Band 10, Heft 16

Spin angular momentum associated with circular polarization is a fundamental and important aspect of photons both in classical and quantum optics. The interaction of this optical spin with matter and structures results in many intriguing optical effects and state-of-the-art applications covered under the emerging subject of spin optics. Distinct from longitudinal optical spin along the mean wavevector, transverse spin, the corresponding vector of which is perpendicular to the mean wavevector, prevails and plays a significant role in confined electromagnetic waves such as focused beams, guided waves, and evanescent waves. In the optical near-field, these transverse spins are generated owing to the spatial variation of the kinetic momentum of confined electromagnetic waves, where the spin and orbital angular momenta are strongly coupled, leading to many interesting topological spin structures and properties. Several reviews on optical transverse spins have been published in recent years in which their concepts and the various configurations producing them were introduced systematically. Here, we introduce in this review the underlying physics and dynamics of transverse spin and the resultant topological structures and properties such as the photonic skyrmions and merons. We term this sub-area ‘spin photonics’, its scope being to cover the design and research of spin structures in strongly confined electromagnetic fields with unique properties and applications. The concepts and framework reviewed have importance in optics, topological photonics, metrology, and quantum technologies and may be used to extend spin-dynamics concepts to fluidic, acoustic, and gravitational waves.

The lead halide perovskite material shows its unique photoelectric properties, the resulting conversion efficiency of perovskite solar cells. However, the efficiency comes to a bottleneck owing to that mechanism research fall behind the device research. Multiple excitons dynamics play an important role, especially in the lifetime and binding energy of multiple excitons. A long multiexciton lifetime is beneficial to the application for light-emitting devices and photovoltaic devices. Large multiexciton binding energy means a large Stokes shift in exciton absorption, thus avoiding the loss of linear absorption. To conclude, discussions are presented regarding views of current multiple excitons research in terms of the biexciton lifetime and biexciton binding energy that should be considered for further advances in materials and devices.

In recent years, new functionality and unprecedented wavefront control has been enabled by the introduction of bianisotropic metasurfaces. A bianisotropic metasurface is characterized by an electric response, a magnetic response, and an electro-magnetic/magneto-electric response. In general, these metasurfaces consists of an array of metallic or dielectric particles located within a subwavelength thick host medium, and are approximated and modeled as infinitely-thin, idealized sheet boundaries defined along a surface. An appropriate sheet boundary condition which effectively models the tangential field discontinuity due to the array of magnetoelectric inclusions is the Generalized Sheet Transition Condition or GSTC. Several forms of the GSTC appear in literature. Here, we present each interpretation and show how they are related. Synthesis approaches unique to each form are overviewed. By utilizing the GSTC in metasurface design, new possibilities emerge which are not possible with conventional design techniques incorporating only electric or only magnetic responses. Since the metasurfaces are designed using bianisotropic boundary conditions, they must be realized using particles which contain magnetoelectric responses. This review article discusses the design of metasurfaces using the GSTC, and the bianisotropic particles used to realize GSTC’s. Further, it discusses new and recent applications that have emerged due to bianisotropy, and future prospects in metasurface design using bianisotropic boundary conditions. The intent is to provide a comprehensive overview of metasurface design involving bianisotropy and for this review article to serve as a starting point for engineers and scientist that wish to introduce bianisotropy into metasurface design.

Two-dimensional (2D) materials with excellent optical properties and complementary metal-oxide-semiconductor (CMOS) compatibility have promising application prospects for developing highly efficient, small-scale all-optical modulators. However, due to the weak nonlinear light-material interaction, high power density and large contact area are usually required, resulting in low light modulation efficiency. In addition, the use of such large-band-gap materials limits the modulation wavelength. In this study, we propose an all-optical modulator integrated Si waveguide and single-layer MoS 2 with a plasmonic nanoslit, wherein modulation and signal light beams are converted into plasmon through nanoslit confinement and together are strongly coupled to 2D MoS 2 . This enables MoS 2 to absorb signal light with photon energies less than the bandgap, thereby achieving high-efficiency amplitude modulation at 1550 nm. As a result, the modulation efficiency of the device is up to 0.41 dB μm −1 , and the effective size is only 9.7 µm. Compared with other 2D material-based all-optical modulators, this fabricated device exhibits excellent light modulation efficiency with a micron-level size, which is potential in small-scale optical modulators and chip-integration applications. Moreover, the MoS 2 -plasmonic nanoslit modulator also provides an opportunity for TMDs in the application of infrared optoelectronics.

