10-08-2013, 12:48 PM
Study of Laser Cooling
Introduction
Laser cooling and trapping of neutral atoms is a rapidly expanding area of physics
research that has seen dramatic new developments over the last decade. These include the ability
to cool atoms down to unprecedented kinetic temperatures (as low as one micro Kelvin) and to
hold samples of a gas isolated in the middle of a vacuum system for many seconds. This unique
new level of control of atomic motion is allowing researchers to probe the behaviour of atoms in
a whole new regime of matter where De‟Broglie wavelengths are much larger than the Bohr
radius. Undoubtedly one of the distinct appeals of this research is the leisurely and highly visible
motion of the laser cooled and trapped atoms.
What is Laser Cooling?
Laser cooling involves atom trapping and cooling, a method where a number of atoms are
confined in a specially shaped arrangement of electric and magnetic fields. Shining particular
wavelengths of laser light at the atoms slows them down, thus cooling them. As this process is
continued, they all are slowed and have the same energy level, forming an unusual arrangement
of matter known as a Bose-Einstein condensate.
Working Principle of Laser Cooling
The primary force used in laser cooling and trapping is the recoil when momentum is
transferred from photons scattering off an atom. This radiation-pressure force is analogous to that
applied to a bowling ball when it is bombarded by a stream of ping pony balls. The momentum
kick that the atom receives from each scattered photon is quite small; a typical velocity change is
about 1 cm/s. However, by exciting a strong atomic transition, it is possible to scatter more than
107 photons per second and produce large accelerations (104 g). The radiation-pressure force is
controlled in such a way that it brings the atoms in a sample to a velocity near zero ("cooling"),
and holds them at a particular point in space ("trapping").
MAGNETO OPTICAL TRAP / BEC Apparatus
Although optical molasses will cool atoms, the atoms will still diffuse out of the region if
there is no position dependence to the optical force. Position dependence can be introduced in a
variety of ways. Here we will only discuss how it is done in the "magneto-optical trap" (MOT),
also known as the "Zeeman shift optical trap," or "ZOT." The position-dependent force is created
by using appropriately polarized laser beams and by applying an inhomogeneous magnetic field
to the trapping region. Through Zeeman shifts of the atomic energy levels, the magnetic field
regulates the rate at which an atom in a particular position scatters photons from the various
beams and thereby causes the atoms to be pushed to a particular point in space. In addition to
holding the atoms in place, this greatly increases the atomic density since many atoms are pushed
to the same position.
Overview Of The Trapping Apparatus
Figure 3.7 shows a general schematic of the trapping apparatus. It consists of two Vortex
diode lasers, two saturated absorption spectrometers, a trapping cell, and a variety of optics. The
optical elements are lenses for expanding the laser beams, mirrors and beam splitters for splitting
and steering the beams, and wave plates for controlling their polarizations. To monitor the laser
frequency, a small fraction of the output of each laser is split off and sent to a saturated
absorption spectrometer. An electronic error signal from the trapping laser's saturated absorption
spectrometer is fed back to the laser to actively stabilize its frequency. The trapping cell is a
small vacuum chamber with an ion vacuum pump, a rubidium source, and windows for
transmitting the laser light.
Conclusion
Laser cooling is the modern technique of cooling, in which we can get Bose-Einstein
condensate which is nearer to almost absolute zero temperature. At this temperature atoms of the
gases have zero kinetic energy and they are almost fixed. Therefore all the atoms of the gas have
the same behavioral characteristics. This unique characteristic has made laser cooling more
important and focused area for research & latest development for scientists and physicist.