Engineering dynamical photon blockade with Liouville exceptional points.

We investigate the dynamical blockade in a nonlinear cavity and demonstrate the connection between the correlation function g(2)(t) and system parameters in the entire nonlinear region. Utilizing the Liouville exceptional points (LEPs) and quantum dynamics, a near-perfect single-photon blockade (1PB) can be achieved. By fine-tuning system parameters to approach the second-order LEP (LEP2), we improved single-photon statistics in both weak and strong nonlinearity regimes, including a significant reduction of g(2)(t) and a pronounced increase in the single-photon occupation number. In the strong nonlinearity region, the maximum photon population may correspond to stronger antibunching effect. Simultaneously, the time window and period of blockade can be controlled by selecting detuning based on the LEP2. Furthermore, the 1PB exhibits robustness against parameter fluctuations, and this feature can be generalized to systems for implementing single-photon sources with nonharmonic energy levels.

Manipulation of path state based on spatiotemporal dielectric metasurface.

In this work, a spatiotemporal metasurface is proposed to manipulate the path of photons flexibly. The spatial modulation is induced by the rectangle silicon units aligned on silica in a manner with a phase gradient only for y-polarized photons, and the temporal modulation is contributed by the pumps of constructing Kerr dynamic gratings. By quantizing designed metasurfaces, the analytical solutions of output photon states can be derived correspondingly. Reversal design could be implemented by tailoring the profile of higher harmonics to infer the intensity of pumps, size of meta-atoms, and initial state. The path-polarization entanglement and correlations of output photons are realized, and then a CNOT gate is obtained by utilizing the deflection of the photon path. This work provides a scheme to deal with the spatiotemporal metasurfaces and expands the applications of metasurfaces in the quantum realm.

Strain-dependent magnetic ordering switching in 2D AFM ternary V-based chalcogenide monolayers.

The lack of macroscopic magnetic moments makes antiferromagnetic materials promising candidates for high-speed spintronic devices. The 2D ternary V-based chalcogenides (VXYSe4; X, Y = Al, Ga) monolayers are investigated based on the density-functional theory and Monte Carlo simulations. The results reveal that the Néel temperature of the VGa2Se4 monolayer is 18 K with zigzag2-antiferromagnetic (AFM) spin ordering. Also, the magnetic ordering of V ions in VAl2Se4 and VAlGaSe4 monolayers prefer zigzag1-AFM coupling with Néel temperature of 47 K and 33 K, respectively. The magnetic anisotropy calculations demonstrate that the easy magnetization axis of the VXYSe4 monolayers is parallel to the y axis. In addition, the VXYSe4 monolayers can be adjusted from the AFM state to the ferromagnetic (FM) state under biaxial stretching, which can be attributed to the competition between d-p-d superexchange and d-d direct exchange caused by the variation of bond length. The transition temperature of VXYSe4 monolayers can be elevated above room temperature with the help of compression strain. In particular, the in-plane magnetic anisotropy is a robust characteristic regardless of the magnitude of the applied biaxial strain. These explorations not only enrich the family of AFM monolayers with excellent stability but also provide distinctive ideas for the performance control of AFM materials and their applications in nanodevices.

Ferroelectricity and High Curie Temperature in a 2D Janus Magnet.

The breaking of the out-of-plane symmetry makes a two-dimensional (2D) Janus monolayer a new platform to explore the coupling between ferroelectricity and ferromagnetism. Using density functional theory in combination with Monte Carlo simulations, we report a novel phase-switchable 2D multiferroic material VInSe3 with large intrinsic out-of-plane spontaneous electric polarization and a high Curie temperature (Tc). The structural transition energy barrier between the two phases is determined to be 0.4 eV, indicating the switchability of the electric polarizations and the potential ferroelectricity. Carrier doping can boost the Curie temperature above room temperature, attributing to the enhanced magnetic exchange interaction. A transition from the ferromagnetic (FM) state to the antiferromagnetic (AFM) state can be induced by carrier doping in octahedra-VInSe3, while FM coupling is well-preserved in tetrahedron-VInSe3, which can be regulated to be either an XY or Ising magnet at an appropriate carrier concentration. These findings not only enrich the family of high-Tc low-dimensional monolayers but also offer a new direction for the design and multifunctional application of multiferroic materials.

