The Science Directorate at NASA Langley Research Center (LaRC) emphasizes an end-to-end approach where technology development, mission development, calibration and validation activities, and data analyses are carried out with the goal of deriving scientific information from space-based observations for decision support. This work includes scientific leadership in space-based missions; technology development; laboratory, surface and sub-orbital (aircraft) measurements; research and analyses projects; atmospheric science data stewardship; applied sciences research in developing decision support tools; and education and outreach activities, all in support of NASA’s Science Mission Directorate (SMD). The LaRC Science Directorate strengths include Earth observation, interdisciplinary research, Earth system modeling, data processing systems, and advanced technology development, with an overall focus on atmospheric science and climate. The Directorate also provides significant support to the SMD Applied Sciences program, particularly in areas of air quality management, energy forecasting, and aviation safety. This presentation will provide an introduction to the LaRC Science Directorate vision, goals, organizational structure and ongoing research and present key examples of basic and applied research, as well as data center and education/outreach activities. More information on the LaRC Science Directorate can be found at URL: http://science.larc.nasa.gov/.

Location: Physics Bldg., Room 401

It is now recognized that arrays of classical waveguides are suitable elements for realizing a number of condensed matter and quantum optical effects like Bloch oscillations, Anderson localization, Hong-Ou-Mandel two photon interference, entanglement, quantum walks. Since various interactions can be controlled by design of the array such structures are especially useful to study physical effects in a region previously unrealized. This talk would focus on some of these developments with special emphasis on quantum fields in such structures.

Location: Physics Bldg., Room 401

Jaime successfully defended his Masters thesis on April 18, 2012.

TITLE:
Determination of Planetary Boundary Layer Height from Ground Based Wind Profiler and Lidar Measurements using the Covariance Wavelet Transform (CWT)

ABSTRACT:
This thesis documents the application of the Covariance Wavelet Transform (CWT) to lidar and, for the first time to our knowledge, wind profiler data to examine the possibility of accurate and continuous planetary boundary layer height (PBLH) measurements on short temporal resolution (one and fifteen minute averages respectively). Comparisons between PBLHs derived from the Elastic Lidar Facility (ELF) through application of the CWT and daytime radiosonde launches from Beltsville and RFK Stadium as part of the September 2009 NOAA/ARL and NCEP field study show an R2 = 0.84 correlation. PBLHs from ELF aided in diagnosing issues with the automatic PBLH calculation from Aircraft Communications Addressing and Reporting System (ACARS) profiles in the Real-Time Mesoscale Analysis used by plume dispersion modelers.

The lowest two kilometers of the atmosphere are only probed infrequently in time and sparsely in the United States. In particular, the 00Z and 12Z launches of radiosondes are particularly ill-posed to obtain PBLH mixing, and depth at the peak of the heating cycle in the daytime. This is reflected by comparisons between PBLHs from ELF and PBLHs from radiosonde launches from Dulles International Airport and Aberdeen Proving Ground at 00 and 12Z which show a R2 = 0.39 correlation.

Determining the mixing in the PBL was one goal of a study of the spatial and diurnal variations of the PBL height over Maryland for July 2011, during NASA’s Earth Venture mission DISCOVER-AQ. A semi-automated PBLH detection algorithm utilizing the CWT for wind profiler data was developed. This algorithm was tested on data from the 915 MHz wind profiler at Beltsville, Maryland, and compared against PBLHs derived from ground based radiosondes measured at Beltsville. Comparisons were also done between PBLHs derived from ground based lidars at UMBC and Beltsville. Results from the comparison show an R2 = 0.87, 0.89, and 0.91 correlation between the radiosonde PBLHs and the lidars and wind profiler PBLHs, respectively.

Accurate determination of the PBLH by applying the CWT to lidar and wind profilers will allow for improved air quality forecasting and understanding of regional pollution dynamics.

