Session H37: SPS Undergraduate Research III

Fabrication of Micromirror Templates Using a Focused CO2 Laser

Micromirrors are an important component in quantum optics, for example in Fabry-Perot microcavities, where light can be recirculated within small volumes. Recently, a CO2 laser method has been demonstrated as a way to fabricate micromirror templates with an exceptionally high surface quality. However, these templates typically vary in size significantly, which is undesirable in many applications, for example when arrays are needed. Here we address this problem by implementing a feedback method, which uses the light emitted by the sample during the ablation process. By measuring the intensity of the light emitted and correcting, in real time, the laser intensity, we can control the feature size to be less than 5 percent.   

High-power, narrow-bandwidth laser based on a tapered amplifier

We have constructed a high-power, tunable, narrow-bandwidth, cw laser based on a semiconductor tapered amplifier at a wavelength of 780 nm. Bandwidth narrowing to less than 1 MHz is accomplished with an intra-cavity interference filter, while 10 GHz of continuous tuning is possible with a cavity mirror mounted on a PZT stack. The laser output has a slope efficiency of 0.5 W/A, optical output power of 0.7 W, and will be used for laser cooling of atomic rubidium.   

Depth of focus in digital holography using spatial partially coherent light

Digital holography is a powerful, but young, imaging technology that has a vast array of applications; its strength lies in the ability to numerically focus on any plane within a sample from a single hologram and to use both amplitude and phase information from the intensity field to reconstruct the sample's 3D characteristics on a computer. The quality of many holograms, however, is compromised by speckle and other interference noise associated with the high-coherence lasers often used to illuminate the sample. Speckle noise may be diminished by lowering the coherence length of the source. In our experiments, partially coherent light was created by directing a laser beam through a rotating ground glass. We aimed to discern whether the coherence of the source could be systematically altered by changing the position of the ground glass within the focused laser beam. We anticipated that altering the coherence length would also systematically change the depth of focus. Initial results support our hypotheses.   

Photocatalytic properties of nanostructured TiO2 surfaces

Photocatalytic chemical reactions are actively explored for direct production of chemical fuels from sun light through electrolysis or for the clean-up of organic pollutants through photocatalysis. Titanium dioxide is a prototypical photocatalyst which has been studied extensively. However, there are still unanswered questions regarding the relationship between surface morphology and photocatalytic properties. In this study, we used ion beam assisted surface nanopatterning and UV-catalysis to investigate the dependence of photoreactivity on surface nanostructures. Energetic argon gas ions were used to induce self-formation of nanopatterns on TiO2 surfaces and the structure formation was characterized by atomic force microscopy. The influence of the surface structure on the photochemical properties was assessed through photocatalytic degradation of methyl orange in aqueous solution with a flat sample and a nanopatterned sample of TiO2, respectively. The resulting absorbance spectrums were then compared.   

Pushing the Limits of Nanoscale Imaging in Atomic Force Microscopy

The invention of scanning probe microscopy has revolutionized the field of nanotechnology. Atomic force microscopy is a branch of scanning probe microscopy in which an extremely sharp tip ($\sim $50 nm diameter) is held in contact with a sample surface. By maintaining constant force between the tip and the sample, the topography of a sample surface can be measured with high precision. The lateral resolution in this technique is however limited by the size of the tip. In this presentation, we present a method to deconvolve the effect of tip size and obtain higher resolution than presented by the experimentally obtained topographic image.   

Scanning Tunneling Microscopy of Fe Doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$

We will present a low temperature scanning tunneling microscopy (STM) study of the high-temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (Bi-2212) which has been intentionally doped with magnetic (Fe) impurities in order to locally disrupt superconductivity around the impurities. We examine spatial variations in the density of states in the vicinity of Fe impurities, and compare our results with previous STM studies of Ni doped Bi-2212. Notable differences between Fe and Ni impurities include differences in the number and energy locations of the impurity peaks. Our analysis shows that Fe is a weaker magnetic impurity than Ni and that the particle-hole symmetry present in the spectra of Ni impurities is not as obvious in Fe impurities. By studying how these impurities interact with superconductivity in Bi-2212 we hope to understand more about the superconducting mechanism in high-temperature superconductors.   

