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Graduate Student Jamie Shaw, Department of Physics, University of Connecticut, "Prospects for Laser Cooling and Trapping Aluminum Monochloride", PhD Dissertation Defense10:00am
11/18
Graduate Student Jamie Shaw, Department of Physics, University of Connecticut, "Prospects for Laser Cooling and Trapping Aluminum Monochloride", PhD Dissertation Defense
Friday, November 18th, 2022
10:00 AM - 12:00 PM
Storrs Campus GS-119
Graduate Student Jamie Shaw, Department of Physics, University of Connecticut
Prospects for Laser Cooling and Trapping Aluminum Monochloride
The advent of laser cooling has ushered in a platform for studying light-matter interactions with unprecedented control. Over the last 20 years, the techniques in laser cooling have become refined enough to enable specific diatomic and polyatomic molecules to be directly laser cooled into the ultracold regime. In this dissertation talk, I will introduce a diatomic species new to the field of laser cooling which promises many advantages over those used in current experiments. Using three novel, high power, UV laser systems developed for this purpose, I will provide initial probes of the molecular structure critical for laser cooling and demonstrate the first signs of optical forces through radiative deflection of a molecular beam.
Webex URL: https://uconn-cmr.webex.com/meet/dam17012 -
Graduate Student Robert McCutcheon, Department of Physics, University of Connecticut, "Propagation of Light through an Atomic Medium with Zero Index of Refraction and Enhanced Refractive Index via Four-Wave Mixing ", PhD Dissertation Defense1:00pm
8/25
Graduate Student Robert McCutcheon, Department of Physics, University of Connecticut, "Propagation of Light through an Atomic Medium with Zero Index of Refraction and Enhanced Refractive Index via Four-Wave Mixing ", PhD Dissertation Defense
Thursday, August 25th, 2022
01:00 PM - 03:00 PM
Other Online
Graduate Student Robert McCutcheon, Department of Physics, University of Connecticut
Propagation of Light through an Atomic Medium with Zero Index of Refraction and Enhanced Refractive Index via Four-Wave Mixing
In media with zero index of refraction, the wavelength of light stretches to infinity, there is no spatial phase change, and the electric and magnetic fields become decoupled. Full transmission of light is possible, even through media of arbitrary shape and size. I will discuss these phenomena and several applications. However, these rely on a stable zero-index response, but it is possible that this response is fragile -- even the presence of light may destroy its own zero index. I will show how a zero index can be obtained in an atomic ensemble, then present a model for calculating the propagation of light through this medium. Simulation results show whether even a single photon can affect the index. I will also discuss work on enhanced refractive index in atomic media. In the most straightforward cases, with and without coherence effects, the index cannot be significantly enhanced in realistic cases due to the practical limitation placed by the atomic density, but direct frequency dependence introduced via four-wave mixing is a possible way around this.
Zoom URL: https://harvard.zoom.us/j/97502338548?pwd=OEZ4V2pnVWJhN3JhaXd3RDJSMHVLQT09 -
Graduate Student Aditi Mahabir, Department of Physics, University of Connecticut, "Negative electronic compressibility in multiband electron gas models", MS Dissertation Defense12:00pm
8/8
Graduate Student Aditi Mahabir, Department of Physics, University of Connecticut, "Negative electronic compressibility in multiband electron gas models", MS Dissertation Defense
Monday, August 8th, 2022
12:00 PM - 02:00 PM
Other Online
Graduate Student Aditi Mahabir, Department of Physics, University of Connecticut
Negative electronic compressibility in multiband electron gas models
In this thesis, I present the results of my investigation of the effects of two electronic bands on the negative electronic compressibility (NEC) in a two-dimensional electron gas (2DEG). We use a simple homogeneous model with Coulombic interactions and first-order multi-band coupling to examine the role of effective mass and relative permittivity in relation to the critical carrier density, where compressibility turns negative. We demonstrate that the population of a second band, along with the presence of inter-band coupling, can dramatically change the cross-over carrier density. This model suggests a method for identifying multi-band electronic systems using precise bulk electronic properties measurements despite the difficulty in confirming such systems. We apply our results to the observed NEC in the 2D electron gas at the interface of LaAlO3 and SrTiO3 (LAO/STO) and determine that, for the known parameters of LAO/STO, the system is likely a realization of a two-band 2D electron gas. Furthermore, we provide general limits on the inter-band coupling with respect to the electronic band population.