Graphdiyne has an inborn band gap energy, where the minimal band gap is about from 0.46 to 1.22 eV, which shows great potential in ultrafast laser generation. In this work, we fabricate a graphdiyne-decorated microfiber and demonstrate its saturable absorption characteristics experimentally. This device is used as a saturable absorber to generate the conventional soliton and noise-like pulse in an erbium-doped mode-locked fiber laser. The conventional soliton with a spectral bandwidth of 2.45 nm can switch into noise-like pulse with 37.14 nm bandwidth by adjusting the pump power and the polarization controller. For the noise-like pulse, the mode-locked state has excellent stability with 2.17 nJ maximum pulse energy and 283 fs coherent peak duration. This work indicates that graphdiyne could be a remarkable nonlinear photonic device to explore the dynamics of various mode-locked pulses.

There are many reports in the literature of bound states in the continuum (BICs) in systems with up–down mirror symmetry. Semiconductor-based technology requires bulk semiconductor substrates, which impose symmetry breaking in the vertical direction. In this paper, we explore the possibility of realizing BICs in a high refractive index subwavelength one-dimensional grating placed on a substrate with a refractive index that varies from 1 to almost the refractive index of the grating, while the refractive index above the grating is 1. We demonstrate that in gratings with broken up–down mirror symmetry not only symmetry-protected BICs can arise, but also Friedrich–Wintgen (FW) and interference-based (IB) BICs with diverging quality factors. The limit of the refractive index difference between the grating and the substrate supporting the BIC was found to be as little as 0.03. We also present a study of configurations composed of a finite number of grating stripes, with refractive indices corresponding to GaAs in the grating and Al-rich AlGaAs in the substrate. We demonstrate that such an all-semiconductor configuration enables Q -factors above 10 4 when composed of fewer than 20 periods and nearly exponential Q -factor growth with increasing numbers of grating periods. The results of this study pave the way for a new class of micro- and nano-optical cavities realised in standard all-semiconductor technology and relying on the high quality factor induced by BIC.

We study the light emission of plasmonic-luminescent hybrid nanostructures consisting of Ag nanoparticles (NPs) embedded in europium oxide (EuO X ). The Ag NPs present a bidimensional organization in the nanostructures and they optically behave as oblate spheroids. The photoluminescence (PL) spectral response of the nanostructures evolves from a narrow red emission characteristic of Eu 3+ ions in absence of Ag NPs to a broad blue-green emission band associated with Eu 2+ ions when the layer of Ag NPs is present. This behavior is not related to a change in the Eu 2+ /Eu 3+ ratio, which is verified by compositional analysis. Instead, a detailed investigation of the PL emission of the nanostructures suggests that the coupling of the Ag NPs to the Eu 2+ ions present in the EuO X layer, which manifests itself in an efficient sensitization of these ions, enhances their broad visible emission. In particular, the longitudinal mode of the Ag NPs surface plasmon is considered to be responsible for the efficient energy transfer for the non-normal incidence excitation PL configuration used. Finally, the use of a capping amorphous Al 2 O 3 layer allows improving the robustness of hybrid nanostructures and further enhances their PL emission. These findings provide a new path to actively control the selective excitation of Eu 2+ and Eu 3+ ions via a controlled coupling with the surface plasmon resonance modes of the Ag NPs and points to these nanostructures as promising building blocks for the development of integrable white light sources.

