Quasicrystals: Making invisible materials

All-dielectric photonic quasicrystals may act as zero-refractive-index homogeneous materials despite their lack of translational symmetry and periodicity, stretching wavelengths to infinity and offering applications in light wavefront sculpting and optical cloaking.Comment: 5 pages, 2 figure

A molecular overlayer with the Fibonacci square grid structure

Quasicrystals differ from conventional crystals and amorphous materials in that they possess long-range order without periodicity. They exhibit orders of rotational symmetry which are forbidden in periodic crystals, such as five-, ten-, and twelve-fold, and their structures can be described with complex aperiodic tilings such as Penrose tilings and Stampfli-Gaehler tilings. Previous theoretical work explored the structure and properties of a hypothetical four-fold symmetric quasicrystal-the so-called Fibonacci square grid. Here, we show an experimental realisation of the Fibonacci square grid structure in a molecular overlayer. Scanning tunnelling microscopy reveals that fullerenes (C ) deposited on the two-fold surface of an icosahedral Al-Pd-Mn quasicrystal selectively adsorb atop Mn atoms, forming a Fibonacci square grid. The site-specific adsorption behaviour offers the potential to generate relatively simple quasicrystalline overlayer structures with tunable physical properties and demonstrates the use of molecules as a surface chemical probe to identify atomic species on similar metallic alloy surfaces

Mesoscale engineering of photonic glass for tunable luminescence

The control of optical behavior of active materials through manipulation of microstructure has led to the development of high-performance photonic devices with enhanced integration density, improved quantum efficiencies and controllable colour output. However, the achievement of robust light-harvesting materials with tunable, broadband and flatten emission remains a long-standing goal, owing to the limited inhomogeneous broadening in ordinary hosts. Here, we describe an effective strategy for management of photon emission by manipulation of mesoscale heterogeneities in optically active materials. Importantly, this unique approach enables control of dopant-dopant and dopant-host interactions at the extended mesoscale. This allows generating intriguing optical phenomena such as high activation ratio of dopant (close to 100 %), dramatically inhomogeneous broadening (up to 480 nm), notable emission enhancement, and moreover, simultaneously extending emission bandwidth and flattening spectral shape in glass and fiber. Our results highlight that the findings connect the understanding and manipulation at the mesoscale realm to functional behavior at the macroscale, and the approach to managing the dopants based on mesoscale engineering may provide new opportunity for construction of robust fiber light source.National Natural Science Foundation of China (Grant IDs: 11474102, 51202180), the Chinese Program for New Century Excellent Talents in University (Grant ID: NCET-13-0221), Guangdong Natural Science Funds for Distinguished Young Scholar (Grant ID: S2013050014549), Fundamental Research Funds for the Central University, Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, World Premier International Research Center Initiative (WPI), MEXT, JapanThis is the author accepted manuscript. It is currently under an indefinite embargo pending publication by Nature Publishing Group

Ultrafast spectroscopy of excitons in semiconducting carbon nanotubes

Ultrafast relaxation dynamics of photoexcitations in semiconducting single walled carbon nanotubes (S-NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed NT films and D 2O solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. Taking into account the PB spectral shift observed for NTs in solution, the PA and PB bands are seen to decay together by following a power law in time of the form (t) -α, with α in the range of 0.7 to 1. The PL emission of S-NTs in D 2O solution shows a polarization degree that agrees with that of the transient photoinduced dichroism. We conclude that the primary photoexcitations in S-NTs are excitons that are confined along the NTs. From the average PL polarization degree and the transient polarization memory decay, we estimate the PL lifetime of isolated NTs in solution is of order 500ps. This relatively long PL lifetime is dominated by non-radiative decay processes, which when coupled with the tiny PL emission quantum efficiency indicates a very small radiative recombination rate, in good agreement with recent theories that include electron correlation

Exciton dynamics in single-walled nanotubes: Transient photoinduced dichroism and polarized emission

Ultrafast relaxation of photoexcitations in semiconducting single-walled carbon nanotubes (S-NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed thin NT films and D 2O solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. The PA and PB decay dynamics as a function of time, t, follow a power law, (t) -α with α in the range of 0.7 to 1. Whereas the PA bands in S-NTs in solution uniformly decay, the PB bands, in contrast, have different decay dynamics across the spectrum, which originates from an ultrafast spectral shift. Nevertheless the dynamics of the PA and PB bands for NTs in solution are the same when the spectral shift is accounted for, indicating a common origin. In addition S-NTs in D 2O solution show polarized PL emission bands in the mid infrared spectral range that follow almost exactly the infrared absorption peaks of the isolated NTs, as well as their transient PB spectrum. The PL emission shows a degree of polarization that agrees with that of the transient photoinduced dichroism. We therefore conclude that the primary photoexcitations in S-NTs are not free carriers, rather they are excitons that are confined along the nanotubes. We found that the transient relaxation kinetics of the excitons depend on the NT form. The fastest exciton dynamics (with sub-picosecond lifetime) characterizes the annealed film, whereas the slowest dynamics (with lifetime of tens of ps) characterizes the isolated NTs in D 2O solution. From the polarization memory decay we could estimate the diffusion constant, D, and the diffusion length, L D, of the excitons along the nanotube. For the annealed films at room temperature we found D ≈ 100 cm 2 s -1 and L D ≈ 100 nm. From the average PL polarization degree, which remains constant across the PL spectrum, and the transient polarization memory decay, we estimate the PL lifetime in NT solution to be of the order of 500 ps. This relatively long PL lifetime is dominated by nonradiative decay processes, which when coupled with the minute PL emission quantum efficiency indicates a very small radiative recombination rate. The weak radiative transition strength is consistent with recent excited state calculations that include electron-hole interaction, which predict that excitons in NTs are basically dark. ©2005 The American Physical Society

Ultrafast spectroscopy of excitons in semiconducting carbon nanotubes

Ultrafast relaxation dynamics of photoexcitations in semiconducting single walled carbon nanotubes (S-NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed NT films and D 2O solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. Taking into account the PB spectral shift observed for NTs in solution, the PA and PB bands are seen to decay together by following a power law in time of the form (t) -α, with α in the range of 0.7 to 1. The PL emission of S-NTs in D 2O solution shows a polarization degree that agrees with that of the transient photoinduced dichroism. We conclude that the primary photoexcitations in S-NTs are excitons that are confined along the NTs. From the average PL polarization degree and the transient polarization memory decay, we estimate the PL lifetime of isolated NTs in solution is of order 500ps. This relatively long PL lifetime is dominated by non-radiative decay processes, which when coupled with the tiny PL emission quantum efficiency indicates a very small radiative recombination rate, in good agreement with recent theories that include electron correlation