  Non-degenerate 2-photon excitation (ND-2PE) of a fluorophore with two laser
beams of different photon energies offers an independent degree of freedom in
tuning of the photon flux for each beam. This feature takes advantage of the
infrared wavelengths used in 3-photon microscopy to achieve an increased
penetration depth, while preserving a relatively high degenerate 2-photon
excitation (D-2PE) cross section, exceeding that achievable with 3-photon
excitation. Here, using spatially and temporally aligned Ti:Sapphire laser and
optical parametric oscillator beams operating at near infrared (NIR) and
short-wavelength infrared (SWIR) optical frequencies, respectively, we provide
a practical demonstration that the emission intensity of a fluorophore excited
in the non-degenerate regime in a scattering medium is more efficient than the
commonly used D-2PE.