Two-dimensional (2D) organic–inorganic perovskites have shown interesting optical properties due to the natural quantum-well structures. The repetition of soft organic and hard inorganic intercalations also renders 2D perovskites rich phonon dynamics. Here, we investigated the coherent acoustic phonons in (PEA) 2 PbI 4 perovskite films by time-resolved Brillouin spectroscopy. The coherent acoustic phonons were launched indirectly in perovskite films by exciting Au nanoplates which were used as optoacoustic transducers. A longitudinal sound velocity ν  = 1937 ± 31 m/s, and an elastic modulus E  = 9.84 GPa along the cross-plane direction of perovskites were obtained from analysis of the Brillouin oscillation frequency. Following a bead-spring model, we calculated a spring constant k  ≈ 1.709 N m −1 for PEA cations which is comparably small for perovskites. We also demonstrated that coherent acoustic phonons are sensitive to differentiate structural heterogeneity and monitor dynamic phase evolution in perovskite films. Domains of PbI 2 -rich and PbI 2 -poor phases were identified. Under light stimulus, PbI 2 -poor phases were gradually disappearing and PbI 2 -rich phases became crystallized. The observations of structural and elastic heterogeneity and dynamic phase evolution using coherent acoustic phonons provide a toolbox for submicroscale elastic characterization of perovskites.

Topological quantum optics that manipulates the topological protection of quantum states has attracted special interests in recent years. Here we demonstrate valley photonic crystals implementing topologically protected transport of the continuous frequency entangled biphoton states. We numerically simulate the nonlinear four-wave mixing interaction of topological valley kink states propagating along the interface between two valley photonic crystals. We theoretically clarify that the signal and idler photons generated from the four-wave mixing interaction are continuous frequency entangled. The numerical simulation results imply that the entangled biphoton states are robust against the sharp bends and scattering, giving clear evidence of topological protection of entangled photon pairs. Our proposal paves a concrete way to perform topological protection of entangled quantum states operating at telecommunication wavelengths.

We have grown arrays of silver nanowires in pores of anodic alumina membranes (metamaterials with hyperbolic dispersion at λ  ≥ 615 nm), spin coated them with the dye-doped polymer (HITC:PMMA), and studied the rates of radiative and nonradiative relaxation as well as the concentration quenching (Förster energy transfer to acceptors). The results were compared to those obtained on top of planar Ag films and glass (control samples). The strong spatial inhomogeneity of emission kinetics recorded in different spots across the sample and strong inhibition of the concentration quenching in arrays of Ag nanowires are among the most significant findings of this study.

The development of novel and cost-effective THz emitters, with properties superior to current THz sources, is an active and important field of research. In this work, we propose and numerically demonstrate a simple yet effective approach of realizing terahertz sources working in continuous-wave form, by incorporating the new physics of bound state in the continuum (BIC) into thermal emitters. By deliberately designing the structure of slotted disk array made of high-resistivity silicon on top of a low index dielectric buffer layer supported by a conducting substrate, a quasi-BIC mode with ultra-high quality factor (∼10 4 ) can be supported. Our results reveal that the structure can operate as an efficient terahertz thermal emitter with near-unity emissivity and ultranarrow bandwidth. For example, an emitter working at 1.3914 THz with an ultranarrow linewidth less than 130 MHz, which is roughly 4 orders of magnitude smaller than that obtained from a metallic metamaterial-based thermal emitter, is shown. In addition to its high monochromaticity, this novel emitter has additional important advantages including high directionality and linear polarization, which makes it a promising candidate as the new generation of THz sources. It holds a great potential for practical applications where high spectral resolving capability is required.

Recently, multiscale three-dimensional (3D) structures consisting of micrometer-scale structure and nanometer-scale structure have received some attention from scientists in the field of surface-enhanced Raman scattering (SERS). In this work, micrometer-scale grating structure and nanometer-scale zinc oxide nano spikes (ZnO NSs) structure are successfully introduced into the SERS substrate with silver nanoparticles (Ag NPs) as the surface plasmon. The optimized particle-in-multiscale 3D substrate (PDMS/grating/ZnO NSs/Ag NPs) presents high sensitivity with an ultralow limit of detection of 1 × 10 −11  M and a high enhancement factor of 7.0 × 10 8 for Rhodamine 6G (R6G) as the probe molecule. It benefits from the electromagnetic field enhancement from the excellent optical capture capability of grating/ZnO NSs structure and abundant electromagnetic hot spots. The quantitative analysis ability of the SERS substrate can be indicated from the good linear correlation between the logarithmic Raman intensity and the molecular concentration. At the same time, this SERS substrate exhibits excellent homogeneity and reproducibility, which have low relative standard deviations (4.43%) of the Raman intensities at 613 cm −1 peaks for R6G as the probe molecule. In addition, this SERS substrate can realize in-situ detection of Raman signal due to its excellent light transmission and flexibility. The particle-in-multiscale 3D structure as SERS substrate exhibits the vast potential in practical applicability for qualitatively and quantitatively chemical and biomedical analysis.

