Hanwei Gao

Northwestern University, Evanston, IL, United States

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Publications (12)117.82 Total impact

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    ABSTRACT: This chapter introduces a new type of engineered surface -- plasmonic crystals with three-dimensional (3D) unit cells -- for biosensing. Nanostructured metal films exhibit surface plasmon (SP) resonances with exceptional sensitivities but are limited to bulk refractive index (RI) sensing. Here we describe the biosensing capabilities of two different plasmonic crystal structures that support both surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs): (i) a nano-pyramidal grating and (ii) a 3D nanohole array. These plasmonic crystals not only exhibit bulk RI sensitivities comparable to commercial SP sensors but also have surface sensitivities that can be tuned by manipulating the free-space light excitation angle θ. We show how tailoring the surface sensitivity of the plasmonic crystals optimizes real-time measurements of biomolecular interactions.
    12/2012;
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    ABSTRACT: This paper reports a new type of diffractive microlens based on finite-areas of two-dimensional arrays of circular nanoholes (patches). The plasmonic microlenses can focus single wavelengths of light across the entire visible spectrum as well as broadband white light with little divergence. The focal length is determined primarily by the overall size of the patch and is tolerant to significant changes in patch substructure, including lattice geometry and local order of the circular nanoholes. The optical throughput, however, depends sensitively on the patch substructure and is determined by the wavelengths of surface plasmon resonances. This simple diffractive lens design enables millions of broadband plasmonic microlenses to be fabricated in parallel using soft nanolithographic techniques.
    Nano Letters 10/2010; 10(10):4111-6. · 13.03 Impact Factor
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    ABSTRACT: This paper describes three-dimensional (3D) nanohole arrays whose high optical transmission is mediated more by localized surface plasmon (LSP) excitations than by surface plasmon polaritons (SPPs). First, LSPs on 3D hole arrays lead to optical transmission an order of magnitude higher than 2D planar hole arrays. Second, LSP-mediated transmission is broadband and more tunable than SPP-enhanced transmission, which is restricted by Bragg coupling. Third, for the first time, two types of surface plasmons can be selectively excited and manipulated on the same plasmonic substrate. This new plasmonic substrate fabricated by high-throughput nanolithography techniques paves the way for cutting-edge optoelectronic and biomedical applications.
    Nano Letters 08/2010; 10(8):3173-8. · 13.03 Impact Factor
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    ABSTRACT: This paper describes how angle-dependent resonances from molded plasmonic crystals can be used to improve real-time biosensing. First, an inexpensive and massively parallel approach to create single-use, two-dimensional metal nanopyramidal gratings was developed. Second, although constant in bulk dielectric environments, the sensitivities (resonance wavelength shift and resonance width) of plasmonic crystals to adsorbed molecular layers of varying thickness were found to depend on incident excitation angle. Third, protein binding at dilute concentrations of protein was carried out at an angle that optimized the signal to noise of our plasmonic sensing platform. This angle-dependent sensitivity, which is intrinsic to grating-based sensors, is a critical parameter that can assist in maximizing signal to noise.
    Nano Letters 07/2010; 10(7):2549-54. · 13.03 Impact Factor
  • Wei Zhou, Hanwei Gao, Teri W Odom
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    ABSTRACT: The angle-dependent optical properties of rhombic plasmonic crystals are described. We show that by extending the capabilities of soft interference lithography, subwavelength periodic patterns with arbitrary 2D Bravais lattices can be generated. In addition, we demonstrate that by lowering the plasmonic crystal lattice symmetry, degenerate conditions can be lifted and more plasmon bands can be excited within a fixed wavelength range. Degeneracies were also removed by changing the polar and azimuthal angles of excitation and visualized in dispersion diagrams. Anticrossings between different plasmon bands were observed to depend significantly on the local refractive index and the excitation direction.
    ACS Nano 02/2010; 4(2):1241-7. · 12.03 Impact Factor
  • Hanwei Gao, Wei Zhou, Teri W. Odom
    Advanced Functional Materials 02/2010; 20(4). · 10.44 Impact Factor
  • Hanwei Gao, Wei Zhou, Teri W. Odom
