Fluorescence intensity multi-photon excitation

Fluorescence emission is just one, but probably the most convenient, of several methods available for observing multi-photon absorption. In a multi-photon excited fluorescence process, the fluorescence intensity Iji does not increase linearly with increasing of the excitation intensity, lac- Instead, Iji and la are related by... 

The basic idea of this method is to investigate the dependence of the fluorescence intensity on the excitation intensity because this relation determines the order of the non-linearity [8, 38-40]. For example, a quadratic dependence corresponds to a two-photon absorption process and a cubic dependence refers to a three-photon absorption. By means of a tunable excitation light source, one can also map the dispersion of the multi-photon transition, the resulting multi-photon excitation... 

The total fluorescence intensity saturated around a few hundreds of mJ/cm2 which corresponds to the irradiation condition where the new plasma-like emission was observed. Above this value fluorescence intensity decreased, which is accompanied with the recovery of the relative intensity of excimer emissions. This means that a quite efficient deactivation channel of excitation intensity opens in this energy range, and the contribution of Si -Si annihilation is depressed. This suggests that fragmentation reactions to diatomic radicals are not induced by the annihilation process. Multi-photon absorption processes via the Si states and chemical intermediates should be involved, although no direct experimental result has as yet been obtained. 

High intensity and monochromaticity, resulting in a high spectral intensity, are ideal tools for spectroscopic investigations, especially for fluorescence measurements with low quantum yields, for the study of multi-photon processes and excited states, and for Raman spectroscopy. For example, important biomolecules like nucleic acids have an extremely low fluorescence quantum yield at room temperature. 

According to Eqs. (l)-(3),the generated SHG intensity depends on the square of the incident fight intensity, while the generated THG intensity will depend on the third power of the incident fight intensity. Similar to multi-photon induced fluorescence process, this nonlinear dependence allows localized excitation and is ideal for intrinsic optical sectioning in scanning laser microscopy. Usually the third-order nonlinear susceptibility is much weaker than the... 

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