Compensating losses in polariton propagation with synthesized complex frequency excitation.

Surface plasmon polaritons and phonon polaritons offer a means of surpassing the diffraction limit of conventional optics and facilitate efficient energy storage, local field enhancement and highsensitivity sensing, benefiting from their subwavelength confinement of light. Unfortunately, losses severely limit the propagation decay length, thus restricting the practical use of polaritons. While optimizing the fabrication technique can help circumvent the scattering loss of imperfect structures, the intrinsic absorption channel leading to heat production cannot be eliminated. Here, we utilize synthetic optical excitation of complex frequency with virtual gain, synthesized by combining the measurements made at multiple real frequencies, to compensate losses in the propagations of phonon polaritons with dramatically enhanced propagation distance. The concept of synthetic complex frequency excitation represents a viable solution to the loss problem for various applications including photonic circuits, waveguiding and plasmonic/phononic structured illumination microscopy.

Active control of nonreciprocal near-field radiative heat transfer with a drift-current biased graphene/α-MoO3 heterostructure.

The interaction between the drift-current biased graphene plasmonics and the hyperbolic phonon polaritons of α-MoO3 provides a promising way to manipulate near-field radiation heat transfer (NFRHT). Through examination of the drift biased graphene/α-MoO3 heterostructure, it has been discovered that drift-current applied to the graphene effectively enhances photon tunneling. Consequently, they dynamically modulate the coupling effect of the two excitations, thereby offering a reliable pathway for the modulation of NFRHT. Furthermore, the influencing mechanism of vacuum gaps on nonreciprocal NFRHT with different drift-current rates is revealed, and it is discovered that the vacuum gaps can filter the nonreciprocal surface plasmon polaritons with high nonreciprocity. Our findings make it possible to manipulate nanoscale thermal rectification and noncontact thermal modulation.

Spin-dependent and tunable perfect absorption in a Fabry-Perot cavity containing a multi-Weyl semimetal.

Spin-dependent absorption has been widely studied in metamaterials and metasurfaces with chirality since it develops significant applications in multiplexed holograms, photodection, and filtering. Here, the one-dimensional photonic crystal Fabry-Perot (FP) cavity containing a multi-Weyl semimetal (mWSM) defect is proposed to investigate the spin-dependent perfect absorption. Results denote that the distinct refractive indices of right hand circularly polarized (RCP) and left hand circularly polarized (LCP) waves are present due to the nonzero off-diagonal term of mWSM, thus supporting the perfect absorption of RCP and LCP waves at distinct resonant wavelengths. The different perfect absorption wavelengths of RCP and LCP waves reveal the spin-dependent perfect absorption. By altering the Fermi energy, tilt degree of Weyl cones, Weyl nodes separation, topological charge, and thickness of the mWSM layer, the perfect absorption wavelength of RCP and LCP waves can be regulated conveniently. Particularly, the linear tunable perfect absorption wavelength with thickness of the mWSM layer supports the accurate determination of perfect absorption wavelength at distinct mWSM thicknesses. Our studies develop simple and effective approaches to acquire the spin-dependent and adjustable perfect absorption without the external magnetic field, and can find practical applications in spin-dependent photonic devices.

Coexistence of Dirac points and nodal chains in photonic metacrystal.

Gapless topological phases, i.e. topological semimetals, come in various forms such as Weyl/Dirac semimetals, nodal line/chain semimetals, and surface-node semimetals. However, the coexistence of two or more topological phases in a single system is still rare. Here, we propose the coexistence of Dirac points and nodal chain degeneracies in a judiciously designed photonic metacrystal. The designed metacrystal exhibits nodal line degeneracies lying in perpendicular planes, which are chained together at the Brillouin zone boundary. Interestingly, the Dirac points, which are protected by nonsymmorphic symmetries, are located right at the intersection points of nodal chains. The nontrivial Z2 topology of the Dirac points is revealed by the surface states. The Dirac points and nodal chains are located in a clean frequency range. Our results provide a platform for studying the connection between different topological phases.

Efficient second-harmonic generation based on off-Γ merging bound states in the continuum.

