Hello my friends,
On a broad conceptual level I can grok how atomic cooling by laser is possible; my layman's understanding of what happens inside a laser cavity paints for me a picture of homogenous atoms being pumped into excited states then dumping their energy into nearby neighbors... It makes some sense to me that such a scenario could be arranged where the atoms are left with less energy than when they began.
But how? That is to say - what are the "rules" of this? It must be the case that only certain materials can be laser cooled - and furthermore - that only certain wavelengths work with certain materials.
I refuse to believe that this phenomenon can only be exhibited in exotic, expensive materials with exotic, expensive lasers.
Cesium atoms? In a gaseous or perhaps plasma state? With magnetic traps and high-performance lasers? Swell. Swell, but doesn't help me experiment at home.
How about solid matter? Sapphire. I believe some big-time laser dudes have cooled sapphire to cryogenic temperatures with laser radiation. Presumably with a very specific wavelength. Presumably with some really specific arrangements of pulses...(?)
OK, OK - here's some fellas who cooled a "semiconductor membrane". http://www.electronics-cooling.com/2012 ... ser-light/
The author of the article doesn't mention the wavelength used, or even whether it was CW or pulsed laser radiation, but the photograph sure makes me want to believe that the cooling was performed with some 532nm photons, possibly even with CW radiation?
In the end, is that what I really want to know - Does CW laser radiation have any place in atomic cooling? Am I going to have to build a pulsed laser if I want to cool the atoms that make up solid material?
I'd love to hear from anyone who has insight into atomic cooling and would be interested in sharing their knowledge.
Many thanks and much respect!
Laser cooling!!!!! Please share your knowledge?
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Dinesh
Laser cooling!!!!! Please share your knowledge?
Actually, you may be mixing two different phenomena. The atoms inside a laser cavity are not cooled by laser cooling. If they were, the laser would not lase.
Let's start with the atoms inside a laser cavity. Actually, let's start with atoms inside a cavity; this would be a sealed box with a light source and some atoms in it. The light source is assumed to uniformly illuminate the inside of the box, creating a uniform light field. This light field consists of light that has specific energy states, so that there may be light of energy E_1, E_2 etc. The atom is assumed to have several specific "energy states" so that the atom would increase it's energy by definite amounts from a specific energy state to a given energy state. Let's say that the difference is also E_1, E_2 etc. The actual mechanism by which it increases it's energy is not important right now, it's just important to note that the energy increase or decrease exists between two specific levels. It's also important to realise that the energy of the light corresponds to an energy difference in the atom, so light of energy E_1 possesses the energy E_1 while, for the atom, E_1 is the difference between two energy states of the atom. Different atoms have different energy differences, so one particular atom may have an energy state difference of E_1 and another may have an energy difference of, say, E_4 (a different amount of energy from E_1). What would be the processes that would occur in such a system? In 1917 Einstein analysed this process and came up with three possibilities: stimulated absorption, spontaneous emission, and stimulated emission.
In the first case, stimulated absorption, if the light in the box contained light of a specific energy E_1 and if there were atom (or atoms) in the box whose energy differences were also E_1, then the atom would absorb this (exact) amount of energy and so go to a higher energy state. Note that this higher energy state is higher than the ground state - higher than "room temperature", if you like. The original lower state is not "cooled", it's at "room temperature" to begin with and gets "hotter". In the second case, spontaneous emission, the atom emits this energy of E_1 spontaneously (it doesn't do it as a result of an interaction, it just emits on it's own at random) and goes back to it's ground state. These processes happen at random so there is this constant dance of atoms absorbing energy and re-emitting energy. The number of events where the atoms absorb energy depends on how many atoms are in the ground state to begin with, and the number of events where the atoms are emitting energy depends on how many atoms are in the higher state at any given time. In the third case, stimulated emission, the atom is already in a higher state when it reacts with the light with the same energy. In this situation, the atom then emits light of the same frequency, which then adds the the light that caused the original emission. This emitted light has the same frequency, energy, direction and polarisation as the light that caused the event. So, now you have two units of light that are exactly the same, ie the light is coherent. Normally, most of the atoms are in the lower ground state and so the number of spontaneous emissions drown out the number of stimulated emissions, so you basically get fluorescence.
