Lasers TPEF with

FIGURE 8.2 Dependence of the TPEF signal on the laser spectral bandwidth for TL (filled circles) and SOD-compensated (open circles) pnlses of the laser pulse. The linear relation between TPEF and bandwidth is observed only for TL (i.e., MllPS-compensated, pulses). (FromXi et al. Opt. Comm. 281(7) 1841-1849, 2008. Used with permission.)... 

All these phenomena can occur simultaneously within the same material, as illustrated by the spectral response of an oriented polymer doped with DCM dye (4-dicyanomethylene-2-methyl-6-p-dimethylamino-styryl-4H-pyran) under 1.06 iJ,m laser irradiation (Figure 1.1). The two sharp signals at 532 and 354 nm are coherent emission induced by SHG and THG, whereas the broad band is incoherent emission of two-photon excited fluorescence (TPEF). 

Two-photon absorption can be measured by several techniques. Two of them are two-photon excited fluorescence (TPEF) and nonlinear transmission (NLT). Pulsed lasers are most often used because TPA is a third-order nonlinear optical process and therefore is most efficient at very high intensities. In the nonresonant TPA, two photons combine to bridge an energy gap larger than the energies of each photon individually, and the transition occurs without the presence of the intermediate state. This can be viewed as being due to a virtual state created by the interaction of the photons with the molecule. 

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