Ultracompact devices engineered for second-harmonic generation (SHG) hold a significant promise across a diverse spectrum of applications. Here, we propose a merging bound state in the continuum at an off-Γ point in a reciprocal space with the anisotropic materials lithium niobate. Such a merging BIC yields a profound reduction in radiative loss and scattering losses while concurrently exhibiting a substantial enhancement in the quality factor. As a result, we achieved a noteworthy SHG efficiency (η = 3.7%) at the incident angle θ = 10° when the pump intensity I0 = 2 kW/cm2, outperforming alternative nanostructures designed for SHG. This research contributes valuable insights into the feasibility of metadevices founded on the principles of nanoengineering applied to traditional nonlinear crystals. Such advancements hold a considerable promise for the development of compact, high-performance SHG devices across a range of applications.

Asymmetry spin-polarization Fermi arc and topological chiral beam splitter with the triply degenerate point in metamaterial.

Dirac points (DPs) and Weyl points (WPs) have received much attention in photonic crystals (PhCs) and three-dimensional (3D) metamaterials research due to the robust surface states and Fermi arcs. In this work, two pairs of triply degenerate points (TDPs) have been proposed in a 3D metamaterial by breaking the time reversal symmetry (T) with an external magnetic field. Based on these TDPs, two pairs of asymmetric surface states with spin-polarization are revealed, and a topological chiral beam splitter is demonstrated showing the different propagating directions of the right-handed polarization (RCP) and left-handed polarization (LCP) lights. Remarkably, we can achieve unidirectional propagation with RCP or LCP even excited by a linear source owing to the asymmetry surface state. Our work provides a new, to the best of our knowledge, platform to study spin-polarization surface states and the enhanced spin photonic Hall effect in the metamaterials.

Second harmonic generation by matching the phase distributions of topological corner and edge states.

Second harmonic generation (SHG) in topological photonic crystals is chiefly concerned with frequency conversion between the same topological states. However, little attention has been paid to the effect of coupling between different topological states on the SHG. In this study, we propose a method for achieving optimal SHG in a topological cavity by matching the phase distributions of the electric fields of the topological corner state (TCS) and topological edge state (TES). Our results show that the intrinsic efficiency can be improved when the phase distributions of the fundamental wave within the TCS and the second harmonic wave within the TES have the same symmetry. Otherwise, conversion efficiency will be greatly inhibited. With this method, we achieved an optimal intrinsic efficiency of 0.165%. Such a platform may enable the development of integrated nanoscale light sources and on-chip frequency converters.

Multi-topological state via the Brillouin zone overlap for nonlinear frequency conversion.

Multiband topological edge states (TESs) or topological corner states (TCSs) in photonic crystals provide effective ways to manipulate the nonlinear frequency conversions. However, the deliberate design and the limited number of multibands lead to the difficulty of experimental realization of the topological nonlinear frequency conversion or higher harmonic generation. Here, we propose an effective method to achieve multiple TESs and TCSs by combining the Brillouin zones of multiple different systems. It is shown that the spectra of the subsystems disperse into different energy levels due to the inter-system hopping. Based on this approach, we construct a topological photonic crystal based on the Brillouin zone overlapped SSH model, which enables the overlapped TCSs to participate in nonlinear frequency conversion. Our scheme can provide a significant way to realize the topological nonlinear frequency conversion with double resonances or multiple resonances.

Dirac-Weyl semimetal in photonic metacrystals.

Dirac-Weyl semimetal is a novel type of topological phase that features the coexistence of Dirac and Weyl points in momentum space. In this study, a photonic Dirac-Weyl semimetal is proposed by introducing screw rotation symmetries into a spatial inversion symmetry-lacking system. A realistic metacrystal structure is designed for experimental consideration. The screw rotation symmetries are crucial for the existence of Dirac points, whose Z2 topology is revealed by the (010) surface states. Meanwhile, two pairs of ideal Weyl points at the same frequency are protected by D2d point group symmetries. The Dirac points and Weyl points reside in a clean frequency interval. The proposed photonic Dirac-Weyl semimetal provides a versatile platform for exploring the interaction between Dirac and Weyl semimetals and exploiting possible photonic topological devices.

Sensitivity-Tunable Terahertz Liquid/Gas Biosensor Based on Surface Plasmon Resonance with Dirac Semimetal.

In this paper, we study the sensitivity-tunable terahertz (THz) liquid/gas biosensor in a coupling prism-three-dimensional Dirac semimetal (3D DSM) multilayer structure. The high sensitivity of the biosensor originates from the sharp reflected peak caused by surface plasmon resonance (SPR) mode. This structure achieves the tunability of sensitivity due to the fact that the reflectance could be modulated by the Fermi energy of 3D DSM. Besides, it is found that the sensitivity curve depends heavily on the structural parameters of 3D DSM. After parameter optimization, we obtained sensitivity over 100°/RIU for liquid biosensor. We believe this simple structure provides a reference idea for realizing high sensitivity and a tunable biosensor device.

