Ultraviolet anti-Stokes photoluminescence in InxGa1-xN/GaN quantum-well structures

Ultraviolet anti-Stokes photoluminescence (PL) is observed in InxGa1-xN/GaN multiple quantum wells. The observed anti-Stokes PL exhibits a quadratic dependence on the excitation energy density. Anti-Stokes PL excitation spectrum is proportional to the optical absorption spectrum of the InxGa1-xN quantum wells. Time-resolved PL measurement shows that a decay of the anti-Stokes PL is slower than that of the GaN PL under the excitation above the band gap of the GaN barrier, and it is half the time constant of the InxGa1-xN PL decay. A two-step two-photon absorption process is directly observed by means of two-color pump-and-probe experiment. It is considered that the anti-Stokes PL is caused by a two-step two-photon absorption process involving a localized state in the InxGa1-xN quantum wells as the intermediate state, and that the second absorption step is provided by photon recycling of the InxGa1-xN PL

Exciton localization and decomposition dynamics in cuprous halide nanocrystals

We report temporal changes of luminescence and absorption (differential transmission) of excitons in nanometer-size semiconductor crystals (nanocrystals) of CuCl embedded in NaCl or in glass, and CuBr nano- crystals embedded in glass. In CuCl nanocrystals in NaCl, an exciton relaxes nonradiatively to some localized state. In CuCl and CuBr nanocrystals in glass, the temporal changes of the differential transmission have a longer decay component in addition to the fast decay component which agrees with the luminescence decay. This result suggests exciton decomposition and the existence of an electron or a hole remainder in the nano- crystals. The decay time of the longer decay component increases by the accumulation of photoexcitation. This phenomenon indicates persistent trapping of carriers in the glass matrix, which is concerned with persistent spectral hole burning in nanocrystals

New descriptions of lattice SU(N) Yang-Mills theory towards quark confinement

We give new descriptions of lattice SU(N) Yang-Mills theory in terms of new lattice variables. The validity of such descriptions has already been demonstrated in the SU(2) Yang-Mills theory by our previous works from the viewpoint of defining and extracting topological degrees of freedom such as gauge-invariant magnetic monopoles and vortices which play the dominant role in quark confinement. In particular, we have found that the SU(3) lattice Yang-Mills theory has two possible options, maximal and minimal: The existence of the minimal option has been overlooked so far, while the maximal option reproduces the conventional SU(3) Cho-Faddeev-Niemi-Shabanov decomposition in the naive continuum limit. The new description gives an important framework for understanding the mechanism of quark confinement based on the dual superconductivity.Comment: Cover+18 pages, 1 figure; version to appear in Phys. Lett.

Significance of vertical carrier capture for electroluminescence efficiency in InGaN multiple-quantum well diodes

The electroluminescence (EL) spectral intensity has been investigated in the high-brightness green InGaNmultiple-quantum-well light emitting diode (LED) in comparison with the single-quantum-well LED overa wide temperature range and as a function of injection current. It is found that the EL variation patternwith temperature and current is dramatically improved when the number of active wells increases as a resultof enhanced carrier capture. The importance of vertical carrier capture processes is pointed out to explainthe anomalous EL intensity variations at low temperatures