Nonreciprocal toroidal dipole resonance and one-way quasi-bound state in the continuum.

Bound states in the continuum (BICs) provide an alternative way of trapping light at nanoscale. Although the last 10 years have witnessed tremendous progress on BICs from fundamentals to applications, nonreciprocal BICs and their potential applications have not been fully exploited yet. In this study, we demonstrated a one-way quasi-BIC by leveraging an all-dielectric magneto-optical (MO) metasurface. We show that the key point for achieving a one-way quasi-BIC is to excite a magnetization-induced leaky resonance. Here we adopt the longitudinal toroidal dipole (TD) resonance characterized by a vortex distribution of head-to-tail magnetic dipoles parallel to the plane of the MO metasurface. We show that, by breaking the time-reversal symmetry, at critical conditions, the TD resonance can be enhanced in the forward channel and perfectly canceled in the time-reversed channel, resulting in a one-way quasi-BIC. The demonstrated phenomena hold significant promise for practical applications such as magnetic field optical sensing, nonreciprocal optical switching, isolation, and modulation.

Polarization resonance with sub-wavelength switching of random light self-interfering in open-end cavity.

Following recent work [Sci. China Phys. Mech. Astron.66, 274213 (2023)10.1007/s11433-023-2097-9] that revealed the sub-wavelength scale resonance phenomenon in scalar random beams counterpropagating in an open-end cavity, we extend the analysis to the vectorial domain and show a similar effect for the polarization properties. We found that, in contrast with the changes in the scalar properties, being of harmonic nature, changes in polarization involve alternating regions of constant values followed by sharp and complex changes.

Enhancing Faraday and Kerr rotations based on the toroidal dipole mode in an all-dielectric magneto-optical metasurface.

The magneto-optical Faraday and Kerr effects are widely used in modern optical devices. In this Letter, we propose an all-dielectric metasurface composed of perforated magneto-optical thin films, which can support the highly confined toroidal dipole resonance and provide full overlap between the localized electromagnetic field and the thin film, and consequently enhance the magneto-optical effects to an unprecedented degree. The numerical results based on the finite element method show that the Faraday and Kerr rotations can reach -13.59° and 8.19° in the vicinity of toroidal dipole resonance, which are 21.2 and 32.8 times stronger than those in the equivalent thickness of thin films. In addition, we design an environment refractive index sensor based on the resonantly enhanced Faraday and Kerr rotations, with sensitivities of 62.96 nm/RIU and 73.16 nm/RIU, and the corresponding maximum figures of merit 132.22°/RIU and 429.45°/RIU, respectively. This work provides a new, to the best of our knowledge, strategy for enhancing the magneto-optical effects at nanoscale, and paves the way for the research and development of magneto-optical metadevices such as sensors, memories, and circuits.

Compact topological polarization beam splitter based on all-dielectric fishnet photonic crystals.

Conventional polarization beam splitters (PBSs) suffer energy loss and signal distortion due to backscattering caused by disturbances. Topological photonic crystals provide backscattering immunity and anti-disturbance robustness transmission owing to the topological edge states. Here, we put forward a kind of dual-polarization air hole-type fishnet valley photonic crystal with a common bandgap (CBG). The Dirac points at the K point formed by different neighboring bands for transverse magnetic and transverse electric polarizations are drawn closer via changing the filling ratio of the scatterer. Then the CBG is constructed by lifting the Dirac cones for dual polarizations within a same frequency range. We further design a topological PBS using the proposed CBG via changing the effective refractive index at the interfaces which guide polarization-dependent edge modes. Based on these tunable edge states, the designed topological PBS (TPBS) achieves efficient polarization separation and is robust against sharp bends and defects, verified by simulation results. The TPBS's footprint is approximately 22.4 × 15.2 µ m 2, allowing high-density on-chip integration. Our work has potential application in photonic integrated circuits and optical communication systems.