Deep learning (DL) has emerged as a promising tool for photonic inverse design. Nevertheless, despite the initial success in retrieving spectra of modest complexity with nearly instantaneous readout, DL-assisted design methods often underperform in accuracy compared with advanced optimization techniques and have not proven competitive in handling spectra of practical usefulness. Here, we introduce a tandem optimization model that combines a mixture density network (MDN) and a fully connected (FC) network to inversely design practical thin-film high reflectors. The multimodal nature of the MDN gives access to infinite candidate designs described by probability distributions, which are iteratively sampled and evaluated by the FC network to allow for rapid optimization. We show that the proposed model can retrieve the reflectance spectra of 20-layer thin-film structures. More interestingly, it reproduces with high precision the periodic structures of high reflectors derived from physical principles, even though no such information is included in the training data. Improved designs with extended high-reflectance zones are also demonstrated. Our approach combines the high-efficiency advantage of DL with the optimization-enabled performance improvement, enabling efficient and on-demand inverse design for practical applications.

Artificially tailoring the polarization and phase of light offers new applications in optical communication, optical tweezers, and laser processing. Valley topological physics provides a novel paradigm for controlling electromagnetic waves and encoding information. The proposed fiber has the inner and outer claddings possessing opposite valley topological phases but the same refractive indices, which breaks through the polarization constraints of the traditional fiber. Robust valley edge states exist at the domain walls between the inner and outer claddings because of bulk edge correspondence. The valley topological fiber modes exhibit the unprecedented radial and azimuthal polarization with high-order azimuthal index. Those topological modes are robust against the disorder of the fiber structure. These results enable guide and manipulate the optical polarization and angular momentum in fiber with high fidelity. The proposed fiber has the potential to become a powerful optical spanner for the application of bio-photonics.

Metallic plasmonic probes have been successfully applied in near-field imaging, nanolithography, and Raman enhanced spectroscopy because of their ability to squeeze light into nanoscale and provide significant electric field enhancement. Most of these probes rely on nanometric alignment of incident beam and resonant structures with limited spectral bandwidth. This paper proposes and experimentally demonstrates an asymmetric fiber tip for broadband interference nanofocusing within its full optical wavelengths (500–800 nm) at the nanotip with 10 nm apex. The asymmetric geometry consisting of two semicircular slits rotates plasmonic polarization and converts the linearly polarized plasmonic mode to the radially polarized plasmonic mode when the linearly polarized beam couples to the optical fiber. The three-dimensional plasmonic modulation induces circumference interference and nanofocus of surface plasmons, which is significantly different from the nanofocusing through plasmon propagation and plasmon evolution. The plasmonic interference modulation provides fundamental insights into the plasmon engineering and has important applications in plasmon nanophotonic technologies.

Dynamic structural color based on tunable optical resonance plays a key role in applications including encryption visualization, camouflage and colorimetric sensing. However, the current design requires either complex growth processes of the high-quality tunable materials or complicated circuit designs. This work makes a humidity-swelling hydrogel layer for metal–insulator–metal (MIM) structure in the dynamic multi-color display. Here, polyvinyl alcohol (PVA) hydrogel structure is patterned through grayscale e-beam lithography and the controlled PVA thickness leads the programmable reflective resonance covering the entire visible range. By varying the ambient humidity between 9.8 and 90.1% RH, the reflective resonance of the structure is tailored across a wavelength range over 100 nm. Our materials platform of humidity-sensitive hydrogel resist presents a novel approach of the stepwise and reversible optical tunability for photonic devices.

