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Fundamentals of Fiber Lasers
Published in Johan Meyer, Justice Sompo, Suné von Solms, Fiber Lasers, 2022
Johan Meyer, Justice Sompo, Sune von Solms
Laser cooling makes it possible to bring clouds of atoms or ions to extremely low temperatures. This has applications in fundamental research and for industrial purposes. Particularly, in biological and medical research, optical tweezers can be used for trapping and manipulating small particles, such as bacteria or parts of living cells.
A Solution in Search of a Problem or Many Problems with the Same Solution? Applications of Lasers
Published in Mario Bertolotti, The History of the Laser, 2004
The principle of laser cooling is the transfer of momentum by a photon to an atom. The atom absorbing the photon receives a push in the direction in which the photon was travelling. In the subsequent re-emission of a photon, the excited atom recoils. If the emission is spontaneous the direction of the re-emitted photon is random. A series of absorptions and re-emissions transfer momentum to the atom in the direction of the laser beam, while the recoils average to zero. The result is that an atom which is propagating against the light beam is slowed, very much like a cyclist riding against the wind.
Laser Coolingl Trapping and Control of Atoms
Published in Yu. N. Kulchin, Modern Optics and Photonics of Nano and Microsystems, 2018
The velocity of thermal cesium atoms is ~100m/s. Laser cooling lowers the temperature of atoms to ~10nK and thereby reduces their thermal velocity to several centimeters per second. To provide a long time for the interaction of an atom with a microwave field, the construction of an atomic fountain was proposed (Fig. 27) [16]. As can be seen from Fig. 27, the cooling laser beams propagate vertically and pass twice through the microwave cavity: from below upwards and from top to bottom. In an atomic fountain, cesium atoms are first cooled by vertical laser beams, and then trapped by a magneto‐optical trap in which they are further cooled. The resulting cloud of ultracold atoms serves as a source of slow atoms in the fountain. A cooled cloud of cesium atoms is directed from below by a laser pulse, vertically upwards, as a result of which the atoms begin to move along ballistic trajectories in the gravitational field of the Earth. Moving upwards, the atoms cross the probing field of the RF resonator and rise to a height of about one meter. Due to the gravity field, the velocity of atoms gradually decreases to zero, and they begin to fall down, passing through the resonator again. Thus, a cloud of cesium atoms twice cuts off the resonator, where the microwave frequency is used to read the frequency of oscillations of their atomic transition. At the same time, there are always atoms in the resonator moving only in one direction, which makes the transition line more specific. As a result, the duration of the frequency measurement in the atomic fountain increases by two orders of magnitude in comparison with the frequency standard on the thermal atomic beam and is ~1s. The microwave transition is registered by recording the fluorescence of cesium atoms initiated by a probe laser beam. The large interaction time makes it possible to realize the relative accuracy of frequency measurement in atomic fountains ~6 ⋅ 10−16. If a cesium atom fountain is placed in a cryogenic chamber, it is possible reduce the effect of thermal radiation from the walls of the chamber on the heating of atoms. This made it possible to increase the accuracy of the frequency measurement by an order of magnitude. Improved frequency control leads to the fact that the atomic clock based on the atomic fountain becomes one of the most accurate hours of the world.
Laser cooling of trapped ions in strongly inhomogeneous magnetic fields
Published in Molecular Physics, 2023
Richard Karl, Yanning Yin, Stefan Willitsch
In Doppler laser cooling, photon scattering from a red-detuned laser beam generates a directional force that cools the motion of a counter-propagating atom [40]. This force, termed scattering force in this manuscript, is the product of the photon momentum and the scattering rate which depends on the internal properties of the atom via the spontaneous decay rates on selected spectroscopic transitions and on the external dynamics via the Doppler and Zeeman shifts and position-dependent laser parameters. In a hybrid trap composed of a RF trap and a magnetic trap, micromotion and magnetic field gradients can lead to timescales of the external dynamics that are comparable to the timescales of the internal population dynamics. Therefore, it is not valid to assume steady-state level populations of the ion as it is often done in laser cooling models based on a continuous friction force [41–43].