Near-field optical microscopy can provide a resolution greater than wavelength of light. In operation, light is passed down a tapered optical fiber and emerges at a fine tip whose diameter is smaller than the wavelength of light. This tip is brought into contact with a fluorescently labeled specimen and is scanned over the surface. Only a fluorophore in the immediate vicinity of the tip is maximally excited. However, there is a low level of excitation in the far field as the light disperses away from the tip. This low level background can be removed by confocal aperture in the imaging optics. We are exploring the alternative strategy of eliminating this background fluorescence by the use of multiple photon excitation. In this scheme, fluorophorr, excitation is proportional to the square or higher powers of the excitation illumination and therefore will essentially only occur in the immediate vicinity of the tip. We are proposing to investigate whether we can get sufficient levels of two-photon excitation in the vicinity of a subwavelength tip to enable images with a good signal to noise ratio to be obtained.
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