  Optical activity is the ability of chiral materials to rotate
linearly-polarized (LP) electromagnetic waves. Because of their intrinsic
asymmetry, traditional chiral molecules usually lack isotropic performance, or
at best only possess a weak form of chirality. Here we introduce a knotted
chiral meta-molecule that exhibits optical activity corresponding to a 90{\deg}
polarization rotation of the incident waves. More importantly, arising from the
continuous multi-fold rotational symmetry of the chiral torus knot structure,
the observed polarization rotation behavior is found to be independent of how
the incident wave is polarized. In other words, the proposed chiral knot
structure possesses two-dimensional (2-D) isotropic optical activity as
illustrated in Fig. 1, which has been experimentally validated in the microwave
spectrum. The proposed chiral torus knot represents the most optically active
meta-molecule reported to date that is intrinsically isotropic to the incident
polarization.
