Browsing by Subject "absorption"
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Item An analysis of AERONET aerosol absorption properties and classifications representative of aerosol source regions(AGU, 2012-09-06) Giles, D. M.; Holben, B. N.; Eck, Thomas; Sinyuk, A.; Smirnov, A.; Slutsker, I.; Dickerson, R. R.; Thompson, A. M.; Schafer, J. S.Partitioning of mineral dust, pollution, smoke, and mixtures using remote sensing techniques can help improve accuracy of satellite retrievals and assessments of the aerosol radiative impact on climate. Spectral aerosol optical depth (τ) and single scattering albedo (ω₀) from Aerosol Robotic Network (AERONET) measurements are used to form absorption (i.e., ω₀ and absorption Ångström exponent (αabs)) and size (i.e., extinction Ångström exponent (αext) and fine mode fraction of τ) relationships to infer dominant aerosol types. Using the long-term AERONET data set (1999–2010), 19 sites are grouped by aerosol type based on known source regions to (1) determine the average ω₀ and αabs at each site (expanding upon previous work), (2) perform a sensitivity study on αabs by varying the spectral ω₀, and (3) test the ability of each absorption and size relationship to distinguish aerosol types. The spectral ω₀ averages indicate slightly more aerosol absorption (i.e., a 0.0 < δω₀ ≤ 0.02 decrease) than in previous work, and optical mixtures of pollution and smoke with dust show stronger absorption than dust alone. Frequency distributions of αabs show significant overlap among aerosol type categories, and at least 10% of the αabs retrievals in each category are below 1.0. Perturbing the spectral ω₀ by ±0.03 induces significant αabs changes from the unperturbed value by at least ∼±0.6 for Dust, ∼±0.2 for Mixed, and ∼±0.1 for Urban/Industrial and Biomass Burning. The ω₀440nm and αext440–870nmrelationship shows the best separation among aerosol type clusters, providing a simple technique for determining aerosol type from surface- and future space-based instrumentation.Item Broad band terahertz time domain spectroscopy on polymers and organic electro-optic polymers(2009-01-01) Valdes, Nestor Noriel; Hayden, L. Michael; Physics; Physics, AppliedSeveral polymers and two organic polymer composites were studied using terahertz time domain spectroscopy and analyzed using Duvillaret's method and a dynamic range analysis to determine their credible bandwidth. Terahertz time domain spectroscopy is a technique that allows us to determine the real index of refraction and the absorption of nonconductive materials in the terahertz band. These polymers are used as hosts to build organic composites for terahertz generation through optical rectification and detection via electro-optic (EO) sampling. Knowledge of their optical parameters is important when considering them as hosts for emitters and detectors. Also, the knowledge of these parameters over a wide bandwidth is important in the determination of the degree of phase matching that these materials can have, for a given center wavelength of an optical pump pulse. Broad bandwidth and a clear spectrum were achieved with an air-plasma emitter and an organic EO polymer sensor. In this thesis a bandwidth that goes from 0.7 to 9 THz for the real index of refraction and for the absorption coefficient for the majority of the polymers, is reported.Item Neuron Mid-Infrared Absorption Study for Direct Optical Excitation of Neurons(IEEE, 2011-10-09) Guo, Dingkai; Chen, Xing; Vadala, Shilpa; Leach, Jennie; Choa, Fow-SenNeuron optical excitations are important for brain-circuitry explorations and sensory-neuron-stimulation applications. To optimize the stimulation, we identify neuron mid-IR absorption peaks in this study and discuss their meanings and delivery methods of mid-IR photonsItem Spatial and temporal variability of column-integrated aerosol optical properties in the southern Arabian Gulf and United Arab Emirates in summer(AGU, 2008-01-15) Eck,Thomas; Holben, B. N.; Reid, J. S.; Sinyuk, A.; Dubovik, O.; Smirnov, A.; Giles, D.; O'Neill, N. T.; Tsay, S.-C.; Ji, Q.; Al Mandoos, A.; Ramzan Khan, M.; Reid, E. A.; Schafer, J. S.; Sorokine, M.; Newcomb, W.; Slutsker, I.A mesoscale network of 14 AERONET Sun photometers was established in the UAE and adjacent Arabian Gulf from August through September 2004 as a component of the United Arab Emirates Unified Aerosol Experiment (UAE²). These measurements allowed for spatial, temporal and spectral characterization of the complex aerosol mixtures present in this environment where coarse mode desert dust aerosols often mix with fine mode pollution aerosols largely produced by the petroleum industry. Aerosol loading was relatively high with 2-month averages of aerosol optical depth (AOD) at 500 nm (τa500) ranging from 0.40 to 0.53. A higher fine mode fraction of AOD was observed over Arabian Gulf island sites with Angstrom exponent at 440–870 nm (α440–870) of 0.77 as compared to an average of 0.64 over coastal sites and 0.50–0.57 at inland desert sites. During pollution events with α440–870 > 1 the retrieved fine mode radius was larger over an island site than a desert site probably because of hygroscopic growth over the humid marine environment. For these same pollution cases, single scattering albedo (ωo) at all wavelengths was ∼0.03 higher (less absorption) over the marine environment than over the desert, also consistent with aerosol humidification growth. At an inland desert location, the ωo at 440 nm remained relatively constant as Angstrom exponent varied since the fine mode pollution and coarse mode dust were both strong absorbers at short wavelengths. However, at longer wavelengths (675–1020 nm) the dust was much less absorbing than the pollution resulting in dynamic ωo as a function of α440–870.Item Ultrafast Carrier Dynamics of Monolayer WS2 via Broad-Band Time-Resolved Terahertz Spectroscopy(American Chemical Society, 2019-11-19) Gustafson, Jon K.; Cunningham, Paul D.; McCreary, Kathleen M.; Jonker, Berend T.; Hayden, L. MichaelWe report on the ultrafast carrier dynamics of monolayer WS2 using broad-band time-resolved terahertz spectroscopy (TRTS). We find that upon photoexcitation, there is an increase in the conductivity. We attribute this photoinduced increase in conductivity to ultrafast positive trion formation. Trion formation in two-dimensional materials is a consequence of the enhanced Coulombic interactions between electrons and holes. We find that trions in monolayer WS2 contribute to the photoconductivity in three ways: a Drude response, a broad resonance response, and a dissociation response at the trion binding energy. This is the first direct measurement of the trion binding energy in the far-infrared. Our results provide a comprehensive understanding of how trions behave in two-dimensional systems and should be broadly useful in studying the carrier dynamics of other two-dimensional systems.