In the process of trying to make phosphorescent lifetime measurements from a very week signal, I have observed that all of my microscope objectives seem to luminesce in the near-infrared. My research involves using two-photon excitation of phosphorescent probes with very small two-photon action cross-sections.  Consequently my excitation source is a gated Ti:Sapphire (@ 800 nm) such that the excitation reaches the sample (or objective in this case) for 20 microseconds out of every two milliseconds.  The laser power reaching the objective is about 400 mW while the gating is on.  Using a spectrometer, I have confirmed that this luminescence extends from ~ 700nm to greater than 850 nm.  The presence of the anti-stokes shifted light implies some sort of multi-photon absorption mechanism.  However, I observe this luminescence whether the laser is mode locked or not.  I have also observed that the luminescence process appears to have a lifetime of ~ 60-80 uS.  Has anyone else ever observed this?  I have observed this with other lenses (not microscope objectives) as well.  Is it possible that the high laser power used is causing a multi-step absorption process followed by luminescence from the metal dopants in the high index glass in the objective?

Any help would be greatly appreciated. 

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Arnold D. Estrada.

Doctoral Candidate
Department of Biomedical Engineering
The University of Texas at Austin
Biomedical Engineering Bldg, 1.324
107 W. Dean Keaton Street
Austin, TX 78712

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