Dirac cones with zero refractive indices in phoxonic crystals.

In this paper, simultaneous zero refractive indices (ZRIs) for both sound and light are realized on the basis of a 2D triangular lattice phoxonic crystal (PxC) with C6v symmetry. For the phononic mode, accidental phononic Dirac degeneracy at the center of Brillouin zone (BZ) occurs at a relatively high frequency which leads to the failure of the efficient medium theory; hence, it is no longer applicable to the realization of acoustic ZRI. We thus turn to a low-frequency phononic Dirac cone located at K point, the corner of the BZ, which shows in-phase pressure field oscillations in expanded unit cells. Using zone folding, we further reveal the cause for the characteristic of acoustic ZRI. For the photonic mode, a low-frequency photonic Dirac-like cone can be achieved by adjusting the geometric parameter due to the high contrast permittivity between scatterers and the matrix. When the phononic and photonic low-frequency Dirac dispersions coexist, the PxC can be mapped into a zero-index material for both sound and light at the same time. The new mechanism for simultaneously controlling sound and light helps to achieve acousto-optic synchronous cloaking and unidirectional transmission, which are numerically demonstrated.

Manipulating the optical beam width in topological pseudospin-dependent waveguides using all-dielectric photonic crystals.

We propose a width-tunable topological pseudospin-dependent waveguide (TPDW) which can manipulate the optical beam width using a heterostructure of all-dielectric photonic crystals (PhCs). The heterostructure can be realized by introducing a PhC featuring double Dirac cones into the other two PhCs with different topological indices. The topological pseudospin-dependent waveguide states (TPDWSs) achieved from the TPDW exhibit unidirectional transport and immunity against defects. As a potential application of our work, using these characteristics of TPDWSs, we further design a topological pseudospin-dependent beam expander which can expand a narrow beam into a wider one at the communication wavelength of 1.55 µm and is robust against three kinds of defects. The proposed TPDW with widely adjustable width can better dock with other devices to achieve stable and efficient transmission of light. Meanwhile, all-dielectric PhCs have negligible losses at optical wavelengths, which provides the prospect of broad application in photonic integrated devices.

Angle-selective chiral absorption induced by diffractive coupling in metasurfaces.

Here we report that a simple chiral metasurface with twisted metallic cut-wire arrays enables highly efficient and continuously tunable chiral absorption over a broad spectral range by scanning the incidence angle over a few degrees. The angle-selective chiral absorption results from the surface plasmon resonance (SPR) excited by diffractive effects of the metasurface. The diffraction-assisted chiral metasurface provides a straightforward strategy for achieving dynamically tunable chiral devices and offers intriguing possibilities for various applications in on-chip chiral detectors/emitters, chiral spectrometers, chiral lasers, and so on.

Enhancement of lateral Casimir force on a rotating particle near hyperbolic metamaterial.

Enhancement of weak Casimir forces is extremely important for their practical detection and subsequent applications in variety of scientific and technological fields. We study the lateral Casimir forces acting on the rotating particles with small radius of 50 nm as well as that with large radius of 500 nm near the hyperbolic metamaterial made of silicon carbide (SiC) nanowires. It is found that the lateral Casimir force acting on the small particle of 50 nm near hyperbolic metamaterial with appropriate filling fraction can be enhanced nearly four times comparing with that acting on the same particle near SiC bulk in the previous study. Such enhancement is caused by the coupling between the resonance mode excited by nanoparticle and the hyperbolic mode supported by hyperbolic metamaterial. The results obtained in this study provide an efficient method to enhance the interaction of nanoscale objects.

Mechanical modulation of spontaneous emission of nearby nanostructured black phosphorus.