Theoretical \& Experimental Design \& Optimization of Multilayer Mirrors for Soft X-Ray Reflection

The reflection of soft X-rays is relevant for the development of ultra fast attosecond cameras, X-ray lithography, and the study of solar storms. Soft X-rays are typically absorbed, as opposed to reflected, due to all materials' absorptive nature. Co/C-am multilayers composed of 40 layers were deposited on Si \& SiO2 substrates by Magneton Sputtering technique and were characterized by grazing-incidence diffraction. Theoretical interfacial roughness and layer thicknesses were simulated using the commercial software IMD, while experimental values were estimated by fitting the reflectivity data. Low reflectivity values were observed at the locations of Bragg peaks. However, through deposition optimization, reflectivity values could potentially reach values above 60\%.   

Domain Coarsening Within the Ising Model on the Hyperbolic Plane

In spite of its simplicity, both the dynamics and equilibrium statistics of the Ising model are nontrivial. Previous studies in this REU have shown that the dynamics of metastable decay is very different in the hyperbolic plane than in the Euclidean plane. Here we perform Monte Carlo simulations of domain coarsening in the hyperbolic plane. We find that domain coarsening occurs more slowly than in the Euclidean plane.   

Quantum chaos: an introduction via chains of interacting spins-1/2

We discuss aspects of quantum chaos by focusing on spectral statistical properties and structures of eigenstates of quantum many-body systems. Quantum systems whose classical counterparts are chaotic have properties that differ from those of quantum systems whose classical counterparts are regular. One of the main signatures of what became known as quantum chaos is a spectrum showing repulsion of the energy levels. We show how level repulsion may develop in one-dimensional systems of interacting spins-1/2 which are devoid of random elements and involve only two-body interactions. We present a simple recipe to unfold the spectrum and emphasize the importance of taking into account the symmetries of the system. In addition to the statistics of eigenvalues, we analyze also how the structure of the eigenstates may indicate chaos. This is done by computing quantities that measure the level of delocalization of the eigenstates.   

Solving the Feynman--Gell-Mann Equation for the Electron

There are very few cases for which the Dirac equation can be solved exactly. Moreover, the techniques familiar from undergraduate quantum mechanics provide little help in solving its linear differential equations. Working instead with a two-component formalism, the transformed Dirac equation can be solved for cases of constant electric and magnetic fields for an electron. This approach was recommend in the famous 1958 paper by Feynman and Gell-Mann and has the form: $ (\imath\hbar\frac{\partial}{\partial t}-V)^2=-\hbar^2 c^2|\nabla-\imath\frac{e}{\hbar}\vec{A}|^2-e\hbar c \vec{\sigma}\cdot (\vec{B}c+\imath \vec{E})+(mc^2)^2 $, when acting on a two component spinor $\varphi$. We will show the solution of this equation for the cases of parallel $E$ \& $B$ fields, perpendicular fields, perhaps oblique fields and finish with a discussion of the Hydrogen atom. Through taking the nonrelativistic limits, these solutions can be verified for well-known conditions.   

Second harmonic generation and non-linear corrections to the high frequency susceptibility of a multiferroic material

We consider the effects of non-linear (second order) corrections to the high-frequency susceptibilities of a material that is simultaneously ferroelectric and a canted antiferromagnet (multiferroic). The non-linear corrections introduce a second harmonic term in the magnetic, electric, and multiferroic susceptibilities. Using the Landau-Lifshitz equation of motion for the magnetic components and the Landau-Khalatnikov relaxation equation for the electric polarization we theoretically compute the non-linear corrections to the susceptibilities for the optic antiferromagnetic mode, the acoustic mode, and the electric susceptibilities up to second order. Using realistic material parameters we find that the corrections have either a noticeable or negligible effect on the first order susceptibility values.   

