Wave and defect dynamics in nonlinear photonic quasicrystals

Quasicrystals are unique structures with long-range order but no periodicity. Their properties have intrigued scientists ever since their discovery¹ and initial theoretical analysis^2,3. The lack of periodicity excludes the possibility of describing quasicrystal structures with well-established analytical tools, including common notions like Brillouin zones and Bloch's theorem. New and unique features such as fractal-like band structures^4-7 and 'phason' degrees of freedom⁸ are introduced. In general, it is very difficult to directly observe the evolution of electronic waves in solid-state atomic quasicrystals, or the dynamics of the structure itself. Here we use optical induction^9-11 to create two-dimensional photonic quasicrystals, whose macroscopic nature allows us to explore wave transport phenomena. We demonstrate that light launched at different quasicrystal sites travels through the lattice in a way equivalent to quantum tunnelling of electrons in a quasiperiodic potential. At high intensity, lattice solitons are formed. Finally, we directly observe dislocation dynamics when crystal sites are allowed to interact with each other. Our experimental results apply not only to photonics, but also to other quasiperiodic systems such as matter waves in quasiperiodic traps¹², generic pattern-forming systems as in parametrically excited surface waves¹³, liquid quasicrystals¹⁴, and the more familiar atomic quasicrystals.