Energetic and temporal aspects of the laser cooling of a stored ionic population are numerically investigated. The process which causes the cooling is described as a Raman scattering. Adapted formalisms and algorithms are developed. The cooling limit is computed and the optimal conditions to decrease the temperature are discussed.
Limits for the laser cooling of a trapped ion and some experimental observations are summarized. The final temperature is low when a dipole‐forbidden line is two‐photon excited, e.g. by way of an electronic Raman process. Now, the two light modes also support dispersive interaction by stimulated Raman‐like scattering. A simplified model in terms of the vibrational Raman effect identifies processes analogous to those known from atoms in a standing light wave, and it reveals novel parametric interaction of ion vibration and light. In the diabatic limit, when dephasing by spontaneous decay is small, the ion approaches the dynamics of an electron in a free‐electron laser.
A new method is analyzed for 3-D cooling of solids based on near-resonant, stimulated Raman scattering on a narrowband transition together with optical pumping on a broadband transition. Estimates of achievable cooling rates indicate that Raman cooling offers significant improvement over anti-Stkes fluorescence cooling at cryogenic temperatures and should enable the attainment of sub-Kelvin termperatures, starting from ambient conditions in bulk samples. Also the method is not restricted to ions with small Stokes shifts, and should therefore lead to diversification of the material platforms used for optical refrigeration.
A longitudinal laser cooling of ion beams at about 5% of the velocity of light has been performed at the Heidelberg Test Storage Ring with various cooling schemes employing the spontaneous force. For a 7.29-MeV 9Be+ beam with an initial longitudinal temperature of 2700 K, the main characteristics of laser cooling in a storage ring are discussed. When undamped, the transverse betatron oscillations of the coasting ions limit the longitudinal temperature after laser cooling to typically 1 K. After damping the transverse motion by precooling the ions with an electron cooler, longitudinal temperatures of below 30 mK have been obtained in the subsequent laser cooling. In this case, the longitudinal ion-beam temperature can be understood as an equilibrium of the laser cooling rate with the heating rate due to intrabeam scattering. Moreover, single binary Coulomb collisions between the (still transversely hot) ions can cause such longitudinal velocity changes that ions are lost out of the critical capture range of the laser cooling force. In these two ways, intrabeam scattering imposes a substantial limit on the temperature or number of laser cooled ions in a storage ring. In our experiments, this process presently limits the ratio between the density-dependent Coulomb energy and the longitudinal thermal energy spread to a value on the order of 1, where liquid rather than gaseous behavior of the ion beam is expected to set in.
The interaction of laser-induced and intrabeam forces has been studied in a dense stored beam of 100-keV 7Li+ ions. A fraction of the ions (\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}4}$) exist in the metastable 1s2s 3S state. Using this state as a laser spectroscopic probe, we observe fast longitudinal heating in the injected, nonequilibrium distribution due to Coulomb scattering. Laser cooling of the metastable ions is ineffective during this heating period. The metastable fraction of the equilibrated beam is subsequently laser cooled to a longitudinal temperature of \ensuremath{\sim}1 mK, the lowest temperature ever reported in a stored ion beam.