The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope has been surveying the sky since August 2008. The LAT detects gamma rays from ~20 MeV to >300 GeV. The field of view is large (~2.4 sr) and provides all-sky coverage every 3 hours. The LAT data provide a rich multiwavelength resource. They also allow searches for new activity in the notoriously variable gamma-ray sky. LAT flare advocates monitor the data daily to track sources of interest and to watch for new gamma-ray outbursts. I will discuss highlights of the transient gamma-ray sky as viewed by the LAT with an emphasis on results for objects in the Galaxy.

Location: Physics Bldg., room 420

Colin successfully defended his dissertation on April 8, 2010.

TITLE:
Electro-optic polymers for terahertz applications

ABSTRACT:
The rapid development of technologies employing terahertz (THz) radiation has led to numerous industrial and scientific applications. Current THz technologies are limited in their frequency response because of phonon absorbance and poor phase matching in crystalline emitters and detectors, or are limited to high-power bench-top pump laser systems for air-plasma generation and detection. In contrast, amorphous electro-optic (EO) polymer composite materials have the potential for broad bandwidth, spectral gap-free THz emission and detection while requiring a relatively low pump laser power.

In this thesis a theoretical description of THz radiation emission and detection using EO polymers is reviewed, including the effects of laser spectral bandwidth, pulse distortion, and material properties of the EO media. This model is used as a guide to improve the response of a THz system employing electro-optic polymer emitters and detectors. EO polymer composites that have been engineered for terahertz applications are described. These materials show a progression of improvements for use in THz systems, including higher EO coefficients, increased photostability, and reduced aggregation and dimerization.

A study of in-plane (longitudinal) and parallel-plate (transverse) poling of EO polymers for use as THz sensors is presented, including a theoretical description of detection sensitivity for each device. In-plane poling allows access to the full optical nonlinearity of the EO polymer, potentially increasing detection sensitivity by a factor of 2.4 over parallel-plate poled devices. A transmission ellipsometric EO measurement technique is developed for the in-plane poling device and is used in the experimental comparison of the two devices.

EO polymer composites are employed as terahertz emitters and sensors in systems using a communication-wavelength pump laser. A 15 THz wideband response is achieved using the ALTB203/APC composite, and is compared to the organic crystal DAST and to the THz system model. Increasing emitter thickness is studied through stacking multiple EO polymer emitter films. Frequency-dependent terahertz index and absorption of the emitter and sensor films are included into the THz model for a more accurate representation of the terahertz system response.

Given proper phase-matching and low absorption, EO polymer materials can potentially be used in a waveguide geometry to generate broadband THz radiation. Coupling these devices with currently-available ultra-fast fiber lasers could lead to the development of field-deployable, compact, inexpensive THz systems.

Li successfully defended her PhD proposal on Monday, April 12, 2010.

TITLE:
Aerosol Absorption Measurements Using Satellite Remote Sensing

ABSTRACT:
Aerosols, the solid or liquid particles suspended in the atmosphere, directly affect the
energy balance of the Earth’s climate system by scattering and absorbing solar radiation.
Aerosol absorption can warm the atmosphere and cool the Earth’s surface, hence aerosols
can affect the atmospheric temperature profile, boundary layer evolution, convection,
cloud formation, and precipitation, particularly over the regions where significant amount
of aerosol absorption occurs.

Much research has focused on studying aerosol absorption properties. One method of
studying aerosol absorption properties is called the critical reflectance technique. The
critical reflectance technique has the unique advantage of providing continuous
measurements and global coverage when it is applied to satellite data. Specifically, it uses
satellite data from two days (a clean day and a polluted day with the same observing
geometry) to retrieve aerosol single scattering albedo (SSA) - the ratio of the aerosol
scattering coefficient to the sum of the aerosol scattering and absorption coefficient.

Using the critical reflectance technique we have investigated how sensitive the retrieved
SSA is to the following factors: aerosol optical depth (AOD), the real part of the
refractive index, detector zenith angle, AOD difference between the polluted day and the
clean day, and changing aerosol types between both days. In addition, we applied this
technique to Moderate Resolution Imaging Spectroradiometer (MODIS) data over South
Africa and South America. The validation results show that SSA from MODIS retrieval
is in good agreement with AERONET SSA within one standard deviation.

