We compare the experimental absolute two-photon absorption (2PA) cross sections and spectra of various organic
molecules with those obtained from the few (two or three) essential states model, assuming certain parameter values
that are either previously calculated theoretically or measured. We study conditions under which the two- or three-level
models are applicable for quantitative description of 2PA, and estimate the corresponding maximum realistic attainable
peak 2PA cross sections and spectral widths. Based on our observations we formulate the steps required to increase the
intensity and broaden the spectral coverage of instantaneous 2PA by optimizing intrinsic molecular parameters such as
transition dipole moments, permanent dipole moments and excited states energies.
We present a new approach to modeling of homogeneous line shape in two-photon absorption (2PA) spectra of
chromophores with large permanent dipole moment difference between the ground- and excited electronic states
using numerical solution of stochastic two-level density matrix equation of motion. Good agreement with
experimental 2PA line shapes is obtained for S1←S0 transition of Styryl 9M, which allows us to estimate that the
permanent dipole moment difference varies in this chromophore within the S1←S0 band in the range, Δμ = 12 - 25
We measure and quantitatively analyze two-photon absorption (2PA) spectra and cross sections in a series of Oxazine
molecules with varying chemical structures.
We compare values of the permanent dipole moment difference (Δμ) obtained by three different methods: (a) 2PA cross
sections using two-level approximation; (b) Stark spectroscopy, and (c) solvatochromic shifts. The first two methods
give coinciding Δμ values, thus justifying two-level approximation, while solvatochromic shifts measurements yield
systematically higher Δμ values. Possible reasons for such a discrepancy are discussed.
We present a detailed study of the local environmental sensitivity of the commercially available laser dye, Styryl-9M.
Positions of the one-photon and two-photon absorption maxima and
two-photon absorption minimum of the dye are
sensitive to the solvent polarity. In aqueous solution its absorption and fluorescence spectra consist of two peaks whose
relative strength depend on the surrounding pH-s. The dye shows one of the highest two-photon absorption cross
sections, 700-1300 GM at the peak, among widely available compounds. Comparison of the linear and nonlinear
properties shows that its maximum cross section can be described by an effective two-level model. Based on the
properties of Styryl-9M we propose a new method of sensing local environment polarity in solutions and biological phantoms. We show that the dye is a promising candidate for two-photon biological imaging and microscopy.