Low-intensity Regime Research Articles

Unveiling the Hanbury Brown and Twiss effect through Rényi entropy correlations

Adopting a quantum information perspective, we analyse the correlations in the thermal light beams used to demonstrate the Hanbury Brown and Twiss effect. We find that the total correlations measured by the Rényi mutual information match the normalized intensity correlations in the regime of low source intensity. Genuine quantum correlations in the form of discord are relevant in such regime but get washed out with increasing source intensity. This provides a new angle on the issue about the nature—quantum versus classical—of the effect.

Flow and mixing characteristics of swirling double-concentric jets subject to acoustic excitation

Characteristic flow modes, flow evolution processes, jet spread width, turbulence properties, and dispersion characteristics of swirling double-concentric jets were studied experimentally. Jet pulsations were induced by means of acoustic excitation. Streak pictures of smoke flow patterns, illuminated by a laser-light sheet, were recorded by a high-speed digital camera. A hot-wire anemometer was used to digitize instantaneous velocity instabilities in the flow. Jet spread width was obtained through a binary edge identification technique. Tracer-gas concentrations were measured for information on jet dispersions. Two characteristic flow patterns were observed: (1) synchronized vortex rings appeared in the low excitation intensity regime (the excitation intensity less than one) and (2) synchronized puffing turbulent jets appeared in the high excitation intensity regime (the excitation intensity greater than one). In the high excitation intensity regime, the “suction back” phenomenon occurred and therefore induced in-tube mixing. The jet spread width and turbulent fluctuation intensity exhibited particularly large values in the high excitation intensity regime at the excitation Strouhal numbers smaller than 0.85. At the excitation Strouhal numbers >0.85, the high-frequency effect caused significant decay of jet breakup and dispersion—the jet spread width and fluctuation intensity decreased sharply and may, at very high Strouhal numbers, asymptotically approach values almost the same as the values associated with unexcited jets. Exciting the jets at the high excitation intensity regime, the effects of puffing motion and in-tube mixing caused breakup of the jet in the near field and therefore resulted in a small Lagrangian integral time and small length scales of fluctuating eddies. This effect, in turn, caused drastic dispersion of the central jet fluids. It is possible that the excited jets can attain 90 % more improvements than the unexcited jets. We provide a domain regarding excitation intensity and Strouhal number to facilitate identification of characteristic flow modes.

A NOTE ON AN INTEGRATION BY PARTS FORMULA FOR THE GENERATORS OF UNIFORM TRANSLATIONS ON CONFIGURATION SPACE

An integration by parts formula is derived for the first-order differential operator corresponding to the action of translations on the space of locally finite simple configurations of infinitely many points on ℝd. As reference measures, tempered grand canonical Gibbs measures are considered corresponding to a non-constant non-smooth intensity (one-body potential) and translation invariant potentials fulfilling the usual conditions. It is proven that such Gibbs measures fulfill the intuitive integration by parts formula if and only if the action of the translation is not broken for this particular measure. The latter is automatically fulfilled in the high temperature and low intensity regime.

Estimation of a Low-Intensity Filtered Poisson Process in Additive White Gaussian Noise

A Poisson-Gauss process is defined as the sum of a filtered Poisson process (a Poisson process passed through a linear filter) and a white Gaussian noise. This model describes the electrical signal at the output of an optical measuring unit consisted of a photodetector and its following amplifying circuits. The intensity of the Poisson process in this model is proportional to the received optical power. With the observations of a Poisson-Gauss process, three estimation problems are considered: minimum mean squared error estimation of the Poisson process at every fixed but arbitrary time, minimum mean squared error estimation of the Poisson intensity, and the maximum likelihood estimation of the intensity. The solutions to these problems are presented in terms of a complicated functional of the observed Poisson-Gauss process which is hard to compute for an arbitrary value of the Poisson intensity; however, under a low-intensity regime, nonlinear filtering schemes are developed to efficiently compute this functional. This special case provides a signal processing framework for single photon detection, a technology dedicated to measurement of low optical powers.

