Degenerate two-photon-absorption spectral studies of highly two-photon active organic chromophores

Abstract

Degenerate two-photon absorption (TPA) spectral properties of five AFX chromophore solutions have been studied using a single and spectrally dispersed sub-picosecond white-light continuum beam. In a specially designed optical configuration, optical pathways inside the sample solution for different spectral components of the focused continuum beam were spatially separated from each other. Thus, the nondegenerate TPA processes coming from different spectral components can be eliminated, and the direct nonlinear absorption spectrum attributed to degenerate TPA processes can be readily obtained. Using this new technique, the complete TPA spectra for these five highly two-photon-active compounds (AF-380, AF-350, AF-295, AF-270, and AF-50) were obtained in the spectral range from 600 to 950 nm on an absolute scale of TPA cross section. The relationship between the molecular structures and their TPA spectral behaviors are discussed. In general the measured TPA spectra are not identical with the linear absorption spectra on the scale of absorbed photon(s) energy. Moreover, for some sample (such as AF-380), the TPA spectrum is totally different from the linear spectrum, which implies the difference of molecular transition pathways and selection rules for one- and two-photon excitation processes. At high excitation intensity levels (>or=15 GW/cm(2)), the saturation behavior of TPA transition can be observed obviously in AF-350 and AF-380 solutions that exhibit much higher nonlinear absorptivity than the other chromophores investigated.

Two-Photon Excitation of Fluorogenic Probes for Redox Metabolism: Dramatic Enhancement of Optical Contrast Ratio by Two-Photon Excitation

“…While using a standard sample (rhodamine 6G) with known two-photon cross section, we can ignore most of the parameters in this equation, and the δ new (two-photon cross section of the unknown dye) can be reduced to: 13 Assuming that the fluorescence quantum efficiencies of twophoton and one-photon excitation are the same, we can calculate the two-photon cross section of the unknown sample considering the fluorescence quantum yields of the unknown sample and standard. Because solutions with high concentrations of the fluorogenic probes were used (40 mM), self-quenching could reduce the fluorescence quantum yields.…”

“…Therefore, a relative method was used to determine the two-photon cross section values employing the standard AF-350 (δ ) 206 GM at 775 nm in tetrahydrofuran, THF). 13 First, the nonlinear transmission, T standard of the standard was measured. Using eq 2, the standard value was determined.…”

Optical Pulse Interactions in Nonlinear Excited State Materials

“…By combining "related" diagrams one can get description of many existing nonlinear materials. For example, one can describe the five-level model of 60 C -toluene solution 43 -nonlinear RSA material -by merging diagrams B 0 ∪ B 1 ∪ B 2 ; or describe the fivelevel model of a chromophore from AFX group 32 exhibiting TPA assisted excited state absorption by combining diagrams B 1 ∪ B 2 ∪ B 3 which is depicted on Fig. 3(TPA+ESA).…”

“…For the AFX group of nonlinear chromophores, 31,32 such simulation is based on the solution of the coupled system of propagation and rate equations with the latter described by a phenomenological five-level absorption model 31 depicted in Fig.2. Propagated light may decrease due to TPA, which excites electrons from the ground level N 0 to the singlet excited state 1 N , and/or due to ESA from singlet and/or triplets states 1 N , 2 N .…”

Dispersive white light continuum single Z-scan for rapid determination of degenerate two-photon absorption spectra

“…With this arrangement, different spectral components of the WLC beam are spatially separated from each other at the sample position and only degenerate TPA from the same spectral component can take place. By comparing the transmission of the sample at the focus to that of the pure solvent, the attenuation of different spectral components can be determined [28][29][30]. In this method the obtained 2PA spectrum is although of degenerate nature, it represents the relative, but not the absolute value of the 2PA cross section.…”