Metalens for generating multi-channel polarization-wavelength multiplexing metasurface holograms.

We demonstrate multi-channel metasurface holograms, where the pixels of holographic images are represented by the focal points of metalens, leading to the nanoscale resolution. The required phase profiles are implemented by elaborately arranging the hybrid all-dielectric meta-atoms with specific orientation angles. For verification, two-channel single-color images are reconstructed on the focal plane of the metalens by polarization control. Alternatively, three-channel color holograms are exhibited by manipulating the incident wavelengths. More uniquely, the metalens can be further engineered to generate polarization-wavelength multiplexing color holograms in six channels. Our work provides an effective approach to reconstructing holographic images and enables potential applications including color display, information engineering, and optical encryption.

Dual-band tunable and strong circular dichroism in a metal-graphene hybrid zigzag metasurface.

Most active chiral metasurfaces operate in a single band and have an unidirectionally tunable circular dichroism (CD). Here, we propose a zigzag metasurface composed of a Z-shaped metallic strip and a L-shaped graphene strip to realize the dual-band tunable and strong CD. Two strong CD values of -0.88 and 0.88 are found at f1 = 0.86 THz and f2 = 1.23 THz, respectively. The strengths and resonant frequencies of these two CD signals can be tuned by varying graphene's Fermi energy (EF). Strikingly, the CD value at 0.86 THz undergoes a continuous adjustment in a large range from 0.79 to -0.88 when EF increases from 0.32 eV to 1.00 eV, implying that the proposed metasurface supports the switching of CD signal between on-, off- and reverse-states. Based on the strong CD signals, the capability of the metasurface as a biosensor to detect Avian Influenza viruses is demonstrated. This work will advance the development of broadband tunable chiral-optical devices.

Dynamic and complete terahertz wavefront manipulation via an anisotropic coding metasurface.

A reconfigurable anisotropic coding metasurface composed of a graphene layer and anisotropic Jerusalem-cross metallic layer is proposed for dynamic and complete multi-channel terahertz wavefront manipulation. By controlling the Fermi energy of graphene, continuous amplitude modulation is realized for the coding elements with certain phase responses. By arranging anisotropic phase coding elements with a specific coding sequence and changing the Fermi energy of graphene, the proposed metasurface can dynamically control multi-channel reflection beams with designed power distribution and simultaneously manipulate the scattering pattern from diffusion to mirror scattering under x- and y-polarized incidence, respectively. Compared with the dynamic phase modulation metasurface, such a tunable metasurface uses three degrees of freedom, including the polarization, phase, and amplitude responses to fully control the reflected wavefronts, which may have promising applications in tunable terahertz multi-functional holograms and multi-channel information communication.

Ultra-thin 2-bit anisotropic Huygens coding metasurface for terahertz wave manipulation.

In this work, we design an ultrathin 2-bit anisotropic Huygens coding metasurface (AHCM) composed by bilayer metallic square-ring structures for flexible manipulation of the terahertz wave. Based on the polarized-dependent components of electric surface admittance and magnetic surface impedance, we confirm that both the electric and magnetic resonances on coding meta-atoms are excited, so as to provide a full phase coverage and significantly low reflection. By encoding the elements with distinct coding sequences, the x- and y-polarized incident waves are anomalously refracted into opposite directions. More uniquely, we also demonstrate that the designed AHCM can be utilized as a transmission-type quarter-wave plate. The proposed metasurface paves a new way toward multifunctional terahertz wavefront manipulation.

Enhanced circular dichroism in Ge2Sb2Te5-loaded metasurface.

