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The band structures of the photonic crystal slabs play a significant role in manipulating the flow of light and pre-dicting exotic physics in photonics. In this letter, we show that the key features of photonic band structures can be achieved experimentally by the polarization- and momentum-resolved photoluminescence spectroscopy utilizing the light emission properties of SiNx. The two-dimensional spectra clearly reveal the energy-momentum dispersion of band structures which is in perfect agreement with the simulation results. The isofrequency contours can be measured easily by adding a bandpass filter with a desired photon energy. Furthermore, it is convenient to observe clearly and directly the optical singularity -- the optical bound states in the continuum featured by dark point in three-dimensional photoluminescence spectra. The polarization-resolved isofrequency contours clearly show that this dark point is the center of an azimuthally polarized vortex. Finally, the helical topological edge states can be easily observed in photonic topological insulators with deformed hexagonal lattices. Our work provides a simple and effective approach for exploring topological photonics and other intriguing phenomena hidden in the photonic crystal slabs., 6 pages, 5 figures

As an inherent characteristic of light, polarization plays important roles in information storage, display and even encryption. Metasurfaces, composed of specifically designed subwavelength units in a two-dimensional plane, offer a great convenience for polarization manipulation, yet improving their integrability and broadband fidelity remain significant challenges. Here, based on the combination of various subwavelength cross-nanofins (CNs), a new type of metasurface for multichannel hybrid polarization distribution in near-field is proposed. Sub-wavelength CN units with various waveplate (WP) functionalities, such as frequency-division multiplexing WP, half-WP and quarter-WP are implemented with high efficiency in broadband. High-resolution grayscale image encryption, multi-image storage and rapid polarization detection are demonstrated by encoding the WP pixels into single, double and four channels, respectively. All these applications possess good fidelity in an ultrabroad wavelength band from 1.2 to 1.9 µm, and the high degree of integrability, easy fabrication and multifunction make the CN-shaped WP pixels a promising candidate in optical device miniaturization, quantum applications and imaging technologies.

Recently, multidimensional metasurfaces, which can modulate multiple optical parameters of input and output lights simultaneously, have aroused intensive interest. Here, we demonstrate multidimensional switchable images and 3D integrated beam splitters based on hyperbolic metamaterials (HMMs). On one hand, a switchable image controlled by output helicity and input wavelengths is achieved by arranging HMMs with different polarization conversion performance. On the other hand, polarization-multiplexed broadband beam splitter is generated by spatially engineering the subunit with broadband half-plate performance. By integrating the multiplexed beam splitter with a filter metamaterial, this multidimensional beam splitter can further realize the separation of output light by space and wavelength. This cascaded multilayer metamaterial achieves a brand new optical functionality and offers more inspiring possibilities for the design of multifunctional optical devices in the future.

Kirigami, with facile and automated fashion of three-dimensional (3D) transformations, offers an unconventional approach for realizing cutting-edge optical nano-electromechanical systems. Here, we demonstrate an on-chip and electromechanically reconfigurable nano-kirigami with optical functionalities. The nano-electromechanical system is built on an Au/SiO2/Si substrate and operated via attractive electrostatic forces between the top gold nanostructure and bottom silicon substrate. Large-range nano-kirigami like 3D deformations are clearly observed and reversibly engineered, with scalable pitch size down to 0.975 μm. Broadband nonresonant and narrowband resonant optical reconfigurations are achieved at visible and near-infrared wavelengths, respectively, with a high modulation contrast up to 494%. On-chip modulation of optical helicity is further demonstrated in submicron nano-kirigami at near-infrared wavelengths. Such small-size and high-contrast reconfigurable optical nano-kirigami provides advanced methodologies and platforms for versatile on-chip manipulation of light at nanoscale., The authors present on-chip and electromechanically reconfigurable nanokirigami with optical functionalities. 3D deformations are achieved via attractive electrostatic forces between a gold nanostructure layer and silicon substrate, resulting in optical reconfigurations with high modulation contrast and small unit size.

The interactions of photonic spin angular momentum and orbital angular momentum, i.e., the spin-orbit coupling in focused beams, evanescent waves or artificial photonic structures, have attracted intensive investigations for the unusual fundamental phenomena in physics and potential applications in optical and quantum systems. It is of fundamental importance to enhance performance of spin-orbit coupling in optics. Here, we demonstrate a titanium dioxide (TiO2)–based all-dielectric metasurface exhibiting a high efficient control of photonic spin Hall effect (PSHE) in a transmissive configuration. This metasurface can achieve high-efficiency symmetric spin-dependent trajectory propagation due to the spin-dependent Pancharatnam-Berry phase. The as-formed metadevices with high-aspect-ratio TiO2 nanofins are able to realize (86%, measured at 514 nm) and broadband PSHEs in visible regime. Our results provide useful insights on high-efficiency metasurfaces with versatile functionalities in visible regime.

