We introduce an application of thermionic emission in a PMT photocathode. Because of the nonlinear dependence of thermionic emission on absorbed laser power, a conventional PMT is found to produce a virtual pinhole effect that rejects unfocused light at least as strongly as a physical pinhole. This virtual pinhole effect is exploited in a scanning transmission confocal microscope equipped with a cw laser source. Because the area of the PMT photocathode is large, signal descanning is not required and thermionic detection acts as a self-aligned pinhole. Our technique of thermionic-detection autoconfocal microscopy is further implemented with graded-field contrast to obtain enhanced phase-gradient sensitivity in unlabeled samples, such as rat hippocampal brain slices.
© 2008 Optical Society of America
Original Manuscript: February 27, 2008
Revised Manuscript: May 3, 2008
Manuscript Accepted: May 11, 2008
Published: June 12, 2008
Vol. 3, Iss. 7 Virtual Journal for Biomedical Optics
Daryl Lim, Kengyeh K. Chu, and Jerome Mertz, "Autoconfocal microscopy with a cw laser and thermionic detection," Opt. Lett. 33, 1345-1347 (2008)