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Historically, nanophotonics deals with a control of light at the nanoscale being closely connected with the rapid advances in plasmonics – the physics of surface plasmon polaritons supported by metal–dielectric interfaces. Properly engineered nanostructures allow the subwavelength propagation of light and its strong confinement in nanowaveguides and nanocavities, making possible the field enhancement and lasing. Spaser was suggested as a special type of nanolaser with a very small footprint that can be modulated quickly thus becoming a good candidate for on-chip optical data processing. However, recent developments in the physics of high-index dielectric nanoparticles and resonant dielectric metasurfaces allowed to advance the field of nanophotonics and introduce novel nonplasmonic nanostructures and nanolasers empowered by topology and interference effects. Here we present first some examples of experimentally realized spasers, and then discuss the recent developments in the cutting-edge high-index dielectric nanostructures employed for nonplasmonic nanolasers based on Mie resonances, anapole states, bound states in the continuum, and the physics of topological phases.

In atomically thin semiconductors, localized exciton (XL) coupled to light provides a new class of optical sources for potential applications in quantum communication. However, in most studies, XL photoluminescence (PL) from crystal defects has mainly been observed in cryogenic conditions because of their sub‐wavelength emission region and low quantum yield at room temperature. Hybrid‐modality of cavity‐spectroscopy to induce and probe the XL emissions at the nanoscale in atomically thin semiconductors is presented. By placing a WSe2 monolayer on the two extremely sharp Au tips in a bowtie antenna with a radius of curvature of <1 nm, tensile strain of ≈0.3% is effectively induced in a <30 nm region to create robust XL states. The Au tip then approaches the strained crystal region to enhance the XL emissions and probe them with tip‐enhanced photoluminescence (TEPL) spectroscopy at room temperature. Through this triple‐sharp‐tips cavity‐spectroscopy with <15 nm spatial resolution, TEPL enhancement as high as ≈4.0 × 104 by the Purcell effect is achieved, and peak energy shifts of XL up to ≈40 meV are observed. This approach combining nano‐cavity and ‐spectroscopy provides a systematic way to induce and probe the radiative emission of localized excitons in 2D semiconductors offering new strategies for dynamic quantum nano‐optical devices. [ABSTRACT FROM AUTHOR]

The fabrication of high-quality metallic powders has been attracted considerable attention in terms of producing advanced materials through additive manufacturing (AM) technology. The efficiency of AM products mainly dependent on the initial powder, therefore, characterisation of pre-alloyed powders become important. In this study, we explored the fabrication of Al0.5CoCrFeMnNi high-entropy alloy (HEA) powders by gas atomisation process and investigated their powder characteristics according to particle size distribution. X-ray diffraction results revealed the formation of major face-centred cubic and minor body-centred cubic phase by the addition of Al content. The detailed characterisations such as scanning electron microscopy, Auger electron spectroscopy and XPS analysis was carried out to examine the chemical composition of HEA powders. The relationship between secondary dendrite arm spacing and cooling rate was evaluated. The results demonstrated that produced alloy powders are spherical in shape with smooth surfaces, compositional homogeneity, indicating the gas-atomised powders are widely acceptable for additive manufacturing applications. [ABSTRACT FROM AUTHOR]

High quality factor and small mode volume in nanocavities enable the demonstration of efficient nanophotonic devices with low power consumption, strong nonlinearity, and high modulation speed, due to the strong light–matter interaction. In this review, we focus on recent state-of-the-art nanocavities and their applications. We introduce single nanocavities including semiconductor nanowires, plasmonic cavities, and nanostructures based on quasi-bound states in the continuum (quasi-BIC), for laser, photovoltaic, and nonlinear applications. In addition, nanocavity arrays with unique feedback mechanisms, including BIC cavities, parity-time symmetry coupled cavities, and photonic topological cavities, are introduced for laser applications. These various cavity designs and underlying physics in single and array nanocavities are useful for the practical implementation of promising nanophotonic devices. [ABSTRACT FROM AUTHOR]

Holographic stereography providing binocular depth cues is one of the most promising technologies for 3D displays. However, conventional holographic stereograms based on micrometer‐scale pixels suffer from multiple diffraction orders and narrow viewing angles. Optical metasurfaces with sub‐wavelength‐scale features have recently been leading amongst the state‐of‐the‐art technologies in 3D holograms but employing only monocular depth cues. Here, a novel method is presented based on optical metasurfaces for obtaining a binocular holographic stereopsis. The demonstrated optical metasurface is an ensemble of several hologram pieces, which produce the different 2D projections of the target 3D structure depending on the observation direction, and displays the holographic stereogram of 25 × 25 × 25 µm3 over a wide viewing angle of more than ±30°. A Gerchberg–Saxton algorithm modified with a spatial Fourier filter calculates the phase and amplitude distribution of meta‐atoms. The results will open avenues to advanced eyeglasses‐free 3D displays that can provide rich and well‐defined depth cues. [ABSTRACT FROM AUTHOR]

