Multiple-polarization-sensitive photodetector Based on a plasmonic metasurface.

On-chip polarization-sensitive photodetectors are highly desired for ultra-compact optoelectronic systems. It has been demonstrated that polarization-sensitive photodetection can be realized using intrinsic chiral and anisotropy materials. However, these photodetectors can only realize the detection of either circularly polarized light (CPL) or linear polarized light (LPL) and are not applicable to multiple-polarization-sensitive photodetection. Herein, we experimentally demonstrate a metasurface-integrated semiconductor to realize multiple-polarization-sensitive photodetection at visible wavelengths. This device is composed of a MoSe2 monolayer on an H-shaped plasmonic nanostructure. The geometric chirality and anisotropy of the H-shaped nanostructure result in CPL and LPL resolved optical responses. By integrating a plasmonic metasurface with monolayer MoSe2, we converted polarization-sensitive optical absorption to the polarization-sensitive photocurrent of the device through the photoconductive effect. Polarization-sensitive photocurrent responses to both CPL and LPL are systematically investigated, which demonstrate a high photocurrent circular dichroism (CD) of 0.35 at a wavelength of 810 nm and photocurrent linear polarization (LP) of 0.4 at a wavelength of 633 nm. Our results provide a potential pathway to realize multiple-polarization-sensitive applications in medicine analysis, biology, and remote sensing.

Reconfigurable Micro/Nano-Optical Devices Based on Phase Transitions: From Materials, Mechanisms to Applications.

In recent years, numerous efforts have been devoted to exploring innovative micro/nano-optical devices (MNODs) with reconfigurable functionality, which is highly significant because of the progressively increasing requirements for next-generation photonic systems. Fortunately, phase change materials (PCMs) provide an extremely competitive pathway to achieve this goal. The phase transitions induce significant changes to materials in optical, electrical properties or shapes, triggering great research interests in applying PCMs to reconfigurable micro/nano-optical devices (RMNODs). More specifically, the PCMs-based RMNODs can interact with incident light in on-demand or adaptive manners and thus realize unique functions. In this review, RMNODs based on phase transitions are systematically summarized and comprehensively overviewed from materials, phase change mechanisms to applications. The reconfigurable optical devices consisting of three kinds of typical PCMs are emphatically introduced, including chalcogenides, transition metal oxides, and shape memory alloys, highlighting the reversible state switch and dramatic contrast of optical responses along with designated utilities generated by phase transition. Finally, a comprehensive summary of the whole content is given, discussing the challenge and outlooking the potential development of the PCMs-based RMNODs in the future.

Angular dependent terahertz emission from the interplay between nanocrystal diamond film and plasmonic metasurface.

Composite structures integrated with metasurfaces and nonlinear films have emerged as alternative candidates to enhance nonlinear response. The cooperative interaction between the two components is complicated. Herein, a split-ring resonator (SRR)-type metasurface was fabricated on a free-standing nanocrystal diamond (NCD) film utilizing electron beam lithography, electron beam evaporation, and a lift-off process. The terahertz (THz) radiation from the SRR-NCD under normal incidence originates from the high-order magnetic resonance of SRR because the NCD film cannot produce detectable THz radiation at this incident angle. As increasing the incident angle, the contribution of the THz radiation from the NCD film gradually increases until reaching 40° incident angle limitation. The results indicate that this angular-dependent THz radiation is induced by the interplay between the NCD film and SRR. This study offers a new approach to investigate nonlinear processes in composite structures.

Dynamic and Polarization-Independent Wavefront Control Based on Hybrid Topological Metasurfaces.

Exceptional points (EPs), known as non-Hermitian singularities, have been observed and investigated in parity-time symmetric metasurfaces. However, the chirality and tunability in non-Hermitian metasurfaces still need to be explored. Here, we propose a dynamic topological metasurface with the meta-atom consisting of two orthogonally oriented nanorods, which are placed on the phase change material Ge2Sb2Te5 (GST) and SiO2 dielectric layer, respectively. When GST is converted from the amorphous state (a-GST) to the crystalline state (c-GST), an EP can be dynamically switched from the "ON" state to the "OFF" state in a parameter space. Moreover, based on the topologically protected phase and amplitude modulations of the cross-polarization component, the phase-only hologram and amplitude-only hologram are engineered in the a-GST case and concealed in the c-GST case. Finally, we explore the 2D-chiral symmetry of meta-atoms and further propose two spin-selective meta-deflectors and a hybrid meta-deflector operating with arbitrary polarizations. The GST-based hybrid metasurface offers richer possibilities to realize various wavefront controls.

Active multiband varifocal metalenses based on orbital angular momentum division multiplexing.

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.

Interior Melting of Rapidly Heated Gold Nanoparticles.

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.

Electromechanically reconfigurable optical nano-kirigami.

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.