In this work, magneto-Raman mapping was performed on monolayer MoS2 at 4 K, which exhibited a prominent magnetic field-induced modulation of phonon intensity. Interestingly, structural microinhomog...

Strain engineering provides an important strategy to modulate the optical and electrical properties of semiconductors for improving devices performance with mechanical force or thermal expansion difference. Here, we present the investigation of the local strain distribution over few-layer MoS2 bubbles, by using scanning photoluminescence and Raman spectroscopies. We observe the obvious direct bandgap emissions with strain in the few-layer MoS2 bubble and the strain-induced continuous energy shifts of both resonant excitons and vibrational modes from the edge of the MoS2 bubble to the center (10 µm scale), associated with the oscillations resulted from the optical interference-induced temperature distribution. To understand these results, we perform ab initio simulations to calculate the electronic and vibrational properties of few-layer MoS2 with biaxial tensile strain, based on density functional theory, finding good agreement with the experimental results. Our study suggests that local strain offers a convenient way to continuously tune the physical properties of a few-layer transition metal dichalcogenides (TMDs) semiconductor, and opens up new possibilities for band engineering within the 2D plane.

The atomic mechanisms of the melting process of gold nanoparticles were investigated by molecular dynamics simulations. The melting under high heating rate is found to be much different from the ne...

A metasurface is one of the most effectual platforms for the manipulation of complex optical fields. One of the current challenges in the field is to develop active or reconfigurable functionalities to extend its operation band which is limited by its intrinsic resonant nature. Here we demonstrate a kind of active Fano-resonant asymmetric metasurface in the near-infrared (NIR) region with heterostructures made of a layer of asymmetric split-ring resonators and a thin layer of phase-change material (PCM). In the asymmetric metasurface, significant tunability in the frequency, Q-factor and strength of the Fano resonance are all achieved by precisely controlling the phase transition of the contained PCM Ge2Sb2Te5 (GST), together with changing the geometric asymmetry of the split-ring resonators. Moreover, we provide a complete transition process of the optical properties for GST and an optimized modulation on the active Fano-resonant metasurface. Our approach to dynamically control a Fano-resonant metasurface paves the way to realizing various active photonic meta-devices involving PCM.

Normal melting invariably starts from surfaces or interfaces due to the weaker bonding constraints in these regions. However, we show that melting can be initiated from the interior of gold nanoparticles with high heating rates. We find that melting starts from the surface with the formation of a premelting layer, as usual, but that the premelting layer does not extend to the interior under certain conditions. Instead, liquid nucleation occurs in the core of the nanoparticle. This unexpected interior melting is connected to the slower melting kinetics, which is related to heat transfer near the premelted surface. The required conditions for interior melting are a suitable size of the nanoparticle and a sufficiently fast heating rate. The present results point to a novel melting regime in nanoparticles. We note that the time scales are now accessible using ultrafast tools such as X-ray lasers that can probe dynamical structure changes, suggesting opportunities for experiments.

Metalenses as miniature flat lenses exhibit a substantial potential in replacing traditional optical component. Although the metalenses have been intensively explored, their functions are limited by poor active ability, narrow operating band and small depth of field (DOF). Here, we show a dielectric metalens consisting of TiO2 nanofins array with ultrahigh aspect ratio to realize active multiband varifocal function. Regulating the orbital angular momentum (OAM) by the phase assignment covering the 2π range, its focal lengths can be switched from 5 mm to 35 mm. This active optical multiplexing uses the physical properties of OAM channels to selectively address and decode the vortex beams. The multiband capability and large DOFs with conversion efficiency of 49% for this metalens are validated for both 532 nm and 633 nm, and the incidence wavelength can further change the focal lengths. This non-mechanical tunable metalens demonstrates the possibility of active varifocal metalenses.

In this Letter, we present the experimental observation of trilayer exciton emission in the WSe2/WS2/MoS2 heterostructures by photoluminescence spectroscopy at a low temperature of ∼10 K. It is found that the trilayer exciton has a lower exciton binding energy than that in the WSe2/MoS2 heterobilayer due to the insertion of monolayer WS2, which is manifested by the blueshift of this trilayer exciton peak relative to the bilayer exciton. Through the measurements of the exciton peak position as a function of excitation power, the trilayer excitons exhibited the density-dependent repulsive interaction among them, and the reduction of binding energy ∼8 meV is deduced. Moreover, the trilayer exciton has a longer lifetime than the bilayer exciton, resulting from the reduction of electron–hole wavefunction overlap.

