Band 10, Heft 7

Nickel oxide (NiO) is one of the very few p -type semiconducting oxides, the study of which is gaining increasing attention in recent years due to its potential applicability in many emerging fields of technological research. Actually, a growing number of scientific works focus on NiO-based electrochromic devices, high-frequency spintronics, fuel cell electrodes, supercapacitors, photocatalyst, chemical/gas sensors, or magnetic devices, among others. However, less has been done so far in the development of NiO-based optical devices, a field in which this versatile transition metal oxide still lags in performance despite its potential applicability. This review could contribute with novelty and new forefront insights on NiO micro and nanostructures with promising applicability in optical and optoelectronic devices. As some examples, NiO lighting devices, optical microresonators, waveguides, optical limiters, and neuromorphic applications are reviewed and analyzed in this work. These emerging functionalities, together with some other recent developments based on NiO micro and nanostructures, can open a new field of research based on this p -type material which still remains scarcely explored from an optical perspective, and would pave the way to future research and scientific advances.

Recently, quantum dots (QD) and quantum rods (QRs) have become extremely popular in displays and lighting applications. Liquid crystal displays (LCDs) equipped with quantum dot enhancement films (QDEFs) offer extended color saturation, increasing said saturation from 60 to 70% to more than 100% of the NTSC color gamut. A plethora of research dealing with EL/PL properties and the device-based performance of these materials has been published. The tunable emission wavelength and the narrow emission bandwidth are the key features of quantum dots and perovskite nanoparticles that primarily depend on the nanoparticle size and material composition. QRs, in contrast, have a core–shell structure and emit polarized light that can roughly double the efficiency of modern displays. However, blue emission for QRs, because of the large bathochromic redshift during shell growth, is a serious problem. Besides photoluminescence, electroluminescence is also important for display applications. These QD-LEDs show a lower turn ON voltage in comparison to organic LEDs, which is very important for high-resolution displays. The solution-processed narrower emission QD-LEDs have already achieved efficiency and a brightness comparable to vacuum-deposited phosphorescent organic LEDs (OLEDs). However, the blue-emitting nanoparticles and their short operational lifetime are the key obstacles in the progression of these devices. Furthermore, recently the display and lighting industry are trying to reduce the short-wavelength emissions, particularly in the spectral region below 455 nm, which has a much greater impact on human ocular health and circadian rhythm. Thus, industries are aiming at blue light in the spectral range of 460–475 nm. This spectral range is very challenging for nanomaterials because of the limited choice of materials. In this review, we summarize the recent progress made in the blue-emitting nanomaterials with a different morphology and composition. This includes recent developments in low Cd materials. Both the PL and EL properties of these materials have been discussed depending on the NP’s shape and material composition. This review also aims to discuss the various device architectures employing blue-emitting NPs, any recent achievements and future challenges.

Metasurfaces have recently gained extensive interest because of their extraordinary optical behavior as artificial material interfaces with ultrahigh compactness. In this framework, dielectric platforms have newly become very promising for nonlinear nanophotonics, providing opportunities, especially for ultrafast optical switching, and high harmonic generation, opening the research field of nonlinear metaoptics. Up to now, nonlinear metaoptics have been mostly explored using single metasurfaces. However, in a long-term vision, the stacking of optical metasurfaces, very challenging in terms of fabrication, is one key goal of this research field. Here, we demonstrate a three-layer metasurface in the AlGaAs-on-insulator platform, which improves the second harmonic generation efficiency by more than one order of magnitude with respect to its one-layer counterpart. Our achievement paves the way toward phase-shaping multilayer and multifunctional all-dielectric metasurfaces.

