Two-photon excited fluorescence TPEF

Two-photon excited fluorescence (TPEF) is one of the most useful ways to quantify TPA in optical materials competing with nonlinear absorption. While nonlinear transmission yields absolute quantities describing TPA, TPEF requires knowledge of the collection efficiency if absolute data must be acquired. Thus, the use of standards is the method of choice to determine 5 in case of TPEF. 

Figure 3.47. Normalized single-photon excitation (SPE) and two-photon excitation (TPE) spectra (left) and single-photon excited fluorescence (SPEF) and two-photon excited fluorescence (TPEF) spectra (right) of 85 in THE In the TPE spectrum, the wavelength data are divided by 2, considering the TPA characteristic. (From Ref. [447] with permission of the American Chemical Society.)...

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. 

Multiphoton microscopy is another technique applied to the study of the nonlinear optical response in porous silicon (Palestino et al. 2009). As an example of its use, two-photon-excited fluorescence (TPEF) emission and second harmonic generation (SHG) from glucose oxidase (GOX) adsorbed on porous silicon were detected simultaneously. 

Before going into the details of various materials and their third-order NLO properties, it would serve well to have an idea of the characterization techniques used for their study. To study the effect of the real part of third-order susceptibility, Z-scan measurements, degenerate four-wave mixing (DFWM), optical heterodyne detection of optical Kerr effect (OHD-OKE), and differential optical Kerr effect (DOKE) detection are employed. For the study of TPA, techniques such as nonlinear transmission (NLT) method, two-photon excited fluorescence (TPEF) method, and Z-scan measurements are used. The observables from the above-mentioned techniques vary depending on the inherent limitations of the technique. The nature of the light source employed like the central wavelength of the laser. 

TOL, toluene THF, tetrahydrofurane DMSO, dimethylsulfoxide CH3CN, acetonitrile CHC13, chloroform. TPEF, two-photon excited fluorescence WLC, white light continuum NLT, nonlinear transmission. cfs, femtosecond ps, picosencond ns, nanosecond. 

DMSO, dimethylsulfoxide TOL, toluene EtOH, ethanol THF, tetrahydrofiirane CH2C12, methylene chloride. TPEF, two-photon excited fluorescence NLT, nonlinear transmission fs, femtosecond ns, nanosecond. Wavelength used for excitation maximum is localized at other wavelength. 

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