Plasmonic nano-structures: Design, modeling, and characterization

Zhengtong Liu, Purdue University

Abstract

Nanometer-scale plasmonic structures have a wide variety of applications in optics and photonics, with examples such as nano-lithography, biomedical sensing, photovoltaics, integrated photonic circuits, optical data storage, near-field scanning optical microscopy, and quantum information processing. Their optical characteristics, such as resonance wavelength, local field enhancements and effective permittivities and permeabilities, are of critical importance for performance optimization. In this report, three types of plasmonic devices have been studied: superlens, nanoantennas, and integrated superlens-nanoantenna devices. Numerical and analytical modeling methods have been developed to model and optimize these devices, and actual devices have been fabricated and experimentally characterized. Spatial harmonic analysis, also known as Fourier modal method or rigorous coupled-wave analysis, is a meshless method to solve Maxwell’s Equations rigorously for periodic structures. Computer codes of spatial harmonic analysis have been developed in this study and used for modeling nanoantenna arrays and metallic gratings. Excellent match between numerical and experimental results has been achieved for multiple incident angles and in a wide spectra range both above and below diffraction threshold. The two-dimensional spatial harmonic analysis modeling tool has been staged on-line at NanoHub and is freely available to the public. Superlens is a slab of metal sandwiched between two dielectric layers. Such a slab can form images of objects in close vicinity without suffering from the diffraction limit that is intrinsic in conventional imaging systems, therefore has great potentials in applications such as nano-lithography where sub-wavelength resolution is highly desired. The superlens based on the original idea works at very limited wavelengths determined by the permittivities of metal and the surrounding dielectrics. This study shows that by tuning the thicknesses of the metal and dielectric layers it is possible to make a superlens that works at an arbitrary predefined wavelength in a wide spectral range. Numerical analysis of single- and multi- layer superlens show that the designs of superlens depend on the objects therefore need to be optimized for specific cases. Nanoantennas are paired nano-particles with a small gap between them. Under resonance conditions nanoantennas can create strong local fields inside the gaps, also known as hot-spots, which are particularly useful in applications such as surface enhanced Raman scattering, surface enhanced fluorescence, and optical trapping. In this study transmittance and reflectance spectra of nanoantenna arrays are measured to study their resonance features. Then, finite element method and spatial harmonic analysis techniques are employed to model the electrodynamics of the nanoantennas. Both numerical techniques show good agreement with each other, as well as with the experimental results. These numerical methods are then used to investigate the near fields of nanoantennas and verify that nanoantennas can generate significantly enhanced fields inside their gaps. The relations between nanoantenna dimensions, resonance wavelengths, and field enhancements are systematically studied. A new device combining a superlens and a nanoantenna array is proposed, and it is demonstrated by numerical modeling in this study that this device can translate the hot-spots generated by the nanoantenna array to the other side of the superlens. The translation makes good use of the hot-spots while avoiding undesired effects, such as quenching, that happen when molecules are too close to metal surfaces. This device has great potentials in the areas of surface-enhanced Raman scattering, surface-enhanced fluorescence, and optical tweezers.

Degree

Ph.D.

Advisors

Kildishev, Purdue University.

Subject Area

Electrical engineering|Nanotechnology|Optics

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