3,862 research outputs found
The Functional Significance of Black-Pigmented Leaves: Photosynthesis, Photoprotection and Productivity in Ophiopogon planiscapus ‘Nigrescens’
Black pigmented leaves are common among horticultural cultivars, yet are extremely rare across natural plant populations. We hypothesised that black pigmentation would disadvantage a plant by reducing photosynthesis and therefore shoot productivity, but that this trait might also confer protective benefits by shielding chloroplasts against photo-oxidative stress. CO2 assimilation, chlorophyll a fluorescence, shoot biomass, and pigment concentrations were compared for near isogenic green- and black-leafed Ophiopogon planiscapus ‘Nigrescens’. The black leaves had lower maximum CO2 assimilation rates, higher light saturation points and higher quantum efficiencies of photosystem II (PSII) than green leaves. Under saturating light, PSII photochemistry was inactivated less and recovered more completely in the black leaves. In full sunlight, green plants branched more abundantly and accumulated shoot biomass quicker than the black plants; in the shade, productivities of the two morphs were comparable. The data indicate a light-screening, photoprotective role of foliar anthocyanins. However, limitations to photosynthetic carbon assimilation are relatively small, insufficient to explain the natural scarcity of black-leafed plants
REAM intensity modulator-enabled 10Gb/s colorless upstream transmission of real-time optical OFDM signals in a single-fiber-based bidirectional PON architecture
Reflective electro-absorption modulation-intensity modulators (REAM-IMs) are utilized, for the first time, to experimentally demonstrate colorless ONUs in single-fiber-based, bidirectional, intensity-modulation and direct-detection (IMDD), optical OFDM PONs (OOFDM-PONs) incorporating 25km SSMFs and OLT-side-seeded CW optical signals. The colorlessness of the REAM-IMs is characterized, based on which optimum REAM-IM operating conditions are identified. In the aforementioned PON architecture, 10Gb/s colorless upstream transmissions of end-to-end realtime OOFDM signals are successfully achieved for various wavelengths within the entire C-band. Over such a wavelength window, corresponding minimum received optical powers at the FEC limit vary in a range as small as <0.5dB. In addition, experimental measurements also indicate that Rayleigh backscattering imposes a 2.8dB optical power penalty on the 10Gb/s over 25km upstream OOFDM signal transmission. Furthermore, making use of on-line adaptive bit and power loading, a linear trade-off between aggregated signal line rate and optical power budget is observed, which shows that, for the present PON system, a 10% reduction in signal line rate can improve the optical power budget by 2.6dB. © 2012 Optical Society of America
Minimal model for active nematics: quasi-long-range order and giant fluctuations
We propose a minimal microscopic model for active nematic particles similar
in spirit to the Vicsek model for self-propelled polar particles. In two
dimensions, we show that this model exhibits a Kosterlitz-Thouless-like
transition to quasi-long-range orientational order and that in this
non-equilibrium context, the ordered phase is characterized by giant density
fluctuations, in agreement with the predictions of Ramaswamy {\it et al.}
[Europhys. Lett. {\bf 62}, 196 (2003)].Comment: Submitted to Phys. Rev. Lett. 4 pages, 4 figure
On the "generalized Generalized Langevin Equation"
In molecular dynamics simulations and single molecule experiments,
observables are usually measured along dynamic trajectories and then averaged
over an ensemble ("bundle") of trajectories. Under stationary conditions, the
time-evolution of such averages is described by the generalized Langevin
equation. In contrast, if the dynamics is not stationary, it is not a priori
clear which form the equation of motion for an averaged observable has. We
employ the formalism of time-dependent projection operator techniques to derive
the equation of motion for a non-equilibrium trajectory-averaged observable as
well as for its non-stationary auto-correlation function. The equation is
similar in structure to the generalized Langevin equation, but exhibits a
time-dependent memory kernel as well as a fluctuating force that implicitly
depends on the initial conditions of the process. We also derive a relation
between this memory kernel and the autocorrelation function of the fluctuating
force that has a structure similar to a fluctuation-dissipation relation. In
addition, we show how the choice of the projection operator allows to relate
the Taylor expansion of the memory kernel to data that is accessible in MD
simulations and experiments, thus allowing to construct the equation of motion.
As a numerical example, the procedure is applied to Brownian motion initialized
in non-equilibrium conditions, and is shown to be consistent with direct
measurements from simulations
Sher 25: pulsating but apparently alone
The blue supergiant Sher25 is surrounded by an asymmetric, hourglass-shaped
circumstellar nebula, which shows similarities to the triple-ring structure
seen around SN1987A. From optical spectroscopy over six consecutive nights, we
detect periodic radial velocity variations in the stellar spectrum of Sher25
with a peak-to-peak amplitude of ~12 km/s on a timescale of about 6 days,
confirming the tentative detec-tion of similar variations by Hendry et al. From
consideration of the amplitude and timescale of the signal, coupled with
observed line profile variations, we propose that the physical origin of these
variations is related to pulsations in the stellar atmosphere, rejecting the
previous hypothesis of a massive, short-period binary companion. The radial
velocities of two other blue supergiants with similar bipolar nebulae, SBW1 and
HD 168625, were also monitored over the course of six nights, but these did not
display any significant radial velocity variations.Comment: 9 pages, 7 figures. Accepted for publication in MNRA
Continuous theory of active matter systems with metric-free interactions
We derive a hydrodynamic description of metric-free active matter: starting
from self-propelled particles aligning with neighbors defined by "topological"
rules, not metric zones, -a situation advocated recently to be relevant for
bird flocks, fish schools, and crowds- we use a kinetic approach to obtain
well-controlled nonlinear field equations. We show that the density-independent
collision rate per particle characteristic of topological interactions
suppresses the linear instability of the homogeneous ordered phase and the
nonlinear density segregation generically present near threshold in metric
models, in agreement with microscopic simulations.Comment: Submitted to Physical Review Letter
Absorbing Phase Transitions of Branching-Annihilating Random Walks
The phase transitions to absorbing states of the branching-annihilating
reaction-diffusion processes mA --> (m+k)A, nA --> (n-l)A are studied
systematically in one space dimension within a new family of models. Four
universality classes of non-trivial critical behavior are found. This provides,
in particular, the first evidence of universal scaling laws for pair and
triplet processes.Comment: 4 pages, 4 figure
Coherence properties of modeless lasers
International audienceMost of classical light sources show a close similarity between their first and second order correlation functions (resp. g(1) and g(2)) functions. We present here the original coherence properties of a peculiar type of laser named modeless laser or Frequency Shifted Feedback (FSF) laser where the g(1) and g(2) functions show a different behaviour. We calculate and evidence experimentally the first and second order correlation functions of modeless lasers, through measurements of the homodyne beat signal and interferometric autocorrelation of a dye FSF laser at the output of a Michelson interferometer. Whereas the degree of first-order coherence vanishes beyond the coherence length of the FSF source, the degree of second-order coherence exhibits periodic revivals far beyond the coherence length, with a period equal to the cavity roundtrip time. Our observations are in good agreement with the theoretical treatment of Yatsenko et al. (Opt. Comm. 282 (2009) 300) [1]
- …