Session A37: Research Collaboration between Mentors and Undergraduate Students

Atomic, molecular and optical physics at Bethel

An example of the close connection between research and advanced labs at Bethel University is the recent realization of cold lithium atoms in a magneto-optical trap (MOT). Several aspects of the cooling and trapping research took root in the laboratory components of the Optics and Lasers upper-level courses. These included a wavelength meter with sub-picometer accuracy and precision, stabilized laser diodes and molecular and atomic spectroscopy. Work on the MOT began in 2008 and has involved students (a total of 12, including several post-General Physics sophomores) working during summers, course projects and senior research. Lithium MOTs offer challenges (e.g.\ low vapor pressure) and advantages in an undergraduate lab with respect to the more common rubidium systems. Lasers for lithium are at 671~nm, a more practical red color that can still take advantage of inexpensive laser diodes and broadband optical coatings. Its relatively simple atomic structure makes lithium amenable for stringent comparisons between theory and experiment. Recent high precision absolute frequency measurements using an atomic beam disagree. Cold-atom spectroscopy of lithium could help resolve questions about the atomic structure of lithium.   

Lithium in a Magneto-Optical Trap

We recently cooled and trapped $\sim$10$^7$ neutral $^7$Li atoms in a magneto-optical trap. Our laser source is a home-built external cavity diode laser at 671~nm and a semiconductor tapered amplifier. Acousto-optic modulators are used to generate five different laser detunings that are necessary for repumping between hyperfine states in the trapping and slowing laser beams. The laser is locked by phase-sensitive detection of fluorescence produced by a frequency-modulated laser beam normally incident to the lithium atomic beam. The laser is tuned to the $2S_{1/2} (F=2)\rightarrow2P_{3/2}(F')$ D2 transition. Two coils of wire with $\sim$100~A of current flowing through them in an anti-Helmholtz orientation generate the magnetic field gradient whose magnitude increases from zero with distance from the center of the trap. We describe a few preliminary measurements on the trapped atoms such as temperature, atom number, and loading/unloading times. Eventually, the cold lithium will be used to demonstrate single-photon cooling in an optical dipole trap. Additionally, we plan to use the trap to do high-resolution, cold atom spectroscopy.   

Undergraduate Research in Theoretical Physics at Goucher College: Thermal Properties of Extreme Type-II Superconductors in High Magnetic Fields

The subject matter of this faculty/student collaborative research involves the complex theoretical task of calculating the thermodynamic properties of strongly coupled extreme type-II superconductors starting from the high-field limit of the Landau level pairing scheme. In these systems, the low temperature and high magnetic field regime of the phase diagram fundamentally differs from the familiar low-field mixed phase, primarily by the appearance of gapless quasiparticle excitations in the energy spectrum. This theoretical/computational research was performed over the past three years in collaboration with Goucher College undergraduate students majoring in physics, mathematics and computer science. These students used their analytical and computational skills to develop computer programs and to calculate numerically the thermal properties of realistic superconducting materials. I will describe how this project created an intellectual environment where students developed an awareness of theoretical physics research and its impact on emerging technologies as well as its possible contribution to the solutions for the global energy challenge.   

Thermal properties of Extreme Type-II Superconductors in High Magnetic Fields

The subject matter of our investigation involves the complex theoretical task of calculating the specific heat of a two-gap extreme type-II superconductor. In extreme type-II superconductors, at low temperatures and high magnetic fields, Landau level quantization of electronic energies results in the appearance of gapless excitations at highly symmetrical points on the Fermi surface. The careful measurements of thermal properties like specific heat in the superconducting mixed state at low temperatures and high magnetic fields can reveal this novel gapless behavior. We present a detailed theoretical and numerical study of a realistic, disordered superconductor in high magnetic field and compare our calculated specific heat to available experimental data for the well-known two-gap superconductors NbSe$_{2}$ and LuNi$_{2}$B$_{2}$C. We also discuss the alterations to the location and number of gapless excitations caused by the inclusion of off-diagonal electronic pairing.   

Undergraduate Studies on Compressible Flows and Shock Waves

The Bethel University physics department believes advanced lab projects and undergraduate research experiences are crucial in the development of our students, particularly those that pursue graduate studies in physics, engineering and other applied fields. Open-ended advanced lab projects are key components in several upper level physics courses. Student project work in a specific course is often enhanced by student experiences in other upper level physics courses or other research experiences. For example, projects in {\em Fluid Mechanics} (PHY420) are often enriched by experiences that students bring from projects in {\em Optics} (PHY330) and {\em Computer Methods in Physics} (PHY350). We present examples from recent undergraduate projects on compressible flows and shock waves. Special attention is given to a project involving the design, construction, and initial testing of a small supersonic blowdown tunnel. This facility was initially constructed as part of a project in {\em Fluid Mechanics} (fall 2010). Subsequent student research projects have included high-speed video shadowgraph imaging (summer 2011) and the development of a MATLAB GUI to allow for side-by-side comparisons between simulation and ongoing experiments with the tunnel (fall 2011).   