Realization of Ultra-Scaled MoS2 Vertical Diodes via Double-Side Electrodes Lamination.

Schottky diode is the fundamental building blocks for modern electronics and optoelectronics. Reducing the semiconductor layer thickness could shrink the vertical size of a Schottky diode, improving its speed and integration density. Here, we demonstrate a new approach to fabricate a Schottky diode with ultrashort physical length approaching atomic limit. By mechanically laminating prefabricated metal electrodes on both-sides of two-dimensional MoS2, the intrinsic metal-semiconductor interfaces can be well retained. As a result, we demonstrate the thinnest Schottky diode with a length of 2.6 nm and decent rectification behavior. Furthermore, with a diode length smaller than the semiconductor depletion length, the carrier transport mechanisms are investigated and explained by thickness-dependent and temperature-dependent electrical measurements. Our study not only pushes the scaling limit of a Schottky diode but also provides a general double-sided electrodes integration approach for other ultrathin vertical devices.

Low threshold optical bistability based on topological edge state in photonic crystal heterostructure with Dirac semimetal.

The special band structure of three-dimensional Dirac semimetal (3D DSM) makes it show strong nonlinear optical characteristics in the terahertz region, which provides a new way to develop terahertz nonlinear devices with low threshold. In this paper, we theoretically study the optical bistability (OB) of transmitted light in a multilayer structure with 3D DSM embedded in two one-dimensional photonic crystals (1D PhC). The topological edge state (TES) excited by the 1D PhC heterostructure significantly enhances the local electric field near the nonlinear 3D DSM, which provides a positive condition for the realization of low threshold OB. Through parameter optimization, we obtain a threshold electric field with an incident electric field of 106 V/m levels. Furthermore, the influences of the Fermi energy and thickness of 3D DSM and the angle of the incident light on the hysteretic behavior as well as the threshold of OB are clarified. 3D DSM-based optical devices with intrinsic OB provide a building block for future integrated optical and all-optical networks.

Higher-order nodal ring photonic semimetal.

The intriguing discovery of higher-order topology has tremendously promoted the development of topological physics. Three-dimensional topological semimetals have emerged as an ideal platform for investigating novel topological phases. Consequently, new proposals have been theoretically revealed and experimentally realized. However, most existing schemes are implemented on the acoustic system, while similar concepts are rarely launched in photonic crystals due to the complicated optical manipulation and geometrical design. In this Letter, we propose a higher-order nodal ring semimetal protected by C2 symmetry originating from C6 symmetry. The higher-order nodal ring is predicted in three-dimensional momentum space with desired hinge arcs connected by two nodal rings. Fermi arcs and topological hinge modes generate significant marks in higher-order topological semimetals. Our work successfully proves the presence of a novel higher-order topological phase in photonic systems that we will strive to apply practically in high-performance photonic devices.

Simultaneous slow light and sound rainbow trapping in phoxonic crystals.

In this paper, we use a phoxonic crystal (PxC) which can control the topological states of light and sound by breaking inversion symmetry and thus make it possible to achieve rainbow trapping of light and sound simultaneously. It is shown that topologically protected edge states can be obtained at the interfaces between PxCs with different topological phases. Thus, we designed a gradient structure to realize the topological rainbow trapping of light and sound by linearly modulating the structural parameter. In the proposed gradient structure, the edge states of light and sound modes with different frequencies are respectively trapped at different positions, owing to near zero group velocity. The topological rainbows of light and sound are simultaneously realized in one structure, which open a new, to the best of our knowledge, view and provide a feasible platform for the application of the topological optomechanical devices.

Lossy-mode-resonance sensor based on perovskite nanomaterial with high sensitivity.

Lossy-mode-resonance (LMR) is a surface plasmon resonance (SPR)-analogue optical phenomenon, which is sensitive to the surrounding environment variations and can be considered as an important detection signal in biochemical sensors. Compared with the SPR sensor which can only operate under transverse magnetic (TM)-polarized light, the LMR sensor shows a more excellent application prospect and can operate in both TM- and transverse electric (TE)-polarized light. In this work, a CH3NH3PbBr3-based LMR configuration is proposed to apply in optical sensors. When the incident light is in TM mode, the preferred way to improve the performance of the LMR sensor is optimizing the thickness of the matching layer, and the highest sensitivity of 11382 refractive index unit (RIU-1) is achieved, which is more than 200 times larger than that of the conventional Au-based SPR sensor; when the incident light is in TE mode, it is more advantageous to improve the properties of LMR sensor by optimizing the thickness of CH3NH3PbBr3 layer, and a high sensitivity of 21697 RIU-1 is obtained. With such high sensitivity, we believe that the CH3NH3PbBr3-based LMR sensor will find potential applications in biology, medicine, chemistry and other fields.

