Standard semiconductor lasers operate in a limited wavelength range, below about 4 microns. Quantum cascade lasers (QCLs) that operate in the mid-IR and far-IR have important applications to medicine, environmental sensing, and national security. While short pulse lasers (~100 fs) are available for standard semiconductor lasers, that is not the case for QCLs. Standard passive modelocking is hard to do in QCLs because of their long coherence times and short gain recovery times. We propose a fundamentally different approach, based on the self-induced-transparency (SIT) effect, that turns these weaknesses into strengths. Solitons, modelocking, and SIT are all reviewed at the beginning of the talk.

Location: Physics Bldg., Room 401

A long-standing goal of physical chemistry is the understanding of how chemical reactions occur and the prediction of cross sections and thermal rate constants. Within the Born-Oppenheimer approximation this requires, firstly, the ability to determine the potential energy of the few-atom system as a function of geometry, and, consequently, the ability to solve the equations of motion for the rearrangement of the nuclei.

The F+H₂→FH+H reaction is one of the most-studied elementary chemical reactions, because increasingly more-sophisticated experimental and theoretical techniques have been brought to bear on this system. Because the F atom has an unfilled 2p shell, more than one electronic state will contribute to this reaction.

We shall present a simple introduction to concepts which underly both the determination of the FHF potential energy function and then, subsequently, the quantum solution of the equations of motion for transfer of the H atom. We will then discuss our recent work on this, and related, hydrogen exchange reactions.

Location: Physics Bldg., Room 401

Steven successfully defended his PhD proposal on April 26, 2011.

TITLE:
Measurement of Cloud Droplet Distribution Parameters: a polarized, multi-angle eye toward cloud microphysics

ABSTRACT:
From radiative forcing effects to a cloud's evolution toward the onset of precipitation, the droplet size distribution (DSD) in a cloud has tremendous influence on the cloud's development and its impact on Earth climate systems.

Most common remotely sensed droplet size information is centered on retrieving the effective radius of the droplet distribution and, therefore, is largely focused on the radiative effects of the cloud.

Effective radius is but a small part of the picture where cloud microphysics and the connection between cloud evolution and atmospheric aerosols are the focus. For more microphysically motivated studies, higher moments of the DSD are needed. Techniques exist for retrieving the effective variance through the use of the polarized signal in the light scattered from the droplet field. This extends our view into the DSD parameter space but is limited to somewhat narrow droplet distributions.

The UMBC Laboratory for Aerosols, Clouds and Optics (LACO) has developed, and employed in the field, several generations of polarimetric remote sensing cameras to measure droplet distribution information and is currently developing next generation multi-angle, polarimetric imaging instrumentation for aircraft and satellite remote sensing of clouds and aerosols.

This proposal discusses the use of LACO imaging polarimeters for droplet size spectrum measurement and the development of techniques to extend the droplet size retrieval to wider spectra using multi-angle information in future generation imaging polarimeters.

Quantum mechanics permits the existence of unique correlations, or entanglement, between individual particles. For a pair of entangled photons, this means that performing a measurement on one photon appears to affect the state of the other. The ability of entangled particles to act in concert is preserved even when they are separated by large distances and serves as a resource for numerous applications. For example, distributing entangled photon pairs over fiber-optic cables enables secure communication between two remote parties or could offer the possibility of interconnecting quantum computers. The vast transparency band of the installed global fiber-optic network, consisting of over a Gigameter of optical fiber cables, presents a particularly attractive opportunity for this task. The bond between entangled photons is, however, very fragile and could be lost. How far could one send entangled photons while still maintaining the connection between them?

We investigate, theoretically and experimentally, how inherent defects and miniscule imperfections in fiber-optic cables degrade entanglement between two photons transmitted over fibers. We show that the loss of entanglement could be either gradual or surprisingly abrupt. We describe relation between local and non-local effects and suggest a novel non-local way to compensate for adverse effects that occur during propagation in fibers. The richness of the observed phenomena suggests that fiber-based entanglement distribution systems could serve as natural laboratories for studying entanglement decoherence.

A brief introduction to the topic of the talk is available on the front page of AT&T Labs website: www.research.att.com

Location: Physics Bldg., Room 401

Gergely successfully defended his PhD proposal on April 28, 2011.

TITLE:
Polarized Imaging Nephelometer Development and Applications for Field and Aircraft

ABSTRACT:
The goal of this research project is to analyze, build, characterize and demonstrate a new airborne instrument for measuring aerosol phase matrix: the LACO/UMBC polarized imaging nephelometer (PI-Neph). We have built a laboratory prototype of this imaging nephelometer concept and the robust airborne version is under construction, funded by the CALIPSO mission (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) under the DEVOTE project (Development and Evaluation of satellite ValidatiOn Tools by Experimenters).

Project success will provide airborne capability to measure phase matrix elements of aerosol particles with an unprecedented angular range at multiple wavelengths. The PI-Neph measurements during the DEVOTE campaign will be analyzed to provide a pilot study showing comparison to RSP (Research Scanning Polarimeter) and HSRL (High Spectral Resolution Lidar) retrievals and to other in situ measurements, especially integrating nephelometers.

Further characterization and testing will prepare the instrument for future aircraft campaigns serving the validation of CALIPSO and RSP retrievals, potentially other aerosol remote sensing instruments and eventually the future ACE mission (Aerosol/Cloud/Ecosystems). A series of in situ experimental flights and ground experiments will enable the use of resultant data for comparison to the new generation of remote sensors. The in situ measurements of aerosol light scattering will enable retrievals of aerosol size and shape distribution and refractive index, which in turn can be compared to lidar, polarimeter or combined polarimeter-lidar retrievals.

Laboratory and/or field studies of polarized phase function of aerosols versus relative humidity will aid in remote sensing of aerosol fields near clouds to help uncover the principles of the aerosol indirect effect on Earth’s climate. The next couple of years of this PhD work will provide the essential bridge between the development, verification and science applications of the PI-Neph, moving toward a proven satellite sensor calibration / validation capability.