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Open Access Publications from the University of California

Scholarly Works (24 results)

  • Thesis
  • Peer Reviewed
UC Irvine Electronic Theses and Dissertations (2016)

The objective of this dissertation is to develop a novel method to measure nanoscale optical field distribution using optical induce forces. We demonstrate the application of Atomic Force Microscopy (AFM) for detecting nearfield optical induced force including axial force and lateral force. We develop a method that detects laterally and axially induced optical forces by different flexure and torsional modes which can enable simultaneous multi-channel detection of nanoscale optical forces along different orientations.

We have mapped optical near-fields with nanometer resolution, limited only by the AFM probe geometry. By detecting the optical force between a gold coated AFM probe and its image dipole on a glass substrate, we profile the electric field distributions in axial force of tightly focused laser beams with different polarizations. The experimentally recorded focal force maps agree well with theoretical predictions based on the dipole-dipole interaction model. We experimentally estimate the aspect ratio of the apex of gold coated AFM probe using only optical forces. We also show that the optical force between a sharp gold coated AFM probe and a spherical gold nanoparticle of radius 15 nm, is indicative of the electric field distribution between the two interacting particles.

We also show a controlled method to fabricate tips that have high enhancement factor, which is desirable for the linear and non-linear response of the molecule response.

Photon Induced Force Microscopy (PIFM) allows for background free, thermal noise limited mechanical imaging of optical phenomenon over wide range of wavelengths from visible to RF with detection sensitivity limited only by AFM performance. The use of an AFM cantilever as a multi-channel detector paves the way for simultaneous PiFM detection of molecular responses with different incident field polarizations.

