RESUMO
Fires can pose a threat to forest ecosystems when those ecosystems are not fire-adapted or when forest community conditions make them vulnerable to wildfires. Thus, investigating fire-prone environmental conditions is urgently needed to create action plans that preserve these ecosystems. In this sense, climate variables can determine the environmental conditions favorable for forest fires. Our study confirms that vapor pressure deficit (VPD) is an essential climate indicator for forest fires, as it is related to maximum temperatures and low humidity, representing the stress conditions for vegetation prone to fires. This study explores the extent to which ENSO phases can modulate climatic conditions that lead to high VPD over Guanajuato, a semi-arid region in central Mexico, during the dry season (March-April-May). Using fire occurrence data from MODIS (2000-2019) and Landsat 5 (1998-1999), we developed a climatic probability model for the occurrence of forest fires using VPD estimated from ERA5 reanalysis for each ENSO phase. We found that VPD and the occurrence of forest fires were higher during El Niño than under Neutral and La Niña years, with a higher risk of forest fire occurrence in Guanajuato's southern region. This study concludes that it is necessary to implement regional and local fire management plans, especially where the largest number of natural protected areas is located.
Assuntos
Incêndios , Incêndios Florestais , Ecossistema , Monitoramento Ambiental , Florestas , ProbabilidadeRESUMO
Optical spectroscopy techniques are crucial for the evaluation and use of quantum dots (QDs) in life and materials science. In that context, the fluorescence quantum yield (Φf) is an essential parameter in the assessment of the luminescent features of QDs. The fluorescence quantum yield can be defined as the ratio of the number of emitted photons to the number of absorbed photons by a luminescent material. In this chapter, we describe absolute and relative methods to measure the fluorescence quantum yield of QDs in solution phase. The advantages and limitations of the techniques are reviewed.
Assuntos
Pontos Quânticos/química , Espectrometria de Fluorescência/métodos , Fluorescência , LuminescênciaRESUMO
We report an alternative random laser (RL) architecture based on a flexible and ZnO-enriched cellulose acetate (CA) fiber matrix prepared by electrospinning. The electrospun fibers, mechanically reinforced by polyethylene oxide and impregnated with zinc oxide powder, were applied as an adsorbent surface to incorporate plasmonic centers (silver nanoprisms). The resulting structures - prepared in the absence (CA-ZnO) and in the presence of silver nanoparticles (CA-ZnO-Ag) - were developed to support light excitation, guiding and scattering prototypes of a RL. Both materials were excited by a pulsed (5 Hz, 5 ns) source at 355 nm and their fluorescence emission monitored at 387 nm. The results suggest that the addition of silver nanoprisms to the ZnO- enriched fiber matrix allows large improvement of the RL performance due to the plasmon resonance of the silver nanoprisms, with ~80% reduction in threshold energy. Besides the intensity and spectral analysis, the RL characterization included its spectral and intensity angular dependences. Bending the flexible RL did not affect the spectral characteristics of the device. No degradation was observed in the random laser emission for more than 10,000 shots of the pump laser.
RESUMO
We report the observation and analysis of anti-Stokes coherent random laser (RL) emission from zinc oxide (ZnO) powders excited by one-, two- or three-photon femtosecond laser radiation. The ZnO powders were produced via a novel proteic sol-gel, low-cost and environmentally friendly route using coconut water in the polymerization step of the metal precursor. One- and two-photon excitation at 354 nm and 710 nm, respectively, generated single-band emissions centred at about 387 nm. For three-photon excitation, the emission spectra showed a strong ultraviolet (UV) band (380-396 nm) attributed to direct three-photon absorption from the valence band to the conduction band. The presence of an intensity threshold and a bandwidth narrowing of the UV band from about 20 to 4 nm are clear evidence of RL action. The observation of multiple sub-nanometre narrow peaks in the emission spectra for excitation above the RL threshold is consistent with random lasing by coherent feedback.
RESUMO
An essential parameter to evaluate the light emission properties of fluorophores is the fluorescence quantum yield, which quantify the conversion efficiency of absorbed photons to emitted photons. We detail here an alternative nonfluorescent method to determine the absolute fluorescence quantum yield of quantum dots (QDs). The method is based in the so-called Thermal Lens Spectroscopy (TLS) technique, which consists on the evaluation of refractive index gradient thermally induced in the fluorescent material by the absorption of light. Aqueous dispersion carboxyl-coated cadmium telluride (CdTe) QDs samples were used to demonstrate the Thermal Lens Spectroscopy technical procedure.
Assuntos
Pontos Quânticos/química , Espectrometria de Fluorescência/métodos , Compostos de Cádmio/química , Telúrio/químicaRESUMO
We developed a new scheme for obtaining coherent random lasing based on a chip consisting of a polymer film doped with Rhodamine 6G, having as scatterers butterfly-like TiO(2) nanomembranes (TiO(2)-NM) supported on a glass substrate. The feedback mechanism for laser action is due to the multiple scattering of light by TiO(2)-NM rather than provided by localized variations of the refractive index in the polymer film. The above-threshold multiple spikes signature indicative of random laser emission with coherent feedback is confirmed. As nanomembranes are foreseen as new MEMS/NEMS building blocks, a new generation of combined active/passive photonic devices can be envisaged.
RESUMO
La utilización de la ecografía tanto en escala de grises como Doppler color resulta una práctica de mucha utilidad en la caracterización de diversas entidades a nivel del escroto y su contenido. Para su realización se utilizaron transductores lineales con frecuencia entre 7,5-10 MHz en cortes transversales y sagitales de testículo y epidídimo, incluyendo imágenes comparativas del tamaño y ecogenicidad de ambos testículos. Este ensayo iconográfico intenta revisar el aporte del ultrasonido en la patología escrotal benigna más frecuente
Assuntos
Humanos , Masculino , Epididimo , Escroto , Testículo , Calcinose , Criptorquidismo , Cistos , Epididimite , Orquite , Doenças Testiculares , Hidrocele Testicular , Torção do Cordão Espermático , Ultrassonografia Doppler em Cores , VaricoceleRESUMO
La utilización de la ecografía tanto en escala de grises como Doppler color resulta una práctica de mucha utilidad en la caracterización de diversas entidades a nivel del escroto y su contenido. Para su realización se utilizaron transductores lineales con frecuencia entre 7,5-10 MHz en cortes transversales y sagitales de testículo y epidídimo, incluyendo imágenes comparativas del tamaño y ecogenicidad de ambos testículos. Este ensayo iconográfico intenta revisar el aporte del ultrasonido en la patología escrotal benigna más frecuente (AU)