni films exhibit an enhanced SPR over the pure Ag film with a narrower full width at half-maximum. Ag films with a Ge seed layer have also been prepared under the same conditions. The surface roughness can be reduced to less than 0.7 nm, but narrowing of the SPR curve is not observed due to increased absorptive damping in the Ge seed layer. Our results show that Ni acts as a roughness-diminishing growth layer for the Ag film while at the same time maintaining and enhancing the plasmonic properties of the combined structures. This points toward its use for low-loss plasmonic devices and optical metamaterials applications.

Phase retrieval is the key to quantitative x-ray phase-contrast imaging. To retrieve the phase image of an x-ray wave field, in general one needs multiple phase-contrast images. We have made a new observation of phase-attenuation duality for soft tissues, and we show how only a single phase-contrast image is needed for successful phase retrieval based on this duality. The phase-retrieval formula based on a single phase-contrast image of inhomogeneous soft tissue is derived and presented. We show the striking enhancement of the tissue contrast in simulated phase images that this new approach produces.

Miniaturization of optical structures makes it possible to control light at the nanoscale, but on the other hand it imposes a challenge of accurately handling numerous unit elements in a miniaturized device with aperiodic and random arrangements. Here, we report both the new analytical model and experimental demonstration of the photon sieves with ultrahighcapacity of subwavelength holes (over 34 thousands) arranged in two different structural orders of randomness and aperiodicity. The random photon sieve produces a uniform optical hologram with high diffraction efficiency and free from twin images that are usually seen in conventional holography, while the aperiodic photon sieve manifests sub-diffraction-limit focusing in air. A hybrid approach is developed to make the design of random and aperiodic photon sieve viable for high-accuracy control of the amplitude, phase and polarization of visible light. The polarization independence of the photon sieve will also greatly benefit its applications in optical imaging and spectroscopy.

Traditional objective lenses in modern microscopy, based on the refraction of light, are restricted by the Rayleigh diffraction limit. The existing methods to overcome this limit can be categorized into near-field (e.g., scanning near-field optical microscopy, superlens, microsphere lens) and far-field (e.g., stimulated emission depletion microscopy, photoactivated localization microscopy, stochastic optical reconstruction microscopy) approaches. However, they either operate in the challenging near-field mode or there is the need to label samples in biology. Recently, through manipulation of the diffraction of light with binary masks or gradient metasurfaces, some miniaturized and planar lenses have been reported with intriguing functionalities such as ultrahigh numerical aperture, large depth of focus, and subdiffraction-limit focusing in far-field, which provides a viable solution for the label-free superresolution imaging. Here, the recent advances in planar diffractive lenses (PDLs) are reviewed from a united theoretical account on diffraction-based focusing optics, and the underlying physics of nanofocusing via constructive or destructive interference is revealed. Various approaches of realizing PDLs are introduced in terms of their unique performances and interpreted by using optical aberration theory. Furthermore, a detailed tutorial about applying these planar lenses in nanoimaging is provided, followed by an outlook regarding future development toward practical applications.

In this letter, we propose and experimentally demonstrate a color filter based on an annular aperture geometry working in the visible range. The device is built by configuring an array of annular apertures in a gold film suitable for transmission measurement. We show effective fine tuning of resonance peaks through precise geometric control of the aperture dimensions. Selective transmission through annular apertures of various sizes leads to continuous color tuning of transmitted electromagnetic waves. This may find potential for application in high-definition displays, optical filters, ultrafast switching, and bio-sensing.

Zinc oxide (ZnO) nanorods coated with silver (Ag) film on a polyethylene terephthalate (PET)flexible substrate were used as the photo anode for water splitting. The hybrid nanostructures were prepared via low-temperature hydrothermal growth and electron beam evaporation. The effects of plasmonic enhanced absorption, surface recombination inhibition and improved charge transport are investigated by varying the Ag thickness. Light trapping and absorption enhancement are further studied by optimizing the curvature of the PET substrates. The maximum short circuit current density (JSC, 0.616 mA cm -2) and the photoelectron conversion efficiency (PCE, 0.81%) are achieved with an optimized Ag film thickness of 10 nm and substrate bending radius of 6.0 mm. The maximum JSC and PCE are seven times and ten times, respectively, higher than those of the bare ZnO nanorods on flexible substrates without bending. The overall PEC performance improvement is attributed to the plasmonic effects induced by Ag film and improved charge transport due to inhibition of ZnO surface charge recombination. Enhanced light trapping (harvesting) induced by bending the PET substrates further improved the overall efficiency.