In this study, we investigate the spontaneous emission of a quantum emitter nearby black phosphorus (BP) sheet. The spontaneous emission can be modulated mechanically by rotating the BP sheet when the quantum emitter is placed parallel to the sheet. The spontaneous emission is dependent on the electron doping and rotation angle of BP with respect to the x-axis. The Purcell factor decreases with the increase in rotation angle under smaller electron doping. The Purcell factor increases with the increase in rotation angle under larger electron doping. The spontaneous emission of quantum emitter nearby two types of BP ribbon arrays tailored along armchair (type I) and zigzag (type II) directions is studied in detail. The spontaneous emission of quantum emitter parallel to type I is enhanced compared with that parallel to BP sheet. The spontaneous emission decreases remarkably for the quantum emitter parallel to type II compared with that parallel to BP sheet. The spontaneous emission can be flexibly modulated by rotating BP ribbon arrays mechanically in two types. The results obtained in this study provide a new method to actively modulate the spontaneous emission.

Strong coupling between excitons and magnetic dipole quasi-bound states in the continuum in WS2-TiO2 hybrid metasurfaces.

Enhancing the light-matter interactions in two-dimensional materials via optical metasurfaces has attracted much attention due to its potential to enable breakthrough in advanced compact photonic and quantum information devices. Here, we theoretically investigate a strong coupling between excitons in monolayer WS2 and quasi-bound states in the continuum (quasi-BIC). In the hybrid structure composed of WS2 coupled with asymmetric titanium dioxide nanobars, a remarkable spectral splitting and typical anticrossing behavior of the Rabi splitting can be observed, and such strong coupling effect can be modulated by shaping the thickness and asymmetry parameter of the proposed metasurfaces, and the angle of incident light. It is found that the balance of line width of the quasi-BIC mode and local electric field enhancement should be considered since both of them affect the strong coupling, which is crucial to the design and optimization of metasurface devices. This work provides a promising way for controlling the light-matter interactions in strong coupling regime and opens the door for the future novel quantum, low-energy, distinctive nanodevices by advanced meta-optical engineering.

Design of phoxonic virtual waveguides for both electromagnetic and elastic waves based on the self-collimation effect: an application to enhance acousto-optic interaction.

The dual beam guides for transverse-electric and transverse-magnetic polarizations of electromagnetic (EM) wave and elastic wave in defect-free phoxonic crystals are reported. The realization for phoxonic virtual waveguides relies on dual flat equifrequency contours (EFCs) enabling self-collimation for EM and elastic waves. As a possible application of our work, the enhanced acousto-optic (AO) interaction in this kind of defect-free phoxonic waveguide, just as it does in defect-based waveguides, is further studied. Results show that obvious shifts of the transmission peaks of EM waves exist for both polarizations during one period of the elastic wave, and single-phonon exchange dominates the AO interaction. This kind of phoxonic virtual waveguide provides an effective platform to enhance AO interaction and exhibits some advantage over defect-based waveguides by properly manipulating the photonic and phononic dispersion surfaces.

Ultra-narrowband perfect absorption of monolayer two-dimensional materials enabled by all-dielectric subwavelength gratings.

Monolayer two-dimensional materials (2DMs) have excellent optical and electrical properties and show great application potential in photodetectors. However, the thickness at the atomic scale leads to weak light absorption, which greatly limits the responsivity of corresponding photodetectors. Here we propose an all-dielectric sub-wavelength zero-contrast grating structure that enables a monolayer of MoS2 with ultra-narrow bandwidth perfect light absorption. The absorption enhancement can be attributed to the critical coupling of guided mode resonances from two specific order diffractions in the structure, as confirmed by the planar waveguide theory and coupled mode theory. Such absorption enhancement can be generalized to any other absorptive atomically thin films, and the wavelength of perfect absorption can be tuned by scaling the dimension of the photonic structure. Our results offer a promising photonic approach to realize ultra-highly sensitive narrow-band photodetectors by using atomically thin materials.

Controlling the spin-selective absorption with two-dimensional chiral plasmonic gratings.