Circular dichroism (CD) is originally obtained from three-dimensional spiral structures by simultaneously exciting electric and magnetic resonances. To simplify construction, multilayer stacked asymmetric structures and the symmetric structures relying on oblique incidence are proposed for enhancing CD. Herein, we achieved the enhancement of dual-waveband CD by adding a Ge2Sb2Te5 (GST) layer on the top of a Z-shape gold array in a normally incident system. Benefited from the polarization selective excitations of electric and magnetic dipole resonances, the CD in a simple planar structure is immensely enhanced from near zero to 0.73 at 1.58 µm. Furthermore, the CD strengths is dynamically tuned by controlling the phase of GST. With the GST phase transition from amorphous (a-GST) to crystalline state (c-GST), CD magnitudes are switched by about 0.73 and 0.27 at dual wavebands respectively. The enhancement of CD by adding a layer on a simple planar array offers a new method for designing planar metasurfaces with strong chirality.

Integrating ferromagnetism and ferroelectricity in an iron chalcogenide monolayer: a first-principles study.

Two-dimensional (2D) ferro-type materials have received great attention owing to the remarkable polarization effect in optoelectronics and spintronics. Using the first-principles method, the coupling between ferromagnetism and ferroelectricity is investigated in a multiferroic Janus 1T-FeSSe monolayer, which has a strong Stoner ferromagnetic ground state. The magnetic anisotropy energy (MAE) is apparently impacted by the out-of-plane asymmetry donated ferroelectricity, which is reflected by the asymmetry of the Z-MAE image. The easy magnetization axis of Janus FeSSe is the +y axis with a large MAE of 0.59 meV, rooting in unpaired d electrons of Fe atoms. The transformation of band splitting and Fermi surface can be effectively engineered by different magnetic polarization directions. The ferromagnetic (FM) coupling of the FeSSe monolayer is very robust under external strain within the range of -6% to 6%, while the strength of magnetic moment of Fe atoms and polarization are easily strain-engineered, the intrinsic mechanism of which can be elaborated by the GKA rules that depend on angles and distances. This multiferroic FeSSe monolayer provides a new platform for exploring the coupling of 2D ferromagnetism and ferroelectricity and designing low-dimensional multiferroic electronics.

Colossal In-Plane and Out-of-Plane Shift Photocurrents in Single-Layer Two-Dimensional α-Antimonide Phosphorus.

For materials lacking inversion symmetries, an interband transition induced by a photon may result in excited electrons (holes) experiencing a spatial shift leading to generation of directional photocurrents. This phenomenon known as bulk photovoltaic effect (BPVE) shift photocurrent (SPC) has recently attracted immense attention owing to its potential in generating photovoltages that are not restricted by Shockley-Queisser limitations imposed by materials' electronic band gaps. The BPVE was recently reformulated in a quantum mechanics viewpoint as the change in the geometrical phase upon photoexcitation and can now be promptly calculated from Bloch wave functions generated by first-principles calculations. The SPC of an electron (hole) is robust against crystal defects and impurities both in the interior and the surface and can be less dissipative and ultrafast. Herein, an emergence of colossal SPC in a pristine two-dimensional (2D) single-layer α-SbP crystal is predicted from first-principles calculations. An external electric field is further applied on the 2D crystal, and a large SPC enhancement is achieved. The locations of the SPC peaks due to both in-plane and out-of-plane responses suggest that α-SbP can generate a large photocurrent both in visible-light and ultraviolet regions. Single-layer 2D α-SbP is thus an excellent material for strong SPC. This finding is thus expected to open a pathway to exploring efficient photovoltaic devices based on monolayer α-SbP and similar materials.

Design and simulation of a GST-based metasurface with strong and switchable circular dichroism.