However, if you could arrange that there are a lot more atoms in the higher state to begin with, then the number of stimulated emission events would drown out the number of spontaneous emissions. This state, where there are more atoms in the higher state than in the lower, is called 'population inversion' (for obvious reasons!) Basically, the result would be a burst of coherent radiation. If you now placed mirrors in the front and back of this box, and then arranged to pump radiation into the box such that, as soon as an stimulated emission event happened in an atom, you pumped that atom back to it's higher state, you'd the get the light passing through the system creating a burst of coherent radiation; the mirror then passes this coherent radiation back into the system and the atoms are now once again in a population inversion state, so the process repeats. Every pass of the light between the mirrors (while ensuring a population inversion state is re-created) will enhance the radiation and so the original radiation is amplified many fold. You have Light Amplification by Stimulated Emission of Radiation (LAbSEoR - no that sounds wrong somehow!! ). Note again, this does not "cool" the atoms to below the ground state, it exploits the "heating" of atoms by creating "hot" atoms to begin with and allowing them to "cool" to "room temperature" in a controlled way.
In laser cooling, the important fact to realise is that heat is a result of kinetic energy of atoms, which is an effect of the speed of an atom. The faster an atom is traveling, the more kinetic energy it has and so the greater is the heat of the system. So, if you could slow down the atoms in a box full of atoms, you'd reduce the heat of the system and so lower the temperature of the system. In other words you'd cool the box by robbing the atoms of their speed. The other thing to realise is that light has momentum, according to Maxwell's equations. So, an obvious mechanism arises to cool down a box with atoms in it. Simply arrange for all the atoms to be going in the opposite way to a beam of light. The light hits the atoms and, having more momentum than the atoms, robs the atoms of their momentum and so slows them down. It's a little more complicated than this, but this is the basic principle. If you could arrange it so that the atoms would lose some of the momentum, then you'd effectively have a "brake" for the atoms. If you can arrange for the atoms to be "trapped" so that it's slowed down to begin with, the momentum transfer is even more potent. This is done by trapping the atoms in a magnetic field.
But, when the light hits the atoms, the atoms also undergo stimulated absorption, which is not an effect you want since this increases the energy of the atom and you need to decrease that energy in order to slow it down. So, you want it to undergo spontaneous emission so that it can divest itself of this extra energy. You want the atom to absorb a specific amount of energy that it can divest this energy from itself through a spontaneous emission process. This exact energy is called the "resonant energy" and if the atom can be arranged to only absorb this resonant energy, it will get rid of that energy quickly. If it absorbs non-resonant energy, it won't get rid of the energy and so won't slow down. The way this is done is by hitting the atom with light that has a frequency just below resonant. Since the atom sees a beam of light approaching it, the atom sees a Doppler-shifted beam of light. If the Doppler shift is just enough for the atom to see the resonant energy it needs, the atom re-emits this light in the form of spontaneous emission. Now the momentum is absorbed along the direction of motion of the atom by the light (going in opposite directions but along a straight line) while the emitted photon (the spontaneous emission ) goes off at a random direction, so the net effect is that more often than not, the atom slows down along it's direction of motion.
It's also important that the atom divest itself of the energy it gained by stimulated absorption, so it doesn't gain net energy. Thus it's important that the atom have specific energy states that can allow it to undergo stimulated absorption and then, almost immediately, to under spontaneous emission. This is, in effect, the opposite of the laser mechanism. In a laser, you want to supress the spontaneous emission rate, while in laser cooling, you want it to undergo spontaneous emission to rid itself of the excess energy
Let's start with the atoms inside a laser cavity. Actually, let's start with atoms inside a cavity; this would be a sealed box with a light source and some atoms in it. The light source is assumed to uniformly illuminate the inside of the box, creating a uniform light field. This light field consists of light that has specific energy states, so that there may be light of energy E_1, E_2 etc. The atom is assumed to have several specific "energy states" so that the atom would increase it's energy by definite amounts from a specific energy state to a given energy state. Let's say that the difference is also E_1, E_2 etc. The actual mechanism by which it increases it's energy is not important right now, it's just important to note that the energy increase or decrease exists between two specific levels. It's also important to realise that the energy of the light corresponds to an energy difference in the atom, so light of energy E_1 possesses the energy E_1 while, for the atom, E_1 is the difference between two energy states of the atom. Different atoms have different energy differences, so one particular atom may have an energy state difference of E_1 and another may have an energy difference of, say, E_4 (a different amount of energy from E_1). What would be the processes that would occur in such a system? In 1917 Einstein analysed this process and came up with three possibilities: stimulated absorption, spontaneous emission, and stimulated emission.