Flux Exclusion Superconducting Quantum Metamaterial: Towards Quantum-level Switching

Nonlinear and switchable metamaterials achieved by artificial structuring on the subwavelength scale have become a central topic in photonics research. Switching with only a few quanta of excitation per metamolecule, metamaterial's elementary building block, is the ultimate goal, achieving which will open new opportunities for energy efficient signal handling and quantum information processing. Recently, arrays of Josephson junction devices have been proposed as a possible solution. However, they require extremely high levels of nanofabrication. Here we introduce a new quantum superconducting metamaterial which exploits the magnetic flux quantization for switching. It does not contain Josephson junctions, making it simple to fabricate and scale into large arrays. The metamaterial was manufactured from a high-temperature superconductor and characterized in the low intensity regime, providing the first observation of the quantum phenomenon of flux exclusion affecting the far-field electromagnetic properties of the metamaterial.

Stimulateurs et défibrillateurs cardiaques et interférences électromagnétiques au travail

Extensive livestock grazing is a global land use activity that has numerous social, cultural, and environmental benefits. Many marginal zones grazed for centuries are now suffering from abandonment, which has been found to be interrelated with species and functional diversity loss, and has potential consequences for the provision of ecosystem services such as soil fertility and soil carbon storage. We sampled soil and plant species abundance in a total of 64 plots (5 × 5 m), in wet and dry habitats under different grazing regimes (from moderate grazing to grazing abandonment for at least 30 years) in Central Spain. We tested the influence of grazing intensity and water availability on different variables related to soil fertility and also on soil carbon storage. In addition, Community weighted mean and Rao quadratic entropy were calculated for two effect traits (specific leaf area and plant height), in order to assess the ultimate effects of these functional attributes on multifunctionality (i.e., the provision of several simultaneous ecosystem functions). We found that grazing abandonment generally produces a reduction in soil organic matter, total nitrogen, available potassium and phosphorus, aggregate stability and soil carbon storage. In addition, grazing abandonment had a negative effect on soil texture with loss of fine particles (clay and silt) and water holding capacity, but only in dry habitats and in longer-term abandoned situation. In general, soil multifunctionality was higher in wet than dry habitats, decreased with grazing abandonment in both habitats. Soil multifunctionality was indirectly mediated by the functional structure of size and leaf traits of the local plant communities, but our results suggested that macroscale grazing effects, i.e. translocation of nutrients by grazers among habitats via dung and urine deposition, might be important mechanisms that influence soil multifunctionality at the local level. Agri-environmental policies should be aware of the risk of widespread grazing abandonment and take advantage of the benefits of low-intensity grazing regimes for ecosystem services such as soil fertility and stability, and soil carbon storage.

Management pressure drives leafhopper communities in vineyards in Southern Switzerland

Abstract. 1. The effects of the current changes in traditional agricultural practices in the Alps on the biodiversity affecting ecosystem functions and services are little known. Vineyards are among the oldest anthropogenic environments of high cultural and natural value that shape the landscape of large areas in Central and Southern Europe. In several mountain regions of the Alps, vineyards are a valid alternative to the landscape homogenisation that has followed post‐cultural land abandonment and agriculture intensification. Key unanswered questions remain regarding the relative contribution of several factors that influence biodiversity, and the level in management pressure with regard to taxonomic and functional diversity enhancement.2. To answer these questions, we sampled leafhoppers (Auchenorrhyncha) as a model taxon using different standard techniques along 24 vine transects within 8 vineyard complexes in Southern Switzerland. Each transect included one vine row, vine canopy, its interrow and the adjacent slope; the latter two were permanently grass‐covered. Data were analysed using a four‐step approach.3. Environment (five variables) and Management (four variables) accounted for most of the variance in the leafhopper assemblage. Pesticide use (insecticide and herbicide) and slope mowing are the most important management predictors of leafhopper species composition.4. With increasing management pressure (i.e. pesticide and mowing), the number of indicator species and particularly the specialists (i.e. stenotopic and oligotopic species) decreases dramatically.5. To promote taxonomic and functional complexity of communities in vineyard systems, we suggest low management pressure with moderate use of pesticide and a low intensity regime of slope mowing.