Circular dichroism (CD) is required in the applications of biological detection, analytical chemistry, etc. Here, we numerically demonstrated large-range switchable CD by controlling the phase change of Ge2Sb2Te5 (GST) in a zigzag array. At the amorphous state of GST (a-GST), the strong and dual-waveband CD effects are realized via the selective excitations of electric, magnetic, and toroidal resonances. With the transition from a-GST to crystalline state GST, CD strengths are tailored dynamically in large ranges. In detail, the CD magnitudes change by about 0.93 and the modulation depths exceed 94% at dual wavebands. The strong CD effects and large-range switch capability in the GST-based metasurfaces will boost the development of active chiroptical devices.

Tunable dual-band metamaterial absorber in the infrared range based on split-ring-groove array.

In this paper, we present a tunable dual-band perfect metamaterial absorber working in the infrared band by integrating a metallic split-ring-groove resonator array with a liquid crystal (LC) layer atop a metal substrate. By varying the height of the central nanodisks, the absorptivity of the dual-band absorption peaks can be simultaneously adjusted. The dual-band resonance frequencies of the proposed absorber exhibit continuous tunability by adjusting the refractive index of the LC, which can be controlled by applying external voltage. The mechanism of the perfect absorption is attributed to the gap plasmonic resonance coupling regime. The presented absorber exhibits good tolerance to incidence angles up to 60° and shows polarization dependent performance, which may offer promising applications in sensing, modulator, and optical absorption switching in the infrared regime.

Theoretical studies on the two-photon absorption of II-VI semiconductor nano clusters.

Semiconductor clusters, ZnnOn, ZnnSn, and CdnSn (n = 2-8), were optimized and the corresponding stable structures were acquired. The symmetry, bond length, bond angle, and energy gap between HOMO and LUMO were analyzed. According to reasonable calculation and comparative analysis for small clusters Zn2O2, Zn2S2, and Cd2S2, an effective method based on density function theory (DFT) and basis set which lay the foundation for the calculation of the large clusters have been obtained. The two-photon absorption (TPA) results show that for the nano clusters with planar configuration, sizes play important role on the TPA cross section, while symmetries determine the TPA cross section under circumstance of 3D stable structures. All our conclusions provide theoretical support for the development of related experiments.

Controlling Tamm phonons using hBN and a distributed Bragg reflector for narrowband refractive index sensing.

Optical Tamm state with sharp reflection dip provides the sensing potential combined with high sensitivity. In this paper, we numerically demonstrate that narrowband refractive index sensing can be realized in a distributed Bragg reflector (DBR) structure with hexagonal boron nitride (hBN). Here, we show that the sensitivity and narrowband properties can not only be regularly governed by different analyte thickness but also exhibit dependence on the number of DBR pairs and the thickness of the hBN layer. With varying the analyte index and optimized analyte thickness, the deep reflectance dip can be sustained with the sensitivity (figure of merit, FOM) close to 3.02 µm/RIU (1093/RIU). In addition, the different analyte categories can be detected through adjusting the thickness of the analyte-filled cavity. High sensitivity, combined with ultra-high FOM originated from strong Tamm phonon mode, offers a promising platform to detect the smallest variation of the refractive index.

Dynamic control of THz polarization modulation and multi-channel beam generation using a programmable metasurface.

Polarization modulation and multichannel beam generation are crucial in multichannel communication and high-resolution imaging at THz frequency. In this work, we present a polarization-reprogrammable coding metasurface composed of VO2/Au composite concentric rings (CCRs). Owing to the phase-change property of VO2, the CCR is designed as a digital coding element for the polarization conversion. When VO2 remains insulator state at room temperature, the y-polarized incident wave is transformed into x-polarized wave, which can be regarded as digital state 0. When VO2 converts into metal state at critical temperature (68 °C), the polarization of reflected wave stays unchanged, corresponding to digital state 1. Any desired linear polarization state of reflected beam is achieved by taking advantage of different coding sequences in a programmable manner. Furthermore, by combining phase gradient with polarization coding states, we propose an anisotropic programmable metasurface to control the multi-channel reflected beams dynamically. By arranging distinct coding sequences, we show that the EM reflected beams can be manipulated flexibly. The proposed programmable metasurface paves new ways towards THz polarization manipulation, signal detection and information communication.

