It turns out that the Doppler effect occurs not just for sound waves, but for all types of waves, including light waves. Atoms moving towards light with a certain frequency experience a Doppler shift in the frequency, making it appear to have a higher frequency to the atom, and vice versa. This is important because atoms are essentially transparent to most frequencies of light, except at a few narrow ranges of frequencies, called resonant frequencies. Each element has a different set of these resonant frequency bands, which make up the element's so-called atomic spectra. If the light passing by an atom happens to fall within one of its resonant frequency bands, atoms will absorb the energy of the light and undergo the electronic transition that corresponds to this frequency, jumping from a lower energy level to a higher energy level. Importantly, since light has momentum as well as energy, the atom will also experience a "kick" backwards as the momentum of the light transfers to the atom, like what happens when billiard balls collide and transfer their momentum to each other. After a small amount of time, the atom will drop back down to the lower energy state by spontaneously emitting light of the same frequency in a random direction, experiencing another momentum kick in the opposite direction of the emitted light as it does so.

With all of these ideas, we can now explain the principle behind Doppler cooling! In Doppler cooling, we shine light that has a frequency slightly below a specific element's resonant frequency on a large sample of these atoms. Since this light is slightly below resonance, it will not affect the atom unless the atom moves towards the light, in which case the frequency of the light is Doppler shifted up until it hits the atom's resonant frequency. This causes the atom to absorb the light, where it experiences a momentum kick against its direction of motion, thus slowing down the atom. Although the atom will later re-emit a photon and feel another momentum kick in a random direction, this random momentum kick averages itself out over a large sample of atoms, and the net effect of this light will be to slow all of the atoms down. And that's all there is to it, the atoms are cooled with light!

In the next and final blog post of this series, we will describe how principles that we have just learned about and a few new ones come together to allow for trapping of atoms with light! As always, feel free to comment any feedback you have about this post, or suggestions for future topics for this blog. Also, if you want to get an email every time I release a new blog, make sure to sign up for email updates below!

References

[1] Bardi, Jason Socrates. "Focus: Landmarks: Laser Cooling of Atoms." Physics 21 (2008): 11. (https://physics.aps.org/story/v21/st11)

[2] Wineland, Dl, and Hans Dehmelt. "Proposed 10^14 delta upsilon less than upsilon laser fluorescence spectroscopy on t1+ mono-ion oscillator iii." Bulletin of the American Physical Society. Vol. 20. No. 4. CIRCULATION FULFILLMENT DIV, 500 SUNNYSIDE BLVD, WOODBURY, NY 11797-2999: AMER INST PHYSICS, 1975.

[3] Hänsch, Theodor W., and Arthur L. Schawlow. "Cooling of gases by laser radiation." Optics Communications 13.1 (1975): 68-69.

[4] Wineland, David J., Robert E. Drullinger, and Fred L. Walls. "Radiation-pressure cooling of bound resonant absorbers." Physical Review Letters 40.25 (1978): 1639.