Aluminum Nitride Nanofibers fabricated using Electrospinning and Nitridation

Aluminum Nitride (AlN) and other nitride semiconductors are important materials in the fields of optoelectronics and electronics. AlN nanofibers were synthesized using electrospinning and subsequent heating under N$_{2}$ and NH$_{3}$ atmospheres. The precursor solution for electrospining contains aluminium nitrate and cellulose acetate. The electrospun nanofibers were heated in N$_{2}$ to eliminate the polymer and produce Al$_{2}$O$_{3}$, and then nitridized at a temperature of 1200\r{ }C under NH$_{3}$ flow. Scanning Electron Microscopy (SEM) observations demonstrate the production of fibers with diameters ranging from a few nanometers to several micrometers. X-Ray Diffraction and UV-VIs analyses show the production of AlN nanofibers with hexagonal wurzite structure and a band gap of approximately approximately 6 eV. Current-Voltage measurements on a single AlN fiber with gold electrodes suggest the formation of a Schottky contact The fabrication method and results from the fibers characterization will be presented.   

Formation of Nanostructures at Gold Surfaces Exposed to Femtosecond Laser Pulses

The evolution of free-standing gold film irradiated by ultrashort laser pulses was simulated using molecular dynamics. The spatially non-uniform deposition of laser energy was modeled by a two-dimensional temperature profile applied during time of electron-ion energy exchange. Our simulations show that the ultrafast two-dimensional heating results in the melting and pressurization of a thin surface layer. Due to a non-uniform stress distribution, this molten layer expands to form a jet-like protrusion at the laser pulse's focal point. Above some critical stress, many voids start to nucleate forming a foam-like material covered by a thin liquid shell/cupola. The still expanding cupola may rupture forming a rim around the newly-developed crater. All these processes lead to complicated surface morphology, which becomes frozen at the nanosecond time scale. Geometrical characteristics of simulated surface profiles, including crater depth and size of frozen bubbles, agree well with experiment. Our simulations help to provide better insight into the atomistic mechanisms of nanostructure formation.   

Gold coated nanoparticles on silicon substrate

The study of Gold nanoparticles is very captivating because of their significance and applications as catalysts in restructuring technologies that are used for manufacturing medicine, energy production, transportation, computation, communication, and environmental changes. In this experiment, I have analyzed the morphology of gold nanoparticles using different techniques. A sputter coating technique was used to deposit gold on silicon substrate. During the process, depositions were performed using varying plasma coating times and voltages. The Gold nanoparticles were then analyzed using the Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The AFM and SEM data revealed that the coating surface morphology was dependent upon deposition conditions.   

Simulations of surface plasmons launched on gold nanogratings

A gold nanograting with a dielectric coating containing fluorescent molecules can enhance the intensity of fluorescence at certain frequencies due to the excitation of surface plasmons. Fluorescence is enhanced by two mechanisms: (1) an enhanced electromagnetic field at the excitation frequency of the fluorophores and (2) surface plasmon modes providing extra decay channels for fluorophores. Previous studies on corrugated film gratings show that coupling to higher diffraction order plasmons occur with lower efficiency. We find that for wire gratings, fluorophores couple to higher order plasmon modes on both sides of the gold wires with uniform efficiency. We also measure directional enhanced fluorescence on both the active (reflection) and substrate (transmission) side of the gratings. In this work, gold nanogratings with a dielectric coating were modeled using COMSOL Multiphysics software, which solves Maxwell's equations in the region of the grating. As the thickness of the dielectric layer containing the fluorophores is increased, the plasmon modes shift. The behavior of the gratings was simulated as a function of height of the gold wires and thickness of the dielectric coating. These simulations will inform future experiments.