Near-field absorption enhancement

Already, several plasmonic effects have been explored within the contexts of thin-film silicon, dye-sensitized, and organic solar cells. The phenomena that have been observed and exploited - principally near-field absorption and light scattering -are described below. Together, these findings suggest strategies for further efficiency enhancements in water photoelectrolysis. 

A nano-light-source generated on the metallic nano-tip induces a variety of optical phenomena in a nano-volume. Hence, nano-analysis, nano-identification and nanoimaging are achieved by combining the near-field technique with many kinds of spectroscopy. The use of a metallic nano-tip applied to nanoscale spectroscopy, for example, Raman spectroscopy [9], two-photon fluorescence spectroscopy [13] and infrared absorption spectroscopy [14], was reported in 1999. We have incorporated Raman spectroscopy with tip-enhanced near-field microscopy for the direct observation of molecules. In this section, we will give a brief introduction to Raman spectroscopy and demonstrate our experimental nano-Raman spectroscopy and imaging results. Furthermore, we will describe the improvement of spatial resolution... 

Osawa, M., Surface enhanced infrared absorption, In Near Field Optics and Plasmon Polar itons Kawata, S., Ed. Springer, Berlin, 2001, 163 187... 

Maier, L, Morgan, M. R. A., Lindner, W., and Pirtner, F. (2008). Optical Resonance-Enhanced Absorption-Based Near-Field Immunochip Biosensor for Allergen Detection. Anal. Chem. 80 2694-2703. 

The enhancement of surface plasmon absorption of metal nanoparticles may be a result of strong near-field coupling in the close-packed copper-silver nanostructure. The effect is more considerable at the spectral range outside of the copper interband absorption that is why it is not evident at the LSPA band of silver nanoparticles. At th e fi equency range near the LSPA band of copper nanoparticles, near-field coupling is not suppressed by die interband absorption so much and the LSPA enhancement is well seen. 

High enhancement of the copper localized surface plasmon absorbency was recorded at the two-layer planar system consisted of copper and silver nanoparticles prepared with successive vacuum evaporation. The result obtained may be caused by strong near-field coupling in the close-packed binary system. The effect may be used for the development of high-absorptive coatings and spectral selective nanoelements in the visible and near infrared spectral ranges. 

The TERS strategy is not suitable for IR absorption spectroscopy (IRAS). This is because a broadband light source is used in IRAS and it is difficult to create a constant enhancement over the entire spectral range. Another problem stems from the much longer wavelength of IR compared to visible light. In other words, the far-field excitation area will be much larger than in the case of TERS, and the SNR will deteriorate. In order to overcome these problems, s-SNOM is commonly used for near-field IRAS [32]. 

Techniques that might be used to enhance the far-red/near-IR absorption of ALA-induced PpIX appear to be quite limited. PpIX is very photolabile and is easily converted into photoproducts that include at least one chlorin (known as photoprotoporphyrin) which has been identified as the hydroxyaldehyde chlorin derivative of PpIX [147]. All such photoproducts of PpIX absorb light at wavelengths longer than PpIX itself. For example, the hydroxyaldehyde absorbs strongly in the red in the vicinity of 667 nm, while PpIX absorbs around 635 nm. The relevance of these PpIX photoproducts to ALA-PDT as currently practiced is not yet clear. While it is relatively easy to produce PpIX photoproducts in vitro, their yields and photostability in vivo under conditions of clinical ALA-PDT are difficult to determine [148,149]. If treatment conditions that enhance the yield of ALA-induced PpIX photoproducts can be identified, then it might be possible to increase both the photosensitizing efficiency of ALA-PDT and the effective depth of the treatment field. 

DUV spectroscopy includes several techniques, such as absorption and scattering spectroscopy, vibrational spectroscopy, photoluminescence, and plasmonically enhanced spectroscopy. This spectroscopic technique has been applied to materials that specifically interact with photons with DUV energies. The methods are not confined to component analysis, but have also been extended to microscopy (Chap. 7) and nanoscopy (near-field microscopy. Chap. 8), although the field is still in its infancy. 

We note that surface-enhanced infrared absorption can also be used for near-field probing of chemical constitution with a local resolution of less than 100 nm (Knoll and Kellmann 1999). In fact, fortunately one finds in many different cases, surface enhancement effects caused by the scanning tip, which should allow one to use a variety of traditionally far-field spectroscopies in the near-field regime. 

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