Probing Multiphoton Photophysics Using Two-Beam Action Spectroscopy.

Abstract

Multiphoton absorption (MPA) is an enabling technology for many applications. However, due to the low probability of MPA processes, their accurate characterization remains a challenge. Here we introduce a new technique, two-beam constant emission intensity (2-BCEIn) spectroscopy, that offers substantial advantages over other existing methods that use the generation of optical emission for the characterization of absorptive nonlinearities. We use 2-BCEIn to study nonlinear absorption in solutions of crystal violet lactone (CVL) over a range of excitation wavelengths in which the dominant nonlinear absorption process transitions from two-photon absorption (750 nm) to three-photon absorption (830 nm). At an excitation wavelength of 800 nm, both two-photon absorption and three-photon absorption contribute substantially to the nonlinear fluorescence excitation (NFE) signal, although the dynamic range of the NFE data is not sufficient to quantify the contributions of each process. 2-BCEIn spectroscopy enables the direct measurement of the local exponent at each emission intensity. 2-BCEIn measurements made at several different emission intensities demonstrate unambiguously that the nonlinear excitation of CVL at 800 nm cannot be described solely as the sum of a two-photon process and a three-photon process. A kinetic model that includes intrapulse excited-state absorption reproduces the features of the 2-BCEIn measurements and enables the determination of the ratio of the three-photon absorption cross section to the two-photon absorption cross section. Such information cannot easily be extracted from conventional NFE measurements. These results demonstrate the power and versatility of two-beam action spectroscopies for elucidating the complex photophysics of multiphoton absorption processes.