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    ABSTRACT: Surface plasmons are responsible for a variety of phenomena, including nanoscale optical focusing, negative refraction, and surface-enhanced Raman scattering. Their characteristic evanescent electromagnetic fields offer opportunities for sub-diffraction imaging, optical cloaking, and label-free molecular sensing. The selection of materials for such applications, however, has been traditionally limited to the noble metals Au and Ag because there has been no side-by-side comparison of other materials. This feature article describes recent progress on manipulating surface plasmons from ultraviolet to near-infrared wavelengths using plasmonic crystals made from 2D nanopyramidal arrays. A library of plasmon resonances is constructed in the form of dispersion diagrams for a series of unconventional and new composite plasmonic materials. These resonances are tuned by controlling both intrinsic factors (unit cell shape, materials type) and extrinsic factors (excitation conditions, dielectric environment). Finally, plasmonic crystals with reduced lattice symmetries are fabricated as another means to tailor resonances for broadband coupling.
    Advanced Functional Materials 12/2009; 20(4):529 - 539. · 10.44 Impact Factor
  • Min Hyung Lee, Hanwei Gao, Teri W Odom
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    ABSTRACT: This letter describes the optical properties of quasi one-dimensional (1D) Au nanoslit arrays on a microscale pitch. The transmission spectra exhibited multiple minima that were well characterized by 1D surface plasmon polariton Bloch wave modes. We found that all higher order modes showed a linear response to small changes of refractive index (RI) with sensitivities up to 560 nm per RI unit, which is comparable to that of two-dimensional nanohole arrays. By calibrating the RI response of the nanoslit arrays, we could use the multiple modes to determine the RI of unknown, nonabsorbing solutions.
    Nano Letters 07/2009; 9(7):2584-8. · 13.03 Impact Factor
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    ABSTRACT: This Letter describes the dispersion properties of plasmonic lattices consisting of near-infinite arrays and superlattices (patches) of nanoholes in thin gold films. Zero-order, optical transmission spectra of the plasmonic lattices revealed that compared to the near-infinite arrays, the superlattices showed new resonances that depended on the separation between patches. Theoretical calculations were in good agreement with the measurements and identified the origins of the additional transmission peaks. Surface plasmon dispersion diagrams were also measured, and the superlattices were found to exhibit satellite-bands around the primary (intra-patch) surface plasmon polariton modes that were not present in the near-infinite nanohole arrays.
    Chemical Physics Letters 01/2009; 483(4):187-192. · 2.15 Impact Factor
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    ABSTRACT: Surface plasmon polaritons (SPPs) are responsible for exotic optical phenomena, including negative refraction, surface enhanced Raman scattering, and nanoscale focusing of light. Although many materials support SPPs, the choice of metal for most applications has been based on traditional plasmonic materials (Ag, Au) because there have been no side-by-side comparisons of the different materials on well-defined, nanostructured surfaces. Here, we report a platform that not only enabled rapid screening of a wide range of metals under different excitation conditions and dielectric environments, but also identified new and unexpected materials for biosensing applications. Nanopyramidal gratings were used to generate plasmon dispersion diagrams for Al, Ag, Au, Cu, and Pd. Surprisingly, the SPP coupling efficiencies of Cu and Al exceeded widely used plasmonic materials under certain excitation conditions. Furthermore, grazing angle excitation led to the highest refractive index sensitivities (figure of merit >85) reported at optical frequencies because of extremely narrow SPP resonances (full-width-at-half-minimum <6 nm or 7 meV). Finally, our screening process revealed that Ag, with the highest sensitivity, was not necessarily the preferred material for detecting molecules. We discovered that Au and even Pd, a weak plasmonic material, showed comparable index shifts on formation of a protein monolayer.
    Proceedings of the National Academy of Sciences 12/2008; 105(51):20146-51. · 9.81 Impact Factor
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    Small 01/2008; 3(12):2029-33. · 7.82 Impact Factor
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    Hanwei Gao, Joel Henzie, Teri W Odom
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    ABSTRACT: This paper provides direct evidence for the role of surface plasmons in the enhanced optical transmission of light through metallic nanoscale hole arrays. Near-field optical images directly confirmed the presence of surface plasmons on gold nanohole arrays with interhole spacings larger than the surface plasmon wavelength. A simple interference model provides an intuitive explanation of the two types of fringe wavelengths observed in the near-field optical images. Far-field spectroscopy revealed a surface plasmon band that contributed a factor > 8 to the transmission enhancement. Furthermore, silicon nanohole arrays did not exhibit any features in the near-field, which demonstrates that metallic materials are necessary for enhanced light transmission through nanohole arrays.
    Nano Letters 09/2006; 6(9):2104-8. · 13.03 Impact Factor