    Cover page: Force Measurement of Nanoscale Optical Nearfield Distribution
    • Article
    UC Irvine Previously Published Works (2021)
    In the framework where the strong coupling is dynamical, the QCD sector may confine at a much higher temperature than it would in the Standard Model, and the temperature-dependent mass of the QCD axion evolves in a non-trivial way. We find that, depending on the evolution of $\Lambda_{\mathrm{QCD}}$, the axion field may undergo multiple distinct phases of damping and oscillation leading generically to a suppression of its relic abundance. Such a suppression could therefore open up a wide range of parameter space, resurrecting in particular axion dark-matter models with a large Peccei-Quinn scale $f_a\gg 10^{12}~\mathrm{GeV}$, i.e., with a lighter mass than the standard QCD axion.
      Cover page: Resurrecting Low-Mass Axion Dark Matter Via a Dynamical QCD Scale
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2021)
      In the framework where the strong coupling is dynamical, the QCD sector may confine at a much higher temperature than it would in the Standard Model, and the temperature-dependent mass of the QCD axion evolves in a non-trivial way. We find that, depending on the evolution of ΛQCD, the axion field may undergo multiple distinct phases of damping and oscillation leading generically to a suppression of its relic abundance. Such a suppression could therefore open up a wide range of parameter space, resurrecting in particular axion dark-matter models with a large Peccei-Quinn scale fa ≫ 1012 GeV, i.e., with a lighter mass than the standard QCD axion.
        • Article
        • Peer Reviewed
        UC Santa Barbara Previously Published Works (2017)
        Progress in the molecular design and processing protocols of semiconducting polymers has opened significant opportunities for the fabrication of low-cost plastic electronic devices. Recent studies indicate that field-effect transistors and organic solar cells fabricated using narrow-bandgap regioregular polymers with translational symmetries in the direction of the backbone vector often outperform those containing analogous regiorandom polymers. This review addresses the cutting edge of regioregularity chemistry, in particular how to control the spatial distribution in the molecular structures and how this order translates to more ordered bulk morphologies. The effect of regioregularity on charge transport and photovoltaic properties is also outlined.
          Cover page: Regioregular narrow-bandgap-conjugated polymers for plastic electronics
          • Article
          • Peer Reviewed
          UC Santa Barbara Previously Published Works (2017)
          Progress in the molecular design and processing protocols of semiconducting polymers has opened significant opportunities for the fabrication of low-cost plastic electronic devices. Recent studies indicate that field-effect transistors and organic solar cells fabricated using narrow-bandgap regioregular polymers with translational symmetries in the direction of the backbone vector often outperform those containing analogous regiorandom polymers. This review addresses the cutting edge of regioregularity chemistry, in particular how to control the spatial distribution in the molecular structures and how this order translates to more ordered bulk morphologies. The effect of regioregularity on charge transport and photovoltaic properties is also outlined.
            Cover page: Regioregular narrow-bandgap-conjugated polymers for plastic electronics.
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2021)
            The authors have requested that this preprint be removed from Research Square.
              Cover page: The Impact of Policy and COVID-19 Prevention Strategies in China: A Qualitative StudyCreative Commons 'BY-NC-ND' version 4.0 license
              • Article
              UC Irvine Previously Published Works (2021)
              One signature of an expanding universe is the time-variation of the cosmological abundances of its different components. For example, a radiation-dominated universe inevitably gives way to a matter-dominated universe, and critical moments such as matter-radiation equality are fleeting. In this paper, we point out that this lore is not always correct, and that it is possible to obtain a form of "stasis" in which the relative cosmological abundances $\Omega_i$ of the different components remain unchanged over extended cosmological epochs, even as the universe expands. Moreover, we demonstrate that such situations are not fine-tuned, but are actually global attractors within certain cosmological frameworks, with the universe naturally evolving towards such long-lasting periods of stasis for a wide variety of initial conditions. The existence of this kind of stasis therefore gives rise to a host of new theoretical possibilities across the entire cosmological timeline, ranging from potential implications for primordial density perturbations, dark-matter production, and structure formation all the way to early reheating, early matter-dominated eras, and even the age of the universe.
                Cover page: Stasis in an Expanding Universe: A Recipe for Stable Mixed-Component Cosmological Eras
                • Article
                • Peer Reviewed
                UC Irvine Previously Published Works (2022)
                One signature of an expanding universe is the time variation of the cosmological abundances of its different components. For example, a radiation-dominated universe inevitably gives way to a matter-dominated universe, and critical moments such as matter-radiation equality are fleeting. In this paper, we point out that this lore is not always correct and that it is possible to obtain a form of "stasis"in which the relative cosmological abundances ωi of the different components remain unchanged over extended cosmological epochs, even as the universe expands. Moreover, we demonstrate that such situations are not fine-tuned but are actually global attractors within certain cosmological frameworks, with the universe naturally evolving toward such long-lasting periods of stasis for a wide variety of initial conditions. The existence of this kind of stasis therefore gives rise to a host of new theoretical possibilities across the entire cosmological timeline, ranging from potential implications for primordial density perturbations, dark-matter production, and structure formation all the way to early reheating, early matter-dominated eras, and even the age of the Universe.
                  • Article
                  • Peer Reviewed
                  UC Irvine Previously Published Works (2024)
                  Many theories of physics beyond the Standard Model predict the existence of large or infinite towers of decaying states. In a previous paper [K. R. Dienes et al., Phys. Rev. D 105, 023530 (2022)PRVDAQ2470-0010], we pointed out that this can give rise to a surprising cosmological phenomenon that we dubbed "stasis"during which the relative abundances of matter and radiation remain constant across extended cosmological eras even though the universe is expanding. Indeed, such stasis epochs are universal attractors, with the universe necessarily entering (and later exiting) such epochs for a wide variety of initial conditions. Matter/radiation stasis is therefore an important and potentially unavoidable feature of many BSM cosmologies. In this paper we extend our arguments to universes containing significant amounts of vacuum energy, and demonstrate that such universes also give rise to various forms of stasis between vacuum energy and either matter or radiation. We also demonstrate the existence of several forms of "triple stasis"during which the abundances of matter, radiation, and vacuum energy all simultaneously remain fixed despite cosmological expansion. We further describe several close variants of stasis which we call "quasi-stasis"and "oscillatory stasis,"and discuss the circumstances under which each of these can arise. Finally, we develop a general formalism for understanding the emergence of stasis within BSM cosmologies irrespective of the number or type of different energy components involved. Taken together, these results greatly expand the range of theoretical and phenomenological possibilities for the physics of the early universe, introducing new types of cosmological eras which may play an intrinsic and potentially inevitable role within numerous BSM cosmologies.
                    Cover page: Stasis, stasis, triple stasis: A theoretical study of cosmological stasisCreative Commons 'BY' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UCLA Previously Published Works (2021)
                    Understanding the particular mechanisms by which vulnerability and capability factors influence cognitive reactivity (CR) can contribute to an enhanced capacity to adequately react to depression. However, few studies have explored the CR model. The main aim of the present study was to develop a model that specifies the predictive effects of CR for depression among individuals at high risk for first-episode and recurrent depression. A national cross-sectional, online study using convenience sampling was conducted among 587 vulnerable healthy individuals and 224 depressed patients in China. A battery of indices, including measures of CR, social support, resilience, self-compassion, life events, neuroticism, sleep condition, and negative emotion, were collected. A structural equation model was applied to analyse the data. The final first-episode and recurrent depressive symptoms of the CR models showed good model fit. According to the models, 45%-52% of the variance in depressive symptom was predicted by CR. Social support, self-compassion, resilience, and positive life events directly influenced CR, with β values ranging from -0.18 to -0.24 (P < 0.01). Neuroticism, negative emotion, poor sleep conditions, and negative life events also directly and positively influenced CR (P < 0.01). The relationship between these negative or positive contributing factors and depression was also indirectly influenced by CR (P < 0.01). Our findings demonstrate the role of CR in the prevention and treatment of depression. The first-episode and recurrent depressive symptoms of the CR models considering both vulnerabilities and capabilities of CR in the psychopathology of depression provide a theoretical basis for interventions that reduce CR in high-risk populations.
                      Cover page: Cognitive reactivity among high‐risk individuals at the first and recurrent episode of depression symptomology: A structural equation modelling analysis
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