1,278 research outputs found
Efficiency of energy funneling in the photosystem II supercomplex of higher plants
The investigation of energy transfer properties in photosynthetic
multi-protein networks gives insight into their underlying design
principles.Here, we discuss excitonic energy transfer mechanisms of the
photosystem II (PS-II) CSM supercomplex, which is the largest
isolated functional unit of the photosynthetic apparatus of higher
plants.Despite the lack of a decisive energy gradient in CSM, we
show that the energy transfer is directed by relaxation to low energy states.
CSM is not organized to form pathways with strict energetic
downhill transfer, which has direct consequences on the transfer efficiency,
transfer pathways and transfer limiting steps. The exciton dynamics is
sensitive to small structural changes, which, for instance, are induced by the
reorganization of vibrational coordinates. In order to incorporate the
reorganization process in our numerical simulations, we go beyond rate
equations and use the hierarchically coupled equation of motion approach
(HEOM). While transfer from the peripherical antenna to the proteins in
proximity to the reaction center occurs on a faster time scale, the final step
of the energy transfer to the RC core is rather slow, and thus the limiting
step in the transfer chain. Our findings suggest that the structure of the
PS-II supercomplex guarantees photoprotection rather than optimized efficiency.Comment: 23 pages, 6 figure
Nonlinear optics in the fractional quantum Hall regime
Engineering strong interactions between optical photons is a great challenge
for quantum science. Envisioned applications range from the realization of
photonic gates for quantum information processing to synthesis of photonic
quantum materials for investigation of strongly-correlated driven-dissipative
systems. Polaritonics, based on the strong coupling of photons to atomic or
electronic excitations in an optical resonator, has emerged as a promising
approach to implement those tasks. Recent experiments demonstrated the onset of
quantum correlations in the exciton-polariton system, showing that strong
polariton blockade could be achieved if interactions were an order of magnitude
stronger. Here, we report time resolved four-wave mixing experiments on a
two-dimensional electron system embedded in an optical cavity, demonstrating
that polariton-polariton interactions are strongly enhanced when the electrons
are initially in a fractional quantum Hall state. Our experiments indicate that
in addition to strong correlations in the electronic ground state,
exciton-electron interactions leading to the formation of polaron polaritons
play a key role in enhancing the nonlinear optical response. Besides potential
applications in realization of strongly interacting photonic systems, our
findings suggest that nonlinear optical measurements could provide information
about fractional quantum Hall states that is not accessible in linear optical
response
Bright single photon emission from a quantum dot in a circular Bragg grating microcavity
Bright single photon emission from single quantum dots in suspended circular
Bragg grating microcavities is demonstrated. This geometry has been designed to
achieve efficient (> 50 %) single photon extraction into a near-Gaussian shaped
far-field pattern, modest (~10x) Purcell enhancement of the radiative rate, and
a spectral bandwidth of a few nanometers. Measurements of fabricated devices
show progress towards these goals, with collection efficiencies as high as ~10%
demonstrated with moderate spectral bandwidth and rate enhancement. Photon
correlation measurements are performed under above-bandgap excitation (pump
wavelength = 780 nm to 820 nm) and confirm the single photon character of the
collected emission. While the measured sources are all antibunched and
dominantly composed of single photons, the multi-photon probability varies
significantly. Devices exhibiting tradeoffs between collection efficiency,
Purcell enhancement, and multi-photon probability are explored and the results
are interpreted with the help of finite-difference time-domain simulations.
Below-bandgap excitation resonant with higher states of the quantum dot and/or
cavity (pump wavelength = 860 nm to 900 nm) shows a near-complete suppression
of multi-photon events and may circumvent some of the aforementioned tradeoffs.Comment: 11 pages, 12 figure
Interplay between excitation kinetics and reaction-center dynamics in purple bacteria
Photosynthesis is arguably the fundamental process of Life, since it enables
energy from the Sun to enter the food-chain on Earth. It is a remarkable
non-equilibrium process in which photons are converted to many-body excitations
which traverse a complex biomolecular membrane, getting captured and fueling
chemical reactions within a reaction-center in order to produce nutrients. The
precise nature of these dynamical processes -- which lie at the interface
between quantum and classical behaviour, and involve both noise and
coordination -- are still being explored. Here we focus on a striking recent
empirical finding concerning an illumination-driven transition in the
biomolecular membrane architecture of {\it Rsp. Photometricum} purple bacteria.
Using stochastic realisations to describe a hopping rate model for excitation
transfer, we show numerically and analytically that this surprising shift in
preferred architectures can be traced to the interplay between the excitation
kinetics and the reaction center dynamics. The net effect is that the bacteria
profit from efficient metabolism at low illumination intensities while using
dissipation to avoid an oversupply of energy at high illumination intensities.Comment: 21 pages, 13 figures, accepted for publication in New Journal of
Physic
Optical signatures of quantum delocalization over extended domains in photosynthetic membranes
The prospect of coherent dynamics and excitonic delocalization across several
light-harvesting structures in photosynthetic membranes is of considerable
interest, but challenging to explore experimentally. Here we demonstrate
theoretically that the excitonic delocalization across extended domains
involving several light-harvesting complexes can lead to unambiguous signatures
in the optical response, specifically, linear absorption spectra. We
characterize, under experimentally established conditions of molecular assembly
and protein-induced inhomogeneities, the optical absorption in these arrays
from polarized and unpolarized excitation, and demonstrate that it can be used
as a diagnostic tool to determine the coherent coupling among iso-energetic
light-harvesting structures. The knowledge of these couplings would then
provide further insight into the dynamical properties of transfer, such as
facilitating the accurate determination of F\"orster rates.Comment: 4 figures and Supplementary information with 7 figures. To appear in
Journal of physical chemistry A, 201
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