Knowledge of the emission source strengths of different (particulate and gaseous) atmospheric constituents is one of the principal ingredients upon which the modeling and forecasting of their distribution and impacts depend. Biomass burning emissions are complex and difficult to quantify. However, satellite remote sensing is providing us tremendous opportunities to measure the fire radiative energy (FRE) release rate or power (FRP), which has a direct relationship with the rates of biomass consumption and emissions of major smoke constituents. In this presentation, we will show how the remote-sensing measurement of FRP is facilitating the development of various scientific studies relating to biomass burning, particularly the quantitative characterization of their emission rates and the implications of this unique capability for improving our understanding of smoke impacts on air quality, weather, and climate.

Physics Bldg., room 401

Raymond successfully defended his dissertation on February 5, 2010.

TITLE:
Nonlinear Optical Properties of Novel Forms of Enriched Carbon Disulfide

ABSTRACT:
The ultimate goal of designing molecules with large third-order nonlinearities is to incorporate them into optical switching and limiting devices. To achieve this, the molecules must have large and fast optical nonlinearities. We undertook picosecond and femtosecond studies of carbon disulfide (CS₂) enriched with sulfur, selenium, cadmium telluride nanoparticles and 2-methyl-4-nitroaniline (MNA) for enhanced optical nonlinearities, using the Z-scan technique. In this method, a sample is scanned along the optic axis (chosen as the z- direction) in the focal region of a single focused laser beam. The intensity transmitted through an aperture (nonlinear refraction) or without an aperture (nonlinear absorption) in the far field is recorded as a function of the sample position. Due to the high intensity of the electromagnetic field in this region, the sample tends to behave as a lens of variable focal length and can focus or defocus the beam depending on the sign of the nonlinearity. Our measurements with a 130-fs source indicate that the nonlinear absorption coefficient of CS₂ was enhanced by more than two orders of magnitude (over pure CS₂) through sulfur and selenium enrichment; while their nonlinear refractive indices experienced minimal change. Enhanced nonlinearities of enriched CS₂ molecules may be harnessed for sensor applications.

" While color and brightness patterns are widely used in animal communications, some animals have evolutionarily discovered the use of polarized-light patterns for signaling. In this talk, I will discuss the use of polarization signals in a group of marine crustaceans, the stomatopods (commonly called mantis shrimps). These animals have unusual biological polarizers, some based on principles that are poorly understood and are not used in any artificial polarization systems. Even more unusual is their use of circularly polarized light for communication. Mantis shrimps are the only animals known to sense the handedness of circularly polarized light, and use this sensory modality in their signaling behavior."

Physics Bldg., room 401

A crystal, i. e. an ordered lattice of atoms or molecules is normally assumed to be almost uniformly excited by an incident light on a sub-wavelength scale. An interatomic interaction produces then a uniform local field (different from that of incident laser) at each atom as well. This is a major assumption in the Lorentz-Lorenz theory of interaction of light with dense matter.

We show that at certain critical conditions on the atomic density and dipole strength, a previously unexpected phenomenon emerges: the interacting atoms break the uniformity of interaction, and in a violent switch to a strong non-uniformity, their excitation and local field form nanoscale strata with a spatial period much shorter than that of laser wavelength, thus changing the entire paradigm of light-matter interaction. The most interesting effects can be observed for relatively small 1D-arrays or 2D-lattices if the laser is almost resonant to an atomic quantum transition. The effects include huge local field enhancement at size-related resonances at the laser frequencies near the atomic line, so that the strata are readily controlled by laser tuning. A striking feature is that for the shortest strata, the nearest atomic dipoles counter-oscillate, which is reminiscent of anti-ferromagnetism of magnetic dipoles in Ising model.

Our results also show the formation of "hybrid" modes, whereby at certain atoms in the lattice, their excitation and local field get completely suppressed. Due to those modes, at certain "magic" array size or configuration, the absorption of light at the exact resonant is almost fully canceled. The simplest magic 2D-shape is a six-point star made of 13 atoms (with one atom at the center). The resonant amplitude enhancement enables optical hysteresis and bistability at low light intensities; the tiniest known optical switch can be made of just two atoms. The new phenomenon promises a potential for nanoscale non-conductive computer elements, sensors for detecting bio-molecules, etc.

Physics Bldg., room 401