Janus and Huygens Dipoles: Near-Field Directionality Beyond Spin-Momentum Locking

Abstract

Unidirectional scattering from circularly polarized dipoles has been demonstrated in near-field optics, where the quantum spin-Hall effect of light translates into spin-momentum locking. By considering the whole electromagnetic field, instead of its spin component alone, near-field directionality can be achieved beyond spin-momentum locking. This unveils the existence of the Janus dipole, with side-dependent topologically protected coupling to waveguides, and reveals the near-field directionality of Huygens dipoles, generalizing Kerker's condition. Circular dipoles, together with Huygens and Janus sources, form the complete set of all possible directional dipolar sources in the far- and near-field. This allows the designing of directional emission, scattering, and waveguiding, fundamental for quantum optical technology, integrated nanophotonics, and new metasurface designs.

Not every dipole is the same: the hidden patterns of dipolar near fields

“…3 arise as solutions from one same equation, describing dipole directionality in a simplified planar waveguide scenario [14]. In fact, any linear combination of the three elemental sources results in an infinite range of possible directional dipole sources.…”

“…Both circular and Huygens' dipole appear as solutions of a single equation, yet a third solution completes the set [14]. This novel dipole greatly resembles the Huygens' dipole, in that it too is a combination of linearly polarized and orthogonal dipoles, but its electric and magnetic dipoles are in quadrature phase, that is, with a π/2 phase difference between dipole moment amplitudes.…”

“…By combining electric and magnetic dipoles together, new directional dipolar source behaviours are revealed [14]. The idea of combining electric and magnetic dipoles to obtain distinct functionality is not new.…”

“…This has been used to create engineered surfaces with perfect reflection, and all-dielectric mirrors among other applications [17]. It turns out that the same phenomenon exists in the dipole's near-field [14]. The Huygens dipole exhibits a different symmetry of near-field excitation compared to the circular one: when sandwiched between waveguides, it couples to the modes in the waveguides along the same direction for both (Fig.…”

“…This happens because the electric field of the waveguided mode indeed has a near circular polarization ≈ (1, ±i, 0) whose handedness depends on its propagation direction, a phenomenon named spinmomentum locking [12], [13]. In a more general case, the expression p* • E + m* • B is composed of the sum of six different terms, corresponding to how each component of the dipole excites each mode: this sum may easily interfere destructively for certain dipoles and waveguide modes [14]. This explanation gives an equal focus to both the dipoles and the fields of the specific waveguide, betraying the fact that the directionality is a universal property of the dipole, valid for any waveguide.…”