Surface–plasmon microscopy

Abstract

The imaging of low-contrast samples is a challenging task for optical measuring techniques, especially if high lateral resolution is also required. For example, a heterogeneously organized lipid monolayer transferred from the water surface to a solid substrate¹ still needs an additional contrast enhancement mechanism (the solubility difference for a fluorescing chromophore incorporated between the fluid and the crystalline domains of the monolayer) to be visualized by fluorescence microscopy. The mere thickness or index contrast between the different regions is not sufficient to use either phase contrast or Nomarsky microscopy² or the more recently developed Isoscope ellipsometer³. Here we describe a new microscope technique—surface plasmon microscopy (SPM)— which offers superior contrast without loss of spatial resolution by using plasmon surface polariton (PSP) fields instead of normal light as the illumination source. Such electromagnetic modes travel along a metal–dielectric interface as a bound, non-radiative surface wave, with its field amplitudes decaying exponentially perpen-dicular to the interface. Although photons can be converted into PSPs by means of a plasmon coupler (a grating or a prism in many cases) this 'light' differs considerably from plane electromagneticwaves⁴. PSPs are characterized by first, a pronounced disper-sion (energy and momentum are not linearly related by the speed of light); and second, a field intensity that is concentrated at the interface and strongly enhanced there. Some of these properties make these modes a sensitive measure of interfaces and ultrathin films. If plasmon surface polariton fields are used to illuminate interfacial structures in light microscopy, high contrast without loss of spatial resolution can be obtained owing to the high sensitivity of the plasmon resonance coupling to (for example) small optical thickness variations of thin dielectric coatings.