Nearfield Optics And Nanofibers

Nanoscale optical fibers were first developed for use in near-field optical microscopy, which is a relatively recent technique involving light sources or detectors that are smaller than the wavelength of light [9]. The first method developed for performing these experiments was to place a pinhole in front of the detector, thus effectively reducing the detector size. In a later variation to these pinholes, an excitation probe with dimensions smaller than the wavelength of the light was used for sample interrogation. Betzig and Chich-ester reported the development of one such probe capable of obtaining measurements with a spatial resolution of approximately 12 nm [10]. The probe was constructed by using a micropipet puller to pull a single-mode optical fiber to a tip diameter of 20 nm, and then coating the walls of the fiber with 100 nm of aluminum to confine the excitation radiation to the tip. With this nanoprobe, images of a pattern were reconstructed from a raster scan performed in the illumination mode, with the probe acting as a localized light source.

Due to its extremely high spatial resolution (subwave-length), near-field microscopy has received great interest, and has been used in many applications [9]. For example, a relatively new technique known as near-field surface-enhanced Raman spectroscopy (NF-SERS) has been used for the measurement of single-dye and dye-labeled DNA molecules with a resolution of 100 nm [11-13]. In this work, DNA strands labeled with the dye brilliant cresyl blue (BCB) were spotted onto a SERS-active substrate that was prepared by the evaporation of silver on a nanoparticle-coated substrate. The silver-coated nanostructured substrates are capable of inducing the SERS effect, which can enhance the Raman signal of the adsorbate molecules up to 108 times [24]. NF-SERS spectra were collected by illuminating the sample using the nanoprobe, and detecting the SERS signals

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 6: Pages (53-60)

using a spectrometer equipped with a charge-coupled device (CCD). Raster scanning the fiber probe over the sample and normalizing for surface topography using Rayleigh-scattered light produced a two-dimensional SERS image of the DNA on the surface of the substrate with subwavelength spatial resolution. Near-field optical microscopy promises to be an area of growing research that could potentially provide an imaging tool for monitoring individual cells, and even biological molecules. Single-molecule detection and imaging schemes using nanofibers could open new possibilities in the investigation of the complex biochemical reactions and pathways in biological and cellular systems.

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