Enhanced spin Hall effect due to the redshift gaps of photonic hypercrystals.

We proposed a method for enhancing the spin Hall effect (SHE) of light in the photonic hypercrystal (PHC). PHC is a periodic structure that combines the properties of hyperbolic metamaterials (HMMs) and conventional one-dimensional-photonic crystals (1DPCs). The proposed PHC is composed of Ti3O5 and HMMs, which alternatively consist of Ag and Ti3O5. The giant ratio of reflection coefficients of TE/TM polarizations can be realized due to the redshift gaps of the PHCs, where the band edge of TE polarization shifts toward short wavelengths but the band edge of TM polarization moves toward long wavelengths. It will eventually lead to the enhancement of SHE in this PHC with the redshift gaps. The maximum transverse shift can be close to 15 µm with the optimum thickness and incident angle. The enhancing SHE provides us an opportunity to expand the corresponding applications in the field of optics.

Dynamically reconfigurable topological states in photonic crystals with liquid crystals.

Dynamically tunable and reconfigurable topological states are realized in higher-order topological insulators with the liquid crystal (LC). By changing the loading voltage of the LC, the eigenfrequency of the edge and corner states can be tuned, but even more important is that the edge state and corner state with the same frequency are realized. Based on this reconfigurability of topological states, optical routers and lasers with multiple topological states can be realized. Our results may be applied to topological optical circuits and provide new ideas for optical field localization and manipulation.

Spin-orbit interactions in a nonlinear medium due to a nonlinear-induced geometric phase.

In general, a spin-polarized light beam cannot couple its spin angular momentum (SAM) with intrinsic orbital angular momentum (IOAM) without spin reversal. Here we find that nonlinear media can give the spin-polarized photon an IOAM, as they travel in the media due to the nonlinear susceptibility along the transmission direction, which does not require spin reversal. To characterize this SAM-to-IOAM conversion process, we establish an evolution ray equation for photons carrying IOAM by reference to the Schrödinger equation. We further reveal the inherent physics of such a phenomenon from a full-wave perspective and find that the vortex generation originates from the nonlinear-induced geometric phase.

Fragile topology in double-site honeycomb lattice photonic crystal.

Fragile topology (FT) opens a new direction in topological photonics, but a new type of photonic crystal (PC) with FT remains to be proposed. In this Letter, the double-site honeycomb lattice (DSHL) PC is proposed by rotating the double dielectric rods (DDR) six times, forming unit cell, and then arraying the unit cells in a triangular lattice. Quantum spin Hall effect occurs by manipulating the DDR in the tangential and radial directions of the unit cell. First, the band structures of DSHL PCs with different structural parameters are calculated, and the laws of topological phase transition are analyzed statistically. Then, to prove the FT properties of two groups of topological nontrivial DSHL PCs, the Wannier-center positions of the bulk bands are calculated by the Wilson-Loop method. Finally, the topological edge states and two groups of topological corner states, which are in the same bulk-state bandgap, are realized successfully. The DSHL PC provides good platforms for both the research of topological photonics and the device design and application, which has a broad prospect.

Recent Advances of Spatial Self-Phase Modulation in 2D Materials and Passive Photonic Device Applications.

Optical nonlinearity in 2D materials excited by spatial Gaussian laser beam is a novel and peculiar optical phenomenon, which exhibits many novel and interesting applications in optical nonlinear devices. Passive photonic devices, such as optical switches, optical logical gates, photonic diodes, and optical modulators, are the key compositions in the future all-optical signal-processing technologies. Passive photonic devices using 2D materials to achieve the device functionality have attracted widespread concern in the past decade. In this Review, an overview of the spatial self-phase modulation (SSPM) in 2D materials is summarized, including the operating mechanism, optical parameter measurement, and tuning for 2D materials, and applications in photonic devices. Moreover, some current challenges are also proposed to solve, and some possible applications of SSPM method are predicted for the future. Therefore, it is anticipated that this summary can contribute to the application of 2D material-based spatial effect in all-optical signal-processing technologies.