Multidimensional Image and Beam Splitter Based on Hyperbolic Metamaterials.

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.

Plasmonic Hybrids of MoS2 and 10-nm Nanogap Arrays for Photoluminescence Enhancement.

Monolayer MoS2 has attracted tremendous interest, in recent years, due to its novel physical properties and applications in optoelectronic and photonic devices. However, the nature of the atomic-thin thickness of monolayer MoS2 limits its optical absorption and emission, thereby hindering its optoelectronic applications. Hybridizing MoS2 by plasmonic nanostructures is a critical route to enhance its photoluminescence. In this work, the hybrid nanostructure has been proposed by transferring the monolayer MoS2 onto the surface of 10-nm-wide gold nanogap arrays fabricated using the shadow deposition method. By taking advantage of the localized surface plasmon resonance arising in the nanogaps, a photoluminescence enhancement of ~20-fold was achieved through adjusting the length of nanogaps. Our results demonstrate the feasibility of a giant photoluminescence enhancement for this hybrid of MoS2/10-nm nanogap arrays, promising its further applications in photodetectors, sensors, and emitters.

Author Correction: Universal mechanical exfoliation of large-area 2D crystals.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Universal mechanical exfoliation of large-area 2D crystals.

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.

Independent tuning of bright and dark meta-atoms with phase change materials on EIT metasurfaces.

The realization of tunable metasurfaces is of fundamental importance for boosting the electromagnetic field control ability. Especially, it is important to put forward new modulation methods to further understand their underlying modulation mechanism and expand their application range. In this paper, tunable electromagnetically induced transparency (EIT) metasurfaces based on the phase change material Ge2Sb2Te5 (GST) are proposed and experimentally demonstrated. Different from previous modulation methods of directly introducing the GST film below the metasurfaces, here a two-step lithography method is introduced to combine independent GST strips with bright and dark meta-atoms in the EIT structures, respectively, achieving the independent modulation of the EIT-like spectra. In addition, by applying temporal coupled-mode theory (TCMT), the EIT-like spectra with different GST crystallization levels were analysed and the corresponding characteristic parameters were determined simultaneously. These fitting results reveal that GST strips can modulate the resonances of the bright and dark meta-atoms independently by shifting the resonant frequency and increasing the decay rate, which in turn result in the different modulation features of the EIT-like spectra. This method improves the degree of freedom of active modulation and provides a new route for tunable slow light devices.

Realization of a near-infrared active Fano-resonant asymmetric metasurface by precisely controlling the phase transition of Ge2Sb2Te5.

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.

Diabolical points in coupled active cavities with quantum emitters.

In single microdisks, embedded active emitters intrinsically affect the cavity modes of the microdisks, resulting in trivial symmetric backscattering and low controllability. Here we demonstrate macroscopic control of the backscattering direction by optimizing the cavity size. The signature of the positive and negative backscattering directions in each single microdisk is confirmed with two strongly coupled microdisks. Furthermore, diabolical points are achieved at the resonance of the two microdisks, which agrees well with theoretical calculations considering the backscattering directions. Diabolical points in active optical structures pave the way for an implementation of quantum information processing with geometric phase in quantum photonic networks.

Two Kinds of Metastable Structures in an Epitaxial Lanthanum Cobalt Oxide Thin Film.

Thin films have attracted much interest because they often have novel properties different from those of their bulk counterparts. In this work, we tune two metastable states in three kinds of lanthanum cobalt oxide thin films by electron beam irradiation and record their dynamic transition process in situ in a transmission electron microscope. The lanthanum cobalt oxide thin films exhibit a homogeneous microstructure in the initial state and then transfer to a stripelike superstructure with 3a0 periodicity (a0 is the perovskite lattice parameter), further developing into a superstructure with 2a0 periodicity in dark stripes (brownmillerite structure). To explore the inherent energy discrepancy within the two metastable states, we perform first-principles calculations on a LaCoO3-δ (0 ≤ δ ≤ 0.5) thin film system by geometry optimization. The calculation results suggest that the forming energy of the 3a0 periodicity stripelike structure is a little lower than that of the 2a0 periodicity in the LaCoO3-δ thin film. Our work explains why the two stripelike structures coexist in lanthanum cobalt oxide thin films and extends prospective applications related to oxygen vacancies in thin films.

Diameter-optimized high-order waveguide nanorods for fluorescence enhancement applied in ultrasensitive bioassays.