Webex URL: https://uconn-cmr.webex.com/meet/alb16177 -
Graduate Student Sunil Thapa, Department of Physics, University of Connecticut, "Study Of High Repetition Rate Ultrashort Pulses and its Application on Data Encryption", PhD Dissertation Defense11:00am
8/3
Graduate Student Sunil Thapa, Department of Physics, University of Connecticut, "Study Of High Repetition Rate Ultrashort Pulses and its Application on Data Encryption", PhD Dissertation Defense
Wednesday, August 3rd, 2022
11:00 AM - 01:00 PM
Other Online
Graduate Student Sunil Thapa, Department of Physics, University of Connecticut
Study Of High Repetition Rate Ultrashort Pulses and its Application on Data Encryption
Optical communication progress has been attributed to time division multiplexing (TDM) and dense wavelength division multiplexing (DWDM). These methods use broadband light source and ultrashort pules with high repetition rate to carry information on optical fibers. The solution to high data traffic demands in future will be ultrafast data processing switching components as conventional electro-optical process efficiency and speed has reached it bottleneck. In this PhD dissertation, an erbium doped fiber ring laser with graphene as a saturable absorber has been designed and this ring laser system is able to generate ultrashort pulses with a repetition rate of 50 GHz. This method produces pulses with narrow width compared to conventional rational harmonic mode locking technique. Effects of two-photon absorption (TPA) on all-optical logic operation in quantum-dot semiconductor optical amplifier (QD-SOA) has been carried out. Inclusion of two photon absorption improves the Q-factor for pseudorandom bit generation (PRBS) at high-speed operation. High speed all-optical encryption operation at 320 Gb/s using optical logic gates which is based on QD-SOA with two photon absorption has been demonstrated. Supercontinuum generation using different shape of dispersion varying waveguides has also been simulated. The non-uniform waveguide allows for continuous phase matching to generate broader spectrum.
Webex URL: https://uconn-cmr.webex.com/uconn-cmr/j.php?MTID=m298c1f157dbb8f58bf7ba97b60a152fb -
Graduate Student Donal Sheets, Department of Physics, University of Connecticut, "Advances in Photon-Based Techniques for Correlated Materials and Applied Physics ", PhD Dissertation Defense1:00pm
7/25
Graduate Student Donal Sheets, Department of Physics, University of Connecticut, "Advances in Photon-Based Techniques for Correlated Materials and Applied Physics ", PhD Dissertation Defense
Monday, July 25th, 2022
01:00 PM - 03:00 PM
Storrs Campus GS-119
Graduate Student Donal Sheets, Department of Physics, University of Connecticut
Advances in Photon-Based Techniques for Correlated Materials and Applied Physics
This dissertation reports advances enabling experimental research in three research areas of current interest: synchrotron-based studies of electronic excitations in correlated materials, development of an ultra-sensitive laser-based system for generating and measuring strain waves, and applied research in response to the COVID-19 pandemic.
The first section describes the role of magnetism in identifying correlated electron phases and presents progress using synchrotron-based resonant inelastic X-ray scattering (RIXS) applied to mixed-valent Yb-based materials. In nonmagnetic semiconductor YbB6 we have studied the energy- and angle-dependent scattering, which clearly shows two distinct photon scattering processes. One of these features can be analyzed and interpreted with ab-initio calculations, while the second appears more universal. We will also present measurements of mixed valent YbAl3 and YbInCu4 which show low energy spectral weight ascribed to many-body resonances. Together, our results demonstrate the potential for RIXS to separately assess the localized and itinerant electronic states of correlated f-electron materials.
The second section details the experimental development of an interferometric femtosecond pump-probe apparatus for observing linear and nonlinear evolution of strain waves and elastic properties in films and single crystals. Using a variable repetition rate amplified Yb fiber laser and 16 ns optical delay, this apparatus enables high sensitivity measurements of surface displacement and change in reflectivity across the time axis. Progress towards the measurement of interacting strain waves and soliton generation near structural and magnetic transitions will be described.
Finally, the development of an apparatus to evaluate the filtrations efficiency and breathability of filters and respirators in response to the COVID-19 pandemic will be described. This apparatus can independently evaluate respirator and filter performance across a range of face velocities, pressure drops, and aerosolized particle sizes. This work has led to collaborations in broad public service, addressed high profile but erroneous claims on the efficacy of woven cloth respirators material, and contributed to developing a piezoelectric nanofiber membrane filter for application in biodegradable masks.
