Femtosecond excitation of two-photon absorption in ruby

We report two-photon absorption in ruby in an experiment simple enough to reproduce in an undergraduate optics course. The high peak power of a femtosecond (fs) pulse from an unamplified titanium:sapphire laser is sufficient to drive a two-photon transition to ruby's broad absorption bands, with subsequent emission at 694 nm from the metastable state. This experiment uses pulses with tens of nanojoules rather than the tens or hundreds of millijoules required for nanosecond experiments, and can be performed with trivial instrumentation - eliminating photomultiplier tubes in favor of simple, inexpensive photodiodes with no amplification or signal processing. A two-photon measurement provides excellent educational value for undergraduate students and expands the repertoire of pedagogical optics experiments in an interesting direction.