The structure of the host lattice has a substantial influence on the optical properties of lanthanide-doped luminescent materials. Hexagonal-phase ( β -phase) NaREF 4 (RE = rare earth) is the most commonly used crystal structure for lanthanide-doped upconversion nanoparticles (UCNPs) owing to its high upconversion (UC) efficiency. In this work, we report, for the first time, that more efficient cooperative sensitization upconversion (CSU) can be achieved in cubic-phase ( α -phase) NaREF 4 UCNPs instead of their β -phase counterparts. With the passivation of an inert shell, the UC emission intensity of α -NaYbF 4 :Tb40%@CaF 2 is 10.5 times higher than that of β -NaYbF 4 :Tb40%@NaYF 4 . We propose that the high-symmetry crystal structure of the α phase facilitates the formations of [Yb–Yb] dimers and [Yb–Yb–Tb] clusters, which are particularly beneficial for CSU. Moreover, we prove that such Tb 3+ -based UCNPs are almost impervious to water quenching because of the large energy gap (∼15,000 cm −1 ) that existed in Tb 3+ between its lowest emit-ting level ( 5 D 4 ) and next low-lying level ( 7 F 0 ). Finally, their potential application for single-nanoparticle imaging has also been demonstrated. As expected, the α -core-shell UCNPs measured at the single-nanoparticle level are estimated to be 9-fold brighter than their β -core-shell counterparts. Importantly, the α -NaYbF 4 :Tb40%@CaF 2 UCNPs offer exciting opportunities for realizing single-nanoparticle imaging at ultralow irradiance (30 W/cm 2 ).

The topological properties of photonic microstructures are of great interest because of their experimental feasibility for fundamental study and potential applications. Here, we show that robust guided-mode-resonance states exist in photonic domain-wall structures whenever the complex photonic band structures involve certain topological correlations in general. Using the non-Hermitian photonic analogy of the one-dimensional Dirac equation, we derive essential conditions for photonic Jackiw-Rebbi-state resonances taking advantage of unique spatial confinement and spot-like spectral features which are remarkably robust against random parametric errors. Therefore, the proposed resonance configuration potentially provides a powerful method to create compact and stable photonic resonators for various applications in practice.

Long-wave infrared (LWIR, 6–14 µm) processes enormous potential for chem/biosensing as it covers abundant molecular absorption fingerprints. Waveguides provide an attractive chip-scale miniaturization solution for optical sensors. However, the exploration of waveguide sensors in this wavelength range is limited. Here, an LWIR photonic platform for fast and sensitive on-chip gas sensing is developed using suspended silicon (Si) waveguide supported by subwavelength grating (SWG) metamaterial claddings. This platform provides a viable approach to fully exploit the transparency window of Si. The SWG structure provides a promising solution to engineer the mode profile for strong light–analyte interaction. Propagation loss and bending loss are studied in the broad wavelength range of 6.4–6.8 µm. Functional devices including grating couplers, Y-junctions, and directional couplers are also demonstrated with high performance. Sensing demonstration based on our platform is presented using toluene vapor detection as an example. The corresponding limit of detection reaches 75 ppm. The response and recovery time to 75 ppm toluene are about 0.8 and 3.4 s, respectively. This good performance makes our platform a promising candidate for on-site medical and environmental applications.

Monolayer transition metal dichalcogenides (TMDs) possess large second-order nonlinear responses due to the broken inversion symmetry, which can extend their intriguing applications in nonlinear nanophotonics and optoelectronics. However, the atomic thickness of monolayer TMDs severely decreases the interaction length with free light with respect to bulk materials, leading to rather low second-harmonic generation (SHG) conversion efficiency. Here, we demonstrate a hybrid structure consisting of a monolayer MoS 2 on a suspended perforated silver film, on which the SHG signal emitted from the monolayer MoS 2 is enhanced by more than three orders of magnitude at room temperature. The pronounced SHG enhancement is attributed to the distinct electric field amplification nearby the nanoholes, which is induced by the symmetric surface plasmon polaritons (SPPs) existing in the ultrathin suspended silver grating. Our results reported here may establish the substrate-free engineering of nonlinear optical effects via plasmonic nanostructures on demand.

Coherent perfect absorption (CPA) describes the absence of all outgoing modes from a lossy resonator, driven by lossless incoming modes. Here, we show that for nanoresonators that also exhibit radiative losses, e.g., plasmonic nanoantennas, a generalized version of CPA (gCPA) can be applied. In gCPA outgoing modes are suppressed only for a subset of (guided plasmonic) modes while other (radiative) modes are treated as additional loss channels - a situation typically referred to as perfect impedance matching. Here we make use of gCPA to show how to achieve perfect impedance matching between a single nanowire plasmonic waveguide and a plasmonic nanoantenna. Antennas with both radiant and subradiant characteristics are considered. We further demonstrate potential applications in background-free sensing.

