Browsing by Author "Wen, Fangfang"

Now showing 1 - 7 of 7
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    Absorption Spectroscopy of an Individual Fano Cluster
    (American Chemical Society, 2016) Yorulmaz, Mustafa; Hoggard, Anneli; Zhao, Hangqi; Wen, Fangfang; Chang, Wei-Shun; Halas, Naomi J.; Nordlander, Peter; Link, Stephan; Laboratory for Nanophotonics
    Plasmonic clusters can exhibit Fano resonances with unique and tunable asymmetric line shapes, which arise due to the coupling of bright and dark plasmon modes within each multiparticle structure. These structures are capable of generating remarkably large local electromagnetic field enhancements and should give rise to high hot carrier yields relative to other plasmonic nanostructures. While the scattering properties of individual plasmonic Fano resonances have been characterized extensively both experimentally and theoretically, their absorption properties, critical for hot carrier generation, have not yet been measured. Here, we utilize single-particle absorption spectroscopy based on photothermal imaging to distinguish between the radiative and nonradiative properties of an individual Fano cluster. In observing the absorption spectrum of individual Fano clusters, we directly verify the theoretical prediction that while Fano interference may be prominent in scattering, it is completely absent in absorption. Our results provide microscopic insight into the nature of Fano interference in systems of coupled plasmonic nanoparticles and should pave the way for the optimization of hot carrier production using plasmonic Fano clusters.
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    Charge Transfer Plasmons: Optical Frequency Conductances and Tunable Infrared Resonances
    (American Chemical Society, 2015) Wen, Fangfang; Zhang, Yue; Gottheim, Samuel; King, Nicholas S.; Zhang, Yu; Nordlander, Peter; Halas, Naomi J.; Laboratory for Nanophotonics
    A charge transfer plasmon (CTP) appears when an optical-frequency conductive pathway between two metallic nanoparticles is established, enabling the transfer of charge between nanoparticles when the plasmon is excited. Here we investigate the properties of the CTP in a nanowire-bridged dimer geometry. Varying the junction geometry controls its conductance, which modifies the resonance energies and scattering intensities of the CTP while also altering the other plasmon modes of the nanostructure. Reducing the junction conductance shifts this resonance to substantially lower energies in the near- and mid-infrared regions of the spectrum. The CTP offers both a high-information probe of optical frequency conductances in nanoscale junctions and a new, unique approach to controllably engineering tunable plasmon modes at infrared wavelengths.
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    Coherent Fano resonances in a plasmonic nanocluster enhance optical four-wave mixing
    (National Academy of Sciences, 2013) Zhang, Yu; Wen, Fangfang; Zhen, Yu-Rong; Nordlander, Peter; Halas, Naomi J.; Laboratory for Nanophotonics
    Plasmonic nanoclusters, an ordered assembly of coupled metallic nanoparticles, support unique spectral features known as Fano resonances due to the coupling between their subradiant and superradiant plasmon modes. Within the Fano resonance, absorption is significantly enhanced, giving rise to highly localized, intense near fields with the potential to enhance nonlinear optical processes. Here, we report a structure supporting the coherent oscillation of two distinct Fano resonances within an individual plasmonic nanocluster. We show how this coherence enhances the optical four-wave mixing process in comparison with other doubleresonant plasmonic clusters that lack this property. A model that explains the observed four-wave mixing features is proposed, which is generally applicable to any third-order process in plasmonic nanostructures. With a larger effective susceptibility χ (3) relative to existing nonlinear optical materials, this coherent double-resonant nanocluster offers a strategy for designing high-performance thirdorder nonlinear optical media.
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    Enhancing the photocurrent and photoluminescence of single crystal monolayer MoS2ﾠwith resonant plasmonic nanoshells
    (AIP Publishing LLC., 2014) Sobhani, Ali; Lauchner, Adam; Najmaei, Sina; Ayala-Orozco, Ciceron; Wen, Fangfang; Lou, Jun; Halas, Naomi J.
    Monolayer molybdenum disulfide (MoS2) produced by controlled vapor-phase synthesis is a commercially promising new two-dimensional material for optoelectronics because of its direct bandgap and broad absorption in the visible and ultraviolet regimes. By tuning plasmonic core-shell nanoparticles to the direct bandgap of monolayer MoS2 and depositing them sparsely (<1% coverage) onto the material's surface, we observe a threefold increase in photocurrent and a doubling of photoluminescence signal for both excitonic transitions, amplifying but not altering the intrinsic spectral response.
