Near-field excitation

Figure Cl.5.4. Comparison of near-field and far-field fluorescence images, spectra and lifetimes for the same set of isolated single molecules of a carbocyanine dye at a PMMA-air interface. Note the much higher resolution of the near-field image. The spectmm and lifetime of the molecule indicated with the arrow were recorded with near-field excitation and with far-field excitation at two different excitation powers. Reproduced with pennission from Trautman and Macklin [125].

We also tried measurements to demonstrate that hot spots make significant contributions to surface enhanced Raman scattering [34]. For this purpose, the sample of nanoparticie assembly was doped with Raman active molecules by a spincoating method, and near-field excited Raman scattering from the sample was recorded. We adopted Rhodamine 6G dye as a Raman active material, which is... 

Figure 3.10 (a) Topography of the sample, (b), (c) Near-field excited Raman spectra at dimers 1 and 2, respectively,taken attwodifferentincident polarizations. The peaks marked with are unassigned, (d) Near-field two-photon excitation images of dimers 1 and 2. (e) Near-field Raman excitation images of dimers 1 and 2 obtained for... 

The point is now to estimate the maximum number of photons that can be detected from a burst. The maximum rate at which a molecule can emit is roughly the reciprocal of the excited-state lifetime. Therefore, the maximum number of photons emitted in a burst is approximately equal to the transit time divided by the excited-state lifetime. For a transit time of 1 ms and a lifetime of 1 ns, the maximum number is 106. However, photobleaching limits this number to about 105 photons for the most stable fluorescent molecules. The detection efficiency of specially designed optical systems with high numerical aperture being about 1%, we cannot expect to detect more than 1000 photons per burst. The background can be minimized by careful dean-up of the solvent and by using small excitation volumes ( 1 pL in hydrodynamically focused sample streams, 1 fL in confocal exdtation and detection with one- and two-photon excitation, and even smaller volumes with near-field excitation). 

S. Foteinopoulou, J. P. Vigneron, and C. Vandenbem. Optical near-field excitations on plasmonic nanoparticle-based structures Optics Express, 2007, 15,4253-4267. 

R. Riehn, R. Stevenson, D. Richards, D.J. Kang, M. Blamire, A. Downes, and F. Cacialli, Local probing of photocurrent and photoluminescence in a phase-separated conjugated-polymer blend by means of near-field excitation. Adv. Funct. Mater., 16, 469 (2006). 

Table 1. Single molecule system studied with near-field excitation.

A third variety of intensity fluctuation has been observed in room-temperature single-molecule experiments, namely that due to real-time observation of singlemolecule intersystem crossing (ISC) [32]. The images presented in Fig. 24 were obtained with far-field rather than near-field excitation. In all the other data presented above, whether I(r) plots or images, the bin/pixel time was substantially longer... 

Realization of near-field excitation for single molecule detection... 

Step 1 Under light illumination, an optical near-field is generated on the Ag surface. This optical near-field excites a coherent phonon at the />n-junction, resulting in generation of a DP-CP. By the two-step phonon-assisted process, electrons can be excited to create electron-hole pairs at the /injunction even though the photon energy of the incident light is lower than Eg. 

As we can observe in Fig. 1.7 the magnitude of the SPP momentum k, is always larger than that of a photon in the dielectric medium bounding the metal,, y a>/c this means that the SPP mode has a non-radiative or bound nature and cannot be excited by incident plane-wave light. In the context of photonics, light and SPP modes have to be coupled with different techniques (prism coupling, near-field excitation, diffraction grating...) [4,7,12,14]. 

Assuming that the Au-PL of a surface element of the sample is proportional to Eiocai. d), the relative near-field-excited PL intensity Ipi (d), emerging from the gap region for a flat sample, is obtained by integrating over the whole focal area ... 

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