Development of fluorescence enhancement (FE) platforms based on ZnO nanorods (NRs) has sparked considerable interest, thanks to their well-demonstrated potential in chemical and biological detection. Among the multiple factors determining the FE performance, high-order waveguide modes are specifically promising in boosting the sensitivity and realizing selective detection. However, quantitative experimental studies on the influence of the NR diameter, substrate, and surrounding medium, on the waveguide-based FE properties remain lacking. In this work, we have designed and fabricated a FE platform based on patterned and well-defined arrays of vertical, hexagonal prism ZnO NRs with six distinct diameters. Both direct experimental evidence and theoretical simulations demonstrate that high-order waveguide modes play a crucial role in FE, and are strongly dependent on the NR diameter, substrate, and surrounding medium. Using the optimized FE platform, a significant limit of detection (LOD) of 10-16 mol L-1 for Rhodamine-6G probe detection is achieved. Especially, a LOD as low as 10-14 g mL-1 is demonstrated for a prototype biomarker of carcinoembryonic antigen, which is improved by one order compared with the best LOD ever reported using fluorescence-based detection. This work provides an efficient path to design waveguiding NRs-based biochips for ultrasensitive and highly-selective biosensing.

Quasi-2D Transport and Weak Antilocalization Effect in Few-layered VSe2.

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 observation of WAL effect in VSe2 is challenging due to the relative weak SOC and three-dimensional (3D) transport nature in thick VSe2. Here, we report on the observation of quasi-2D transport and WAL effect in sublimed-salt-assisted low-temperature chemical vapor deposition (CVD) grown few-layered high-quality VSe2 nanosheets. The WAL magnitudes in magnetoconductance can be perfectly fitted by the 2D Hikami-Larkin-Nagaoka (HLN) equation in the presence of strong SOC, by which the spin-orbit scattering length lSO and phase coherence length lϕ have been extracted. The phase coherence length lϕ shows a power law dependence with temperature, lϕ∼ T-1/2, revealing an electron-electron interaction-dominated dephasing mechanism. Such sublimed-salt-assisted growth of high-quality few-layered VSe2 and the observation of WAL pave the way for future spintronic and valleytronic applications.

Enhanced Strong Interaction between Nanocavities and p-shell Excitons Beyond the Dipole Approximation.

Large coupling strengths in exciton-photon interactions are important for the quantum photonic network, while strong cavity-quantum dot interactions have been focused on s-shell excitons with small coupling strengths. Here we demonstrate strong interactions between cavities and p-shell excitons with a great enhancement by the in situ wave-function control. The p-shell excitons are demonstrated with much larger wave-function extents and nonlocal interactions beyond the dipole approximation. Then the interaction is tuned from the nonlocal to the local regime by the wave function shrinking, during which the enhancement is obtained. A large coupling strength of 210 µeV has been achieved, indicating the great potential of p-shell excitons for coherent information exchange. Furthermore, we propose a distributed delay model to quantitatively explain the coupling strength variation, revealing the intertwining of excitons and photons beyond the dipole approximation.

Spin-Selective Transmission in Chiral Folded Metasurfaces.

Controlling the spin angular momentum of light (or circular polarization state) plays a crucial role in the modern photonic applications such as optical communication, circular dichroism spectroscopy, and quantum information processing. However, the conventional approaches to manipulate the spin of light require naturally occurring chiral or birefringent materials of bulky sizes due to the weak light-matter interactions. Here we experimentally demonstrate an approach to implement spin-selective transmission in the infrared region based on chiral folded metasurfaces that are capable of transmitting one spin state of light while largely prohibiting the other. Due to the intrinsic chirality of the folded metasurface, a remarkable circular dichroism as large as 0.7 with the maximum transmittance exceeding 92% is experimentally demonstrated. The giant circular dichroism is interpreted within the framework of charge-current multipole expansion. Moreover, the intrinsic chirality can be readily controlled by manipulating the folding angle of the metasurface with respect to the cardinal plane. Benefiting from its strong chirality and spin-dependent transmission characteristics, the proposed folded metasurface may be applied to a range of novel photon-spin selective devices for optical communication technologies and biophotonics.

Folding 2D Structures into 3D Configurations at the Micro/Nanoscale: Principles, Techniques, and Applications.

Compared to their 2D counterparts, 3D micro/nanostructures show larger degrees of freedom and richer functionalities; thus, they have attracted increasing attention in the past decades. Moreover, extensive applications of 3D micro/nanostructures are demonstrated in the fields of mechanics, biomedicine, optics, etc., with great advantages. However, the mainstream micro/nanofabrication technologies are planar ones; therefore, they cannot be used directly for the construction of 3D micro/nanostructures, making 3D fabrication at the micro/nanoscale a great challenge. A promising strategy to overcome this is to combine the state-of-the-art planar fabrication techniques with the folding method to produce 3D structures. In this strategy, 2D components can be easily produced by traditional planar techniques, and then, 3D structures are constructed by folding each 2D component to specific orientations. In this way, not only will the advantages of existing planar techniques, such as high precision, programmable patterning, and mass production, be preserved, but the fabrication capability will also be greatly expanded without complex and expensive equipment modification/development. The goal here is to highlight the recent progress of the folding method from the perspective of principles, techniques, and applications, as well as to discuss the existing challenges and future prospectives.