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Open Access Publications from the University of California

Scholarly Works (9 results)

  • Thesis
  • Peer Reviewed
UC Irvine Electronic Theses and Dissertations (2019)

Metallic nanosctructures possess electromagnetic properties useful for various applications, such as surface enhanced Raman spectroscopy (SERS), harmonic generation and solar cells. Specifically, because of their ability in confining electromagnetic fields to nanoscopic dimensions, they provide an exquisite platform for conducting studies on molecules placed in their vicinity. In this dissertation, I introduce and study various plasmonic nanostructures to achieve electric and magnetic field enhancement for spectroscopy and microscopy applications.

First, I investigate feasibility of CMOS-compatible optical structures to develop novel integrated spectroscopy systems. I show that local field enhancement is achievable utilizing dimers of plasmonic nanospheres that can be assembled from colloidal solutions on top of a CMOS-compatible optical waveguide. The resonant dimer nanoantennas are excited by modes guided in the integrated silicon nitride waveguide. Specifically, I investigate how the field enhancement depends on dimer location, orientation, distance and excited waveguide mode. However, the field enhancement achievable with using oligomers is limited due to inherent losses of plasmonic particles. Thus, I study a novel structure called two-scale, in which Rayleigh anomaly caused by a 1D set of periodic nanorods is utilized. A thorough study of this structure is carried out by using an effective analytical-numerical model which is also compared to full-wave simulation results. Experimental results comparing enhancements in surface enhanced Raman scattering measurements with and without nanorods demonstrate the effectiveness of a Rayleigh anomaly in boosting the field enhancement.

The other side of the dissertation is dedicated to magnetic field enhancement. I propose various metallic and dielectric nanostructures for local magnetic field enhancement at optical frequencies. I show that dielectric structures can be a good alternative for their plasmonic counterpart due to their low loss. The idea behind each structure is supported by results from full-wave simulations. More importantly, I utilize azimuthally polarized beam as a way of boosting local magnetic field and isolating it from electric field. The magnetic field enhancement of these structures can be utilized in studying magnetic dipole transitions, magnetic imaging and enhanced spectroscopy applications.