Collective nature of light emission by atomic ensembles yields fascinating effects such as superradiance and radiation trapping even at the single-photon level. Photon emission is influenced by virtual transitions which lead to collective Lamb shift. For large samples light emission is also affected by retardation due to the finite value of the speed of light. I will discuss collective emission of a single photon from a cloud of N atoms and show exact analytical solutions of this many-atom quantum mechanical problem for superradiant and trapped states. I will discuss how virtual and nonlocal effects modify evolution of the atomic system yielding peculiar temporal features and new kind of cavity QED. I will also show that fully quantum mechanical treatment of spontaneous emission of weakly excited atomic ensembles is analogous to emission of N classical harmonic oscillators.

Location: Physics Bldg., Room 401

Date: Thursday, April 19, 2012
Time: 8:00 am
Location: PHYS 401

TITLE:
Retrieval of microphysical properties of aerosols from a hybrid multiwavelength lidar dataset

ABSTRACT:
Aerosols continue to pose one of the largest uncertainties in the global models utilized to assess different climate change scenarios. In particular, models still fail to represent the vertical distribution of aerosols in the atmosphere with reasonable accuracy.

Currently, most aerosol measurements and retrievals from either spaceborne or ground-based instruments reflect the contribution of the total atmospheric column and therefore do not provide information on the aerosol vertical distribution. Lidar (light detection and ranging) systems are of particular interest in that respect, as they are able to provide profiles of optical properties of aerosols, such as backscatter and extinction coefficients, with high spatial and temporal resolution. Furthermore, it has been demonstrated that from backscatter and extinction coefficient profiles at multiple wavelengths it is possible to retrieve physical properties of aerosols such as effective radius, surface-area, number and volume distributions, complex index of refraction and single scattering albedo.

Most studies to test and validate the inversion algorithm scheme for multiwavelength lidar data have been restricted to the combination of the elastic and Raman scattering measurements from ground-based systems that were carefully designed to emit and receive signals in the same optical path. For this work I propose a new combination of lidar measurements, comprising airborne HSRL (High Spectral Resolution Lidar at 532 nm + 1064 nm elastic channel - nadir viewing) and ground-based elastic and Raman measurements (at 355 nm - zenith viewing). In this new geometry, HSRL measurements within 5 km of the ground-based systems are regarded as collocated measurements. By combining different lidar retrieval techniques to obtain the optical dataset at multiple wavelengths necessary to retrieve the aerosol microphysical properties, and by adding a horizontally averaged component from the geometry of the problem, I will be exploring the feasibility of utilizing this new methodology for test and validation of the retrieval algorithms in the framework of future field campaigns employing synergistic airborne and ground-based lidar measurements, and also for a future spaceborne multiwavelength lidar mission.

Date: Tuesday, April 24, 2012
Time: 10:00 am
Location: PHYS 401

TITLE:
Surface reactions during the atomic layer deposition of high‑κ dielectrics on GaAs surfaces

ABSTRACT:
Atomic layer deposition (ALD) of high dielectric constant (high‑κ) gate dielectrics on III-V semiconductors has been a subject of great interest and has shown promising applications in metal-oxide-semiconductor field effect transistors (MOSFET). However, the mechanism for the deposition of high‑κ gate dielectrics on III-V semiconductors is still not clear. The purpose of the proposed research is to study the surface reactions of a series of metal organic precursors and H₂O on GaAs surfaces during ALD. The adsorption and reaction of HfO₂ precursors, including tetrakis(dimethylamino)hafnium [Hf(N(CH₃)₂)₄] (TDMAH), tetrakis(diethylamino)hafnium [Hf(N(C₂H₅)₂)₄] (TDEAH) and tetrakis(ethylmethylamino)hafnium [Hf(N(CH₃C₂H₅)₄] (TEMAH), TiO₂ precursor tetrakis(dimethylamino)titanium [Ti(N(CH₃)₂)₄] (TDMAT) and Ta₂O₅ precursor pentakis(dimethylamino)tantalum [Ta(N(CH₃)₂)₅] (PDMAT) and H₂O on GaAs (100) surfaces will be investigated by in-situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). This research will improve our understanding of the surface reaction mechanisms during the ALD of high‑κ gate dielectrics on III-V semiconductors.

