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In [[atomic physics]], '''Raman cooling''' is a sub-recoil cooling technique that allows the cooling of [[atoms]] using optical methods below the limitations of [[Doppler cooling]], limited by the recoil energy of a photon given to an atom. This scheme can be performed in simple [[optical molasses]] or in molasses where an [[optical lattice]] has been superimposed, which are called respectively free space Raman cooling <ref>[http://link.aps.org/doi/10.1103/PhysRevLett.69.1741 ''Laser cooling below photon recoil with three-level atoms''], Mark Kasevich, [[Steven Chu]], Phys. Rev. Lett. '''69''', 1741 (1992)</ref> and Raman side-band cooling.<ref>[http://link.aps.org/doi/10.1103/PhysRevLett.84.439 ''Beyond Optical Molasses: 3D Raman Sideband Cooling of Atomic Cesium to High Phase-Space Density''], Andrew J. Kerman, Vladan Vuletic, Cheng Chin, and Steven Chu, Phys. Rev. Lett. '''84''', 439 (2000)</ref> Both techniques make use of [[Raman scattering]] of laser light by the atoms.
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==Two photon Raman process==
The transition between two [[Hyperfine structure|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 deexcites 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.
 
[[Image:RamanProcess.png|center]]
 
The illustration of this process is shown in the schematic illustration of a two-photon Raman process. It enables the transition between the two levels <math>|g_1\rangle</math> and <math>|g_2\rangle</math>. The intermediate, virtual level is represented by the dashed line, and is red-detuned with respect to the real excited level, <math>|e\rangle</math>. The frequency difference <math>f_2-f_1</math> here matches exactly the energy difference between <math>|g_1\rangle</math> and <math>|g_2\rangle</math>.
 
== Free space Raman cooling==
 
In this scheme, a pre-cooled cloud of atoms (whose temperature is of a few tens of microkelvins) undergoes a series of pulses of Raman-like processes. The beams are counterpropagating, and their frequencies are just as what has been described above, except that the frequency <math>f_2</math> is now slightly [[Laser detuning|red-detuned]] (detuning <math>\Delta</math>) with respect to its normal value. Thus, atoms moving towards the source of the laser 2 with a sufficient velocity will be resonant with the Raman pulses, thanks to the [[Doppler effect]]. They will be excited to the <math>|g_2\rangle</math> state, and get a momentum kick decreasing the modulus of their velocity.  
 
If the propagation directions of the two lasers are interchanged, then the atoms moving in the opposite direction will be excited and get the momentum kick that will decrease the modulus of their velocities. By regularly exchanging the lasers propagating directions and varying the detuning <math>\Delta</math>, one can manage to have all atoms for which the initial velocity satisfies <math>|v|>v_{max}</math> in the state <math>|g_2\rangle</math>, while the atoms such that <math>|v|<v_{max}</math> are still in the <math>|g_1\rangle</math> state. A new beam is then switched on, whose frequency is exactly the transition frequency between <math>|g_2\rangle</math> and <math>|e\rangle</math>. This will [[optical pumping|optically pump]] the atoms from the <math>|g_2\rangle</math> state to the <math>|g_1\rangle</math> state, and the velocities will be randomized by this process, such that a fraction of the atoms in <math>|g_2\rangle</math> will acquire a velocity <math>|v|<v_{max}</math>.
 
By repeating this process several times (eight in the original paper, see references), the temperature of the cloud can be lowered to less than a microkelvin.
 
== 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.   
 
[[Image:RamanSidebandCooling.png|center]]
 
By means of Raman processes, an atom can be transferred to a state where the magnetic moment has decreased by one and the vibrational state has also decreased by one (red arrows on the picture). After that the atoms which are in the lowest vibrational state of the lattice potential (but with <math>m\neq 1</math>) are [[optical pumping|optically pumped]] to the m=1 state (role of the <math>\sigma_+</math> and <math>\pi</math> light beams). Since the temperature of the atoms is low enough with respect to the pumping beam frequencies, the atom is very likely not to change its vibrational state during the pumping process. Thus it ends up in a lower vibrational state, which is how it is cooled.
In order to reach this efficient transfer to the lower vibrational state at each step, the parameters of the laser, i.e. power and timing, should be carefully tuned. In general, these parameters are different for different vibrational states because the strength of the coupling ([[Rabi frequency]]) depends on the vibrational level. Additional complication to this naive picture arises from the [[Recoil temperature|recoil of photons]], which drive this transition. The last complication can be generally avoided by performing cooling in a so-called [[Lamb Dicke regime]]. In this regime the atom is trapped so strongly in the optical lattice that it effectively does not change its momentum due to the photon recoils. The situation is similar to the [[Mössbauer effect]].
 
 
This cooling scheme allows to obtain a rather high density of atoms at a low temperature, using only optical techniques. It is still not sufficient to attain for example [[Bose-Einstein condensation]], but it can be a starting point for such experiments. For instance, the 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.<ref>''Bose-Einstein condensation of Cesium'', Tino Weber, Jens Herbig, Michael Mark, Hanns-Christoph Nägerl, Rudolf Grimm, Science, 299, 232</ref>
 
== References ==
{{Reflist}}
 
[[Category:Atomic physics]]
[[Category:Cooling technology]]

Latest revision as of 13:45, 3 September 2014

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OK, so we've all measured up in a new location and had to issue in space for distance from plant at maximum size, size of reflectors, sturdy mountings for ballasts, fans, extractor fans with tubing and so on. With LED growing none of these elements need to be regarded. LED develop lights can be considerably closer to the plants, and without all that excess equipment required, it seriously is feasible to grow in a small space, discreetly. Invest in a grow light and you are going to be content.

The use of LED's in horticulture indoor gardening food production by means of the growing of aquatic plants and hydroponics and plant propagation will continue to grow in the near future. As far more adaptations of Led lights are made and analysis finds new uses for them in the field of indoor agriculture, growers can appear forward to even bigger harvests. Day-time photo of a single of our greenhouses - TotalGrow fixtures are hung about 4' above the develop bed.

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LED plant lights emit pretty small heat, which suggests that even if they have been left on for various days, it will be attainable for you to touch them devoid of burning your hand. Pick out the led develop light for indoor plants, as an alternative of sunlight to plant development, bring lovely plants for you. It does not only offer the bright light but also assists retain cooler air temperature for germination to take spot especially in the case of lettuce. You will have to also know the coverage of grow lights.

On flowering plants, getting decreased photo period equates to an raise in the plant strain. The LED lights emit a certain kind of wavelength, and develop lights at times will need to emit wavelengths specific to the harvest becoming grown. If you are taking into consideration working with LED grow lights, there are different utilizes exactly where they can benefit you. Indoor growing makes it achievable to develop where it is not at the moment achievable.

Based on their guidance, including Jacob Nelson and Dr. Bugbee's analysis of plant lighting, I made up a spreadsheet calculating the price of working with the lights for five years, utilizing the each day light integral (DLI), the minimum total number of photons important every day that researchers advise for expanding seedlings (four mol/m²/d) and lettuce and herbs (12 mol/m²/d). Lots of develop light advertisements speak about lumen output.” They are not assisting.

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