Thermoacoustic Refrigerator

Thermoacoustic refrigerators have typically been designed to cool an isolated working fluid, necessitating a heat exchanging device to draw heat from the outside target fluid into the cooled working fluid. For example, a thermoacoustic refrigerator designed to chill air might utilize argon as an isolated working fluid, necessitating a heat exchanger to draw heat from the air to the argon. A second heat exchanger would be required to draw heat from the argon to a cold sink. A design team at Utah State University has created a thermoacoustic refrigerator which uses air at atmospheric pressure as its working fluid. By selecting an open-open tube design with pressure nodes at both ends, the design team was able to impose airflow directly through the thermoacoustic stack. In this way, atmospheric air is cooled directly, obviating the need for a heat exchanger to operate between the air and an isolated working fluid

Phase of Kerr-based few-cycle parametric amplification

Kerr instability amplification can amplify over an octave of spectrum, a broad bandwdith supporting few-cycle pulses. However, dispersion management in this regime is crucial to maintain the ultrashort pulse duration. In our simulations, we find that the dispersion of Kerr instability amplification is near zero at the pump wavelength, and can be compensated pre-amplification to generate near-transform-limited amplified few-cycle pulses. We also find the phase of the amplified pulse depends on the seed phase and is independent of the pump, and does not significantly depend on the pump intensity. We discuss chirping the seed pulse to avoid saturation, a route for generating sub-mJ few-cycle pulses from the Kerr nonlinearity

Supercontinuum amplification by Kerr instability

The versatility of optical parametric amplifiers make them excellent sources for next-generation ultrashort strong-field physics experiments, however phase matching considerations limit the available bandwidth. We demonstrate supercontinuum amplification in a four-wave optical parametric amplifier by Kerr instability. Experimentally, we amplify spectra that span nearly an octave without spatial chirp, with two-cycle transform limited pulse duration. We also theoretically explore regimes that demonstrate single-cycle amplification in the near infrared

A Numerical Investigation of Sidebands in High Harmonic Spectra

A Numerical Investigation of Sidebands in High Harmonic Spectra When an atom is subjected to coherent electromagnetic radiation, it is possible for its electrons to be excited by the electric field that is inherent to this radiation. A commonly used source for coherent electromagnetic radiation is a laser, which delivers an electric field that oscillates in time. High harmonic generation (HHG) is the process of applying a strong laser field to a material, such that one of its electrons is accelerated away from the atom, only to be driven back toward it by the oscillating laser field. As the electron is repeatedly accelerated away from and back to this ion, it emits photons that are of odd-integer harmonics of the driving field. In other words, light is emitted at frequencies that are odd integer multiples of the laser frequency. When a second, weaker field is applied to the system, the trajectory of the electron is perturbed at its maximum distance from the ion. This process causes a signal to occur in the harmonic spectrum that is characteristic of the perturbing field energy. This signal is referred to as sidebands due to their shape and the way they appear on either side of the even harmonics. These sidebands have been shown to contain valuable information regarding the optical free-induction decay of complex biomolecules, which can be used to spectroscopically identify these molecules. Numerical simulations of this physical process were performed using the split-step Fourier method of solving the time-dependent Schrödinger equation. The results of these simulations provide insights on how to optimize the signal of the sidebands in future laboratory experiments

Mechanical behavior and microstructure of self-assembling oligopeptide gels

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 97-103).Hydrogels have become widely used in the fields of tissue engineering and drug delivery. One class of hydrogel is formed from synthetic oligopeptides that self-assemble into a solution of beta-sheet filaments. These filaments can aggregate to form a gel suitable for culture of various cell types. Examples of such self-assembling peptides are RAD16-I, RAD16-II, and KLD-12. One limitation of self-assembling peptide hydrogels is their mechanical weakness. Herein are presented computational and experimental studies that elucidate the microstructure and mechanical behavior of these materials. Strategies to enhance their mechanical properties are also examined. Steered molecular dynamics modeling was used to characterize the mechanical interaction between filaments, and a coarse-grained model was developed to extend the system to ordinary time scales. A microindentation assay was developed and used to characterize the mechanical properties of gels. Several strategies for enhancing the gels' mechanical properties were tested. Gel microstructure was observed in thin sections of material with transmission electron microscopy, revealing in detail the loose, disorganized structure of assembled beta-sheet filaments. The results demonstrate that these self-assembling peptide gels are formed from a loosely arranged structure of beta-sheet filaments, not from dense bundles of parallel filaments as was previously proposed. Estimates of gel stiffness based on this loose structure are in approximate agreement with experimental measurements. Among the strategies tested to increase gel stiffness, introducing cross-links and increasing solid concentration proved to be effective approaches.by Nathan Allen Hammond.Ph.D

That Water Stinks! Will Changes in Water Quality Alter Blue Crab Response to Pesticides?

Nathan Hammond, Allen Schaefer, and Sophie Bott are students in Biological Sciences at Louisiana Tech University. Jennifer M. Hill is an Assistant Professor in Biological Sciences at Louisiana Tech University

Do Differing Enrichment Methodologies Affect the Belowground Productivity of Spartina Alterniflora?