I propose to continue studying aerosol absorption properties retrieved from MODIS data
using the critical reflectance technique and its application to climate forcing. I will
quantify the uncertainty of critical reflectance and SSA and extend the SSA retrieval to
all MODIS wavelengths with enough signal and other AERONET stations. I will also
validate retrieval results with AERONET and other available measurements.
Furthermore, to better understand how aerosol absorption affects the Earth’s energy
balance, I will determine the aerosol radiative forcing at the Earth’s surface and the top of
the atmosphere. In addition, I will produce regional SSA maps and provide aerosol
absorption information to aid in aerosol semi-direct effect studies.

Surface concentration of aerosol particles, also known as particulate matter (PM), is a key component determining air quality, especially with small articles (diameter less than 2.5 µm, or PM2.5) which are known to cause respiratory diseases. Local emissions and long-range transport can both contribute to the PM2.5 levels at the surface. In the past decade, satellites remote sensing measurements of global aerosol distributions have become available, continuously providing large-scale “chemical weather” pictures, which can be potentially useful for estimating surface PM2.5 levels. In this presentation, we discuss the possibilities and challenges in using satellite data for air quality applications, in particular, we will address the following questions: (1) What is the relationship between column aerosol optical depth (AOD), which is the quantity measured by satellite, and surface PM2.5 concentrations? (2) How and why this relationship varies with time and location? (3) What is the optimal approach to use model and satellite data for air quality studies? We will present case studies over the U.S. using the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model, satellite data from MODIS and MISR, and surface PM2.5 concentration data from the U.S. EPA and IMPROVE monitoring networks. We will also explore the use of the lidar data from CALIPSO.

Location: Physics Bldg., room 401

Hubble's Diverse Universe is a 40 minute documentary focused on nine African American and Hispanic American astronomers and astrophysicists. They discuss their research and their experiences being minority astronomers. They share their personal stories and give advice about how to succeed in the sciences. The film was conceived and produced by Jarita Holbrook and Romeel Dave' of the University of Arizona. Holbrook will be available for Q & A after the screening. The film has been presented at many institutions during the 2009 International Year of Astronomy. Holbrook who is the fourth African American woman to earn a Ph.D. in astrophysics in the USA appears in the film.

Location: Physics Bldg., room 401

Paul successfully defended his dissertation on April 16, 2010.

TITLE:
Optical Pump Terahertz Probe Studies of Semiconducting Polymers

ABSTRACT:
Optical-pump terahertz-probe spectroscopy (OPTP) has been applied to study charge generation, transport and the evolution of the photo-induced excited states in thin film organic semiconductors, with emphasis on their relevance to photovoltaic technology. In these experiments the response of the photoexcited material to the AC electric field of a terahertz (THz) pulse was measured. From this response, the evolution of the complex conductivity in the far-infrared was monitored. OPTP presents advantages over other techniques by being an all-optical probe of the complex conductivity over nanometer scale distances with sub-picosecond resolution and exhibits particular sensitivity to carrier scattering rates, which typically lay in the THz range. Conductivity models were applied to the extracted conductivity curves in order to determine technologically relevant quantities like the charge carrier mobility and external quantum yield of charge carrier generation.

We observed charge carriers generated on a subpicosecond time scale in thin films of polyhexylthiophene (P3HT). Through application of the Drude-Smith model (DSM) over the 0-2 THz band, we determined a room temperature intrinsic mobility of about 30 cm²/Vs. The temperature dependence of the conductivity dynamics showed signs of thermally activated polaron hopping influenced by torsional disorder. Above and below gap excitation resulted in similar dynamics, both showing that the majority of carriers recombine within 1 ps. We were able to observe charge transfer occurring on a sub-ps timescale to the soluble fullerene, PCBM, for both excited states, demonstrating that narrow gap polymers can be blended with PCBM for photovoltaic applications.