Coherent control of the isomerization of retinal in bacteriorhodopsin in the high intensity regime

Coherent control protocols provide a direct experimental determination of the relative importance of quantum interference or phase relationships of coupled states along a selected pathway. These effects are most readily observed in the high intensity regime where the field amplitude is sufficient to overcome decoherence effects. The coherent response of retinal photoisomerization in bacteriorhodopsin to the phase of the photoexcitation pulses was examined at fluences of 10(15) - 2.5 × 10(16) photons per square centimeter, comparable to or higher than the saturation excitation level of the S(0) - S(1) retinal electronic transition. At moderate excitation levels of ∼6 × 10(15) photons/cm(2) (<100 GW/cm(2)), chirping the excitation pulses increases the all-trans to 13-cis isomerization yield by up to 16% relative to transform limited pulses. The reported results extend previous weak-field studies [Prokhorenko et al., Science 313, 1257 (2006)] and further illustrate that quantum coherence effects persist along the reaction coordinate in strong fields even for systems as complex as biological molecules. However, for higher excitation levels of ∼200 GW/cm(2), there is a dramatic change in photophysics that leads to multiphoton generated photoproducts unrelated to the target isomerization reaction channel and drastically changes the observed isomerization kinetics that appears, in particular, as a red shift of the transient spectra. These results explain the apparent contradictions of the work by Florean et al. [Proc. Natl. Acad. Sci. U.S.A. 106, 10896 (2009)] in the high intensity regime. We are able to show that the difference in observations and interpretation is due to artifacts associated with additional multiphoton-induced photoproducts. At the proper monitoring wavelengths, coherent control in the high intensity regime is clearly observable. The present work highlights the importance of conducting coherent control experiments in the low intensity regime to access information on quantum interference effects along specific reaction coordinates.

Tuning the shape anisotropy of silver nanoparticles using time-delayed laser pulse pair irradiations

Glass-embedded spherical silver nanoparticles were irradiated by pairs of delayed femtosecond laser pulses. The influence of intensity, relative polarization (parallel or orthogonal) and time delay between the pulses was investigated. We found a delay-dependent reversal of the orientation of prolate nanoparticles produced in the low-intensity regime: at very short time delays up to 10 ps between pulse pairs the polarization direction of the second-hitting pulse defines the particles’ symmetry axes; in an intermediate regime between 10 and 20 ps no optical dichroism is found at all; at more than 20 ps delay between the pulses, finally, the transformed nanoparticles are oriented along the polarization direction of the first-hitting pulse. Also, in the quite different situation of the high-intensity regime using parallel-polarized pulse pairs, where normally oblate particles are created, isotropic spectral changes (i.e., no dichroism) after irradiation were observed at delay times around 20 ps. The possible physical background of this apparently very special inter-pulse delay of around 20 ps is discussed.

A mesocosm experiment investigating the effects of substratum quality and wave exposure on the survival of fish eggs

In a mesocosm experiment, the attachment of bream (Abramis brama) eggs to spawning substrata with and without periphytic biofilm coverage and their subsequent survival with and without low-intensity wave exposure were investigated. Egg attachment was reduced by 73% on spawning substrata with a natural periphytic biofilm, compared to clean substrata. Overall, this initial difference in egg numbers persisted until hatching. The difference in egg numbers was even increased in the wave treatment, while it was reduced in the no-wave control treatment. Exposure to a low-intensity wave regime affected egg development between the two biofilm treatments differently. Waves enhanced egg survival on substrata without a biofilm but reduced the survival of eggs on substrata with biofilm coverage. In the treatment combining biofilm-covered substrata and waves, no attached eggs survived until hatching. In all treatments, more than 75% of the eggs became detached from the spawning substrata during the egg incubation period, and <1% of these detached eggs survived within the substratum interstices. Hence, detached eggs contributed little to the reproductive success we observed. The implications of these results on the spawning success of fish species that use freshly inundated spawning substrata in shallow waters are discussed.

Investigation of low-intensity convection regimes in a rectangular cavity with a heat flux on the boundary

The characteristics of heat transfer during natural thermogravitational fluid convection of low intensity in a rectangular cavity heated from below (cooled from above) are investigated. Local convection effects are studied. The dependence of local superheating (supercooling) on the Grashof number and the cavity side ratio is found for single-, two-and three-vortex steady motions. The limits of the convection regimes are estimated.