Webex URL: https://uconn-cmr.webex.com/uconn-cmr/j.php?MTID=m02a409563f5ff7517c0a4ef604b25a17 -
Graduate Student Amani Jayakody, Department of Physics, University of Connecticut, "Perovskite SrCoOx - Surprising Structural and Magnetic Behavior", PhD Dissertation Defense11:00am
7/18
Graduate Student Amani Jayakody, Department of Physics, University of Connecticut, "Perovskite SrCoOx - Surprising Structural and Magnetic Behavior", PhD Dissertation Defense
Monday, July 18th, 2022
11:00 AM - 01:00 PM
Storrs Campus GS-119
Graduate Student Amani Jayakody, Department of Physics, University of Connecticut
Perovskite SrCoOx - Surprising Structural and Magnetic Behavior
The concept of electronic phase separation explains several unusual magnetic and electronic properties of doped perovskite oxides such as cuprates, manganites, and cobaltites. The electronic phase separation observed in La1-ySryCoOx is an important example of the type of magnetic phases that can coexist, although there is an ongoing discussion as to the role of chemical segregation. Hole doping of this system can be achieved by substitution of Sr2+ for La3+ while keeping the full oxygen stoichiometry. An alternative way to do the same hole doping is to vary the oxygen concentration of the perovskite system. Strontium cobalt oxide with varying oxygen concentration (SrCoOx, 2.875 < x < 3) is one of the systems which shows magnetic phase separation for intermediate oxygen values. The end points are SrCoO2.875 which is ferromagnetic with Tc=220 K and SrCoO3 which is also ferromagnetic but with Tc=280K. Samples with intermediate values of oxygen concentration show two phase magnetic behavior between these values. Initial lab-based x-ray diffraction indicated that these compounds had only a single crystalline phase despite the two-part magnetism. We undertook a combined study using both high resolution x-ray powder diffraction measurements and pair distribution function measurements in order to understand possible differences between local and average structures. SrCoO3 remains in the simple cubic perovskite structure from 10--300 K. SrCoO2.875 has a much more complicated structure. At room temperature, it is tetragonal with an expanded unit cell. Upon cooling, the structure appears to undergo a second order transition to a cubic phase: i.e. a higher-symmetry phase at low temperature. This is extremely unusual, the opposite of typical structural transitions, and violates the usual assumption that entropy dominates the free energy changes with temperature in structural phase transitions. I present the structural studies of SrCoOx and discuss their implications.
Webex URL: https://uconn-cmr.webex.com/uconn-cmr/j.php?MTID=mf4a1b82f84d196fe0ec681616f968e14 -
Graduate Student Asanka Amarasinghe, Department of Physics, University of Connecticut, "Cosmological Perturbation Theory in Standard Gravity and Conformal Gravity ", PhD Dissertation Defense10:00am
7/12
Graduate Student Asanka Amarasinghe, Department of Physics, University of Connecticut, "Cosmological Perturbation Theory in Standard Gravity and Conformal Gravity ", PhD Dissertation Defense
Tuesday, July 12th, 2022
10:00 AM - 12:00 PM
Storrs Campus GS-119
Graduate Student Asanka Amarasinghe, Department of Physics, University of Connecticut
Cosmological Perturbation Theory in Standard Gravity and Conformal Gravity
Cosmological perturbations play a vital role in the study of the anisotropy of the cosmic microwave background radiation and in large-scale structure formation. The standard procedure for treating these perturbations is to decompose them into scalar (S), vector (V) and tensor (T) components, and assume (the decomposition theorem) that the three sectors independently satisfy the fluctuation equations. Ordinarily, this procedure is carried out in a convenient gauge in a background geometry with vanishing spatial 3-curvature. We have carried out a general SVT decomposition in a cosmology with an arbitrary spatial 3-curvature and expansion radius in a completely gauge invariant way that involves no choice of gauge at all. We have been able to prove the decomposition theorem in this general case, by first manipulating the fluctuation equations so that the various sectors separate out at a higher derivative level; and then finding appropriate boundary conditions under which the solving of these equations leads to the form that is required of the decomposition theorem. With these boundary conditions we thus justify the use of the decomposition theorem in the standard Einstein gravity-based cosmology. In addition, we establish the decomposition theorem in the alternate conformal gravity theory. In addition, using this same SVT decomposition we have derived a closed form expression for the fluctuation in the temperature of the cosmic microwave background for the arbitrary background with arbitrary spatial 3-curvature and arbitrary expansion radius, with the expression that we obtain being completely gauge invariant and with no choice of gauge being made. Interestingly, the expression that we obtain in the non-vanishing spatial 3-curvature case has no explicit dependence on the spatial 3-curvature. This meshes very well with the standard inflationary universe where there is no dependence on the spatial 3-curvature.
