Pseudo-anapole regime in terahertz metasurfaces

We present the numerical, theoretical, and experimental study of a terahertz metasurface supporting a pseudo-anapole. Pseudo-anapole effect arises when electric and toroidal dipole moments both tend to a minimum, instead of destructive interference between electric and toroidal dipole moments in conventional anapole mode. Such overlap allows resonance suppression of electric type radiation. Thus, it becomes possible to study the multipoles of other families and higher order excitations. We estimate multipoles contribution to the metasurface response via the multipole expansion method. The series is extended with such terms as mean-square radii and multipoles interference. We also study the metasurface geometrical tunability. Via scaling, we demonstrate that it is possible to control the metasurface toroidal and electric responses independently. This in turn proves the fact that these multipoles have different physical origin. Moreover, we demonstrate that proposed metasurface allows excitation of coherent magnetic dipole and electric quadrupole modes which is crucial for planar cavities and lasing spasers in nanophotonics.