Geometric Phase in Twisted Topological Complementary Pair.

Geometric phase enabled by spin-orbit coupling has attracted enormous interest in optics over the past few decades. However, it is only applicable to circularly-polarized light and encounters substantial challenges when applied to wave fields lacking the intrinsic spin degree of freedom. Here, a new paradigm is presented for achieving geometric phase by elucidating the concept of topological complementary pair (TCP), which arises from the combination of two compact phase elements possessing opposite intrinsic topological charge. Twisting the TCP leads to the generation of a linearly-varying geometric phase of arbitrary order, which is quantified by the intrinsic topological charge. Notably distinct from the conventional spin-orbit coupling-based theories, the proposed geometric phase is the direct result of the cyclic evolution of orbital-angular-momentum transformation in mode space, thereby exhibiting universality across classical wave systems. As a proof of concept, the existence of this geometric phase is experimentally demonstrated using scalar acoustic waves, showcasing the remarkable ability in the precise manipulation of acoustic waves at subwavelength scales. These findings engender a fresh understanding of wave-matter interaction in compact structures and establish a promising platform for exploring geometric phase, offering significant opportunities for diverse applications in wave systems.

Controlling the directional excitation of surface plasmon polaritons using tunable non-Hermitian metasurfaces.

In this work, we propose an efficient approach to controlling the directional excitation of surface plasmon polaritons (SPPs) by dynamically modulating the real-part perturbation in a passive parity-time symmetric metasurface. This non-Hermitian system can experience two exceptional points that can induce two unidirectional excitation states of SPPs along opposite directions. Empowered by its superior modulation depth, the energy ratio and energy intensities of two excited SPP states can be effectively manipulated by this non-Hermitian metasurface. To demonstrate these findings, we design and numerically verify non-Hermitian metasurfaces integrated with an Sb2Se3 phase-change material. Our work provides a promising platform for the controllable engineering of SPP excitations, holding significant potential for the development of new plasmonic devices, including on-chip SPP sources, routers and sorters, and integrated optical circuits.

Topotactic fabrication of transition metal dichalcogenide superconducting nanocircuits.

Superconducting nanocircuits, which are usually fabricated from superconductor films, are the core of superconducting electronic devices. While emerging transition-metal dichalcogenide superconductors (TMDSCs) with exotic properties show promise for exploiting new superconducting mechanisms and applications, their environmental instability leads to a substantial challenge for the nondestructive preparation of TMDSC nanocircuits. Here, we report a universal strategy to fabricate TMDSC nanopatterns via a topotactic conversion method using prepatterned metals as precursors. Typically, robust NbSe2 meandering nanowires can be controllably manufactured on a wafer scale, by which a superconducting nanowire circuit is principally demonstrated toward potential single photon detection. Moreover, versatile superconducting nanocircuits, e.g., periodical circle/triangle hole arrays and spiral nanowires, can be prepared with selected TMD materials (NbS2, TiSe2, or MoTe2). This work provides a generic approach for fabricating nondestructive TMDSC nanocircuits with precise control, which paves the way for the application of TMDSCs in future electronics.

Tunable dual quasi-bound states in continuum and electromagnetically induced transparency enabled by the broken material symmetry in all-dielectric compound gratings.

Dual quasi-bound states in continuum (quasi-BICs) enabled by the broken geometric symmetry offer an effective way to design high-quality photonic devices, yet challenged by tunable functionalities. Here we employ the material asymmetry originating from the tunable material property of phase-change materials to design quasi-BICs in all-dielectric compound gratings. We find the even and odd quasi-BICs are modulated by the geometric and material asymmetries, respectively, and this effect is ensured by two different types of structural symmetries in the compound structure. Particularly, tunable electromagnetically induced transparency (EIT) can be achieved by modulating the material asymmetry. Furthermore, we systematically design the compound gratings consisting of the phase-change material of Sb2Se3 to demonstrate tunable dual quasi-BICs and EITs. Analytical calculations and numerical simulations are performed to verify these findings. Our work provides a promising way to enhance the flexibility of realizing quasi-BICs, which may boost tunable applications in nanodevices assisted by quasi-BICs.

Reconfigurable chiral exceptional point and tunable non-reciprocity in a non-Hermitian system with phase-change material.

Non-Hermitian optics has emerged as a feasible and versatile platform to explore many extraordinary wave phenomena and novel applications. However, owing to ineluctable systematic errors, the constructed non-Hermitian phenomena could be easily broken, thus leading to a compromising performance in practice. Here we theoretically proposed a dynamically tunable mechanism through GST-based phase-change material (PCM) to achieve a reconfigurable non-Hermitian system, which is robust to access the chiral exceptional point (EP). Assisted by PCM that provides tunable coupling efficiency, the effective Hamiltonian of the studied doubly-coupled-ring-based non-Hermitian system can be effectively modulated to resist the external perturbations, thus enabling the reconfigurable chiral EP and a tunable non-reciprocal transmission. Moreover, such tunable properties are nonvolatile and require no static power consumption. With these superior performances, our findings pave a promising way for reconfigurable non-Hermitian photonic devices, which may find applications in tunable on-chip sensors, isolators and so on.

Enhanced and unidirectional photonic spin Hall effect in a plasmonic metasurface with S4 symmetry.

In this Letter, we report the significant enhancement of the photonic spin Hall effect (SHE) in a plasmonic metasurface with ${\rm S}_4$ symmetry. We find that an enhanced SHE of reflected light can occur in both horizontally and vertically polarized incident beams, and the maximum transverse displacement can approach half of the beam waist. Such a large displacement is caused by the non-resonant and near-zero pseudo-Brewster angles in the plasmonic metasurface. Owing to ${\rm S}_4$ symmetry, a unidirectional SHE is obtained in the metasurface, i.e., large and tiny transverse displacements are realized for a linearly polarized beam incident from the opposite side. This Letter provides a new, to the best of our knowledge, way to achieve an enhanced photonic SHE and offers more opportunities for designing spin-based nanophotonic devices.

