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- Information compression at the turbulent phase transition in cold-atom gasesPublication . Giampaoli, R.; Figueiredo, J. L.; Rodrigues, J. D.; Rodrigues, José-António; Terças, H.; Mendonça, J. T.The statistical properties of physical systems in thermal equilibrium are blatantly different from their far-from -equilibrium counterparts. In the latter, fluctuations often dominate the dynamics and might cluster in ordered patterns in the form of dissipative coherent structures. Here, we study the transition of a cold atomic cloud, driven close to a sharp electronic resonance, from a stable to a turbulent phase. From the atomic density distribution- measured using a spatially resolved pump-probe technique-we have computed the Shannon entropy on two different basis sets. Information compression, corresponding to a minimum in the Shannon entropy, has been observed at criticality, where the system fluctuations organize into high-order (low-entropy) patterns. Being independent of the representation used, this feature is a property shared by a vast class of physical systems undergoing phase transitions.
- Intercomparison of spectroradiometers and Sun photometers for the determination of the aerosol optical depth during the VELETA-2002 field campaignPublication . Estelles, V.; Utrillas, M. P.; Martinez-Lozano, J. A.; Alcantara, A.; Alados-Arboledas, L.; Olmo, F. J.; Lorente, J.; de Cabo, X.; Cachorro, V.; Horvath, H.; Labajo, A.; Sorribas, M.; Diaz, J. P.; Diaz, A. M.; Silva, A. M.; Elias, T.; Pujadas, M.; Rodrigues, José-António; Canada, J.; Garcia, Y.[ 1] In July 2002 the VELETA-2002 field campaign was held in Sierra Nevada ( Granada) in the south of Spain. The main objectives of this field campaign were the study of the influence of elevation and atmospheric aerosols on measured UV radiation. In the first stage of the field campaign, a common calibration and intercomparison between Licor-1800 spectroradiometers and Cimel-318 Sun photometers was performed in order to assess the quality of the measurements from the whole campaign. The intercomparison of the Licor spectroradiometers showed, for both direct and global irradiances, that when the comparisons were restricted to the visible part of the spectrum the deviations were within the instruments' nominal accuracies which allows us to rely on these instruments for measuring physical properties of aerosols at the different measurement stations. A simultaneous calibration on AOD data was performed for the Cimel-318 Sun photometers. When a common calibration and methodology was applied, the deviation was lowered to much less than 0.01 for AOD. At the same time an intercomparison has been made between the AOD values given by the spectroradiometers and the Sun photometers, with deviations obtained from 0.01 to 0.03 for the AOD in the visible range, depending on the channel. In the UVA range, the AOD uncertainty was estimated to be around 0.02 and 0.05 for Cimel and Licor respectively. In general the experimental differences were in agreement with this uncertainty estimation. In the UVB range the AOD measurements should not be used due to maximum instrumental uncertainties.
- Unfocused laser ignition of high-pressure He–H2-O2 combustible mixturesPublication . Grosso Ferreira, R.; Carvalho, B.; Rodrigues, José-António; Rodrigues, R.; Smith, A.; Marraffa, L.; Lino da Silva, M.We report consistent ignition of high-pressure (p(fill)>20-30 bar) hydrogen-oxygen mixtures diluted with helium in two different combustion vessels, using an unfocused Nd:YAG laser. This corresponds to laser irradiances several orders of magnitude below the minimum ignition energies reported in the literature. This unusual phenomena has led us to try to measure the amount of laser radiation absorbed by the gas medium. By placing a mirror inside a cylindrical vessel and filling it up to 100 bar with He-H-2 or He-O-2 non-combustible mixtures, we obtain the pressure-dependent absorptivity of the combustible He-H-2-O-2 mixture. We find no measurable absorption of the laser signal by the medium, for the overall pressure range, to the experimental apparatus sensitivity (about 1% of the laser irradiance). The exact mechanism for ignition remains henceforth unknown. One possibility could the creation of seed electrons created by autofocusing ionization of dust/impurities in the gas, but this has yet to be experimentally confirmed.
- Photon bubble turbulence in cold atom gasesPublication . Giampaoli, R.; Rodrigues, João D.; Rodrigues, José-António; Mendonça, J. T.Turbulent radiation flow is commonplace in systems with strong, incoherent, light-matter interactions. In astrophysical contexts, photon bubble turbulence is considered a key mechanism behind enhanced radiation transport, and its importance has been widely asserted for a variety of high energy objects such as accretion disks and massive stars. Here, we show that analogous conditions to those of dense astrophysical objects can be obtained in large clouds of cold atoms, prepared in a laser-cooling experiment, driven close to a sharp electronic resonance. By accessing the spatially-resolved atom density, we are able to identify a photon bubble instability and the resulting regime of photon bubble turbulence. We also develop a theoretical model describing the coupled dynamics of both photon and atom gases, which accurately describes the statistical properties of the turbulent regime. This study thus opens the possibility of simulating radiation-dominated astrophysical systems in cold atom experiments.
- Quasi-static and dynamic photon bubbles in cold atom cloudsPublication . Rodrigues, João D.; Giampaoli, Ruggero; Rodrigues, José-António; Ferreira, António V.; Terças, Hugo; Mendonça, José T.Turbulent radiation flow is ubiquitous in many physical systems where light–matter interaction becomes relevant. Photon bubble instabilities, in particular, have been identified as a possible source of turbulent radiation transport in astrophysical objects such as massive stars and black hole accretion disks. Here, we report on the experimental observation of a photon bubble instability in cold atomic gases, in the presence of multiple scattering of light. Two different regimes are identified, namely, the growth and formation of quasi-static structures of depleted atom density and increased photon number, akin to photon bubbles in astrophysical objects, and the destabilisation of these structures in a second regime of photon bubble turbulence. A two-fluid theory is developed to model the coupled atom–photon gas and to describe both the saturation of the instability in the regime of quasi-static bubbles and the low-frequency turbulent phase associated with the growth and collapse of photon bubbles inside the atomic sample. We also employ statistical dimensionality reduction techniques to describe the low-dimensional nature of the turbulent regime. The experimental results reported here, along with the theoretical model we have developed, may shed light on analogue photon bubble instabilities in astrophysical scenarios. Our findings are consistent with recent analyses based on spatially resolved pump–probe measurements.