Effects of excited-state absorption on two-photon absorbing materials

Two-Photon Fluorescence Microscopy and Applications in Angiogenesis and Related Molecular Events

The role of angiogenesis in health and disease have gained considerable momentum in recent years. Visualizing angiogenic patterns and associated events of surrounding vascular beds in response to therapeutic and laboratory-grade biomolecules have become a commonplace in regenerative medicine and the biosciences. To aid imaging investigations in angiogenesis, the two-photon excitation fluorescence microscopy (2PEF), or multiphoton fluorescence microscopy is increasingly utilized in scientific investigations. The 2PEF microscope confers several distinct imaging advantages over other fluorescence excitation microscopy techniques - for the observation of in-depth, three-dimensional vascularity in a variety of tissue formats, including fixed tissue specimens and in vivo vasculature in live specimens. Understanding morphological and subcellular changes that occur in cells and tissues during angiogenesis will provide insights to behavioral responses in diseased states, advance the engineering of physiologically-relevant tissue models and provide biochemical clues for the design of therapeutic strategies. We review the applicability and limitations of the 2PEF microscope on the biophysical and molecular-level signatures of angiogenesis in various tissue models. Imaging techniques and strategies for best practices in 2PEF microscopy will be reviewed.

Dynamics of two-photon-induced three-photon absorption in nanosecond, picosecond, and femtosecond regimes

Based on five-level rate-equation theory, we develop the laser-pulse-duration dependence of two-photon-absorption-induced singlet and triplet excited-state absorptions (ESAs). We present analytical expressions for the effective three-photon absorption coefficients caused by both singlet and triplet ESAs under the pulsed excitation on time scales from femtoseconds to microseconds. We demonstrate that the triplet ESA is predominant with longer laser pulses (microseconds to tens of nanoseconds) and that the resultant nonlinear absorption (NLA) can be adequately interpreted by a simplified four-level model. Under the excitation of picosecond laser pulses, generally speaking, the competition between singlet and triplet ESAs is observable. In this instance, the photodynamics of the system can be understood by a five-level model. In the femtosecond regime, however, a three-level model is validated in the prediction of NLA, because the triplet ESA becomes negligible.

Two-photon-induced excited-state nonlinearities

We report a theoretical investigation of laser-pulse-duration dependence of excited-state nonlinearities induced by two-photon absorption (2PA). We investigate the spatiotemporal characteristics of the transmitted pulses caused by 2PA-induced excited-state absorption, which strongly depends on laser pulse duration. By taking into account 2PA cross-section, excited-state photophysical properties, as well as laser pulse duration, we quantitatively determine the effective fifth-order nonlinearities caused by excited-state absorption and refraction. The results are capable of predicting the magnitude of 2PA-induced excited-state nonlinearities on the time scales where laser pulse duration is less than or comparable to the lifetime of 2PA-induced excited states. We also develop Z-scan theories for quickly yet unambiguously estimation of 2PA coefficient, third-order nonlinear refraction index, and excited-state absorption and refraction cross-sections.

Axial superresolution of two-photon microfabrication

An axial superresolution diffraction theory is developed in a two-photon microfabrication system. This method can improve the axial superresolution of the two-photon microfabrication system. A theoretical analysis of the photosensitive resin is discussed based on the exciting power and the concentration of free radical theory. Simulated results of the two-photon microfabrication verify the method and show that it can provide insight into the microfabrication system.