Figure 1.
Panel (a) schematizes the spin–orbit coupling mechanism at play through a planar chiral polymer system, here conceptually represented by a 2D spiral. Planar (2D) chirality (in its most general form, that is without any rotational invariance, see ref (1)) is characterized by circular polarization conversions that depend on the direction of the probe beam transmitted through the spiral. Panel (b) illustrates the breaking of left- vs right-handed polarization in a Fabry–Perot cavity composed of two usual metallic mirrors but enclosing a 2D chiral medium. Panel (c) describes how a planar chiral system viewed under oblique illumination yields signatures of 3D chirality (i.e., circular dichroism). Two opposite ± θ oblique illumination angles are connected by a simple mirror symmetry in the (x, y) plane and this corresponds to the sequence of transformations detailed as the succession of a flip (a C2 rotation along the y-axis) and of a mirror reflection with respect to the (x, z) plane. The result of this sequence is to show, as detailed in the main text, that the optical activity associated with this extrinsic 3D chirality induced on the planar chiral system at oblique illumination is reversed for opposite incidence angles ± θ.