We observed charge carrier generated on a sub-ps time scale in thin amorphous films of metalated polymers. The time evolution of the conductivity showed that charge carriers recombine and only excitons persist after 100 ps. This characteristic appears to be common to amorphous systems. An intrinsic mobility of 20 cm²/Vs was found for the most promising material.

Broadband (0-6 THz ) studies of the photoconductivity in P3HT suggest that the hole mobility is lower than initially determined. They also bring into question whether the DSM can describe the conductivity effectively or whether delocalized polaron transitions at higher frequencies are the origin of the observed features.

One of the outstanding problems of terahertz (THz) spectroscopy is the measurement of the underlying vibrational spectrum of a molecular solid, where individual vibrational transitions are often merged into broad absorption features by line broadening processes. In this talk I describe how this problem can be addressed using the technique of waveguide terahertz time-domain spectroscopy (THz-TDS). In this technique an analyte molecular film is deposited on one of the inner surfaces of a single-mode metal parallel plate waveguide (PPWG) with a 50 micron gap between the plates. The vibrational spectrum of the film is then measured in a sensitive manner using sub-picosecond THz pulses confined within the PPWG over a relatively long pathlength of a few centimeters. I will show how waveguide THz-TDS (applied at cryogenic temperatures) has proven useful in resolving the underlying vibrational fingerprint spectrum of a variety of explosives solids in the frequency range between 0.2 THz – 4.0 THz, with linewidths of individual transitions as narrow as 7 GHz (0.21 cm 1), and line frequencies determined to a precision of 1 GHz (0.03 cm-1). I’ll also discuss how these highly resolved spectra are currently being modeled using solid state computational methods.

Vincenzo successfully defended his dissertation on April 26, 2010.

TITLE:
Theoretical and experimental study of a new algorithm for factoring numbers

ABSTRACT:
The security of codes, for example in credit card and government information, relies on the fact that the factorization of a large integer number N is a rather costly process on a classical digital computer. Such a security is endangered by the Shor's algorithm which employs entangled quantum systems to find, with a polynomial number of resources, the period of a function which is connected with the factors of N. We can surely expect a possible future realization of such a method for large numbers, but so far the period of Shor's function has been only computed for the number 15.

Inspired by Shor's idea, our work aims to methods of factorization based on the periodicity measurement of a given continuous periodic "factoring function" which is physically implementable using an analog computer.

In particular, we have focused on both the theoretical and the experimental analysis of Gauss sums with continuous arguments leading to a new factorization algorithm. The procedure allows, for the first time, to factor several numbers by measuring the periodicity of Gauss sums performing first-order "factoring" interference processes.

We experimentally implemented this idea by exploiting polychromatic optical interference in the visible range with a multi-path interferometer, and achieved the factorization of seven digit numbers.

The immediate development of this work consists of implementing the same factorization physical principle on entangled systems in order to achieve prime number decompositions of large integers with a polynomial number of resources.

The following Physics students will be inducted into Sigma Pi Sigma, the National Physics Honor Society at the annual Physics Picnic: Benjamin Ecker, James Gerity, Cameron Howard and David Mason.

The UMBC chapter of Sigma Pi Sigma was established in 1980. It exists to honor outstanding scholarship in Physics; to encourage interest in Physics; to promote an attitude of service of its members towards their fellow students, colleagues and the public; and to provide a fellowship of persons who have excelled in Physics. Undergraduate candidates shall have attained at least a 3.25 grade point average on 4.0 system for Physics courses and at least a 3.0 grade point average for cumulative course grades in all courses. The inductees will also be recognized at the 5th Annual CNMS Student Recognition Day on May 7, 2010.

2010 is the 50th anniversary of the construction of the first successful laser. Dr. Anthony M. Johnson, Director of CASPR, Professor of Physics and Professor of Computer Science & Electrical Engineering at UMBC has been named as a "Laser Pioneer" by Laserfest, the official website for the yearlong celebration.

In an associated interview, Dr. Johnson describes some of his motivation to promote underrepresented minorities to pursue careers in science, and recent work to use of lasers for environmental and health purposes.