Laser Cooling Technology

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Definition:
Laser Cooling is the technique in which atomic and molecular samples are cooled down near absolute zero, when its interaction occurs with two or more laser fields. Laser cooling technique used a fact when an atom absorbs light and re-emits photon or particle of light then momentum changes.

Laser cooling techniques combine with atomic spectroscopy with mechanical effect of light to compresses the velocity distribution of an ensemble of particles, that is why it cooling particles.

The first example of laser cooling and also it is the most common method is called Doppler cooling. Laser cooling is not meant to be cooling of lasers, but rather the use of dissipative light forces for reducing the random motion. Depending on the mechanism used, the temperature achieved can be very low.

History:
Laser cooling started in 1975 where it has some possibility of laser cooling. But in 1978, first laser cooling demonstration is done for ions trapped. First 3-D cooling is successful in 1982 at the temperature 240 micro kelvins.

Methods of Laser cooling-
Doppler cooling, resolved sideband cooling, Raman Sideband cooling, Zeeman Slower, EIT-electromagnetically induced transparency cooling and Sisyphus cooling.

DOPPLER COOLING-
Here Light forces are exerted by absorption and spontaneous emission of photons.
Rate of processes depends on the velocity of atom or ion due to the Doppler shift.
For example, a beam of atoms in vacuum chamber can be stopped. It can be cooled with a single frequency laser beam. So optical frequency first chosen to be higher than atomic resonance, so that only the fastest atoms can absorb photons. Laser frequency is reduced so that slower & slower atoms can participate in the interaction. Doppler cooling can also be used in an optical molasses.

Sisyphus cooling – 
It allows to get substantially below the Doppler limit, down to the much lower recoil limit associated with the recoil momentum related to the absorption or emission of a single photon. Even when recoil limit is not the final one: specifically the method of velocity-selective coherent population trapping allows sub-recoil temperatures in Nano kelvin.

Applications of laser cooling -
• For high-resolutions pectroscopic measurements by the elimination of Doppler broadening.
• For studying behavior of ultracold gases, which follow Bose–Einstein condensation.
• It has many uses in quantum optics.
• It has uses in research and applications in quantum information technology (e.g., quantum computing).
• For very precise measurement of gravitational fields .(used e.g. for gravitational physics or
• For oil field exploration, that is based on the Doppler shift of free-falling cooled atoms.
• For lithography with cold atomic beams to form very accurately controlled structures.

Doppler cooling is a technique for laser cooling of small particles (atoms or ions). The basic principle is that absorption and spontaneous emission of photon that lead to light forces. These forces become velocity-dependent through the Doppler effect- an absorption resonance of an atom or ion is shifted, towards lower frequencies when the particle is moving towards the light source. For example, a beam of atoms in a vacuum chamber can be stopped and cooled with laser beam. Optical frequency of this is higher than the atomic resonance, so that only the fastest atoms can absorb photons. Also laser frequency is reduced so that slower and slower atoms participate in the interaction. That is why finally all atoms have a greatly reduced speed. Doppler cooling can also be used in an arrangement called optical mosses where cooling occurs in all three dimensions.
The minimum temperature achievable with Doppler cooling is called Doppler limit.

Sisyphus cooling:
It is a mechanism for laser cooling of atoms or ions by using light forces.

It was understood in 1985, laser cooling experiments with cesium atoms can lead to temperatures well below the Doppler limit that the simple mechanism of cooling is not a sufficient explanation. The mechanism of Sisyphus cooling is somewhat sophisticated. It involves a polarization gradient, as generated e.g. by two counter propagating linearly polarized laser beam with perpendicular polarization directions. Therefore sometimes it called polarization gradient cooling. Essential ingredient is that when atoms in a certain dressed state i.e. reach a position where their potential energy is relatively large, it becomes likely that they are pumped optically into another state for which the potential energy at that position is close to a minimum. Polarization gradient introduces non-conservative light forces which can reduce the average kinetic energy of atoms.
Sisyphus cooling has become important for optical frequency standard, because it makes it possible to cool atoms to very low temperatures. So that linewidth of certain transition becomes very small.

Resolved sideband cooling:
Resolved sideband cooling is a technique allowing cooling of tightly bound atoms and ions beyond the Doppler cooling limit. Preparation of a particle in a definite state with high probability is an essential part of state manipulation experiments in quantum optics and in quantum computing.

Resolved sideband cooling this is a laser cooling technique that can be used to cool strongly trapped atoms to the quantum ground state of their motion. The atoms are usually precooled using Doppler laser cooling. Resolved sideband cooling is used to cool the atoms beyond the Doppler cooling limit.

Example - Raman sideband cooling of Cs atom.