Graphical User Interface for Supersonic Flow and Shock Waves in a Converging-Diverging Nozzle

A graphical user interface (GUI) is developed to study the compressible flow in a converging-diverging (CD) nozzle. Related experiments are carried out with a small supersonic blowdown tunnel in the Bethel University physics department. The tunnel is constructed with two 5-gallon pressure tanks which are connected by a CD nozzle. Flow in the nozzle goes through three stages during the operation of the blowdown tunnel. The first stage is highly transient and culminates with the development of the quasi-static flow condition throughout the nozzle. In the second stage, flow is fully developed with sufficient driving pressure to sustain supersonic flow in the entire divergent section of the nozzle. In the final stage, the driving pressure is no longer sufficient and a normal shock recedes from the exit of the nozzle to the throat. The GUI is created with MATLAB and focuses on modeling the second and third stage of the flow in the nozzle. Modeling in stage two is based on a 1D isentropic flow assumption, whereas stage three is based on 1D isentropic flow along with normal shock relations. Additional functions of the GUI are being implemented to allow for side-by-side comparisons between simulation and ongoing experiments.   

Characterization of Gold Nanoparticles for Radiotherapy Applications

Gold nanoparticles (AuNPs) are able to conjoin with biological molecules and efficiently absorb light for conversion into heat energy. Thus, they are being investigated for treatment of near surface carcinomas. In this research, we synthesize AuNPs using two different approaches to yield variation in particle size and monodispersity. Transmission electron microscopy was used to characterize particle size and shape while UV-Visible spectrophotometry characterized their absorption wavelength. Presently, fibroblast cells are being used to establish protocols for cell growth, exposure to nanoparticles, irradiation, and cell viability. The future direction of this work is to synthesize a variety of nanoparticles in order to determine the optimal shape, size and composition for photothermal radiotherapy treatment.   

Absorption and Agglomeration of Gold Nanoparticles as a means to Probe Cell Metabolic Activity for Radiotherapy Applications

Polylactic-co-glycolic acid (PLGA) surface treatment of gold nanoparticles (AuNPs) enable those particles to cross the cellular wall. Once within the cell, absorption spectral shifts, predicted by the Mie Theory equation, occur because the dielectric constant of the matrix differs from that surrounding the cell. This phenomena will lead to particle agglomeration (due to changes in surface energy of the particles) and shifts in the Mie absorption spectra. Both phenomena are being explored in fibroblast cells as a means to track cell type and cellular metabolic activity. UV-Vis spectrophotometry and scanning electron microscopy (SEM) are incorporated to analyze the resulting particle/cell mixtures. Early results will be presented.   

Research in growth, characterization, and applications of wide bandgap materials at a Primarily Undergraduate Institution

Zinc oxide (ZnO) is a wide bandgap semiconductor that has attracted a great deal of attention with demonstrated applications in ultraviolet (UV) light detection, air-quality monitoring, missile warning systems, gas detection, and utilization as light-emitting diodes. Our undergraduate research group has been characterizing the growth of various ZnO film and nanowire systems, and we have fabricated and characterized ZnO-based devices, such as UV photodetectors, gas sensors, and photocatalysts. The materials and characterization systems with which we are working and our small niche within the broader field combine to address many of the challenges associated with undergraduate research. In this talk, we will discuss these challenges and how we have overcome them. We will also discuss how we have taken small amounts of money and crumbling facilities and produced a strong research group that involves 3-5 undergraduate students per semester, publishes approximately two peer-reviewed articles per year with undergraduate co-authors, and has achieved a steady stream of external funding.   

Photocatalytic activity of ZnO thin films prepared by DC sputter deposition and thermal oxidation

Zinc oxide (ZnO) is a wide-bandgap semiconductor with a broad range of applications, such as photocatalysis. The photocatalytic properties of ZnO result from the highly-oxidizing holes and reducing electrons that are induced on the ZnO surface by ultraviolet (UV) light. The efficiency of electron-hole pair formation is therefore critical for photocatalysis, and thus the optical quality of the films in the UV region is of critical importance. ZnO thin films have been fabricated using DC sputter deposition of Zn-metal films followed by thermal oxidation at different temperatures (300, 600, and 900$^{\circ}$C). Characterization of the optical properties of the resulting ZnO thin films through photoluminescence indicates that increasing oxidation temperature leads to reduced UV excitonic emission. The photocatalytic activities of the films were also characterized by measuring the efficiency of degradation of Rhodamine B dye in solution. The photocatalytic efficiency of the film annealed at a temperature of 300$^{\circ}$C was higher compared to those of the films annealed at temperatures of 600$^{\circ}$C and 900$^{\circ}$C. The increased photocatalytic efficiency is attributed to the increased optical quality of the films that results from lower oxidation temperatures.   