Tuning of mid-infrared absorption through phonon-plasmon-polariton hybridization in a graphene/hBN/graphene nanodisk array.

In this paper, we utilize a heterostructured graphene/hBN/graphene nanodisk array to implement an electrically tunable absorber in and out of the Reststrahlen band (RSB) region of hBN. Tuning of phonon-type resonance absorption in the RSB region is achieved through phonon-plasmon-polariton hybridization. The hybrid phonon mode enabled a 290 nm shift of the resonant wavelength, and the sensitivity of absorption peak to the electrical control is 362.5 nm/eV. Simultaneously, the nearly perfect absorption is obtained in the condition of high chemical potential of graphene. Moreover, the plasmon polaritons are strongly modified by phonon polaritons of hBN, so the FWHM of absorption peaks out of the RSB region reduce to 45-49 nm, and the maximum Q of absorption reaches 220.44 at EF=0.65 eV, which is paving a way toward coherent emission at the atmospheric transparent band. Importantly, graphene-assisted hyperbolic phonon polaritons of hBN will enable future phonon devices with high optical performance and wide tunability.

Electronic Properties of Triangle Molybdenum Disulfide (MoS2) Clusters with Different Sizes and Edges.

The electronic structures and transition properties of three types of triangle MoS2 clusters, A (Mo edge passivated with two S atoms), B (Mo edge passivated with one S atom), and C (S edge) have been explored using quantum chemistry methods. The highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of B and C is larger than that of A, due to the absence of the dangling of edge S atoms. The frontier orbitals (FMOs) of A can be divided into two categories, edge states from S3p at the edge and hybrid states of Mo4d and S3p covering the whole cluster. Due to edge/corner states appearing in the FMOs of triangle MoS2 clusters, their absorption spectra show unique characteristics along with the edge structure and size.

Discrimination of enantiomers through quantum interference and quantum Zeno effect.

Quantum optical methods have great potential for highly efficient discrimination of chiral molecules. We propose quantum interference-based schemes of enantio-discrimination under microwave regime among molecular rotational states. The quantum interference between field-driven one- and two-photon transitions of two higher states is designed to be constructive for one enantiomer but destructive for the other, since a certain transition dipole moment can be set to change sign with enantiomers. Therefore, two enantiomers can evolve into entirely different states from the same ground state. Through strengthening the constructive interference, the quantum Zeno effect is found in one enantiomer and then its excitation is suppressed, which also enables the enantio-discrimination. We simulate the schemes for differentiating between S and R enantiomers of 1, 2-propanediol (C3H8O2) molecules. With the analysis of the phase sensitivity to microwave fields and the effect of energy relaxations, the highly efficient enantio-discrimination of the 1, 2-propanediol molecules may be achieved.

Modulation of the electronic band structure of silicene by polar two-dimensional substrates.

Using the density functional theory (DFT) calculations, we find that  Janus group-III chalcogenide monolayers can serve as a suitable substrate for silicene, and the Dirac electron band properties of silicene are also fully preserved. The maximum opened band gap can reach 179 meV at the Dirac point due to the interaction of silicene and the polar two-dimensional (2D) substrate. In addition, the electronic band structure of the heterostructure can be modulated by applying an electric field where its predicted band gap increases or decreases according to the direction of the applied external electric field. Furthermore, an insight into the electron structures can be understood by analyzing the electron energy-loss (EEL) spectra. From these results, we also predict that heterostructures with polar 2D substrates have broad application prospects in multi-functional devices. Besides, Janus group-III chalcogenide monolayers can be used as good substrates for growing silicene and the modulation of the electronic structure can also be applied to nanodevices and optoelectronic devices.