The importance of grazing intensity and frequency for physiological responses of the tropical seagrass Thalassia hemprichii

Seagrass grazing is an intrinsic disturbance in primarily tropical and subtropical areas. While there is a general parabolic response in seagrass growth to grazing intensity, there is less knowledge on the role of grazing frequency, as well as potential interactions between grazing intensity and frequency. This study experimentally investigated physiological responses in Thalassia hemprichii to simulated (leaf cutting) grazing regimes with different intensities (25% vs. 75%) and frequencies (1 times vs. 3 times) over 35 days in Chwaka Bay (Zanzibar, Tanzania). The results showed that the two high-intensity treatments (75% removal) had 37–41% lower growth rate than the low-intensity/low-frequency treatment, and rhizome sugar and starch content were both affected in a similar way. A 36% lower starch content in the simulated low-intensity/high-frequency regime (25% × 3) compared to the one of low-intensity/low-frequency (25% × 1) also shows an interaction between grazing intensity and frequency. This suggests that high-intensity (and to some extent frequency) grazing regimes, in comparison to low-intensity regimes, could negatively affect T. hemprichii growth, energy reserves, and thereby the ability to deal with additional stress like light limitation or grazing.

全固态内腔光学参量振荡器阈值特性的研究

We derive the threshold pump intensity for a singly resonant intracavity optical parametric oscillator (IOPO) based on a temporal coupled field model. Particular attention is paid to the dependence of the intracavity singly resonant OPO (SRO) threshold intensity on the signal wave output coupling. Meanwhile, a Nd:YAG laser pumped KTiOPO4 (KTP) IOPO for eye-safe laser output is studied experimentally. The experiment is performed with four signal wave output reflectivities of 60%, 70%, 80%, and 90%, respectively. The measured values are in good agreement with the theoretical results. With an output coupler reflectivity of 80%, a peak power of 70 kW at 1572 nm has been obtained at a repetition rate of 3.5 kHz. The pulse width is 4.9 ns. Such investigation is helpful to identifying suitable operational regime of low pump intensity.

Efficient extension and enhancement of high-order harmonics of He+ by combined laser pulses

We theoretically study the interaction of a He+ ion with fundamental laser pulses and different combined laser pulses. For low laser intensity the cut-off energy is extended; however, for higher laser intensity the cut-off of the enhanced harmonics is extended under the combination of two fundamental laser pulses whose wavelengths are 800 nm and 394 nm, respectively. We investigate the yield of ionization as a function of peak laser intensity and find that the yield of ionization is different for low and higher laser intensity regimes. The efficient extension of cut-off energy of enhanced harmonics is demonstrated by a classical analysis.

Efficient visible light detection using individual germanium nanowire field effect transistors

We report photoconductivity (PC) in individual germanium nanowire field effect transistors (GeFETs). PC measurements with a global illumination reveal that GeFETs can be used as a polarization-sensitive nanoscale light detector in the visible range. It is also found that the PC shows sensitive optical response especially in the low intensity regime. We observe a high internal gain in PC in conjunction with strong saturation behavior, which is attributed to the filling of surface trapping states. This mechanism for high internal gain is consistent with spatially resolved scanning photocurrent measurements, whose results confirm that optical absorption is in the linear regime.

Barrier crossing driven by Lévy noise: Universality and the role of noise intensity

We study the barrier crossing of a particle driven by white symmetric Lévy noise of index alpha and intensity D for three different generic types of potentials: (a) a bistable potential, (b) a metastable potential, and (c) a truncated harmonic potential. For the low noise intensity regime we recover the previously proposed algebraic dependence on D of the characteristic escape time, T_{esc} approximately C(alpha)D;{mu(alpha)} , where C(alpha) is a coefficient. It is shown that the exponent mu(alpha) remains approximately constant, mu approximately 1 for 0<alpha<2 ; at alpha=2 the power-law form of T_{esc} changes into the known exponential dependence on 1D ; it exhibits a divergencelike behavior as alpha approaches 2. In this regime we observe a monotonous increase of the escape time T_{esc} with increasing alpha (keeping the noise intensity D constant). The probability density of the escape time decays exponentially. In addition, for low noise intensities the escape times correspond to barrier crossing by multiple Lévy steps. For high noise intensities, the escape time curves collapse for all values of alpha . At intermediate noise intensities, the escape time exhibits nonmonotonic dependence on the index alpha , while still retaining the exponential form of the escape time density.