Date: Wednesday, April 25, 2012
Time: 11:00 am
Location: PHYS 401

TITLE:
Probing the Structure and Morphology of X-ray and Gamma Ray Binaries

ABSTRACT:
High Energy Binary Systems, consisting of a stellar remnant and a companion object, are best characterized by the mass of the companion star and the energy band of the emission (X-ray or gamma ray). The mass transfer mechanism present in High Energy Binary Systems is dependent on the companion’s mass (high mass or low mass in relation to the compact object) and the orbital separation between the two objects. Temporal and spectral data indicate that High Energy Binary Systems show various absorption and emission features that require further research to probe into the mass transfer mechanism, observed luminosity outbursts and other physical properties of the system. I plan to develop a case study of these systems using a detailed temporal and spectral analysis of the poorly understood 4U 1210-64 in addition to archived data on other elusive sources. These data have been and will be collected using the Suzaku and Fermi Space Telescopes. The objective of our research is to understand the mass transfer mechanism present in these systems, characterize the physical properties of the X-ray and gamma ray emission and to further understand the relationship between the orbital period and spin period of these objects (Corbet’s diagram).

Date: Monday, April 30, 2012
Time: 3:00pm
Location: PHYS 401

Title
Optical Measurement on Quantum Cascade Lasers and Mid-IR Semiconductor Materials Using Femtosecond Pulses

Abstract
Ultrafast time-resolved optical technique provides an insight into the carrier dynamics and light-matter interactions in quantum cascade lasers (QCLs) and mid-IR semiconductor materials. The proposed research will use the mid-infrared (mid-IR) fs pulses from a difference frequency generator (DFG) to investigate the carrier dynamics and nonlinearities of QCLs provided by Princeton University, grown by metal-organic chemical vapor deposition (MOCVD). Additionally, fs near and mid-IR pulses are used to excite photoluminescence (PL) to investigate the quality of novel mid-IR semiconductor materials.

QCLs are unipolar devices based on intersubband transitions instead of interband transitions as in most semiconductor lasers. To investigate their carrier dynamics such as resonant tunneling, stimulated emission, and superlattice transport, we need to eliminate electron-hole generation, light and heavy-hole effects, and other interband processes. fs mid-IR pulses provide the resonant photon energy for pump-probe techniques in QCLs. In the pump-probe technique, a strong pump beam is coupled into the active core of a working QCL to perturb the population inversion and the gain from equilibrium. A weak probe beam is used to monitor the gain recovery after the carrier distribution is perturbed by the pump. The dependence of the probe signal on the time delay between the pump and probe gives the time-resolved information on the intersubband transition related carrier mechanisms mentioned above. The pump-probe signal indicates the dephasing mechanisms such as electron-electron (e-e) scattering, electron longitudinal optical (LO) phonon scattering, and carrier heating etc., decreasing the intersubband tunneling rate. Among all the dephasing mechanisms, the LO phonon scattering plays a dominant role in QCLs. This proposed research will apply pump-probe technique in QCLs at different temperatures, because the phonons are largely suppressed at cryogenic temperatures. In addition, QCLs have giant inherent nonlinearities, which give the QCLs a potential in ultrafast pulse generation. The Kerr nonlinearities will be studied by coupling the mid-IR fs pulses into a QCL waveguide.

The second part of this research is to investigate novel mid-IR semiconductor materials using fs pulse excited photoluminescence (PL) and time-resolved photoluminescence (TRPL). Preliminary experimental measurements have been conducted on an InAs/GaSb type-II superlattice sample grown by MOCVD at UMBC.

The results of this research will provide a better understanding of carrier dynamics in QCLs and novel mid-IR materials and would further help the designers and growers to improve the quality and performance of mid-IR devices.