Evaluation of small hole metallic arrays for plasmon-enhanced spectroscopic substrates.

摘要

Light transmission enhancement through metallic subwavelength hole arrays has been studied to understand the coupling mechanism of the transmission behaviors in this dissertation. Surface plasmon modes generated by the periodic hole structure and interactions with the evanescent propagating waveguide modes are attributed to the high transmission. In our experimental research with various hole shape, size and periodicity, the transmission resonances have been measured and characterized for the relationships between the hole geometry and transmission signals. Throughout the study, we have several outstanding achievements in view of light transmission enhancement: (1) photoluminescence effects with optically thin metal film, (2) the relationships between the transmission behaviors and hole shape dependence, (3) significant role of hole's geometry in fluorescence excitation spectrum within small hole arrays.; The subwavelength hole arrays fabricated by electron beam lithography processes and an ion-milling etching system are examined by the scanning electron microscope and atomic force microscope. Our characterized and developed processes carry out well-defined hole shapes and sizes on gold metallic thin films. The hole shapes used for the transmission enhancement observation have circular, square and equilateral triangular geometry on 60 nm and 130 nm of metal thicknesses. The hole diameter for circular or side length for square and triangular shape are prepared in ranges from about 50 nm to 300 nm. The measurement of transmission signal through the metallic hole arrays are collected by objective lenses with an inverted microscope system to a high resolution spectrometer.; From the characterized transmission spectra, the photoluminescence effects with optically thin gold metal films are described by the electron transitions and recombination process between d-bands in the metal and the Fermi level in the conduction band. Furthermore, we presented that the transmission resonance peak wavelengths are inconsistent with the simple plasmon model as determined with only the momentum conservation law. The effects of hole area, hole shape and metal film thicknesses affect significantly the transmission spectra. The red-shift of resonance peaks and the coupling intensity of transmission with various hole size, shape and metal film thicknesses have been observed. Especially, the equilateral triangular hole shape presents strong transmission enhancement compared to the circular and square hole shape.; For the laser spectroscopy experiments, 0.5 wt.% PMMA solution containing the Rhodamine 6G fluorescent molecules was coated onto the metallic hole arrays. The fluorophore solution was mixed into the PMMA solution with a concentration of about 0.5x10-7 M. The PMMA is spin-coated to a thickness on metal of 40 nm. A Nd:YAG 532 nm wavelength laser coupled into a single-mode optical fiber was used for excitation of the fluorescent molecules with the transmission measurement system.; The excitation is achieved with two different orientations of laser light incidence in order to determine the coupling efficiency of surface plasmon modes to propagating waveguide modes. The used hole arrays samples are square with different periodicity, 400 nm and 600 nm, and the hole size is about 110 nm and 120 nm, respectively. The measured spectra from both orientations show the same transmission profiles. The convolution of the bulk fluorescence with the white light transmission signals of the same samples has been characterized for the effects of diffraction from the periodic hole patterns. From the study of fluorescence excitation through the hole arrays, it appears determining that the hole geometry, i.e. hole shape and hole size, is the dominant factor in the fluorescence spectrum.