Laser Cooling with Optical Frequency Combs

The optical cavity that surrounds the gain medium of a laser oscillator is typically capable of supporting a large number of longitudinal modes that are equally spaced in frequency.  Since most gain media support optical amplification over a wider frequency range than the spacing between these modes (given by the free-spectral range of the cavity), a laser of this sort (a mode-locked laser) is perfectly happy to lase in multiple modes at the same time.  Add in a little bit of nonlinearily to such a laser, and all of the available modes can lock together in phase.  For one, brief, shining moment, the electric field of every participating mode can add constructively and the laser emits a pulse of light.  This process then repeats itself at incredibly regular intervals of time, and the resulting spectrum is known as an optical frequency comb.

While it has been recognized for some time that optical frequency combs are useful measurement devices, they can also be used to control the motion of atoms and molecules in ways that are not possible (or practical) with continuous-wave (cw) lasers.  For instance, work in our group has shown that laser cooling and trapping can be accomplished with optical frequency combs in a way that is specifically designed to work for species that can't currently be laser cooled.  That list contains atoms like hydrogen, carbon, oxygen, and nitrogen -- the very building blocks of modern chemistry.  Our next step is to push this technique into the deep ultraviolet and to cool and trap these species for precision spectroscopy and ultracold chemistry.

Another, related way that optical frequency combs can prove usful is in the laser cooling of atomic ions.  We are currently working toward eliminating the need for costly and difficult cw lasers for these species, which currently pose a fairly significant hurdle to working with them.  For trapped ions, the trap is sufficiently tight that only one beam is typically needed to cool in 3D due to the fact that the ion will oscillate back and forth.  However, this introduces some special complications to how the atom "sees" the optical frequency comb, which is a topic we are currently studying.  We are also working on ways to produce state-selective excitation of the hyperfine qubit in 171Yb+ using broadband pulses, which will allow temporally-sensitive detection of fluorescence.