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Browsing by Author "Bartels, Randy, advisor"

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    Advances in single-pixel imaging toward biological applications
    (Colorado State University. Libraries, 2014) Winters, David G., author; Bartels, Randy, advisor; Marconi, Mario C., committee member; Prasad, Ashok, committee member; Bernstein, Elliot R., committee member
    In this work, we discuss two new methods for single-pixel imaging. First, we leverage advances in laser metrology and frequency synthesis to measure small shifts in the center frequency of an optical pulse. Pulses acquire such shifts when probing a transient optical susceptibility, as in impulsive stimulated Raman scattering, which we use to demonstrate the technique. We analyze the limits of this technique with regard to fundamental noise, and predict detection sensitivity in these limiting cases. We then present work on imaging in two dimensions, both x-y and x-z, using single element detectors. We accomplish this by multiplexing spatial frequency projections in time, allowing rapid two dimensional imaging without an imaging detector. As we eliminate the imaging detector, the sensitivity to scattering is dramatically decreased, allowing the method to be used deep in scattering tissue. Results are shown for several geometries and experimental configurations, demonstrating imaging capabilities across a variety of sample types, including fluorescent and biological samples.
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    Advancing impulsive Raman spectroscopy and microscopy for biological applications
    (Colorado State University. Libraries, 2024) Smith, David R., author; Bartels, Randy, advisor; Wilson, Jesse, advisor; Tobet, Stuart, committee member; Jost, Dylan, committee member
    Chemically sensitive, label-free spectroscopy and microscopy is a critical tool for the study of many complex and dynamic biological systems. The development of the impulsive stimulated Raman scattering (ISRS) techniques in this thesis represent important steps forward in addressing the ability to interrogate Raman vibrations in complex and scattering samples, particularly low frequency Raman modes.
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    Bulk and interface vibrational Raman spectroscopy with coherence modulated optical susceptibilities
    (Colorado State University. Libraries, 2010) Wilson, Jesse W., author; Bartels, Randy, advisor; Krapf, Diego, committee member; Marconi, Mario, committee member; Roberts, Jacob, committee member
    The effect on an ultrashort probe pulse of an impulsively prepared vibrational coherence is described by effective linear and nonlinear optical susceptibility perturbations. Linear susceptibility perturbations modulate both the amplitude and phase of a probe pulse. Three spectral interferometry methods are described for measuring this phase modulation, geared toward spectral resolution, noise suppression, and rapid data acquisition. Third-order nonlinear interactions perturbations may be used to acquire surface-specific Raman spectra. While second-order spectroscopy is an established surface-specific technique, odd-order methods have been passed over because the signal is generated in the bulk media. We show that through a surface Fresnel modulation, coherence-modulated third harmonic generation can be used to obtain surface-specific vibrational information. Bulk and interface contributions to the vibrational signal are separated by scanning the interface through the focus of the laser beam.
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    Development and implementation of near-infrared ultrafast laser sources generated by nonlinear fiber propagation
    (Colorado State University. Libraries, 2015) Domingue, Scott R., author; Bartels, Randy, advisor; Krummel, Amber, committee member; Krapf, Diego, committee member; Marconi, Mario, committee member
    This dissertation is broken up into three parts: (I) generating high-quality ultrafast pulses around 1060 nm, (II) using the pulses from part (I) to generate pulses around 1300 nm, and (III) analyzing newly developed experimental theories and methods utilizing these pulses for linear and nonlinear microscopy. The majority of the work in this dissertation is choreographing the dance between nonlinear spectral broadening in optical fiber and the associated complexity in accumulated spectral phase. We have developed and employed several systems which manage to accomplish this task quite elegantly due to our technological contributions, producing high-quality pulses with high oscillator-type pulse energies both at 1060 and 1250 nm. In addition to developing some theory and techniques extending current types of nonlinear microscopy, we have as a capstone an experimental microscope cascading several of our primary source and application technologies to conduct an entirely new form of spectroscopic absorption imaging.
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    Multiphoton spatial frequency modulated imaging
    (Colorado State University. Libraries, 2023) Wernsing, Keith, author; Bartels, Randy, advisor; Squier, Jeff, committee member; Wilson, Jesse, committee member; Borch, Thomas, committee member
    Far-field optical microscopy has seen significant development in the last 20 years in its ability to resolve specimen information beyond the diffraction limit. However, nearly all of these super-resolution techniques are predicated on the use of fluorescence as the contrast mechanism in the sample. While the variety of fluorophores available for labeling a sample are a widely-utilized tool, in many instances non-fluorescent contrast mechanisms also provide valuable information. Multiphoton microscopy is one route to probing non-fluorescent contrast mechanisms. It has the benefit of sampling multiple contrast mechanisms at once, including second- and third-harmonic generation and Raman vibrational characteristics, as well as autofluorescence and labeled fluorescence. However, development of super-resolving techniques for coherent scattering processes like harmonic generation or coherent Raman excitation has lagged behind that of incoherent scattering processes like fluorescence. In this work I present the first technique to simultaneously enhance resolution in both real-state (e.g., fluorescence) and virtual-state (e.g. harmonic generation) molecular excitation mechanisms, known as multiphoton spatial-frequency modulated imaging (MP-SPIFI). Standard SPIFI works by projecting spatial cosine patterns onto the sample and gathering object spatial frequency information. Multiphoton SPIFI generates harmonics of these cosine patterns and therein gathers information beyond the frequency passband of the microscope. We demonstrate our initial results with two-photon fluorescence and SHG. An extensive model is built describing the super-resolved image formation process. We then present a method for extending the native, 1D resolution enhancement into two dimensions for an isotropic enhancement. Finally, we present development of two femtosecond, amplified pulsed laser sources tailored to boost SNR in multiphoton processes, through parabolic pulse amplification, and chirped pulse fiber broadening, in order to deliver the high average power & high peak power required by MP-SPIFI for driving nonlinear processes across a line-focus geometry.
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    Ultrafast quantum coherent control apparatus
    (Colorado State University. Libraries, 2007) Wilson, Jesse, author; Bartels, Randy, advisor; Levinger, Nancy, committee member; Rocca, Jorge J. G., committee member
    In recent years, the availability of ultrafast laser sources has opened up a number of opportunities for exploring molecular dynamics that take place on femtosecond time scales. Coherent control experiments involve creating, manipulating, and measuring these ultrafast phenomena. Such controllable processes include second harmonic generation (SHG), creation of vibrational wavepackets, high-harmonic generation, photodissociation, and more.The foundation to all these experiments is an ultrafast pulse shaper and a high-dimensional search algorithm. Here we present the design and construction of a spectral phase-only pulse shaper, including details on alignment and calibration. We also demonstrate the functionality of the device by producing several pulse profiles that could be potentially useful in coherent control experiments. A covariance matrix analysis evolutionary strategy (CMAES) is also implemented, and demonstrated to optimize SHG in a nonlinear crystal. Finally, recognizing that phase-only shapers cannot produce the full range of temporal shapes available to a given input pulse, we show the design and construction of a pulse shaper which uses only a single linear phase mask to gain control over both spectral phase and amplitude by use of phase gratings.