Investigating the Role of Disorder in the Two-Dimensional Superfluid Transition

The superfluid transition in two-dimensional helium films is an example of a Berezinskii-Kosterlitz-Thouless transition. Characteristic features of the superfluid transition, such as the abrupt onset of superfluidity, have been observed to be significantly altered in disordered two-dimensional systems. We are in the initial stages of experiments aimed at understanding the role of disorder in the two-dimensional superfluid transition. Disorder is introduced into the system by adsorbing helium films to CaF$_2$ surfaces with varying roughness. Quantitative characterization of the roughness of these films is crucial to understanding the superfluid transition in a disordered environment and is our current focus. The overall scope and goals of our experiments will be presented along with some of the challenges of doing low temperature physics in the undergraduate setting. The important role that undergraduates have played in these experiments will also be discussed.   

Characterization of CaF$_{2}$ surfaces using Adsorption-Desorption Isotherms and Atomic Force Microscopy

We are interested in using rough CaF$_{2}$ films to study the superfluid transition in two-dimensional helium systems. These experiments require quantitative information regarding the topography of the CaF$_{2}$ surfaces. The surface roughness of CaF$_{2}$ films is known to increase with film thickness as has been shown with previous atomic force microscopy (AFM) measurements [1]. We have fabricated a series of CaF$_{2 }$samples of different film thicknesses and thus different surface roughnesses. These surfaces were studied using AFM and adsorption-desorption isotherm measurements with liquid nitrogen at T=77 K. The isotherm measurements allow us to determine the pore size distribution of each CaF$_{2 }$film thickness. We find the emergence of hysteretic capillary condensation due to deep pores in the CaF$_{2}$ as the film thickness increases. The development of these deep pores is also seen in our AFM measurements. Our combined results provide a detailed description of CaF$_{2}$ surface roughness which can be utilized in the planned superfluid experiment. [1] D.R. Luhman and R.B. Hallock, Phys Rev. E \textbf{70}, 051606 (2004).   

Surface plasmon microscopy with low-cost metallic nanostructures for biosensing I

The field of plasmonics aims to manipulate light over dimensions smaller than the optical wavelength by exploiting surface plasmon resonances in metallic films. Typically, surface plasmons are excited by illuminating metallic nanostructures. For meaningful research in this exciting area, the fabrication of high-quality nanostructures is critical, and in an undergraduate setting, low-cost methods are desirable. Careful optical characterization of the metallic nanostructures is also required. Here, we present the use of novel, inexpensive nanofabrication techniques and the development of a customized surface plasmon microscopy setup for interdisciplinary undergraduate experiments in biosensing, surface-enhanced Raman spectroscopy, and surface plasmon imaging. A Bethel undergraduate student performs the nanofabrication in collaboration with the University of Minnesota. The rewards of mentoring undergraduate students in cooperation with a large research university are numerous, exposing them to a wide variety of opportunities. This research also interacts with upper-level, open-ended laboratory projects, summer research, a semester-long senior research experience, and will enable a large range of experiments into the future.   

Surface plasmon microscopy with low-cost metallic nanostructures for biosensing II

Due to diffraction, traditional far-field optical microscopy cannot achieve the resolution necessary for applications in nano-scale imaging, sensing, and spectroscopy. However, the manipulation of surface plasmon waves in metallic nanostructures offers a solution. Surface plasmons are evanescent electromagnetic waves sustained by the oscillations of free electrons at the surface of a metal film. As plasmons propagate on the surface, they will probe the surface with high sensitivity and large field intensities. Furthermore, since plasmons are sensitive to within only 10-100 nm from the metallic surface and exhibit large electric field intensities, they have been explored for applications in biosensing and surface-enhanced Raman spectroscopy. We present the development of a microscopy setup for surface plasmon biosensing and the use of reliable, repeatable, low-cost nanofabrication techniques based on template stripping. High-quality nano-patterns are fabricated and used for proof-of-concept biosensing and surface-enhanced Raman spectroscopy experiments. In our customized microscopy setup, holographic illumination with arbitrary laser patterns is made possible by a spatial light modulator. This aids in the careful optical characterization of our nanofabricated samples.