Metasurface provides subwavelength structures for manipulating wavefronts of light. The benefits of subwavelength components offer a continuous modulation of amplitude, phase, and polarization, thus eliminating the production of higher-order images and improving the utilization of light intensity. Despite the rapid progress in this field, multiparameter control of light using single layer metasurface is rarely reported. In fact, multiparameter control of light helps to improve information storage capacity and image fidelity. With simultaneous manipulation of polarization and amplitude at each pixel, it is possible to encode two separate images into one metasurface and reconstruct them under proper conditions. In a proof of concept experiment, we demonstrate an independent display of two binary images at the same position with polarization de-multiplexing from a single metasurface. This unique technology of encoding two images through amplitude and polarization manipulation provides a new opportunity for various applications in, such as encryption, information storage, polarization holograms, optical communications and fundamental physics.

With strong spin–orbit coupling (SOC), ultrathin two-dimensional (2D) transitional metal chalcogenides (TMDs) are predicted to exhibit weak antilocalization (WAL) effect at low temperatures. The ob...

Recent theoretical predictions have indicated that BiTeX (X = Cl, Br, I) type strong spin orbital coupling semiconductor compounds with strong Rashba effects are suitable for use as topological insulators under compressed conditions and novel phenomena have been identified in experiments. The compound BiTeCl is expected to exhibit a topological nontrivial state under pressure. In this study, we obtained measurements based on the resistivity, Hall coefficient, synchrotron X-ray diffraction, and Raman spectroscopy using the Rashba semiconductor BiTeCl crystal at high pressures up to 54.8 GPa. Our experiments indicated that the Rashba semiconductor BiTeCl exhibited a pressure-induced metal to insulator-like transition at 2.8 GPa. A pressure-induced structural phase transition was observed above 5.5 GPa, which was followed by a superconducting transition. The superconducting transition temperature ( T C ) increased to a maximum 4.2 K with pressures up to 32 GPa. Compared with BiTeI, the structural phase transition of BiTeCl occurs at a relatively lower pressure due to the weaker Rashba effects, where the inner chemical pressure is affected by the replacement of I with Cl.

We experimentally and theoretically demonstrate a broadband and polarization-insensitive truncated-cone-type multilayer metamaterial absorber in the near-IR to mid-IR range. Its fabrication process...

Direct growth of graphene film on dielectric substrates is significant for the applications in electronic devices, which can avoid additional complicated and damaging treatments. Despite the advances, a facile way is urgently required to prepare graphene directly on insulators at a low temperature. We presented a gallium (Ga)-assisted atmosphere chemical vapor deposition to synthesize graphene films on silicon nitride (Si3N4). Liquid Ga was dropped on the substrate as a catalyst and flowed away after growth, achieving graphene on Si3N4 (100 nm)/SiO2 (100 nm)/Si and recovering the most of Ga. Multilayer graphene film was formed at the interface between Ga and the substrate. The growth of graphene was investigated by regulating the growth conditions, and an interfacial precipitation mechanism was proposed. Graphene synthesized with recovered Ga droplet is comparable to that prepared using pure Ga, revealing the reusability of Ga catalysts.

Silicon nanotube anodes are notably promising for high-performance lithium-ion batteries due to their outstanding structural stability, but fundamental understanding about their structural evolution during lithiation still remains unclear. Here, the expansion and fracture behavior of lithiated silicon nanotubes is investigated, and the influences of the crystal phase, crystal orientation, inner radius, wall thickness, and thickness–radius ratio are demonstrated. Experiments and simulations demonstrate anisotropic expansion and outer-surface fractures of crystalline silicon nanotubes and isotropic expansion of amorphous ones. The inner holes of nanotubes undergo much less expansion than the outside. The fracture ratio and the maximum hoop stress are positively correlated with both wall thickness and inner radius for crystalline silicon nanotubes. Their competition gives rise to an optimal thickness–outer radius ratio of about 2/3, and the critical diameter reaches 700 nm correspondingly. This research provides significant insight into the lithiation behavior of silicon nanotubes, which could help to design improved silicon anodes.

