A relatively simple direct measurement of the two-photon absorption spectrum for emergent organic chromophores -- important to biological imaging -- measures the spectrum in a single shot by synchronizing an excitation pulse from a monochromatic laser with a probe pulse from a white-light continuum source. Synchronization, however, encounters challenges from the frequency chirping effects inherent to continuum generation.

Collaborators at State University of New York at Buffalo and Wright Patterson Air Force Research Laboratory in Ohio presented a simpler, less time-consuming approach in the July 1 issue of Optics Express. The technique directly measures the degenerate two-photon absorption spectrum by applying a single intense continuum beam from a Ti:sapphire laser oscillator/amplifier system. A white-light continuum is dispersed by a prism or grating. Different spectral components arrive spatially separated at the sample.

Because the spectral components pass through different areas of the sample, each component's degenerate two-photon absorption process can be calculated individually and compiled into relative spectral intensity distributions. This new technique is also useful for those materials that are two-photon absorptive but nonfluorescent.