Raman transition replaces the one-photon transition used in the sideband above by a two-photon process via a virtual level. In the Cs cooling experiment, trapping is provided by an isotropic optical lattice in a magnetic field, which also provides Raman coupling to the red sideband of the Zeeman manifolds. Preparation of cold sample of Cs atoms is carried out in optical molasses in a magneto-optic trap, atoms are allowed to occupy a 2D, near resonance lattice. Lattice is changed adiabatically to a far off resonance lattice, which leaves the sample sufficiently well cooled for sideband cooling to be effective. A magnetic field is turned on to tune the Raman coupling to the red motional sideband, relaxation between the hyperfine states is provided by a pump laser pair, after some time, pumping is intensified to transfer the population to a specific hyperfine state.

Raman cooling:
Raman cooling is a sub-recoil cooling technique that allows the cooling of atoms using optical methods below the limitations, it limited by the recoil energy of a photon given to an atom. This scheme can be performed in simple molasses where an optical lattice has been superimposed, which called respectively free space Raman cooling, and Raman side-band cooling. Both techniques make use of Raman scattering of laser light by the atoms. Bose–Einstein condensation of cesium has been achieved for the first time in an experiment that used Raman side-band cooling as its first step.


Two photon Raman process:
The transition between two hyperfine states of the atom can be triggered by two laser beams: the first beam excites the atom to a virtual excited state (for example because its frequency is lower than the real transition frequency), and the second beam nonexcites the atom to the other hyperfine level. The frequency difference of the two beams is exactly equal to the transition frequency between the two hyperfine levels.

It enables the transition between the two levels & . The intermediate, virtual level is represented by the dashed line, and is red-detuned with respect to the real excited level . The frequency difference here matches exactly the energy difference between and .

Raman side-band cooling:
This cooling scheme starts from atoms in a magneto-optical trap, an optical lattice is then ramped up, such that an important fraction of the atoms are trapped. If the lasers of the lattice are powerful enough, each site can be modelled as a harmonic trap. Since the atoms are not in their ground state, they will be trapped in one of the excited levels of the harmonic oscillator. The aim of Raman side-band cooling is to put the atoms into the ground state of the harmonic potential in the lattice site.

We consider a two level atom, the ground state of which has a quantum number of F=1, such that it is three-fold degenerate with m=-1, 0 or 1. A magnetic field is added, which lifts the degeneracy in m due to the Zeeman effect, Its value is exactly tuned such that the Zeeman splitting between m=-1 and m=0 and between m=0 and m=1 is equal to the spacing of two levels in the harmonic potential created by the lattice.

Zeeman slower: 
It is a scientific apparatus that is commonly used in quantum optics to cool a beam of atoms from room temperature or above to a few kelvins.

At the entrance of the Zeeman slower the average speed of atoms is on the order of a few hundred m/s. The spread of velocity is also in the order of a few hundred m/s. Final speed at the exit of the slower is few 10 m/s with an even smaller spread.

A Zeeman slower consists of a cylinder through which the beam travels, a pump laser that is shown on the beam in the direction opposite to the beam's motion, and a magnetic field produced by a coil that points along the symmetry axis of the cylinder and varies spatially along the axis of the cylinder. The pump laser, which is required to be near-resonant to an atomic or molecular transition, Doppler slows a certain velocity class within the velocity distribution of the beam. Resonant frequency enables lower and lower velocity classes to be resonant with the laser, as atomic or molecular beam propagates along the slower, hence slowing the beam.

Zeeman slower is usually used as a preliminary step to cool the atoms in order to trap them in a magneto optical trap.

Thus it aims at a final velocity of about 10 m/s (depending on the atom used), starting with a beam of atoms with a velocity of a few hundred meters per second. The final speed to be reached is a compromise between the technical difficulty of having a long Zeeman slower and the maximal speed allowed for an efficient loading into the trap.

Electromagnetically induced transparency (EIT) COOLING: 
It is a coherent optical nonlinearity which renders a medium transparent window over a narrow spectral range within an absorption line. Extreme dispersion is also created within this transparency which leads to slow light. It is in essence a quantum interference effect that permits the propagation of light through an otherwise opaque atomic medium.

Observation of EIT involves two optical fields (highly coherent light sources, i.e. laser) which are tuned to interact with three quantum states of a material.

The "probe" field is tuned near resonance between two of the states and measures the absorption spectrum of the transition. A much stronger "coupling" field is tuned near resonance at a different transition.

If the states are selected properly, the presence of the coupling field will create a spectral "window" of transparency which will be detected by the probe.

The coupling laser is sometimes referred to as the "control" or "pump", the latter in analogy to incoherent optical nonlinearities such as spectral hole burning or saturation.

EIT is based on the destructive interference of the transition probability amplitude between atomic states.
It is closely related to EIT are coherent population trapping (CPT) phenomena.

Posted by: Akhilesh Kumar. in Science , Technology | Date: 11/05/2016

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