Disorder-Enhanced Transport in Photonic Quasicrystals

TYPECondensed Matter Seminar
Speaker:Liad Levi
Affiliation:Physics, Technion
Location:Lidow Nathan Rosen (300)

Anderson localization, a fundamental concept in solid-state physics, describes how introducing disorder can transform a conducting crystal into an insulator. This prediction and subsequent experiments have shown that generally disorder works to arrest transport, in periodic systems (crystals) containing disorder as well as in fully random potentials. However, some systems still pose fundamental challenges to this concept, most notably, quasicrystals. Quasicrystals constitute an intermediate phase between fully periodic and fully disordered media: they do not have a unit cell and do not exhibit translation symmetry, but nevertheless possess non-crystallographic rotational symmetry, long-range order and display Bragg diffraction. Although many of the properties of QCs are now well understood, some fundamental questions remain. Perhaps one of the most intriguing questions related to QCs has to do with transport. Opposite to crystals containing disorder, which exhibit Anderson localization, it has been suggested that disorder can enhance transport in QCs. Indirect experiments indicated that indeed, in some regime, increasing disorder could enhance transport.  This increase in transport with disorder is directly opposite to the characteristic behaviour of crystals, wherein transport is reduced with increasing disorder. Thus far, experiments on transport in atomic QCs were carried out through studying the macroscopic conductivity. With the recent progress in photonic lattices, manifesting analogies between light propagating in a waveguide array and an electron in an atomic lattice, it is natural to expect experimental studies of transport in photonic QCs. Here, we study photonic quasicrystals containing disorder, and present the first direct experimental observation of disorder-enhanced transport in QCs. We do that by directly imaging wavepackets propagating through the photonic QC containing disorder. We show that disorder considerably enhances the transport of wavepackets associated with eigenstates in the proximity of a pseudogap (a sharp reduction in the density of states), the region in which the Fermi energy is found in electronic systems.  Enhanced transport occurs because disorder acts to couple highly localized states near the pseudogap and as a result, states become more extended.