Laura, what kind of laser diode would you recommend? Its output frequency should be tunable, right? Thanks.

Ximao, I would recommend you to consult the following publication for saturated absorption laser spectroscopy.

[1] D. W. Preston, “Doppler-free saturated absorption: Laser spectroscopy,” American Journal of Physics, vol. 64, no. 11, p. 1432, 1996.

American journal of Physics is aimed toward undergraduate lab teaching so it should give the sufficient amount of detail to work from there. You need a narrow linewidth tunable laser. The building guide is here:

[2] K. B. MacAdam, A. Steinbach, and C. E. Wieman, “A narrow-band tunable diode laser system with grating feedback, and a saturated absorption spectrometer for Cs and Rb,” American Journal of Physics, vol. 60, no. 12, pp. 1098–1111, 1992.

This here is an experiment in atomic spectroscopy for undergrads:

[3] A. J. Hachtel, J. D. Kleykamp, D. G. Kane, M. D. Marshall, B. W. Worth, J. T. Barkeloo, J. C. B. Kangara, J. C. Camenisch, M. C. Gillette, and S. Bali, “An undergraduate measurement of radiative broadening in atomic vapor,” American Journal of Physics, vol. 80, no. 8, p. 740, 2012.

To learn more about laser frequency locking which is a closely related subject, check those two papers as well:

[4] E. D. Black, “An introduction to Pound-Drever-Hall laser frequency stabilization,” American Journal of Physics, vol. 69, no. 1, p. 79, 2001.

[5] C. E. Liekhus-Schmaltz and J. D. D. Martin, “Understanding Pound-Drever-Hall locking using voltage controlled radio-frequency oscillators: An undergraduate experiment,” American Journal of Physics, vol. 80, no. 3, p. 232, 2012.

Finally the extension of atomic spectroscopy and a good frequency locked laser is trapping atom in a magneto-optical trap which is more involving but can be cheaply implemented in an undergrad laboratory:

[6] C. E. Wieman, G. Flowers, and S. Gilbert, “Inexpensive laser cooling and trapping experiment for undergraduate laboratories,” American Journal of Physics, vol. 63, no. 4, p. 317, 1995.

[7] A. S. Mellish and A. C. Wilson, “A simple laser cooling and trapping apparatus for undergraduate laboratories,” American Journal of Physics, vol. 70, no. 9, p. 965, 2002.

I hope to have given you some leads in your field of interest for an undergrad lab.

For the laser diodes, I have worked with Sacher and would definitely recommend, like Laura to go for Toptica: http://www.toptica.com/products/diode_lasers/research_grade_diode_lasers/tunable_diode_lasers/the_ultimate_tunable_diode_laser_dl_pro.html

If your budget is limited but not your time, you can buy only the laser diode from Toptica which is cheaper, then get a grating, piezo, TEC (peltier) and you can build your own external cavity littrow design laser box.

Jerome, thank you very much for the useful information. I can see you are very experinced in mentoring undergrad search. Besides the IR laser, do you know any laser source that can give xuv lights, for example, ~ 40 eV?

XUV is at the other end of the laser spectrum, I don't really work with them except with frequency doubled laser to get down to 213 nm for super efficient Fiber Bragg Inscription. Is there any XUV laser diodes out there? There is at the research grades but for some you could buy commercially for use, I don't think there is.