Resonance wavelengths

Because of the tunabiUty, dye lasers have been widely used in both chemical and biological appHcations. The wavelength of the dye laser can be tuned to the resonant wavelength of an atomic or molecular system and can be used to study molecular stmcture as well as the kinetics of a chemical reaction. If tunabiHty is not required, a dye laser is not the preferred instmment, however, because a dye laser requires pumping with another laser and a loss of overall system efficiency results. 

Resonant Excitation Excitation by a laser, which is resonant with an electronic transition of the material under investigation, can increase the Raman cross-section by approximately 10. The transitions and thus the resonance wavelengths are specific for the substances. Resonance excitation thus leads to selectivity that can be useful for suppressing bulk bands, but can also complicate the detection of mixtures of substance with different absorption spectra. 

This process occurs only at the silver nanoparticles of which resonance wavelength is in accordance with the incident light wavelength. Therefore, size-specific dissolution is possible (processes bl-b3 in Figure 2). 

Silver nanoparticles can be deposited on Ti02 by UV-irradiation. Deposition of polydisperse silver particles is a key to multicolor photochromism. The nanoparticles with different size have different resonant wavelength. Upon irradiation with a monochromatic visible light, only the resonant particle is excited and photoelectrochemically dissolved, giving rise to a decrease in the extinction at around the excitation wavelength. This spectral change is the essence of the multicolor photochromism. The present photoelectrochemical deposition/dissolution processes can be applied to reversible control of the particle size. 

FIGURE 7.6 (a) V[ position dependency upon the resonance wavelength (top) and 77K absorption spectrum... 

NLO materials which necessitate operation at off-resonance wavelengths, generally in the NIR region ca. 1,000-1,500 nm. 

In order to investigate this effect, ordered arrays of metallic nano-islands were fabricated on glass substrates by a process of natural lithography using monodisperse polystyrene nanospheres. The metal particle dimensions were tailored in order to tune the plasmon resonance wavelength to match the spectral absorption of the fluorophore. The fluorophore, Cy5 dye, which is widely used in optical immunoassays and has a medium quantum efficiency ( 0.3), was used in this preliminary study of the plasmonic enhancement effect. 

Figure 18. Range of resonance wavelengths achievable by varying nanoisland geometry.

The same observations are true for the evanescent wave of the modes in the surrounding medium. Moreover, these changes in the field distributions are accompanied by an increase of the effective refractive indices of the cladding modes, as will be clarified later. For this reason and by virtue of (3.1), the resonance wavelengths of the coated LPG are expected to blue-shift in response to an overlay thickness change. 

The presence of the HRI coating induces an increase in the effective refractive index of the cladding modes and thus a decrease in the resonance wavelengths. In addition, the spatial shift of the cladding mode field profile toward the HRI overlay promotes a decrease of the overlap integral with the core mode and so of the transmission loss peak. The subsequent evaporation of the solvent molecules from the nano-cavities results in a reduction of its refractive index. This explains the partial... 

The mechanism that induces such a change in the resonance wavelength behavior is the transition of the lowest order cladding mode into an overlay guided mode and the consequent mode redistribution. Here, the attention is focused on the analysis of the transition curves to determine the novel sensitivity characteristics of HRI-coated LPGs and to outline the dominant role of the overlay thickness and the mode order. 

To this aim, an analytical form was proposed to model the transition curves. The Lorentzian-Cumulative function was found to be the best fit of the experimentally determined transition curves. This means that each resonance wavelength Aqy related to the cladding mode LP0j can be expressed as ... 

Once modeled the transition curves, it is possible to retrieve an analytical form for the sensitivity characteristics in nano-sized coated LPGs. By using the SRI-derivative of the resonance wavelengths, the SRI sensitivity can be expressed as ... 

Zhou, K. Liu, H. Hu, X., Tuning the resonant wavelength of long period fiber gratings by etching the fiber s cladding, Opt. Commun. 2001, 197, 295 297... 

The resonance wavelength (2rcs) of a LPFG can be roughly estimated by the following phase matching condition ... 

Fig. 8.3 Transducing methods (a) Resonant wavelength shift scheme monitor A2p between the reference resonance (solid) and the resonance under sensing (dash) and (b) intensity variation scheme monitor A/ at a selected wavelength...

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