Multiphoton Processes

Polarized Two-Photon Absorption and Heterogeneous Fluorescence Dynamics in NAD(P)H

“…This again implies differences in the 2PA tensor structures of the two species; circular polarization favors off-diagonal elements, requiring simultaneous action by the applied electric field along two orthogonal axes, whereas linear polarization favors diagonal transition terms, corresponding to two parallel transition moments requiring simultaneous action twice along a single symmetry axis. 22 Individual Transition Tensor Structures. The initial single-photon fluorescence anisotropies in NADH and NADPH were found to be 0.36(±0.07) and 0.35(±0.05) respectively, corresponding to transition dipole moment angles θ M of 20(±10)°for NADH and 16(±9)°for NADPH (see Figure 5 and Table 2).…”

“…In essence, the first photon can be thought of as selecting a nonstationary virtual state that can be caused to undergo transitions to the final state by a second photon with only certain polarizations. 21,22 The polarization dependence of 2PA is most commonly expressed through the polarization ratio Ω, defined as the ratio of the 2PA cross sections for circularly and linearly polarized excitation 23 …”

“…Transforming from the laboratory to molecular frame of reference and performing the necessary orientational averaging is most conveniently achieved using a spherical tensor formalism. 22,24−27 With this approach and assuming a planar transition, valid for NAD(P)H, 2 Ω can be written as…”

“…However, for 2PA, the transition does not involve a simple dipolar rearrangement of electronic charge density but depends on a sum of products of all the allowed single-photon electric dipole transition moments between the ground, virtual, and final states. In essence, the first photon can be thought of as selecting a nonstationary virtual state that can be caused to undergo transitions to the final state by a second photon with only certain polarizations. , The polarization dependence of 2PA is most commonly expressed through the polarization ratio Ω, defined as the ratio of the 2PA cross sections for circularly and linearly polarized excitation …”

Metabolic Profiling of Live Cancer Tissues Using NAD(P)H Fluorescence Lifetime Imaging

“…A pulsed excitation source is a key feature of the TCSPC technique and represents an important difference between fluorescence lifetime and routine confocal imaging [50]. Many well equipped microscopy facilities will now offer two-photon imaging platforms, which rely on the use of a pulsed laser operating in the red to near-infrared portion of the spectrum to excite the target molecule using the near-simultaneous arrival of two photons each carrying half the energy of the single photon transition [51]. Consequently, FLIM electronics can be added directly to these existing systems without the requirement for a new dedicated laser.…”