Vortex-induced quasi-shear polaritons

Hyperbolic shear polaritons(HShPs)emerge with widespread attention as a class of polariton modes with broken symmetry due to shear lattices.We find a mechanism of generating quasi-HShPs(q-HShPs).When utilizing vortex waves as excitation sources of hyperbolic materials without off-diagonal elements,q-HShPs will appear.In addition,these asymmetric q-HShPs can be recovered as symmetric modes away from the source,with a critical transition mode between the left-skewed and right-skewed q-HShPs,via tuning the magnitude of the off-diagonal imaginary component and controlling the topological charge of the vortex source.It is worth mentioning that we explore the influence of parity of topological charges on the field distribution and demonstrate these exotic phenomena from numerical and analytical perspectives.Our results will promote opportunities for both q-HShPs and vortex waves,widening the horizon for various hyperbolic materials based on vortex sources and offering a degree of freedom to control various kinds of polaritons.

Anisotropic Fermat’s principle for controlling hyperbolic van der Waals polaritons

Transformation optics(TO)facilitates flexible designs of spatial modulation of optical materials via coordinate transformations,thus,enabling on-demand manipulations of electromagnetic waves.However,the application of TO theory in control of hyperbolic waves remains elusive due to the spatial metric signature transition from(+,+)to(−,+)of a two-dimensional hyperbolic geometry.Here,we proposed a distinct Pythagorean theorem,which leads to establishing an anisotropic Fermat’s principle.It helps to construct anisotropic geometries and is a powerful tool for manipulating hyperbolic waves at the nanoscale and polaritons.Making use of absolute instruments,the excellent collimating and focusing behaviors of naturally in-plane hyperbolic polaritons in van der Waals𝛼α–MoO_(3)layers are demonstrated,which opens up a new way for polaritons manipulation.

Photonic crystal slabs with maximal chiroptical response empowered by bound states in the continuum

To enhance the strength of chiral light–matter interaction for practical applications,the chirality and quality factors(Q-factors)of current methods need to be strengthened simultaneously.Here,we propose a design of photonic crystal slabs(Ph Cs)supporting chiral bound states in the continuum(BICs)of transverse electric(TE)and transverse magnetic(TM)modes,exhibiting maximal chiroptical responses with high Q-factors and near-unity circular dichroism(CD=0.98).Different from the past,the Ph Cs we employed only have reduced in-plane symmetry and can support simultaneously chiral quasi-BICs(q-BICs)of TE and TM mode with two-dimensional ultra-strong external and internal chirality.Based on the temporal coupled-mode theory,two analytical expressions of CD of chiral q-BICs response are revealed,which are consistent with the simulation results.Furthermore,we elucidate these results within the charge-current multipole expansion framework and demonstrate that the co-excitation of higher-order multipole electric/magnetic modes is responsible for near-perfect CD.Our results may provide more flexible opportunities for various applications requiring high Q-factors and chirality control,such as chiral lasing,chiral sensing,and enantiomer separation.