The turn-off fluorescent photoswitches for information encryption are constantly being developed. However, there are no reports about time-switchable (fluorescence lifetime-switchable) encryption to overcome the limitations of tunable encoding numbers in spectrally and temporally encoded libraries. Based on the double-exponential fitting of fluorescence lifetime, we propose, a fatigue-free and highly flexible switch between the amplitude-weighted average fluorescence lifetime ( τm ) and the intensity-weighted average fluorescence lifetime ( τi ), which will realize the supermultiplexed fluorescence lifetime switchable encryption. The potentially enormous library of different fluorescent lifetime combinations would facilitate the development of information security.

Natural materials typically interact weakly with the magnetic component of light which greatly limits their applications. This has led to the development of artificial metamaterials and metasurfaces. However, natural atoms, where only electric dipole transitions are relevant at optical frequencies, can cooperatively respond to light to form collective excitations with strong magnetic, as well as electric, interactions together with corresponding electric and magnetic mirror reflection properties. By combining the electric and magnetic collective degrees of freedom, we show that ultrathin planar arrays of atoms can be utilized as atomic lenses to focus light to subwavelength spots at the diffraction limit, to steer light at different angles allowing for optical sorting, and as converters between different angular momentum states. The method is based on coherently superposing induced electric and magnetic dipoles to engineer a quantum nanophotonic Huygens’ surface of atoms, giving full 2π phase control over the transmission, with close to zero reflection.

Hot electrons generated in metallic nanostructures have shown promising perspectives for photodetection. This has prompted efforts to enhance the absorption of photons by metals. However, most strategies require fine-tuning of the geometric parameters to achieve perfect absorption, accompanied by the demanding fabrications. Here, we theoretically propose a Ag grating/TiO 2 cladding hybrid structure for hot electron photodetection (HEPD) by combining quasi-bound states in the continuum (BIC) and plasmonic hot electrons. Enabled by quasi-BIC, perfect absorption can be readily achieved and it is robust against the change of several structural parameters due to the topological nature of BIC. Also, we show that the guided mode can be folded into the light cone by introducing a disturbance to become a guided resonance, which then gives rise to a narrow-band HEPD that is difficult to be achieved in the high loss gold plasmonics. Combining the quasi-BIC and the guided resonance, we also realize a multiband HEPD with near-perfect absorption. Our work suggests new routes to enhance the light-harvesting in plasmonic nanosystems.

Difference-frequency generation (DFG) is elemental for nonlinear parametric processes such as optical parametric oscillation and is instrumental for generating coherent light at long wavelengths, especially in the middle infrared. Second-order nonlinear frequency conversion processes like DFG require a second-order susceptibility χ (2) , which is absent in centrosymmetric materials, e.g. silicon-based platforms. All-optical poling is a versatile method for inducing an effective χ (2) in centrosymmetric materials through periodic self-organization of charges. Such all-optically inscribed grating can compensate for the absence of the inherent second-order nonlinearity in integrated photonics platforms. Relying on this induced effective χ (2) in stoichiometric silicon nitride (Si 3 N 4 ) waveguides, second-order nonlinear frequency conversion processes, such as second-harmonic generation, were previously demonstrated. However up to now, DFG remained out of reach. Here, we report both near- and non-degenerate DFG in all-optically poled Si 3 N 4 waveguides. Exploiting dispersion engineering, particularly rethinking how dispersion can be leveraged to satisfy multiple processes simultaneously, we unlock nonlinear frequency conversion near 2 μm relying on all-optical poling at telecommunication wavelengths. The experimental results are in excellent agreement with theoretically predicted behaviours, validating our approach and opening the way for the design of new types of integrated sources in silicon photonics.

A three-dimensional (3D) nanoscale optical router is a much-desired component in 3D stacked optical integrated circuits. However, existing 3D routers based on dielectric configurations suffer from large footprints and nanoscale routers based on plasmonic antennas only work in a 2D in-plane scene. Here, we propose and experimentally demonstrate cross-layered all-optical 3D routers with nanoscale footprints. Optical slot antenna pairs are used to realize the routing of plasmonic signals between different layers for arbitrary direction in a broadband wavelength range. The routers are also integrated with waveguide directly for exploring further applications. Based on these router elements, a 3D network of optical butterfly interconnection is demonstrated for multi-directional all-optical data communication. The proposed configuration paves the way for optical cross-layer routing on the nanoscale and advances the research and applications for 3D plasmonic circuits with high integration density in the future.