J. R. Sambles et al s conversion via the excitation of the surface plasmon stepped integrally [16]. This then allows determination hence we may record with suitable polarizers a surface of the assumed isotropic relative permittivities and plasmon resonance maximum. This has only very thicknesses of the organic overlayers. A range of different recently been examinated in detail and compared with experiments have also been performed with inorganic theory by developing Chandezon's model further for the overlayers, again to study their dielectric properties. In broken symmetry situation. The maximum p to s a sense this is relatively unexciting; of more interest are conversion occurs with a twist angle of 45 and studies where changes in the overlayer occur. For with the maximum groove depth Results for maximum example the progressive laser-induced desorption of p to s conversion using a surface plasmon on a silver organic films, predeposited onto the active metal layer, coated grating are illustrated in figure 12. These may be studied [17], or the inverse, the condensation observations are particularly exciting because prism through the attractive potential between a volatile coupling, at least for isotropic media, can never provide organic and a metal film may be readily observed[18] the required symmetry breaking, and this therefore opens In the latter case this leads, with careful experimentation. up a new range of potential devices to the determination of the bonding potential of organics on to the metal layer[19]. A variant of monitoring changes in the thickness of overlayers is the study, again 6. Applications by measuring the shift in the resonance position, of the This then brings us to examine the possible exploitation effective relative permittivity of the overlayer as a of this novel surface mode in devices. Perhaps we need consequence of exposure to gas[20]. This technique, with first address the question of why it is of interest at all. the appropriate overlayers has applications in optical gas The basic answer is that the momentum of the surface sensing. Extending this idea, solutions rather than gases plasmon, which is readily monitored by coupling may be placed adjacent to the metal film and then incident radiation to it, is easily changed by thin layers changes in the region of the surface plasmon decay lengt of material deposited on the metal surface[ 13] or by may once again be readily monitored. This opens up small changes in the dielectric constant of the material potential for immunoassay using antigen protein films adjacent to the metal on the metal layer which bind to specific antibodies in One of the simplest studies that may be undertaken is solution[21]. As the antibodies bind to the antigen that of the chemical contamination of the metal the surface plasmon resonance is shifted in angle and a supporting the surface plasmon. For example it is simple, direct optical measure of antigen concentration can be by monitoring the surface plasmon resonance of silver in obtained. In the same context, of fluids adjacent to the the Kretschmann-Raether geometry to observe the active metal surface there are a large range of studies, progressive growth of silver sulphide on exposure to the from optical examination of electrochemical proces- atmosphere. Kovacs[ 14] performed this experiment and ses [22], to studies of liquid crystal alignment [23, 24] and o y monitoring the shift in resonance angle over many more complex processes such as the kinetics of adsoption days found for his particular environment that 2 nm of of block copolymers from solution[251 silver sulphide formed after about thirty days of Currently there is much interest in the use of optical exposure. This is sufficiently slow to allow most excitation of surface plasmons in these and related areas experiments with silver to be conducted in air without of physics, physical chemistry and biophysics. Added to undue concern over this overlayer formation. On the this is a perceived potential for device application in other hand if the same type of experiment were areas other than just sensors. For instance, fibre conducted with a thin film of aluminium the initial polarizers with very high extinction ratios have already exposure to air results in the rapid formation of a been fabricated in which thin metal layers provide the relatively stable aluminium oxide layer some 3 to 4 nm necessary surface plasmon resonance absorption thus thick[15]. While these studies are intrinsically interesting destroying one polarization component[26]. There is there is more interest in deliberately overcoating the also interest in the use of surface plasmon excitation in surface plasmon supporting metal, the active medium, scanning surface microscopy[27]. Small variations in with other types of layers. For example studies have been overlayers on an active metal film are easily converted performed with organic multilayers deposited using the into large differences in reflectivity by setting the system Langmuir-Blodgett technique. By careful control of the at the angle of the surface plasmon excitation and deposition of these layers, well-defined stepped structures scanning across the sample may be fabricated. For these the angular dependent Another area with potential for the use of surface reflectivity for the Kretschmann-Raether geometry plasmons is non-linear optics. The optical excitation of shows a surface plasmon resonance which steps this surface-travelling wave resonance results in strong progressively to a higher angle as the layer thickness is enhancement of the optical field at the surface supporting