Spin-selective absorption in a two-dimensional (2D) chiral plasmonic grating is observed by excitation of chiral-dependent plasmonic cavity resonance. For the proposed structures, the incident right-handed circularly polarized light is absorbed with nearly 100% efficiency, whereas the incident left-handed circularly polarized light is reflected with same handedness. Moreover, we show that the location of spin-selective absorption can be controlled flexibly by tuning the plasmonic cavity dimension. The intensity of spin-selective absorption can be enhanced as well as suppressed based on Fabry-Perot interference phase relation. Such 2D chiral plasmonic gratings could find many potential applications in novel photon-spin selective devices, such as circularly polarized light detectors/emitters, chiral sensors, chiral cavities, and spin lasers.

Thickness-modulated passivation properties of PEDOT:PSS layers over crystalline silicon wafers in back junction organic/silicon solar cells.

PEDOT: PSS/silicon heterojunction solar cell has recently attracted much attention due to the fact that it can be simply and cost-effectively fabricated. It is crucial to suppress the interfacial recombination rate between silicon (Si) and organic film for improving device efficiency. In this study, we demonstrated a thickness-dependent passivation effect, i.e. the passivation quality over Si substrate was promoted dramatically with increasing the thickness of PEDOT:PSS layer. The effective minority carrier lifetime increased from 32 µs for 50 nm to 360 µs for 200 nm, which corresponds to a change in implied open circuit voltage (V oc-implied) from 545 to 635 mV. Back-junction hybrid solar cells featuring PEDOT:PSS films at rear side were designed to enable adoption of thick PEDOT:PSS layers without having to worry about parasitic absorption, showing a power conversion efficiency (PCE) of 16.3%. Combined with a proper pre-condition on the Si substrate, the back-junction hybrid solar cell with 200 nm PEDOT:PSS layer received an enhanced PCE of 16.8%. In addition, the improved long-term stability for the back-junction device was also observed. The PCE remained 90% (unsealed) after being stored in ambient atmosphere for 30 days and over 80% (sealed) after 150 days.

Heterojunction Hybrid Solar Cells by Formation of Conformal Contacts between PEDOT:PSS and Periodic Silicon Nanopyramid Arrays.

Surface nanotexturing with excellent light-trapping property is expected to significantly increase the conversion efficiency of solar cells. However, limited by the serious surface recombination arising from the greatly enlarged surface area, the silicon (Si) nanotexturing-based solar cells cannot yet achieve satisfactory high efficiency, which is more prominent in organic/Si hybrid solar cells (HSCs) where a uniform polymer layer can rarely be conformably coated on nanotextured substrate. Here, the HSCs featuring advanced surface texture of periodic upright nanopyramid (UNP) arrays and hole-conductive conjugated polymers, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), are investigated. The tetramethylammonium hydroxide etching is used to smooth the surface morphologies of the Si-UNPs, leading to reduced surface defect states. The uniform Si-UNPs together with silane chemical-incorporated PEDOT:PSS solution enable the simultaneous realization of excellent broadband light absorption as well as enhanced electrical contact between the textured Si and the conducting polymer. The resulting PEDOT:PSS/Si HSCs textured with UNP arrays show a promising power conversion efficiency of 13.8%, significantly higher than 12.1% of the cells based on the-state-of-the-art surface texture with random pyramids. These results provide a viable route toward shape-controlled nanotexturing-based high-performance organic/Si HSCs.

Improved optical absorption in visible wavelength range for silicon solar cells via texturing with nanopyramid arrays.

Surface-texture with silicon (Si) nanopyramid arrays has been considered as a promising choice for extremely high performance solar cells due to their excellent anti-reflective effects and inherent low parasitic surface areas. However, the current techniques of fabricating Si nanopyramid arrays are always complicated and cost-ineffective. Here, a high throughput nanosphere patterning method is developed to form periodic upright nanopyramid (UNP) arrays in wafer-scale. A direct comparison with the state-of-the-art texture of random pyramids is demonstrated in optical and electronic properties. In combination with the antireflection effect of a SiNx coating layer, the periodic UNP arrays help to provide a remarkable improvement in short-wavelength response over the random pyramids, attributing to a short-current density gain of 1.35 mA/cm2. The advanced texture of periodic UNP arrays provided in this work shows a huge potential to be integrated into the mass production of high-efficiency Si solar cells.

