Third-harmonic microscopy with a titanium-sapphire laser

We demonstrate third-harmonic microscopy of thin biological objects using a Ti:sapphire laser source. A very simple modification to the collection and detection systems of microscope allows for the detection of the third-harmonic signal near 270 nm. We show that, using a Ti:sapphire laser, we can combine third-harmonic imaging with two-photon excitation fluorescence simultaneously. Compared to other possible laser sources for third-harmonic microscopy, the Ti:sapphire laser provides better optical resolution, better power availability, and is less absorbed in water. We find that the Ti:sapphire laser is the most suitable laser source for third-harmonic microscopy for thin biological specimens.