| Artificially structured electromagnetic materials do not exist in nature and exhibit many remarkable properties conventional materials do not possess which has extensively attracted scientists' attention.These artificial materials show unique application potential in more and more fields,including space communication,electromagnetic stealth technology and so on.Most of these applications are based on their unique ability to manipulate light waves.Here,we will design artificial electromagnetic materials to control the flow of electromagnetic waves based on two-dimensional electromagnetic Helmholtz cavity.We study various electromagnetic modes in a Helmholtz cavity with a perfect electrical conductor(PEC)wall and design a metagraing enabling retroreflection based on Mie scattering theory.First,we study the feature of several whispering-gallery modes after their excitation wavelengths are given based on a rigorous dual series approach and analyze the dependence of the electromagnetic modes on angle of incidence and angular width of the opening.It is found that the WGM is very sensitive to any minor changes of the incident wavelength or angular width of the opening,but is insensitive to variance of angle of incidence within a definite range.Then we find a magnetic symmetric dipole mode(MSDM)in Helmholtz cavities,and investigate the formation mechanism of the mode in physics.Retroreflection can be achieved in a metagrating constructed by a linear array of these Helmholtz cavities based on the MSDM.The retroreflection shows the rather robust behavior to the orientation angle of the Helmholtz cavities and the high retroreflectance can be achieved in a wide wavelength range.The Helmholtz cavities with finite wall thickness support the MSDMs of the same quality as infinite thin one when the interior hollow region is not changed.The metagrating allows the realization of a high-efficiency and broadband retroreflector with low fabrication demand. |
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