13 research outputs found
Ultrafast carrier relaxation in GaN, In_(0.05)Ga_(0.95)N and an In_(0.05)Ga_(0.95)/In_(0.15)Ga_(0.85)N Multiple Quantum Well
Room temperature, wavelength non-degenerate ultrafast pump/probe measurements
were performed on GaN and InGaN epilayers and an InGaN multiple quantum well
structure. Carrier relaxation dynamics were investigated as a function of
excitation wavelength and intensity. Spectrally-resolved sub-picosecond
relaxation due to carrier redistribution and QW capture was found to depend
sensitively on the wavelength of pump excitation. Moreover, for pump
intensities above a threshold of 100 microJ/cm2, all samples demonstrated an
additional emission feature arising from stimulated emission (SE). SE is
evidenced as accelerated relaxation (< 10 ps) in the pump-probe data,
fundamentally altering the re-distribution of carriers. Once SE and carrier
redistribution is completed, a slower relaxation of up to 1 ns for GaN and
InGaN epilayers, and 660 ps for the MQW sample, indicates carrier recombination
through spontaneous emission.Comment: submitted to Phys. Rev.
A Low-Symmetry Cubic Mesophase of Dendronized CdS Nanoparticles and Their Structure-Dependent Photoluminescence
A liquid crystal (LC) phase with P213 symmetry, the lowest so far in a cubic LC, was obtained in a system of CdS quantum dots (QDs) modified with a two-layer corona of aliphatic thiols (inner) and LC aromatic dendrons (outer). We propose that the unusual low symmetry of this cubic mesophase is a result of the multi-layered corona, which prefers to adopt an anisotropic radial profile because of the combination of long and short “bristles.” The anisotropic distribution of dendrons (long bristles) in the P213 phase is thought to facilitate π-π interaction among the aromatic moieties. The interaction gives rise to non-radiative exciton energy-transfer pathways that induce photoluminescence quenching of the CdS QDs. This is believed to be the first example of structure-dependent emission-quenching behavior
Theory of coherent acoustic phonons in InGaN/GaN multi-quantum wells
A microscopic theory for the generation and propagation of coherent LA
phonons in pseudomorphically strained wurzite (0001) InGaN/GaN multi-quantum
well (MQW) p-i-n diodes is presented. The generation of coherent LA phonons is
driven by photoexcitation of electron-hole pairs by an ultrafast Gaussian pump
laser and is treated theoretically using the density matrix formalism. We use
realistic wurzite bandstructures taking valence-band mixing and strain-induced
piezo- electric fields into account. In addition, the many-body Coulomb
ineraction is treated in the screened time-dependent Hartree-Fock
approximation. We find that under typical experimental conditions, our
microscopic theory can be simplified and mapped onto a loaded string problem
which can be easily solved.Comment: 20 pages, 17 figure