Webex URL: https://uconn-cmr.webex.com/uconn-cmr/j.php?MTID=m625d9ffe27f6cfcfd01e3b9956b3d63a -
Title: Cosmological Perturbation Theory in Standard Gravity and Conformal Gravity, Doctoral Dissertation Oral Defense; Asanka Amarasinghe10:00am
7/12
Title: Cosmological Perturbation Theory in Standard Gravity and Conformal Gravity, Doctoral Dissertation Oral Defense; Asanka Amarasinghe
Tuesday, July 12th, 2022
10:00 AM - 11:00 AM
Storrs Campus GS 119
Title: Cosmological Perturbation Theory in Standard Gravity and Conformal Gravity
Cosmological perturbations play a vital role in the study of the anisotropy of the cosmic microwave background radiation and in large-scale structure formation. The standard procedure for treating these perturbations is to decompose them into scalar (S), vector (V) and tensor (T)components, and assume (the decomposition theorem) that the three sectors independently satisfy the fluctuation equations. Ordinarily, this procedure is carried out in a convenient gauge in a background geometry with vanishing spatial 3-curvature. We have carried out a general SVT decomposition in a cosmology with an arbitrary spatial 3-curvature and expansion radius in a completely gauge invariant way that involves no choice of gauge at all. We have been able to prove the decomposition theorem in this general case, by first manipulating the fluctuation equations so that the various sectors separate out at a higher derivative level; and then finding appropriate boundary conditions under which the solving of these equations leads to the form that is required of the decomposition theorem. With these boundary conditions we thus justify the use of the decomposition theorem in the standard Einstein gravity based cosmology. In addition we establish the decomposition theorem in the alternate conformal gravity theory. In addition using this same SVT decomposition we have derived a closed form expression for the fluctuation in the temperature of the cosmic microwave background for the arbitrary background with arbitrary spatial 3-curvature and arbitrary expansion radius, with the expression that we obtain being completely gauge invariant and with no choice of gauge being made. Interestingly, the expression that we obtain in the non-vanishing spatial 3-curvature case has no explicit dependence on the spatial 3-curvature. This meshes very well with the standard inflationary universe where there is no dependence on the spatial 3-curvature. -
Graduate Student Bradley Clarke, Department of Physics, University of Connecticut, "Development of an Amplified Chirp System and its Application to Ultracold Molecules", PhD Dissertation Defense2:00pm
6/28
Graduate Student Bradley Clarke, Department of Physics, University of Connecticut, "Development of an Amplified Chirp System and its Application to Ultracold Molecules", PhD Dissertation Defense
Tuesday, June 28th, 2022
02:00 PM - 04:00 PM
Storrs Campus GS-119
Graduate Student Bradley Clarke, Department of Physics, University of Connecticut
Development of an Amplified Chirp System and its Application to Ultracold Molecules
We have developed a system for producing amplified pulses of frequency-chirped light at 780 nm on nanosecond timescales for use in ultracold 87Rb photoassociation (PA) experiments. The system starts with tunable cw laser light and employs a pair of fiber-based phase modulators, a semiconductor optical amplifier, and a tapered amplifier to generate arbitrary optical frequency chirps with peak powers greater than 1 W. Driving the modulators with an arbitrary waveform generator enables arbitrary chirp shapes, such as one/two-frequency linear chirps, which enhance the rate of PA molecule formation compared to unchirped light. We overcome the optical power limitations of the modulators by duty cycling and avoid unseeded operation of the tapered amplifier by multiplexing the chirped pulses with "dummy" light from a separate diode laser. Despite amplified spontaneous emission (ASE) making up only 3% of the amplified chirp system's output, its presence can be a considerable hindrance to PA experiments. However, advantageous use of ASE has helped to illuminate the mechanisms for 87Rb magneto-optical-trap formed ultracold molecules.
