Applications of two-photon fluorescence

Anisotropy properties of the molecular fluorescence under two-photon excitation reflect the nature of 2PA processes and may provide additional information on the electronic structure of the molecules, including the peculiarities of the 2PA mechanism. In general, the measurements of two-photon fluorescence anisotropy are more sensitive than at one-photon excitation due to a broader range of anisotropy values [13] that in some cases provide extra advantages for practical applications of2PA [65]. 

This review describes some of the recent developments in materials which exhibit enhanced two-photon absorption that can initiate photopolymerization or up-converted emission. Various optical methods including femtosecond time-resolved pump-probe experiments to characterize the two-photon properties are discussed. Finally, the applications of two-photon processes to optical power limiting, up-converted lasing, 3-D data storage, 3-D micro-fabrication, two-photon fluorescence microscopy and bio-imaging, and two-photon photodynamic therapy are presented. 

Due to the need to use expensive and complicated laser pump systems, early development occurred primarily by physicists. Two-photon absorption properties of existing materials were extensively studied in this stage. In early 1990s, some potential apphcations with this technique were demonstrated, such as two-photon fluorescence (2PF) 3D data storage [2] and 2PF microscopy [3]. However, the 2PA efficiency of the materials is the bottleneck for these applications, and this has triggered the effort to search for new materials with higher... 

Applications making use of the nonlinear absorption of dyes are passive Q-switching in solid-state lasers, pulse shaping, pulse intensity measurements of high-power ultrashort pulses, optical isolation between amplifier stages of high power solid-state lasers, and pulse width measurements of ultrashort pulses by the two-photon-fluorescence (TPF) method. 

The two compounds 121 and 122 were studied for applications in two-photon excitation fluorescence microscopy (TPEM). The corresponding Zn complexes showed an enhanced efficiency and a significant increase of the two photon absorption (TPA) cross section <07JA11888>. 

The group of Blanchard-Desce synthesized a family of molecules based on the general scheme displayed in Fig. 16, in order to deduce wide and comprehensive relationships in structure/physical properties (TPA and fluorescence) in the range 700-900 nm for two-photon excited fluorescence-based applications [98,99]. The influence of the conjugated core, of the length or the nature of linkers on TPA and spectroscopic properties was systematically studied. A TPA cross-section of 4200 GM at 710 nm has been measured, with high two-photon fluorescence properties. 

The fate of two-photon excited neutral molecules and atoms is most easily followed in real time by fluorescence monitoring. In the next sections we show that in spite of the small two-photon absorption cross-section, the method can be used for quantitative kinetic measurements. For some applications, it is more straightforward and leads to less ambiguous interpretation than one-photon excitation. 

CHll BIOLOGICAL APPLICATIONS OF SINGLE- AND TWO-PHOTON FLUORESCENCE... 

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