134 research outputs found
Exciton-phonon scattering and photo-excitation dynamics in J-aggregate microcavities
We have developed a model accounting for the photo-excitation dynamics and
the photoluminescence of strongly coupled J-aggregate microcavities. Our model
is based on a description of the J-aggregate film as a disordered Frenkel
exciton system in which relaxation occurs due to the presence of a thermal bath
of molecular vibrations. In a strongly coupled microcavity exciton-polaritons
are formed, mixing superradiant excitons and cavity photons. The calculation of
the microcavity steady-state photoluminescence, following a CW non resonant
pumping, is carried out. The experimental photoluminescence intensity ratio
between upper and lower polariton branches is accurately reproduced. In
particular both thermal activation of the photoluminescence intensity ratio and
its Rabi splitting dependence are a consequence of the bottleneck in the
relaxation, occurring at the bottom of the excitonic reservoir. The effects due
to radiative channels of decay of excitons and to the presence of a
paritticular set of discrete optical molecular vibrations active in relaxation
processes are investigared.Comment: 8 pages, 6 figure
Plasmonic gold nanodiscs fabricated into a photonic-crystal nanocavity
We fabricate and characterise an optical structure consisting of a photonic crystal L3 nanocavity containing two gold nanodisks placed close to a field antinode. We use finite difference time domain (FDTD) modelling to show that the optical properties of the nanocavity are sensitive to the physical separation between the gold nanodisks, and that at reduced separation, the q-factor of a cavity mode polarised parallel to the dimer long-axis is reduced, indicating coupling between the cavity mode and a localised plasmon. Preliminary experimental measurements indeed indicate a damping of the cavity mode in the presence of the dimer; a result consistent with the FDTD modelling. Such a scheme may be used to integrate plasmonic systems into all-optical photonic circuits
Optical properties of nanowires based on a blend of poly (9,9-dioctylfluorene) [PFO] and poly(9,9-dioctyl fluorene-alt-benzothiadiazole) [F8BT]
Near-field and far-field optical microscopy are used to study the optical properties of nanowires based on a blend of 95% poly (9,9-dioctylfluorene) [PFO] doped with 5% poly (9,9-dioctylfluorene-alt-benzothiadiazole) [F8BT]. Single nanowires were imaged and optical investigations revealed that they act as nanoscale optical waveguides. Investigation using polarization-resolved far-field PL spectroscopy revealed emission from the nanowires was strongly anisotropic having preferred axial polarization. This suggests that a significant number of the polymer chains are oriented along the nanowire axis
Band energy control of molybdenum oxide by surface hydration
EPSRC (Grants EP/M009580/1, EP/J017361/1, EP/I01330X/1, and EP/I028641/1), the Royal Society, and the European Research Council. The work benefited from the University of Bath's High Performance Computing Facility, and access to the HECToR supercomputer through membership of the UKs HPC Materials Chemistry Consortium, which is funded by EPSRC (Grant No. EP/F067496) and the UltraFOx grant
Room temperature broadband polariton lasing from a dyeâfilled microcavity
A material system is proposed to generate polariton lasing at room temperature over a broad spectral range. The system developed is based on a boronâdipyrromethene fluorescent dye (BODIPYâG1) that is dispersed into a polystyrene matrix and used as the active layer of a strongly coupled microcavity. It is shown that the BODIPYâG1 exciton polaritons undergo nonlinear emission over a broad range of excitonâcavity mode detuning in the greenâyellow portion of the visible spectrum, with polariton lasing achieved over a spectral range spanning 33 nm. The recorded linewidth of â0.1 nm corresponds to a condensate coherence lifetime of â1 ps. It is proposed that similar effects can be anticipated using a range of molecular dyes in the BODIPY family; a result that paves the way for tunable polariton devices over the visible and nearâinfrared spectral region
Intrinsic optical bistability of thin films of linear molecular aggregates: The one-exciton approximation
We perform a theoretical study of the nonlinear optical response of an
ultrathin film consisting of oriented linear aggregates. A single aggregate is
described by a Frenkel exciton Hamiltonian with uncorrelated on-site disorder.
The exciton wave functions and energies are found exactly by numerically
diagonalizing the Hamiltonian. The principal restriction we impose is that only
the optical transitions between the ground state and optically dominant states
of the one-exciton manifold are considered, whereas transitions to other
states, including those of higher exciton manifolds, are neglected. The optical
dynamics of the system is treated within the framework of truncated optical
Maxwell-Bloch equations in which the electric polarization is calculated by
using a joint distribution of the transition frequency and the transition
dipole moment of the optically dominant states. This function contains all the
statistical information about these two quantities that govern the optical
response, and is obtained numerically by sampling many disorder realizations.
We derive a steady-state equation that establishes a relationship between the
output and input intensities of the electric field and show that within a
certain range of the parameter space this equation exhibits a three-valued
solution for the output field. A time-domain analysis is employed to
investigate the stability of different branches of the three-valued solutions
and to get insight into switching times. We discuss the possibility to
experimentally verify the bistable behavior.Comment: 13 two-column pages, 8 figures, accepted to the Journal of Chemical
Physic
Ultralong-range polariton-assisted energy transfer in organic microcavities.
Non-radiative energy transfer between spatially-separated molecules in a microcavity can occur when an excitonic state on both molecules are strongly-coupled to the same optical mode, forming so-called âhybridâ polaritons. Such energy transfer has previously been explored when thin-films of different molecules are relatively closely spaced (~100 nm). In this letter, we explore strong-coupled microcavities in which thin-films of two J-aggregated molecular dyes were separated by a spacer layer having a thickness of up to 2 ÎŒm. Here, strong light-matter coupling and hybridisation between the excitonic transition is identified using white-light reflectivity and photoluminescence emission. We use steady-state spectroscopy to demonstrate polariton-mediated energy transfer between such coupled states over âmesoscopic distancesâ, with this process being enhanced compared to non-cavity control structures
Nanoscale Mapping of Bromide Segregation on the Cross Sections of 2 Complex Hybrid Perovskite Photovoltaic Films Using Secondary 3 Electron Hyperspectral Imaging in a Scanning Electron Microscope
Mixed halide (I/Br) complex organic/inorganic hybrid perovskite materials have attracted much attention recently because of their excellent photovoltaic properties. Although it has been proposed that their stability is linked to the chemical inhomogeneity of I/Br, no direct proof has been offered to date. Here, we report a new method, secondary electron hyperspectral imaging (SEHI), which allows direct imaging of the local variation in Br concentration in mixed halide (I/Br) organic/inorganic hybrid perovskites on a nanometric scale. We confirm the presence of a nonuniform Br distribution with variation in concentration within the grain interiors and boundaries and demonstrate how SEHI in conjunction with low-voltage scanning electron microscopy can enhance the understanding of the fundamental physics and materials science of organic/inorganic hybrid photovoltaics, illustrating its potential for research and development in âreal-worldâ applications
Strong excitonâphoton coupling in a low-Q all-metal mirror microcavity
Copyright © 2002 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 81 (2002) and may be found at http://link.aip.org/link/?APPLAB/81/3519/1We report the experimental observation of strong excitonâphoton coupling in a planar microcavity composed of an organic semiconductor positioned between two metallic (silver) mirrors. Via transmission and reflectivity measurements, we observe a very large, room temperature Rabi splitting in excess of 300 meV. We show that the Rabi-splitting is enhanced in all-metal microcavities by a factor of more than 2 compared to an organic film positioned between a silver mirror and a dielectric mirror. This enhancement results from the significantly larger optical fields that are confined within all-metal microcavities
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