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    Plasmonic Nanostructures: Optical Nanocircuits, Tunable Charge Transfer Plasmons, and Properties of Fano Resonant Nanoclusters
    (2015-09-22) Wen, Fangfang; Halas, Naomi; Nordlander, Peter; Link, Stephan
    Metallic Nanoparticles have attracted increasing interest due to their abilities to confine and manipulate light at the nanoscale via the excitation of surface plasmons, the collective oscillation of conduction band electrons. Surface plasmons can focus electromagnetic field into a subwavelength dimension and sense the change of the local dielectric environment, promising properties for surface enhanced spectroscopy and sensing applications. New interesting properties emerge, such as the Fano resonance, when clusters of nanoparticles are brought into close proximities. The reduced light scattering within the Fano resonance corresponds to the intense local fields around and within the clusters, a promising feature for the development of ultrasensitive chemical sensors. Cluster of nanoparticles also support a new plasmon resonance known as the charge transfer plasmon (CTP) when their junctions are made conductive. This thesis will focus on exploring new properties of complex plasmonic nanoclusters and applying them in applications of optical nanocircuits, frequency modulation, and surface enhanced Raman scattering. First, this thesis demonstrates the realization of 3D optical nanocircuits using plasmonic dimer antenna composed of two Au nanodisks separated by a gap. Individual antennas are loaded with media of specific geometries and dielectric properties, acting as optical nanocircuits that tune the resonance of the nanoantennas at visible wavelengths. Series and parallel combinations of nanocircuit elements (nanocapacitors, nanoinductors and nanoresistors) can be realized by appropriately loading specific arrangements of dielectric, semiconducting and metallic nanoparticles in the antenna gap. Second, this thesis investigates the CTP in nanowire-bridged dimer nanoantennas. The CTP arises at lower energies and depends sensitively on the junction conductance, offering a new route for achieving tunable plasmon resonances by modifying junction geometries or materials. Third, this thesis examines the complex near field properties of the Fano resonant plasmonic nanoclusters using the surface enhanced Raman scattering (SRES) both from molecules distributed randomly on the structure and from carbon nanoparticles deposited at specific locations within the structure. It is found that the largest SERS enhancement is achieved when the Fano resonance overlaps with the laser excitation wavelength and the specific stokes mode of the analyte. Finally, the plasmonic properties of the Fano nanoclusters are shown to be substantially modified by the addition of carbon nanoparticles. The placement of several carbon nanoparticles in junctions between multiple adjacent Au particles introduces a collective magnetic plasmon mode into the existing Fano dip, giving rise to an additional subradiant mode in the metallodielectric nanocluster.
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    Ternary CuIn7Se11: Towards Ultra-Thin Layered Photodetectors and Photovoltaic Devices
    (Wiley, 2014) Lei, Sidong; Sobhani, Ali; Wen, Fangfang; George, Antony; Wang, Qizhong; Huang, Yihan; Dong, Pei; Li, Bo; Najmaei, Sina; Bellah, James; Gupta, Gautam; Mohite, Aditya D.; Ge, Liehui; Lou, Jun; Halas, Naomi J.; Vajtai, Robert; Ajayan, Pulickel
    2D materials have been widely studied over the past decade for their potential applications in electronics and optoelectronics. In these materials, elemental composition plays a critical role in defining their physical properties. Here we report the first successful synthesis of individual high quality CuIn7Se11 (CIS) ternary 2D layers and demonstrate their potential use in photodetection applications. Photoconductivity measurements show an indirect bandgap of 1.1 eV for few-layered CIS, an external quantum efficiency of 88.0 % with 2 V bias across 2 μm channel with and a signal-to-noise ratio larger than 95 dB. By judicious choice of electrode materials, we demonstrate the possibility of layered CIS-based 2D photovoltaic devices. This study examines this ternary 2D layered system for the first time, demonstrating the clear potential for layered CIS in 2D material-based optoelectronic device applications.
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    Tuning the acoustic frequency of a gold nanodisk through its adhesion layer
    (Nature Publishing Group, 2015) Chang, Wei-Shun; Wen, Fangfang; Chakraborty, Debadi; Su, Man-Nung; Zhang, Yue; Shuang, Bo; Nordlander, Peter; Sader, John E.; Halas, Naomi J.; Link, Stephan; Laboratory for Nanophotonics
    To fabricate robust metallic nanostructures with top-down patterning methods such as electron-beam lithography, an initial nanometer-scale layer of a second metal is deposited to promote adhesion of the metal of interest. However, how this nanoscale layer affects the mechanical properties of the nanostructure and how adhesion layer thickness controls the binding strength to the substrate are still open questions. Here we use ultrafast laser pulses to impulsively launch acoustic phonons in single gold nanodisks with variable titanium layer thicknesses, and observe an increase in phonon frequencies as a thicker adhesion layer facilitates stronger binding to the glass substrate. In addition to an all-optical interrogation of nanoscale mechanical properties, our results show that the adhesion layer can be used to controllably modify the acoustic phonon modes of a gold nanodisk. This direct coupling between optically excited plasmon modes and phonon modes can be exploited for a variety of emerging optomechanical applications.