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Mesophotonics have developed a proprietary range of photonic quasicrystals that provide novel functionality for a broad range of applications. Photonic Quasicrystals are similar to Photonic Crystal but rely on a quasicrystal arrangement of air holes. Quasicrystals possess long-range translation order but short range disorder. Photonic Quasicrystals offer predetermined high symmetry orders not achievable in nature for example 8, 12 or 14 and hence offer the following unique features:
So physically how can we differentiate a photonic crystal and a photonic quasicrystal?
In the case of PCs, the structure has short range as well as long range translational order. This translates to a structure having regular arrangement of holes etched into a dielectric structure. In the long range, i.e. when a laser is diffracted off the top surface of the structure a regular arrangement corresponding to the diffraction peaks of the lattice are visible in the far field.
In the case of PQCs, the structure has short range disorder and long range translational order. In this case, when inspected the structure under an SEM does not possess regular arrangements or arrays of holes but comprises of ensembles and complex tilings of holes and hence the term disorder. On the other hand in the long range, the diffraction off the surface of the PQC gives rise to bright Bragg rings in the far field rather than sharp bright spots when compared to PC.
This is very advantageous in LED applications where this high symmetry can allow much reduced interference effects in the far field and allow even far-field illumination.
The tessellation utilised in one of the PQC structures is clearly visible on the bottom left hand figure where recursive dissections of dodecagons allows the generation of a 12 fold symmetric structure which is termed the square-triangle tiling. The (bottom middle figure) SEM is the corresponding PQC structure is highlighted.
Some other quasicrystal structures are demonstrated (top and bottom right hand side).
Benefits of PQCs over PhCs for LEDs
Comparison between directional properties of a radially symmetric PQC and a regular PC structure (square lattice).
Although extraction efficiencies for regular PCs can be quite high,
Get much more isotropic beam profile with Quasi-crystal, eg fewer far field artefacts.
PQC is Radialy symmetric
Reduces secondary optics