Surface Plasmon Field-Enhanced Diffraction

The local field generated by total internal reflection [23] or by the SPR [14-18] can be diffracted by gaining (or losing) discrete momenta Ak = mg, generated by the periodic surface structure of periodicity A (Fig. 4). The diffraction angle deviates in discrete increments from the specular-reflection angle (i.e., zeroth-order diffraction) in fulfillment of the corresponding momentum match condition  

This implies that the diffraction signal modulated by a unit amount of optical contrast variation dnd increases linearly with the level of initial contrast And. 

The SPR, as well as the SPFS and the SPDS set-ups are built in the Kretschmann-Raether configuration depicted in Fig. 5. A detailed description can be found in a more specific publication [25]. Briefly, the modulated and polarized beam of a laser is reflected off the base of the coupling prism and fed into a photo-diode detector. The prism/sample and the photo-detector are mounted on two co-axial goniometers, respectively, allowing for an independent tuning of the respective angular positions. 

The fluorescence detection unit is mounted towards the base of the prism, rotating together with the prism (sample) at 6, while the photo-diode detecting the reflected light rotates at 26. The fluorescence emission from the sample 

The diffraction experiments can be performed on the same Kretschmann SPR set-up (cf. Fig. 6). To resolve the diffraction orders, the angular acceptance of the photo-diode detector is defined by a 1 mm slit to be At 0.08°. The coaxial goniometers enable an independent tuning of the incident angle of the laser and/or the detection angle. Both motors rotate in a 0/20 fashion for the usual SPR angular scans, whereas only the detector motor rotates when performing diffraction scans. 

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Alternatively, various analytical methods based on SPR phenomenon have been developed, including surface plasmon field-enhanced Raman scattering (SERS) [7], surface plasmon field-enhanced fluorescence spectroscopy (SPFS) [8-11], surface enhanced second harmonic generation (SHG) [12], surface enhanced infrared absorption (SEIRA) [13], surface plasmon field-enhanced diffraction spectroscopy (SPDS) [14-18], Most of these methods take advantage of the greatly enhanced electromagnetic field of surface plasmon waves, in order to excite a chromophoric molecule, e.g., a Raman molecule or a fluorescent dye. Therefore, a better sensitivity is expected. 

Fig. 16. Photographs of the surface plasmon field-enhanced diffraction patterns generated by various PS patterns on Au surfaces (A) parallel lines 40 pm wide, separated by 62.5 pm (B) parallel lines 28 pm wide, separated by 83 pm (C) square grid pattern, bar width 28 pm, grid width 83 pm, [D] hexagonal grid pattern, bar width 8 pm, hexagonal grid width 48 pm.

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