MXene, a new advanced two-dimensional material, has attracted great attention in energy storage, transparent electrodes, and electromagnetic shielding due to its high conductivity, high specific surface area, and hydrophilic surface. Given the solution-processability and tunable work function, MXene also holds great potential for wide-range photodetection and integrated optics. Here, we demonstrate a waveguide integrated Schottky photodetector based on Ti 3 C 2 T x /Si van der Waals heterojunction. Specifically, the barrier of the Schottky photodetector can be adjusted by using simple surface treatment. The work function of the Ti 3 C 2 T x is reduced from 4.66 to 4.43 eV after vacuum annealing, and the barrier height of Ti 3 C 2 T x /p-Si Schottky junction is correspondingly increased from 0.64 to 0.72 eV, leading to 215 nm working wavelength blue-shift. The photodetector exhibits working wavelength tunability in short-wavelength infrared regions due to the engineered Schottky barrier. To our best knowledge, this is the first demonstration of utilizing MXene in waveguide-integrated photodetection, showing the potential applications for various scenarios thanks to the flexible working wavelength range induced by the tunable barrier.

To realize nonreciprocal transmission, it is necessary to break the time-reversal symmetry of the transmission system, but it is very challenging to keep the linear polarized (LP) input and output unchanged in the free space transmission system. Magnetized semiconductor InSb can realize terahertz (THz) nonreciprocal transmission for the two conjugated photonic spin states, but it cannot realize efficient one-way transmission of LP state due to gyro-mirror symmetry. In this work, by introducing a pair of orthogonal uniaxial anisotropies from the meta-gratings on both sides of InSb, both the gyro-mirror and time-reversal symmetries are broken for the LP state, thus making this cascaded grating–InSb–grating structure serves as a high-performance isolator for the LP light. The experiment results indicate isolation of 50 dB at 0.4 THz for the same LP input and output under a weak biased magnetic field of 0.17 T. Moreover, we further illustrate the factors affecting the isolation bandwidth of the device, also demonstrated another broadband structure with the 10 dB isolation bandwidth from 0.2–0.7 THz, and the relative bandwidth achieves 110%. The mechanisms of THz nonreciprocal transmission and polarization manipulation proposed in this work will contribute to the development of efficient THz magneto-optical devices.

Nanoscale nonlinear optics is limited by the inherently weak nonlinear response of conventional materials and the small light–matter interaction volumes available in nanostructures. Plasmonic excitations can alleviate these limitations through subwavelength light focusing, boosting optical near fields that drive the nonlinear response, but also suffering from large inelastic losses that are further aggravated by fabrication imperfections. Here, we theoretically explore the enhanced nonlinear response arising from extremely confined plasmon polaritons in few-atom-thick crystalline noble metal films. Our results are based on quantum-mechanical simulations of the nonlinear optical response in atomically thin metal films that incorporate crucial electronic band structure features associated with vertical quantum confinement, electron spill-out, and surface states. We predict an overall enhancement in plasmon-mediated nonlinear optical phenomena with decreasing film thickness, underscoring the importance of surface and electronic structure in the response of ultrathin metal films.

Tunable high-refractive-index nanostructures are highly desired for realizing photonic devices with a compact footprint. By harnessing the large thermo-optic effect in silicon, we show reversible and wide thermal tuning of both the far- and near-fields of Mie resonances in isolated silicon nanospheres in the visible range. We perform in situ heating in a transmission electron microscope and electron energy-loss spectroscopy to show that the Mie resonances exhibit large spectral shifts upon heating. We leverage the spectral shifts to demonstrate near-field tuning between different Mie resonances. By combining electron energy-loss spectroscopy with energy-dispersive X-ray analysis, we show a reversible and stable operation of single silicon nanospheres up to a temperature of 1073 K. Our results demonstrate that thermal actuation offers dynamic near-field tuning of Mie resonances, which may open up applications in tunable nonlinear optics, Raman scattering, and light emission.