Photolithography is the prevalent microfabrication technology. It needs to meet resolution and yield demands at a cost that makes it economically viable. However, conventional far-field photolithography has reached the diffraction limit, which imposes complex optics and short-wavelength beam source to achieve high resolution at the expense of cost efficiency. Here, we present a cost-effective near-field optical printing approach that uses metal patterns embedded in a flexible elastomer photomask with mechanical robustness. This technique generates sub-diffraction patterns that are smaller than 1/10th of the wavelength of the incoming light. It can be integrated into existing hardware and standard mercury lamp, and used for a variety of surfaces, such as curved, rough and defect surfaces. This method offers a higher resolution than common light-based printing systems, while enabling parallel-writing. We anticipate that it will be widely used in academic and industrial productions. Photolithography is an established microfabrication technique but commonly uses costly shortwavelength light sources to achieve high resolution. Here the authors use metal patterns embedded in a flexible elastomer photomask with mechanical robustness for generation of subdiffraction patterns as a cost effective near-field optical printing approach. [ABSTRACT FROM AUTHOR]

Abstract: Structural colors generated by optical micro‐ or nanostructures have attracted much attention for replacing pigment colors due to their tunability and semipermanency. The advantages of structural colors are now ready to be extended to the spectral range outside that of visible wavelengths. Here, bright structural colors are demonstrated in the near‐ultraviolet (near‐UV) wavelength range based on the plasmonic resonance of an aluminum nanodisk array. Collective plasmonic oscillation of the aluminum nanodisks selectively reflects the near‐UV light with a high reflectance and narrow linewidth. The color of the reflected light is tuned over the full near‐UV range, depending on the structural parameters of the nanodisk array. In particular, aluminum nanodisk arrays can be used as structural color pixels that demonstrate printing of near‐UV images with a high resolution of ≈10 000 pixels per inch. The demonstrated near‐UV structural colors show significant progress toward UV filtering and imaging, anticounterfeit, color multiplexing, and photocatalysis. [ABSTRACT FROM AUTHOR]

Abstract: We propose and experimentally investigate a self-aligned metallic nanocavity that can enhance second harmonic signals from a single KNbO3 nanowire. With pump beams in the spectral proximity of the metallic nanocavity resonance, second harmonic intensity is observed to be >1800 times stronger than that of a typical bare nanowire under the same illumination conditions. By studying spectral features and polarization characteristics, we confirm that the nonlinear enhancement originates from the locally intensified electric field of a surface plasmon-polariton mode. This simple and robust scheme represents a powerful platform to study single nanowire nonlinearity. [Copyright &y& Elsevier]

We demonstrated lasing in localized modes of an optimized photonic amorphous structure with short-range order. The InGaAsP multiple quantum wells embedded in a free-standing InGaAsP slab were employed as an active medium by optical pumping at room temperature. The high index contrast between air and InGaAsP, combined with uniformity of local topology and short-range order, enables strong confinement of light via multiple scattering. We confirmed the characteristic of lasing modes in the complex geometry with numerical simulations based on the real images of fabricated samples. [ABSTRACT FROM AUTHOR]

The authors report an electrically driven, hexapole mode, single-cell photonic crystal laser operating at 1537.8 nm. Electrical current is supplied through a submicrometer-sized current post beneath the cavity center. This wavelength-scale single-cell photonic crystal laser operates in a single mode with threshold current of ∼100 μA at room temperature. Operation in the hexapole mode is confirmed by the near-field profile, far-field polarization, and the finite-difference time-domain computation based on the fabricated cavity structure. [ABSTRACT FROM AUTHOR]

Efficient integration of a single-photon emitter with an optical waveguide is essential for quantum integrated circuits. In this study, we integrated a single-photon emitter in a hexagonal boron nitride (h-BN) flake with a Ag plasmonic waveguide and measured its optical properties at room temperature. First, we performed numerical simulations to calculate the efficiency of light coupling from the emitter to the Ag plasmonic waveguide, depending on the position and polarization of the emitter. In the experiment, we placed a Ag nanowire, which acted as the plasmonic waveguide, near the defect of the h-BN, which acted as the single-photon emitter. The position and direction of the nanowire were precisely controlled using a stamping method. Our time-resolved photoluminescence measurement showed that the single-photon emission from the h-BN flake was enhanced to almost twice the intensity as a result of the coupling with the Ag nanowire. We expect these results to pave the way for the practical implementation of on-chip nanoscale quantum plasmonic integrated circuits. [ABSTRACT FROM AUTHOR]

Dielectric nano-antennas are promising elements in nanophotonics due to their low material loss and strong leaky-mode optical resonances. In particular, light scattering can be easily manipulated using dielectric nano-antennas. To take full advantage of dielectric nano-antennas and explore their new optical applications, it is necessary to fabricate three-dimensional nano-structures under arbitrary conditions such as in non-planar substrates. Here, we demonstrate full-visible-range resonant light scattering from a single dielectric optical nano-rod antenna. The nano-rod antenna was formed by electron beam-induced deposition (EBID), a promising three-dimensional nanofabrication technique with a high spatial resolution. The nano-rods consist of amorphous alloys of C and O, with a width of 180 nm on average and a length of 4.5 μm. Polarization-resolved dark-field scattering measurements show that both transverse-electric and transverse-magnetic mode resonances cover the full visible range as the height of the nano-rod antenna varies from 90 to 280 nm. Numerical simulations successfully reproduce the measured scattering features and characterize the modal properties, using the critical points dispersive dielectric constant of the EBID carbonaceous material. Our deep understanding of resonant light scattering in the EBID dielectric nano-antenna will be useful for near-field measurement or for the implementation of three-dimensional nanophotonic devices. [ABSTRACT FROM AUTHOR]