Extraordinary wave modes in purely imaginary metamaterials beyond the critical angle.

When waves are incident from a high-index medium to a low one, total reflection occurs commonly for the incidence beyond the critical angle. However, this common sense is broken by a purely imaginary metamaterial (PIM), which also supports a real refraction index yet with pure loss and gain elements in their permittivity and permeability. We find that even beyond the critical angle of a lower-index PIM slab, some extraordinary wave modes including laser, anti-laser, perfect attenuator and perfect amplifier can appear. The general conditions of these wave modes are theoretically given out and the underlying mechanisms are revealed. Also, we study the influence of incident polarizations, geometric thickness and the parameters of the PIM slab on these extraordinary wave modes, with more wave propagation behaviors discovered.

Enhancing the Faraday rotation of monolayer black phosphorus by the optical Tamm state at the photonic crystal interface.

Black phosphorus (BP) is a two-dimensional material with a direct bandgap that exhibits in-plane anisotropy, high charge carrier mobility, and excellent optical properties. It also can demonstrate a strong magneto-optical response under an external magnetic field. In this paper, we present a theoretical study to enhance the Faraday rotation of the monolayer BP by the optical Tamm state at the interface between two photonic crystals. The optical Tamm state can increase the Faraday rotation angle significantly through the localization of the electromagnetic field with high transmittance. When the externally applied magnetic field is 5 T, the gain in the Faraday rotation angle can reach 37.37 dB with a transmittance greater than 65%. The Faraday rotation angle can be adjusted proportionally by the external magnetic field while retaining the high transmittance, and the operating frequency also remains unchanged. In addition, the Faraday rotation angle and operating frequency can be adjusted by changing the carrier density and photonic crystal parameters.

Polarization beam splitter based on extremely anisotropic black phosphorus ribbons.

Properly designed black phosphorus (BP) ribbons exhibit extreme anisotropic properties, which can be used to fabricate a high-efficiency transmitter or reflector depending on the linear polarization of excitation. In this study, we design a highly efficient and broad-angle polarization beam splitter (PBS) based on extremely anisotropic BP ribbons around the mid-infrared frequency region with an ultra-thin structure, and study its performance by using transfer matrix calculation and finite element simulation. In the broad frequency range of 80.4 terahertz - 85.0 terahertz (THz) and an wide angle range of more than 50°, the reflectivity and transmissivity of the designed PBS are both larger than 80% and the polarization extinction ratios are higher than 25.50 dB for s-polarization light and 20.40 dB for p- polarization light, respectively. Furthermore, the effect of incident angle and device parameters on the behavior of the proposed PBS is examined. Finally, we show that the operation frequency of this PBS can be tuned by the electron concentration of BP, which can facilitate some practical applications such as tunable polarization splitters or filters, and mid-infrared sensors.

Tunable THz reflection-type polarizer based on monolayer phosphorene.

We demonstrate a tunable reflection-type polarizer in the terahertz (THz) regime formed by inserting a monolayer phosphorene in a Fabry-Perot cavity composed of a metal substrate and a distributed Bragg reflector. The physical mechanism of the polarizer is analyzed through the reflection spectrum, the electric field distribution, the energy flow, and the power dissipation density calculated by transfer-matrix method and finite-difference time-domain method. The results show that the polarization-selected reflection is caused by the in-plane anisotropic absorption of the phosphorene due to its special atomic lattice, and the polarization selection is further enhanced by the cavity resonator. A polarized reflection light can be obtained with a polarizing extinction ratio of more than 20 dB and a total reflectivity around 50% in the designed THz frequency. The operation frequency of the polarizer can be tuned by the angle of the incident light, the doping electron concentration, and the uniaxial strains of the phosphorene. The refection-type polarizer provides many applications such as filters, detectors, and biosensors.

Designing a nearly perfect infrared absorber in monolayer black phosphorus.

Black phosphorus (BP) is a type of 2D layered material with a direct bandgap that displays high carrier mobility and strong in-plane anisotropy; it also exhibits potential as a promising optoelectronic material for IR applications. In this paper, we propose a nearly perfect IR absorber composed of a metal film, a spacer with a monolayer BP inside, and a distributed Bragg reflector (DBR). The electric field is confined inside the resonator generated by the metal film and DBR, and the absorption can be enhanced up to nearly 100%, owing to the strong interaction of BP with the confined field. Our results show that the absorption performance of the proposed structure is not only critically dependent on the electron density but also relies on the position of the BP within the spacer. This dependence can be mitigated because the absorption peak wavelength can be tuned by adjusting the angle of the light and the parameters of the DBR. Moreover, the absorber can be served as a reflective linear polarizer based on the anisotropic absorption properties. Our work can be helpful in designing a narrow perfect absorber and polarization-sensitive devices for IR waves.

Multiple adjustable optical Tamm states in one-dimensional photonic quasicrystals with predesigned bandgaps.

We proposed an approach to get multiple and adjustable optical Tamm states (OTSs) by constructing a structure consisting of a metal layer and one-dimensional photonic quasicrystals with preassigned bandgaps. In the structure, multiple OTSs excited simultaneously in each bandgap were observed. We explored the physics mechanism of the multiple OTSs by analyzing the electric field intensity distribution in the structure. Besides, the results also show that the thickness of the top layer gives one more degree of freedom in designing multiple OTSs. Finally, we demonstrated that one additional OTS can be obtained independently by adding another bandgap to the proposed structure.