Plasma induced laser beam smoothing below the filamentation threshold

This paper deals with a statistical approach for description of the laser field interaction with underdense plasmas and modification of the laser beam temporal coherence during its propagation through a plasma at power well below the filamentation threshold. The main properties of the plasma density perturbations driven by a randomized laser beam are derived from a stochastic wave equation. The laser spectral and angular broadening is shown to occur on a distance that depends essentially on the ratio of the average power in a speckle to the critical power for filamentation. The coherence time of the transmitted light is reduced to the plasma acoustic time of response to the laser. It is typically a few picoseconds. Dedicated diagnostics have been developed for the interaction code PARAX in order to analyze the laser and plasma statistical properties. The effect of the plasma length on the transmitted light coherence is found to be in good agreement with theoretical predictions. Forward stimulated Brillouin scattering is shown to play a key role in the laser coherence loss in this low-intensity regime. The limitations of the analytical model are discussed in terms of the deviation of the electric field distribution from the Gaussian statistics and creation of density-electric field correlations. This regime of laser induced incoherence is especially interesting in that the associated angular broadening is not as deleterious as observed for higher intensities. Moreover, beam smoothing can be achieved in low-density plasmas where energy losses due to absorption and backscattering are not too important.

Small amplitude travelling wave solitons in biased photorefractive crystal

The positive and negative travelling wave solitons are predicted in photorefractive crystal biased by positive electric field in the low intensity regime. The amplitude and width of these solitons do not vary, showing the bright soliton properties, whereas the propagation properties of these solitons differ from previously observed steady-state photorefractive spatial solitons. These travelling wave solitons move on a straight line trajectory. The deflecting angle can be controlled by adjusting the parameter of the system.

Femtosecond excitation dynamics in gold nanospheres and nanorods

Femtosecond visible photoluminescence is detected from gold nanoparticles using time-resolved fluorescence upconversion spectroscopy. We directly compared this fast luminescence from gold nanospheres $(\ensuremath{\emptyset}25\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ with that obtained from nanorods ($\ensuremath{\emptyset}15\ifmmode\times\else\texttimes\fi{}40\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $\ensuremath{\emptyset}15\ifmmode\times\else\texttimes\fi{}27\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) under vis $(3.02\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and $\mathrm{UV}(4.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ excitation. A fast $(\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$ decay was obtained for the nanoparticles and the emission was depolarized. Degenerate femtosecond pump-probe experiments in the low excitation intensity regime demonstrated much slower electron thermalization and/or equilibration dynamics on the time scale of a few hundred femtoseconds. These features strongly indicate a $d$-hole-conduction electron recombination process as the origin of this photoluminescence. A direct comparison of the fast emission spectra from nanorods and nanospheres is used to discuss the emission enhancement mechanism. These results suggest that the classical local field enhancement theory describes quantitatively well many of the emission features of nanorods with respect to those for nanospheres without invoking more complex models.

Vegetation and fire history since the Late Pleistocene from the Trinity Mountains, northwestern California, USA

A 267-cm sediment core spanning the past c. 15 200 cal. yr was recovered from Mumbo Lake, in the Trinity Mountains of northwestern California's Klamath Region. Plant macrofossils and pollen detail local and extra-local vegetation history, while high-resolution charcoal analysis provides details on local fire history. For the first c. 3000 years, climate was colder and drier than present, and supported an open, subalpine parkland vegetation, with low fire frequencies and fuel biomass. From c. 12 100 to 9800 cal. yr BP increasing moisture and soil development led to a woodland community with three new pine species invading the basin. Fire frequencies remained low, but individual fires may have been more intense because of increased fuel loads. Between c. 9800 and 7200 cal. yr BP, climate warmed and dried considerably, allowing for the expansion of oak and other chaparral species. Fire frequencies increased in the early Holocene, but low charcoal accumulation rates suggest a frequent, relatively low-intensity fire regime. From c. 7200 to 3800 cal. yr BP, the climate became cooler and moister again. Many conifer species appeared for the first time, although chaparral species maintained a strong presence. The fire record shows a dramatic increase in charcoal accumulation rates as well as an increase in fire frequency. From c. 3800 cal. yr BP to present, more conifer species enter the record, and abundance of chaparral species gradually diminishes to present levels.