Webex URL: https://uconn-cmr.webex.com/uconn-cmr/j.php?MTID=m9e6a251aaa194fda7c0a8aefa5048d9f
YepiXUFD529 -
Graduate Student Rasika Dahanayake, Department of Physics, University of Connecticut, "Temperature and Solvent induced coil-globule transition of Polypropylene Oxide in solutions and its behavior within self-assembled nanostructures ", PhD Dissertation Defense12:30pm
6/13
Graduate Student Rasika Dahanayake, Department of Physics, University of Connecticut, "Temperature and Solvent induced coil-globule transition of Polypropylene Oxide in solutions and its behavior within self-assembled nanostructures ", PhD Dissertation Defense
Monday, June 13th, 2022
12:30 PM - 02:30 PM
Storrs Campus GS-119
Graduate Student Rasika Dahanayake, Department of Physics, University of Connecticut
Temperature and Solvent induced coil-globule transition of Polypropylene Oxide in solutions and its behavior within self-assembled nanostructures
Bio-compatible amphiphilic polymers are actively studied due to their many biomedical and industrial applications. Polypropylene oxide (PPO) is a polymer that exhibits an interesting thermal responsiveness. The mechanism of temperature induced coil-globule transition hard to assess with traditional methods, therefore molecular dynamics (MD) simulations can provide new insights. Here using all-atom MD simulations, we investigate the coil-globule transition and demonstrate that it is the temperature-induced increase in the sequence length of monomers that are not hydrogen bonded to water that drives the transition. Longer chains can create longer sequences, which serve as nucleation sites for hydrophobic cluster formation causing the transition to occur at lower temperatures.
Coil-globule transition of PPO can also be induced by addition of isobutyric acid (IBA) to aqueous solution. Adding a very small amount of IBA caused PPO to lose hydrogen bonds with water and create more stable hydrogen bonds with IBA which act as a nucleation site for the polymer collapse. With further addition of IBA, PPO expands and resides at the IBA/water interface to retain some fraction of hydrogen bonds with water along with IBA-water hydrogen bonds. In contrast to polyethylene oxide (PEO), where a significant fraction of water is doubly-bonded to polymer, PPO is mostly hydrated via singly bonded water molecules and loses hydration upon a temperature increase. These results illustrate that hydrogen bonding and a polymer's capability to maintain its hydration shell are the key factors in polymer responsiveness to external triggers.
This polymer responsiveness leads to temperature sensitive micelles used in biomedical applications. The structure and properties of cholesterol-polyethylene oxide-cholesterol micelles were studied using atomistic and coarse-grained MARTINI MD simulations. We also investigated using all-atom MD simulations the structure of di-block PPO-PEO and tri-block Pluronic micelles in aqueous solutions and analyzed micelle interactions with co-solvents such as alcohols and IBA. We found that di-block copolymers were able pack much tighter into spherically symmetric micelles compared to triblock copolymers. Depending on their hydrophobicity, co-solvents can localize at the interface or penetrate within the PPO core of micelles that can affect micelle properties and stability.
Webex URL: https://uconn-cmr.webex.com/uconn-cmr/j.php?MTID=m4eb17cf6a55d389d932d3718106547a6 -
Graduate Student Gloria Fonseca Alvarez, Department of Physics, University of Connecticut, "Mapping the Growth of Black Holes: Masses and Spectral Energy Distributions of Quasars"", PhD Dissertation Defense11:00am
6/8
Graduate Student Gloria Fonseca Alvarez, Department of Physics, University of Connecticut, "Mapping the Growth of Black Holes: Masses and Spectral Energy Distributions of Quasars"", PhD Dissertation Defense
Wednesday, June 8th, 2022
11:00 AM - 01:00 PM
Storrs Campus GS-119
Graduate Student Gloria Fonseca Alvarez, Department of Physics, University of Connecticut
Mapping the Growth of Black Holes: Masses and Spectral Energy Distributions of Quasars"
Supermassive black holes reside at the centers of massive galaxies, yet the processes by which these grow are not fully understood. This has been limited by the availability of accurate measurements of black hole (BH) mass and other fundamental properties for diverse samples across cosmic time. In this dissertation, I investigate BH scaling relations used to estimate mass, and use multi-wavelength SEDs to probe the physical processes that govern the inner environments of supermassive black holes. Time-domain observations of active galactic nuclei (AGN) can be used to measure BH mass through reverberation mapping (RM). RM results have established a tight correlation between the radius of the broad-line region (BLR) and the luminosity of AGN. Recent RM campaigns like the Sloan Digital Sky Survey Reverberation Mapping Project (SDSS-RM) show deviations from this correlation that is widely used to estimate mass from single-epoch observations. We find that these deviations are not accounted for by observational biases, but rather depend on the properties of AGN. This can lead to overestimation of BH masses. SDSS-RM has resulted in BH masses for > 100 quasars spanning a wide range of redshifts (0.1 < z < 3). This sample has multi-wavelength coverage from Pan-STARRS, GALEX TDS, WISE and XMM-Newton. We present contemporaneous UV/Optical SEDs for 32 of these quasars and discuss their UV/Optical and X-ray properties. These UV/Optical SEDs, combined with the BH masses from RM, can be used to measure the accretion rates of a large number of quasars.
Webex URL: https://uconn-cmr.webex.com/meet/gvf17001