TY - JOUR

T1 - Momentum-space geometric structure of helical evanescent waves and its implications on near-field directionality

AU - Wei, Lei

AU - Rodriguez Fortuno, Francisco Jose

PY - 2020/1/7

Y1 - 2020/1/7

N2 - In this paper, a momentum-space geometrical structure in helical evanescent electromagnetic waves is revealed. It is shown that for every helical evanescent wave on a helicity-dependent half tangent line in momentum space, the orientation of each of its field, spin, and Poynting vectors is the same. This geometric structure is revealed in a remarkable relation between the far-field and near-field components of the angular spectrum. Any general evanescent wave vector is linked to two points on the kρ=k0 circle of propagating wave vectors via two helicity-dependent tangent lines. Knowing the field of a general dipolar source on the kρ=k0 circle is sufficient to determine its entire evanescent angular spectrum. By applying this concept, we gain insights into near-field directionality by showing that every zero in the angular spectrum is a helicity singularity where two half tangent lines of opposite helicity intersect. A powerful method for the synthetic design of near-field directional sources is also devised, using structured helical illumination to gain full control of the near-field directionality. The results provide fundamental insight into helical evanescent waves and have implications in areas where chiral light-matter interaction plays a central role.

AB - In this paper, a momentum-space geometrical structure in helical evanescent electromagnetic waves is revealed. It is shown that for every helical evanescent wave on a helicity-dependent half tangent line in momentum space, the orientation of each of its field, spin, and Poynting vectors is the same. This geometric structure is revealed in a remarkable relation between the far-field and near-field components of the angular spectrum. Any general evanescent wave vector is linked to two points on the kρ=k0 circle of propagating wave vectors via two helicity-dependent tangent lines. Knowing the field of a general dipolar source on the kρ=k0 circle is sufficient to determine its entire evanescent angular spectrum. By applying this concept, we gain insights into near-field directionality by showing that every zero in the angular spectrum is a helicity singularity where two half tangent lines of opposite helicity intersect. A powerful method for the synthetic design of near-field directional sources is also devised, using structured helical illumination to gain full control of the near-field directionality. The results provide fundamental insight into helical evanescent waves and have implications in areas where chiral light-matter interaction plays a central role.

UR - http://www.scopus.com/inward/record.url?scp=85078150140&partnerID=8YFLogxK

U2 - doi:10.1103/PhysRevApplied.13.014008

DO - doi:10.1103/PhysRevApplied.13.014008

M3 - Article

VL - 13

JO - PHYSICAL REVIEW APPLIED

JF - PHYSICAL REVIEW APPLIED

IS - 1

M1 - 014008

ER -