30,773 research outputs found
Evolution of optical gain properties through three generations of electroluminescent fluorene-based polymers
Conjugated polymer semiconductors combine the processing and mechanical
characteristics of plastics with the desirable optical and electronic properties of
semiconductors. The aim of the research reported in this thesis was to investigate the
evolution of the optical gain properties through three generations of electroluminescent
fluorene-based polymers. Detailed optical, optoelectrical and gain characterisations
were carried out on a range of different electroluminescent polyfluorene-based
polymers.
It was discovered that not all of the polymers were gain media as some were
unable to give ASE. SC006 was found to be the most intriguing material among the rest
of the tested polymers; this third generation polymer was found to be a non ASE
material while achieving a high PLQE of 96% with 1.3ns-long excited state lifetime.
Therefore it was evident that optimised highly efficient light emitting conjugate
polymers for PLEDs are not necessarily effective optical gain media, and high steady
state PLQE and long excited state lifetime are insufficient for good optical gain
properties.
Furthermore, in order to investigate the ASE quenching mechanism in SC006, a
series of solvatochromism studies were carried out on this polymer. The time-resolved
PL characteristics were compared between polymers of second and third generations.
The combination of intermolecular and intramolecular energy transfer process was
found to be responsible for the ASE quenching.
Moreover, the effects of the differences in Yamamoto and Suzuki synthesis routes
on optical gain properties of the first generation statistical and alternating copolymers
were investigated and were found to be insignificant.
Finally, the application of the gain quenching mechanism was demonstrated by an
optical switching process performed on a polymer DFB laser. This enabled complete
control over the laser emission from the polymer laser, thus achieving a minimum of a
thirty fold reduction in the visible light output in the presence of a control pulse
Strange-Beauty Meson Production at Colliders
The production rates and transverse momentum distributions of the
strange-beauty mesons and at colliders are calculated
assuming fragmentation is the dominant process. Results are given for the
Tevatron in the large transverse momentum region, where fragmentation is
expected to be most important.Comment: Minor changes in the discussion section. Also available at
http://www.ph.utexas.edu/~cheung/paper.htm
Following the density perturbations through a bounce with AdS/CFT Correspondence
A bounce universe model, known as the coupled-scalar-tachyon bounce (CSTB)
universe, has been shown to solve the Horizon, Flatness and Homogeneity
problems as well as the Big Bang Singularity problem. Furthermore a scale
invariant spectrum of primordial density perturbations generated from the phase
of pre-bounce contraction is shown to be stable against time evolution. In this
work we study the detailed dynamics of the bounce and its imprints on the scale
invariance of the spectrum. The dynamics of the gravitational interactions near
the bounce point may be strongly coupled as the spatial curvature becomes big.
There is no a prior reason to expect the spectral index of the primordial
perturbations of matter density can be preserved. By encoding the bounce
dynamics holographically onto the dynamics of dual Yang-Mills system while the
latter is weakly coupled, via the AdS/CFT correspondence, we can safely evolve
the spectrum of the cosmic perturbations with full control. In this way we can
compare the post-bounce spectrum with the pre-bounce one: in the CSTB model we
explicitly show that the spectrum of primordial density perturbations generated
in the contraction phase preserves its stability as well as scale invariance
throughout the bounce process.Comment: 19 pages, 4 figure
On-Shell Gauge Invariant Three-Point Amplitudes
Assuming locality, Lorentz invariance and parity conservation we obtain a set
of differential equations governing the 3-point interactions of massless
bosons, which in turn determines the polynomial ring of these amplitudes. We
derive all possible 3-point interactions for tensor fields with polarisations
that have total symmetry and mixed symmetry under permutations of Lorentz
indices. Constraints on the existence of gauge-invariant cubic vertices for
totally symmetric fields are obtained in general spacetime dimensions and are
compared with existing results obtained in the covariant and light-cone
approaches. Expressing our results in spinor helicity formalism we reproduce
the perhaps mysterious mismatch between the covariant approach and the light
cone approach in 4 dimensions. Our analysis also shows that there exists a
mismatch, in the 3-point gauge invariant amplitudes corresponding to cubic
self-interactions, between a scalar field and an antisymmetric rank-2
tensor field . Despite the well-known fact that in 4 dimensions
rank-2 anti-symmetric fields are dual to scalar fields in free theories, such
duality does not extend to interacting theories.Comment: significantly revised, final version published in JHE
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