420 nm light passes through a diffraction grating after being generated by pumping rubidium vapor with 780 nm and 776 nm light. The pump beam appears very dim near the second order diffraction beam.
The first paper from the Photonics and Quantum Optics Lab at Pacific University appeared today in the American Journal of Physics: Collimated blue light generation in rubidium vapor and [PDF]. Congratulations to my student coauthors and our collaborators at the University of Portland!
This is a fairly straightforward experiment to set up if you already do saturated absorption or other rubidium vapor labs anywhere in your curriculum. We used two external cavity diode lasers (780 nm and 776 nm), a warm rubidium vapor cell, and some additional standard optics. A 420 nm bandpass filter is handy, but not necessary for viewing the generated light.
The results are pretty cool to see, and they illustrate frequency conversion in nonlinear optics. With a (barely visible) near-IR beam going into the vapor cell, you see a bright blue/UV beam coming out… it’s almost magic when you see it for the first time. Of course the power in the blue beam is much lower than in the NIR beam, but we are more sensitive to blue so it looks quite bright.
The photo shown here was taken with an open shutter while I swept a piece of paper along the beam. The camera has an internal IR filter which does artificially dim the IR beam. This isn’t too far off from the correct appearance.
Andrew Dawes 