“…[630,633,634] Although topology has been well investigated in various photonic system, such as, photonic crystals, [636][637][638][639] optical waveguides, [640] metamaterials, [628,632] particles, [641] optical cavities, [642] coupled resonators, [643] silicon photonics and optomechanics, [630] 3D TPL printing enable topological photonics exotic and unique properties, which is hard to realize in 2D system. [632,633,[644][645][646][647][648][649][650][651][652][653][654][655][656][657] Extending topological phenomenon from 2D photonic crystals to 3D version, charge-1 and charge-2 Weyl points are observed experimentally in direct printed photonic crystals (Figure 13f and g) [658] or coated 3D printed photonic crystals (Figure 13h). [659] Because of the mathematical analogy between the paraxial approximation of the Helmholtz equation and the Schrodinger equation, [630,633] photonic waveguide system can be used to examine Floquet topological phenomenon, [660,661] by introducing artificial gauge fields (AGFs) and treating the waveguide axis z as time varying dimension (Figure 13i).…”