**3. Discussion**

None of the three optical symmetry planes is directly related to the geometric mirror plane of the helix. However, Figure 3b enables the comparison of geometric and optical symmetries. In order to further analyze the optical response, we show the wavelength-dependent classification of the symmetry planes on top of Figure 4. The three classes correspond to sharply separated wavelength ranges: Class 1 is valid for *λ* ∈ [550, 680] nm. For larger wavelengths *λ* ∈ [680, 1025] nm, the *T*-matrix possesses the symmetry according to planes of class 2. Finally, in the long wavelength regime (*λ* ∈ [1025, 2050] nm), the symmetry is in class 3.

**Figure 4.** Wavelength-dependent classification of symmetry planes of *T*-matrix (top). Absolute value of averaged diagonal *T*-matrix entries corresponding to induced electric dipoles (solid), magnetic dipoles (dashed) and electric quadrupoles (dotted). The classes 3 (green), 2 (red) and 1 (blue) belong to decreasing wavelengths. Changes in symmetry of *T* are due to higher order multipoles.

The analysis in Figure 3b suggests that class 3 (green) is the closest one to the geometric mirror plane. This is further strengthened by the full angular spectrum of the optical chirality coefficient *χ*TT (see Supplementary Materials). Accordingly, we find that the optical response is dominated by the geometric shape for long wavelengths. Obviously, the optics is dominated by the electric dipole moment in this regime which is also shown in Figure 4. Here, the mean of the diagonal entries of submatrices of the *T*-matrix are shown. These are proportional to the electric and magnetic dipole moments as well as to the electric quadrupole moments.

The three symmetry classes of the *T*-matrix occur close to three electric dipole peaks (*λ* = 623, 833, and 1473 nm) and are influenced by the anisotropy of the *T*-matrix. Truly chiral behaviour, as observed here, however, originates not from anisotropy but from coupling between electric and magnetic multipoles [26]. In Supplementary Materials, we elaborate on the complex interplay between these different contributions in the dipolar limit. Here, we limit the discussion to the main aspects of different multipolar contributions.

For large wavelengths with symmetry of class 3, the electric dipoles are much larger then any other induced multipole. In the intermediated regime of symmetry class 2, the magnetic dipole moment significantly increases. For short wavelengths with planes of class 1, the electric quadrupole moment is stronger than the magnetic dipole moment which yields the change in the optical symmetry. Higher order multipoles including mixed electric-magnetic moments are depicted in Supplementary Materials, in which it is shown that the dominant dipolar moments contribute additionally to the

variation of mirror planes. This elaborated study of multipolar resonances underlines again that the chiral response deviates from expectations due to a purely geometrical analysis of the scatterer.