Simultaneous localization of photons and phonons within the transparency bands of LiNbO3 phoxonic quasicrystals.

We report the properties of dual phononic-photonic band gaps and localized modes of eightfold lithium niobate (LiNbO3) phoxonic quasicrystals (PhXQCs). Complete and large phoxonic band gaps are easily achieved despite the low refractive index of LiNbO3 substrate. Point defect intentionally introduced can form localized modes within both forbidden and transparency bands over a wide range of geometric parameters. Further analysis indicates that the localized modes within transparency bands originate from the intrinsic high-order rotational symmetry of quasiperiodic structures, which resemble whispering gallery modes. LiNbO3 PhXQCs provide a good candidate to enhance phononic-photonic interaction and show considerable advantage over the periodic counterparts.

Simultaneous large band gaps and localization of electromagnetic and elastic waves in defect-free quasicrystals.

We report numerically large and complete photonic and phononic band gaps that simultaneously exist in eight-fold phoxonic quasicrystals (PhXQCs). PhXQCs can possess simultaneous photonic and phononic band gaps over a wide range of geometric parameters. Abundant localized modes can be achieved in defect-free PhXQCs for all photonic and phononic polarizations. These defect-free localized modes exhibit multiform spatial distributions and can confine simultaneously electromagnetic and elastic waves in a large area, thereby providing rich selectivity and enlarging the interaction space of optical and elastic waves. The simulated results based on finite element method show that quasiperiodic structures formed of both solid rods in air and holes in solid materials can simultaneously confine and tailor electromagnetic and elastic waves; these structures showed advantages over the periodic counterparts.

Large-Area Nanosphere Self-Assembly by a Micro-Propulsive Injection Method for High Throughput Periodic Surface Nanotexturing.

A high throughput surface texturing process for optical and optoelectric devices based on a large-area self-assembly of nanospheres via a low-cost micropropulsive injection (MPI) method is presented. The novel MPI process enables the formation of a well-organized monolayer of hexagonally arranged nanosphere arrays (NAs) with tunable periodicity directly on the water surface, which is then transferred onto the preset substrates. This process can readily reach a throughput of 3000 wafers/h, which is compatible with the high volume photovoltaic manufacturing, thereby presenting a highly versatile platform for the fabrication of periodic nanotexturing on device surfaces. Specifically, a double-sided grating texturing with top-sided nanopencils and bottom-sided inverted-nanopyramids is realized in a thin film of crystalline silicon (28 µm in thickness) using chemical etching on the mask of NAs to significantly enhance antireflection and light trapping, resulting in absorptions nearly approaching the Lambertian limit over a broad wavelength range of 375-1000 nm and even surpassing this limit beyond 1000 nm. In addition, it is demonstrated that the NAs can serve as templates for replicas of three-dimensional conformal amorphous silicon films with significantly enhanced light harvesting. The MPI induced self-assembly process may provide a universal and cost-effective solution for boosting light utilization, a problem of crucial importance for ultrathin solar cells.

Three-dimensional power splitter based on self-imaging effect in multimode layer-by-layer photonic crystal waveguides.

The self-imaging effect based on symmetrical interference in multimode layer-by-layer photonic crystal waveguides (PhCWs), is numerically studied with finite-difference time-domain simulations. With the properties of twofold images, a kind of three-dimensional (3D) PhCW-based power splitters with an ultracompact size using complete photonic bandgaps is proposed, calculated, and analyzed. The presented structure can be extended for the design of M×N power splitters for 3D photonic integrated circuits applications.