    Cover page: Optical Nanoantennas for Sensors, Microscopy and SpectroscopyCreative Commons 'BY-NC-ND' version 4.0 license
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2017)
    Pseudomonas aeruginosa is an opportunistic, biofilm-forming pathogen. P. aeruginosa produces pyocyanin, a secondary metabolite as part of its quorum sensing signaling system activated during biofilm formation. Self-organized plasmonic nanogaps provide enhancements to Raman scattering signals to achieve 10 ng/mL limit of detection of pyocyanin that enables early detection of biofilm formation. We report, for the first time, in-line biofilm detection in microfluidic channels as early as three hours from the onset of bacterial culturing and quantification spanning five-orders of magnitude of pyocyanin concentration during biofilm formation of PA14.
      Cover page: Self-Assembled Plasmonic NanogapsCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2016)
      We investigate the feasibility of CMOS-compatible optical structures to develop novel integrated spectroscopy systems. We show that local field enhancement is achievable utilizing dimers of plasmonic nanospheres that can be assembled from colloidal solutions on top of a CMOS-compatible optical waveguide. The resonant dimer nanoantennas are excited by modes guided in the integrated silicon nitride waveguide. Simulations show that 100-fold electric field enhancement builds up in the dimer gap as compared to the waveguide evanescent field amplitude at the same location. We investigate how the field enhancement depends on dimer location, orientation, distance and excited waveguide mode.
        Cover page: Electric field enhancement with plasmonic colloidal nanoantennas excited by a silicon nitride waveguide.
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2018)
        We propose a novel multi-length-scale architecture for giant electric field enhancement. We investigate the capability of our structure to boost the electric field analytically and using fullwave simulations and verify our results with surface-enhanced Raman spectroscopy experiment.
          Cover page: Electric Field Enhancement by Two-scale StructureCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2016)
          Advances in understanding chemical and physical driving forces in self-assembly allow the fabrication of unique nanoarchitectures with subwavelength building blocks as the basis for plasmonic and metamaterial devices. Chemical crosslinking of colloidal nanospheres has produced among the smallest gap spacings, necessary to obtain regions of greatly enhanced electric field, hotspots, which are critical to tailor light-matter interactions. However, obtaining uniform electromagnetic response of dense nanoantennas over large area for use in devices remains challenging. In this work, electrohydrodynamic (EHD) flow and chemical crosslinking is combined to form dense, yet discrete, Au nanosphere clusters (oligomers) on a working electrode. EHD provides a long range driving force to bring nanospheres together and anhydride crosslinking yields 0.9 nm gap spacings. Using selective chemistry, nanospheres are simultaneously crosslinked onto a block copolymer template, producing oligomers with a narrower wavelength band width and higher hotspot intensity than monolayer structures produced without a template. We investigate nanoantenna response via full wave simulations, ultraviolet-visible spectroscopy, and surface enhanced Raman scattering (SERS). Nanoantennas exhibit uniform hotspot intensity and gap spacing. Simulations show field enhancements of 600, correlating well with measured average SERS enhancement of 1.4x10^9. Nanoantenna substrates produce a SERS signal with a relative standard deviation of 10% measured over a 1 mm2 area, crucial for nano-optical devices such as optical sensors, among other applications. Understanding long range (EHD flow) and short range (chemical crosslinking) driving forces provides the control for assembling colloidal nanoparticles in architectures for large area plasmonic and metasurface device fabrication.
            Cover page: Bridging the Gap between Crosslinking Chemistry and Directed Assembly of Metasurfaces Using Electrohydrodynamic FlowCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2019)
            We propose a two-scale architecture by simultaneously exploiting both the Rayleigh anomaly and localized surface plasmon resonance in colloidal directly assembled oligomers to achieve giant field enhancement, overcoming limitations by losses and the intrinsic nonlocality of the dielectric response of metal nanoparticles. We show that the total field enhancement in the nanogap hotspots of this two-scale structure is the product of the enhancements of each individual geometrical scale. Under this scheme, chemically assembled metallic oligomers enable strong enhancement of the near-field in resultant few-nanometer gaps at their plasmonic resonance. An extra enhancement factor results from combining the plasmonic resonance enhancement of these oligomers (nanometer scale) with a Rayleigh anomaly caused by a one-dimensional (1D) periodic set of nanorods (micrometer scale) fabricated using a lithographic method. This extra field enhancement cannot be obtained by further reducing the nanoantennas' gaps, since they are already in the few-nm ranges thanks to direct chemical assembly of oligomers. Experimental results verify that surface-enhanced Raman spectroscopy (SERS) signal is enhanced by more than one order of magnitude due to the Rayleigh anomaly as compared to the signal taken by using only metallic oligomers without periodic nanorods. The proposed two-scale structure could serve as a substrate for different spectroscopy techniques including SERS and can improve the sensitivity of spectroscopy for ultrasensitive biomolecule detection and single molecule spectroscopy studies.
              Cover page: Two-Scale Structure for Giant Field Enhancement: Combination of Rayleigh Anomaly and Colloidal Plasmonic Resonance
