Lasing from cold ytterbium atoms
Atoms that interact with the light field inside an optical resonator (a cavity) form a paradigmatic system in quantum optics. From the fundamental perspective, it allows one to study cavity quantum electrodynamics, including the well-known Jaynes-Cummings model for coherent interaction between single atoms and single photons. From the applied perspective, it is the basic arrangement for a laser. Our research lies between these two perspectives: we study the behavior of cold ytterbium atoms inside a high-finesse cavity, looking at both the properties of the light field emitted from the system and at the properties of the atoms, which the light field reveals. We find a wide spectrum of phenomena such as cavity-induced line shifts, bi-stability, and lasing.
The relevant atomic levels and transitions of ytterbium are displayed in Fig. 1. The main spectral lines are the dipole-allowed transitions at 399 nm, with ~30 MHz linewidth, and the much narrower intercombination line at 556 nm, with ~180 kHz linewidth. The optical cavity is resonant with the 556-nm line. The cavity itself has 4.9 GHz free spectral range and ~30,000 finesse.
The atoms are trapped and cooled in a magneto-optical trap (MOT) formed by 6 intersecting laser beams at 399 nm at the center of the cavity. Their temperature, measured spectroscopically, is about 5 mK . When light at 556 nm is coupled into the resonator, strong absorptive and dispersive interaction is observed. It results, for example, in bi-stability when the cavity frequency is scanned . When light from a lateral "pump" beam at 556 nm is scattered into the cavity via the trapped atoms, we observe lasing, marked by a threshold behavior of output vs. input power and a laser-like g(2) correlation function .