In recent years, the magneto-optical properties of two-dimensional transition metal disulfides have attracted more and more attention due to their further device applications in spintronics and valleytronics. However, to our knowledge, the plasmonic effect on the magneto-optical properties of WS2 has not been studied. In this work, monolayer WS2 transferred on SiO2/Si substrate and Au film were investigated respectively using polarized-Raman spectroscopy at 4 K under different magnetic fields. Prominent magnetic field–induced variations in the Raman intensities of WS2 samples were observed, which also exhibited significant differences in the spectral evolution versus magnetic field. The resonance magnetic field was 5 T and 5.5 T for the WS2 on SiO2/Si substrate and Au film, respectively. Remarkably, the magneto-optical Raman intensities of A1′&nbsp, and 2LA(M) modes for WS2 on Au film were reduced to approximately 60% compared with that of WS2 on SiO2/Si. These results suggest that the plasmonic effect–induced charge transfer plays an important role in the magneto-optical Raman effect of WS2.

Metallic high aspect ratio (HAR) nano-architectures provide new opportunities for a series of plasmonic devices due to their additional controllable degrees in height space compared to 2D patterns, but there is no efficient way that suitable for the rapid fabrication of large area HAR structures limited by the processing ability of traditional methods. Here in this work, we have developed a templated electrochemical deposition (ECD) method to fabricate various HAR metallic nano-structures for diversified plasmonic devices. The templated ECD method is based on the ECD filling of the nanopores that are fabricated by electron beam lithography. With this templated ECD method, numbers of HAR architectures including nanorods, nanofins and even mushroom-like structures, which have a line width as small as 100 nm and the aspect ratio up to 10:1, are established over a large scale. What is more, by simultaneously considering the designed layout and edge effect, sub 10 nm nanogap arrays are prepared, whose aspect ratio reaches 100:1 and the gap width reduces to 5 nm. Due to the extreme light confinement ability brought from Fabry–Perot resonance, the HAR nanogaps can be treated as a surface enhanced Raman scattering (SERS) substrate. Finite domain time difference simulation shows that fan-like 10 nm nanogap with a height of 700 nm has the largest light enhancement factor (EF). The configuration optimized nanogap is capable for the sensing of rhodamine 6G with a 10−9 M concentration. And the SERS EF of the nanogap is calculated to be 4 × 106, indicating the ultrasensitive molecular detection ability of the HAR nanogap. The templated ECD method not only brings a new chance for the construction of HAR metallic 3D structures, but also opens up a new horizon for the design of a series of plasmonic devices.

Strain engineering, aiming to tune physical properties of semiconductors, provides a promising paradigm for modern micro/nanoelectronics. Two-dimensional materials (2DMs) are the ideal candidates for the next generation of strain engineered devices because of their intrinsic exceptional mechanical flexibility and strength. However, conventional strain modulation methods in 2DMs cannot satisfy the demand of future device applications, because strained structures by these methods lack consistency, reproducibility, and design flexibility. Here, based on the photoresist degeneration induced by electron irradiation, we present a non-contact approach to accurately and directly write the strains with designed patterns from the nanometer to micrometer scale in 2DMs. Profit from controllable manipulation of the electron beam, the developed strategy offers a capability for constructing tensile, compress, or complex strains in MoSe2 monolayers; hence, unique electronic structures for unique physical properties can be designed. Aside from 2DMs, this approach is also appropriate for other types of materials such as Au, α-Si, and Al2O3. Its flexibility and IC-compatibility allow our strain lithography methodology promising in accelerating the potential applications of 2DMs in extensive fields ranging from nanoelectromechanical systems, high-performance sensing, and nontraditional photovoltaics to quantum information science.

Two-dimensional materials provide extraordinary opportunities for exploring phenomena arising in atomically thin crystals. Beginning with the first isolation of graphene, mechanical exfoliation has been a key to provide high-quality two-dimensional materials, but despite improvements it is still limited in yield, lateral size and contamination. Here we introduce a contamination-free, one-step and universal Au-assisted mechanical exfoliation method and demonstrate its effectiveness by isolating 40 types of single-crystalline monolayers, including elemental two-dimensional crystals, metal-dichalcogenides, magnets and superconductors. Most of them are of millimeter-size and high-quality, as shown by transfer-free measurements of electron microscopy, photo spectroscopies and electrical transport. Large suspended two-dimensional crystals and heterojunctions were also prepared with high-yield. Enhanced adhesion between the crystals and the substrates enables such efficient exfoliation, for which we identify a gold-assisted exfoliation method that underpins a universal route for producing large-area monolayers and thus supports studies of fundamental properties and potential application of two-dimensional materials.