Mariana E. Penny and Stephanie W. Plaisance are students in Environmental Science at Louisiana Tech University. Nathan Hammond is a student in Biological Sciences at Louisiana Tech University. Jennifer M. Hill is an Assistant Professor in Biological Sciences at Louisiana Tech University

Real-Time FTIR for Applications in Attoscience and Beyond

Real-Time FTIR for Applications in Attoscience and Beyond ACME Research Group Nathan Drouillard, supervised by Dr. TJ Hammond February 28th, 2020 Fourier transform infrared spectroscopy (FTIR) is one of the most sensitive spectroscopic techniques, which is useful for measuring weak spectral signatures. This project involves developing the software for a home-built FTIR spectrometer and optimizing the device for attosecond (1 as = 1e-18 s) spectral changes. We convert the interferometric optical signal measured with a photodiode (light sensor) using an analog-to-digital converter (ADC), and store the data into memory using the Python programming language. This signal arises due to interference between two overlapping laser beams in the interferometer. Inherent limitations of this process are that it is relatively slow for large datasets and does not allow for signal analysis in real-time. To overcome these limitations, we have adopted the Python library Bokeh to create a private web server that will allow for real-time observation of the signal in both time and frequency domains (through the Fourier transform). Python is a ubiquitous language, thus making this software quite versatile from a hardware implementation and collaboration point of view. The development of such software is extremely valuable and will be applied to future experiments that involve extracting weak signals in the infrared regime. One possible application is chemical identification, for instance air and water pollutants. The hope is that this could be useful in cleanup efforts to identify water pollutants in the Great Lakes, even in very small quantities

Applying Fourier transform spectroscopy to ultrafast measurements

This research involves understanding the effects of ultrashort laser pulses, which are generally of the order of femtoseconds (1 fs = 10-15 s) or attoseconds (1 as = 10-18 s), on a material and modeling light-material interaction. Attosecond time resolution is necessary to measure electron wave packet motion, with shorter pulses being important because they give us better temporal resolution. Ultrashort pulses can resolve electron motion and electronic transport properties, which have applications in telecommunications, quantum materials, and protein folding. We aim to create ultrashort laser pulses to excite and measure the electron wave packet motion in a femtosecond (or attosecond) time scale. We perform the measurement with an experimental setup similar to Fourier transform infrared spectroscopy (FTIR), a well-developed high-resolution spectroscopic technique. FTIR can measure weak absorption bands in materials; we will combine this idea with ultrashort laser pulses to measure transient absorption properties. We introduce a semiconductor material (or a metal) in to one arm of the Michelson Interferometer to measure the electronic motion inside or off the surface of the material. Measuring and controlling electronic properties at these timescales (which is 6 orders of magnitude faster than what is currently possible) is a crucial step in developing next generation technologies. This research involves understanding the effects of ultrashort laser pulses, which are generally of the order of femtoseconds (1 fs = 10-15 s) or attoseconds (1 as = 10-18 s), on a material and modeling light-material interaction. Attosecond time resolution is necessary to measure electron wave packet motion, with shorter pulses being important because they give us better temporal resolution. Ultrashort pulses can resolve electron motion and electronic transport properties, which have applications in telecommunications, quantum materials, and protein folding. We aim to create ultrashort laser pulses to excite and measure the electron wave packet motion in a femtosecond (or attosecond) time scale. We perform the measurement with an experimental setup similar to Fourier transform infrared spectroscopy (FTIR), a well-developed high-resolution spectroscopic technique. FTIR can measure weak absorption bands in materials; we will combine this idea with ultrashort laser pulses to measure transient absorption properties. We introduce a semiconductor material (or a metal) in to one arm of the Michelson Interferometer to measure the electronic motion inside or off the surface of the material. Measuring and controlling electronic properties at these timescales (which is 6 orders of magnitude faster than what is currently possible) is a crucial step in developing next generation technologies

Stoichiometry control of magnetron sputtered Bi2_2Sr2_2Ca1−x_{1-x}Yx_xCu2_2Oy_y (0≤\lex≤\le0.5) thin film, composition spread libraries: Substrate bias and gas density factors

A magnetron sputtering method for the production of thin-film libraries with a spatially varying composition, x, in Bi2Sr2Ca1-xYxCu2Oy (0<=x<=0.5) has been developed. Two targets with a composition of Bi2Sr2YCu2O_{8.5 + \delta} and Bi_2Sr_2CaCu_2O_{8 + \delta} are co-sputtered with appropriate masks. The target masks produce a linear variation in opposite, but co-linear radial direction, and the rotation speed of the substrate table is sufficient to intimately mix the atoms. EDS/WDS composition studies of the films show a depletion of Sr and Bi that is due to oxygen anion resputtering. The depletion is most pronounced at the centre of the film (i.e. on-axis with the target) and falls off symmetrically to either side of the 75 mm substrate. At either edge of the film the stoichiometry matches the desired ratios. Using a 12 mTorr process gas of argon and oxygen in a 2:1 ratio, the strontium depletion is corrected. The bismuth depletion is eliminated by employing a rotating carbon brush apparatus which supplies a -20 V DC bias to the sample substrate. The negative substrate bias has been used successfully with an increased chamber pressure to eliminate the resputtering effect across the film. The result is a thin film composition spread library with the desired stoichiometry.Comment: 16 pages, 12 figures, 4 tables, submitted to Physica C - Superconductivity (April 15, 2005), elsart.st