Laser cooling process

In the experiment, we try to determine (0 o) +, and 0) by resonant excitation of the ion motion through the application of a periodic driving force (see Section 10.3.3). In practice, the situation is complicated slightly by the fact that the laser-cooled ion(s) is (are) subject to a damping force due to the laser-cooling process. In this case, the equations of motion read... 

The position of a laser-cooled ion is measured by imaging the fluorescence light emitted during the Doppler laser-cooling process onto a CCD camera. There are two fundamental factors that limit the resolution the spatial resolution of the imaging system and the finite ion temperature. 

FIGURE 11.20 First atomic levels involved of singly-ionized calcium (Call) for the ion atomic clock at 729 nm (light continuous line). One of the bold continuous line (397nm) indicates the transition used for the laser cooling process and the second other one (866 nm) permits the back pumping from the Dj/j toward the cooling cycle. 

Laser Cooling The 1997 Nobel Prize in physics was shared by Steven Chu of Stanford University, William D. Phillips of the National Institute of Science and Technology, and Claude N. Cohen-Tannoudje of the College de France for their development and theoretical explanation of laser cooling, a process that can lower the temperature of a gas to a very low value. 

Little basic research has been devoted to Yb3+ ion itself which possesses only one excited state [151]. However, as shown previously, this ion is of special interest for a number of energy transfer and up-conversion processes, which take advantage of the strong 2F7/2 — 2F5/2 absorption band of Yb3+. In addition, efficient lasing around 1.02 can be obtained with this ion and laser cooling effects have been observed. These points are developed in the next section devoted to applications of rare-earth-doped fluoride glasses. 

In spite of these apparent difficulties, the quantum Monte Carlo calculations of cooling rates in ID agree very well with the observations. This indicates that the basic physical interactions aire all understood. What is lacking is a good physical picture of the cooling process, such as the Sisyphus picture that has guided the development of laser cooling so well until now. 

Miyasaka H, Hagihara M, Okada T and Malaga N 1992 Femtosecond laser photolysis studies on the cooling process of chrysene in the vibrationally hot S. state in solution Chem. Phys. Lett. 188 259-64... 

Thus we should expect the increase of the pulse duration with the increase of Pi- This increase is actually observed experimentally (Fig. 6(b)). However, the shape of the measmed pulse is not rectangular. We explain the shape of AI(t,A) by the non-uniform heating of VO2 during laser pulses and the further cooling process. The reasons for this are the small penetration depth of light into the sample and the inhomogeneous thermoconductivity properties of opal-V02 polydomain composite. 

Doppler laser-cooling is an essential ingredient in the SCSI-MS technique. First, it provides the necessary damping force to cool directly and sympathetically the atomic and molecular ions, respectively, such that a cold and strongly-coupled two-ion system is formed. Second, it gives rise to the fluorescence photons used in the detection process. Third, the radiation pressure force can be modulated to excite the common motion of the ions. 

Fig. 9.9 Velocity distribution before dashed) and after solid) Zeeman cooling. The arrow indicates the highest velocity resonant with the slowing laser. (The extra bump at 1700 m/s is from F = atoms, which are optically pumped into F = 2 during the cooling process.) [W. PhUlips, Nobel Lecture 1995]...

The physics behind laser ablation is much more complicated than as explained above. Three characteristic timescales are involved to define the nature of laser interaction with a metallic material [1], They are the electron cooling time lattice heating time Tj, and laser pulse duration tl. For a nanosecond pulse laser, tL 3> Xi. the process is predominantly laser heating. For a picosecond pulse laser, x femtosecond pulse laser, the process is exclusively a laser ablation process. Laser ablation of semiconductor and dielectric materials involves different mechanisms [2]. 

The temperature given by = Ay/2 is not the minimum temperature achievable with laser cooling. One can begin cooling on a broad transition before switching to a more forbidden transition. Thus, atoms can be initially cooled quickly when it is important to scatter many photons/sec. Once cooled to less than 1 mk, a more leisurely cooling process can be used to reach temperatures below 10 K. At these temperatures, the recoil due to a single photon momentum becomes a factor. 

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