Displacement-mediated bound states in the continuum in all-dielectric superlattice metasurfaces

Bound states in the continuum (BICs) are localized states coexisting with extended waves inside the continuous spectrum range, which have infinite lifetimes without any radiation. To extract high-Q quasi-BIC resonances from the symmetry-protected BIC for practical applications, symmetry-breaking approaches are usually exploited, either by slightly breaking the excitation field symmetry or structure symmetry. Here, we introduce an all-dielectric superlattice metasurface that can symmetry-compatibly convert BIC states into high-Q quasi-BIC modes based on the guided-mode resonance coupling by relative displacement tuning. The metasurface is composed of a superlattice of multiple nanobeams, supporting both magnetic mode and toroidal mode with large tunability. Both modes can interact with the incident continuum by mediating the displacement between nanobeams, which empowers dual asymmetric Fano resonances with high Q-factors. The bandwidth of the toroidal mode under y-polarized incidences and that of the magnetic mode under x-polarized incidences can be readily tuned by the local displacement between nanobeams in each unit cell. Such displacement-mediated BIC resonance is promising for various applications such as bio-molecule sensing and low threshold lasing.

Displacement-mediated bound states in the continuum in all-dielectric superlattice metasurfaces

Bound states in the continuum (BICs) are localized states coexisting with extended waves inside the continuous spectrum range, which have infinite lifetimes without any radiation. To extract high-Q quasi-BIC resonances from the symmetry-protected BIC for practical applications, symmetry-breaking approaches are usually exploited, either by slightly breaking the excitation field symmetry or structure symmetry. Here, we introduce an all-dielectric superlattice metasurface that can symmetry-compatibly convert BIC states into high-Q quasi-BIC modes based on the guided-mode resonance coupling by relative displacement tuning. The metasurface is composed of a superlattice of multiple nanobeams, supporting both magnetic mode and toroidal mode with large tunability. Both modes can interact with the incident continuum by mediating the displacement between nanobeams, which empowers dual asymmetric Fano resonances with high Q-factors. The bandwidth of the toroidal mode under TE-polarized incidences and that of the magnetic mode under TM-polarized incidences can be readily tuned by the local displacement between nanobeams in each unit cell. Such displacement-mediated BIC resonance is promising for various applications such as bio-molecule sensing and low threshold lasing.

Morphology Manipulation and Upconversion Luminescence Enhancement of β-NaLuF4: Er3+ Hexagonal Microtubes via Sr2+ Doping

A new and facile strategy to enhance the upconversion luminescence (UCL) emission of NaLuF4: Er3+ microcrystals (MCs) using strontium (Sr) as a dopant has been reported. With the introduction of Sr2+, the products change from long NaLuF4: Er3+ hexagonal microtubes to short hexagonal microtubes and finally to hexagonal microprisms. The growth mechanism is profoundly discussed according to the different reaction time-dependent morphologies. More importantly, the total fluorescence intensity is significantly reinforced by doping Sr2+ ions. When 18% Sr2+ is doped into NaLuF4: Er3+ hexagonal microtubes, the maximum green and red luminescence intensities are about 5.8 and 4.4 times higher than those of Sr2+-free samples, respectively. The influences of Sr2+ ion doping content on the phase, the morphology, and the local crystal field symmetry of the as-synthesized NaLuF4 crystals are investigated.

On the Use of a New Class of Simplified Multi-Window Iris Notch in the Design of Ultra-Compact High-Rejection Waveguide Filters for Satellite Links

This work introduces a simplified multi-aperture iris notch suitable for designing waveguide filters having an extremely improved compactness/rejection ratio, regarding available solutions, and adequate pass-band performances. The proposed iris architecture, analyzed for the first time, exhibits a unique transmission zero in the waveguide mono-mode bandwidth which can be easily located below or above the pass-band. The frequency of this transmission zero is evaluated in terms of the iris dimensions thus providing useful guidelines for designing filters with suitable responses. As a consequence of this simplified topology, any designed filter can be easily manufactured by cutting along its E-field symmetry plane. This strategy greatly improves the filter’s insertion loss regarding classical implementations based on more complicated arrangements with piled thin metallic sheets. Two exemplary filters have been designed and tested to be used in a high-performance X-band SATCOM terminal with an 80% size reduction with respect to the existing systems. Both filters covering the Rx (7.25–75 GHz) and Tx (7.9–8.4 GHz) sub-bands show a reflection of −25 dB with insertion losses below 1 dB in the pass-band, whereas they present a very sharp out-of-band rejection of at least 90 dB, that is, a 600 dB/GHz slope at X band.

Symmetry selective directionality in near-field acoustics

Understanding unidirectional and topological wave phenomena requires the unveiling of intrinsic geometry and symmetry for wave dynamics. This is essential yet challenging for the flexible control of near-field evanescent waves, highly desirable in broad practical scenarios ranging from information communication to energy radiation. However, exploitations of near-field waves are limited by a lack of fundamental understanding about inherent near-field symmetry and directional coupling at sub-wavelengths, especially for longitudinal waves. Here, based on the acoustic wave platform, we show the efficient selective couplings enabled by near-field symmetry properties. Based on the inherent symmetry properties of three geometrically orthogonal vectors in near-field acoustics, we successfully realize acoustic Janus, Huygens, spin sources and quadrupole hybrid sources, respectively. Moreover, we experimentally demonstrate fertile symmetry selective directionality of those evanescent modes, supported by two opposite meta-surfaces. The symmetry properties of the near-field acoustic spin angular momenta are revealed by directly measuring local vectorial fields. Our findings advance the understanding of symmetries in near-field physics, supply feasible approaches for directional couplings, and pave the way for promising acoustic devices in the future.

Impact of Ag2O Content on the Optical and Spectroscopic Properties of Fluoro-Phosphate Glasses

Glasses with the system (84.60-x) NaPO3-5 ZnO-(9.40-x) NaF-x Ag2O-1 Er2O3, (x = 0, 2, 4, and 6) (mol%) were synthesized by the conventional melt-quenching method. The impact of the addition of Ag2O on the physical, thermal, structural, and optical properties of the glasses is discussed. The Judd-Oflet analysis was used to evaluate the radiative properties of the emission transitions of the glasses. The enhancement of luminescence properties due to Ag2O is discussed in terms of consequent changes in the local electromagnetic field, symmetry, and the ligand field around the Er3+ ion. The heat treatment of the glass was performed in order to precipitate Ag nanoparticles (NPs), which form as a layer at the surface of the heat-treated glasses as confirmed using scanning electron microscopy (SEM). The Ag NPs were found to increase the intensity of the emission at 1.5 µm.