              • Article
              • Peer Reviewed
              UC Irvine Previously Published Works (2015)
              Pseudomonas aeruginosa (PA), a biofilm forming bacterium, commonly affects cystic fibrosis, burn victims, and immunocompromised patients. PA produces pyocyanin, an aromatic, redox active, secondary metabolite as part of its quorum sensing signaling system activated during biofilm formation. Surface enhanced Raman scattering (SERS) sensors composed of Au nanospheres chemically assembled into clusters on diblock copolymer templates were fabricated and the ability to detect pyocyanin to monitor biofilm formation was investigated. Electromagnetic full wave simulations of clusters observed in scanning electron microcopy images show that the localized surface plasmon resonance wavelength is 696 nm for a dimer with a gap spacing of 1 nm in an average dielectric environment of the polymer and analyte; the local electric field enhancement is on the order of 400 at resonance, relative to free space. SERS data acquired at 785 nm excitation from a monolayer of benzenethiol on fabricated samples was compared with Raman data of pure benzenethiol and enhancement factors as large as 8×109 were calculated that are consistent with simulated field enhancements. Using this system, the limit of detection of pyocyanin in pure gradients was determined to be 10 parts per billion. In SERS data of the supernatant from the time dependent growth of PA shaking cultures, pyocyanin vibrational modes were clearly observable during the logarithmic growth phase corresponding to activation of genes related to biofilm formation. These results pave the way for the use of SERS sensors for the early detection of biofilm formation, leading to reduced healthcare costs and better patient outcomes.
                Cover page: Surface enhanced Raman scattering for detection of Pseudomonas aeruginosa quorum sensing compoundsCreative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                UC Irvine Previously Published Works (2017)
                Nanoparticles from colloidal solution-with controlled composition, size, and shape-serve as excellent building blocks for plasmonic devices and metasurfaces. However, understanding hierarchical driving forces affecting the geometry of oligomers and interparticle gap spacings is still needed to fabricate high-density architectures over large areas. Here, electrohydrodynamic (EHD) flow is used as a long-range driving force to enable carbodiimide cross-linking between nanospheres and produces oligomers exhibiting sub-nanometer gap spacing over mm2 areas. Anhydride linkers between nanospheres are observed via surface-enhanced Raman scattering (SERS) spectroscopy. The anhydride linkers are cleavable via nucleophilic substitution and enable placement of nucleophilic molecules in electromagnetic hotspots. Atomistic simulations elucidate that the transient attractive force provided by EHD flow is needed to provide a sufficient residence time for anhydride cross-linking to overcome slow reaction kinetics. This synergistic analysis shows assembly involves an interplay between long-range driving forces increasing nanoparticle-nanoparticle interactions and probability that ligands are in proximity to overcome activation energy barriers associated with short-range chemical reactions. Absorption spectroscopy and electromagnetic full-wave simulations show that variations in nanogap spacing have a greater influence on optical response than variations in close-packed oligomer geometry. The EHD flow-anhydride cross-linking assembly method enables close-packed oligomers with uniform gap spacings that produce uniform SERS enhancement factors. These results demonstrate the efficacy of colloidal driving forces to selectively enable chemical reactions leading to future assembly platforms for large-area nanodevices.
                  Cover page: Driving Chemical Reactions in Plasmonic Nanogaps with Electrohydrodynamic Flow.
                  • Article
                  • Peer Reviewed
                  UC Irvine Previously Published Works (2018)
                  Detection of bacterial metabolites at low concentrations in fluids with complex background allows for applications ranging from detecting biomarkers of respiratory infections to identifying contaminated medical instruments. Surface-enhanced Raman scattering (SERS) spectroscopy, when utilizing plasmonic nanogaps, has the relatively unique capacity to reach trace molecular detection limits in a label-free format, yet large-area device fabrication incorporating nanogaps with this level of performance has proven difficult. Here, we demonstrate the advantages of using chemical assembly to fabricate SERS surfaces with controlled nanometer gap spacings between plasmonic nanospheres. Control of nanogap spacings via the length of the chemical crosslinker provides uniform SERS signals, exhibiting detection of pyocyanin, a secondary metabolite of Pseudomonas aeruginosa, in aqueous media at concentration of 100 pg·mL-1. When using machine learning algorithms to analyze the SERS data of the conditioned medium from a bacterial culture, having a more complex background, we achieve 1 ng·mL-1 limit of detection of pyocyanin and robust quantification of concentration spanning 5 orders of magnitude. Nanogaps are also incorporated in an in-line microfluidic device, enabling longitudinal monitoring of P. aeruginosa biofilm formation via rapid pyocyanin detection in a medium effluent as early as 3 h after inoculation and quantification in under 9 h. Surface-attached bacteria exposed to a bactericidal antibiotic were differentially less susceptible after 10 h of growth, indicating that these devices may be useful for early intervention of bacterial infections.
                    Cover page: Longitudinal Monitoring of Biofilm Formation via Robust Surface-Enhanced Raman Scattering Quantification of Pseudomonas aeruginosa-Produced Metabolites
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