In the first case, stimulated absorption, if the light in the box contained light of a specific energy E_1 and if there were atom (or atoms) in the box whose energy differences were also E_1, then the atom would absorb this (exact) amount of energy and so go to a higher energy state. Note that this higher energy state is higher than the ground state - higher than "room temperature", if you like. The original lower state is not "cooled", it's at "room temperature" to begin with and gets "hotter". In the second case, spontaneous emission, the atom emits this energy of E_1 spontaneously (it doesn't do it as a result of an interaction, it just emits on it's own at random) and goes back to it's ground state. These processes happen at random so there is this constant dance of atoms absorbing energy and re-emitting energy. The number of events where the atoms absorb energy depends on how many atoms are in the ground state to begin with, and the number of events where the atoms are emitting energy depends on how many atoms are in the higher state at any given time. In the third case, stimulated emission, the atom is already in a higher state when it reacts with the light with the same energy. In this situation, the atom then emits light of the same frequency, which then adds the the light that caused the original emission. This emitted light has the same frequency, energy, direction and polarisation as the light that caused the event. So, now you have two units of light that are exactly the same, ie the light is coherent. Normally, most of the atoms are in the lower ground state and so the number of spontaneous emissions drown out the number of stimulated emissions, so you basically get fluorescence.
However, if you could arrange that there are a lot more atoms in the higher state to begin with, then the number of stimulated emission events would drown out the number of spontaneous emissions. This state, where there are more atoms in the higher state than in the lower, is called 'population inversion' (for obvious reasons!) Basically, the result would be a burst of coherent radiation. If you now placed mirrors in the front and back of this box, and then arranged to pump radiation into the box such that, as soon as an stimulated emission event happened in an atom, you pumped that atom back to it's higher state, you'd the get the light passing through the system creating a burst of coherent radiation; the mirror then passes this coherent radiation back into the system and the atoms are now once again in a population inversion state, so the process repeats. Every pass of the light between the mirrors (while ensuring a population inversion state is re-created) will enhance the radiation and so the original radiation is amplified many fold. You have Light Amplification by Stimulated Emission of Radiation (LAbSEoR - no that sounds wrong somehow!! ). Note again, this does not "cool" the atoms to below the ground state, it exploits the "heating" of atoms by creating "hot" atoms to begin with and allowing them to "cool" to "room temperature" in a controlled way.
In laser cooling, the important fact to realise is that heat is a result of kinetic energy of atoms, which is an effect of the speed of an atom. The faster an atom is traveling, the more kinetic energy it has and so the greater is the heat of the system. So, if you could slow down the atoms in a box full of atoms, you'd reduce the heat of the system and so lower the temperature of the system. In other words you'd cool the box by robbing the atoms of their speed. The other thing to realise is that light has momentum, according to Maxwell's equations. So, an obvious mechanism arises to cool down a box with atoms in it. Simply arrange for all the atoms to be going in the opposite way to a beam of light. The light hits the atoms and, having more momentum than the atoms, robs the atoms of their momentum and so slows them down. It's a little more complicated than this, but this is the basic principle. If you could arrange it so that the atoms would lose some of the momentum, then you'd effectively have a "brake" for the atoms. If you can arrange for the atoms to be "trapped" so that it's slowed down to begin with, the momentum transfer is even more potent. This is done by trapping the atoms in a magnetic field.
But, when the light hits the atoms, the atoms also undergo stimulated absorption, which is not an effect you want since this increases the energy of the atom and you need to decrease that energy in order to slow it down. So, you want it to undergo spontaneous emission so that it can divest itself of this extra energy. You want the atom to absorb a specific amount of energy that it can divest this energy from itself through a spontaneous emission process. This exact energy is called the "resonant energy" and if the atom can be arranged to only absorb this resonant energy, it will get rid of that energy quickly. If it absorbs non-resonant energy, it won't get rid of the energy and so won't slow down. The way this is done is by hitting the atom with light that has a frequency just below resonant. Since the atom sees a beam of light approaching it, the atom sees a Doppler-shifted beam of light. If the Doppler shift is just enough for the atom to see the resonant energy it needs, the atom re-emits this light in the form of spontaneous emission. Now the momentum is absorbed along the direction of motion of the atom by the light (going in opposite directions but along a straight line) while the emitted photon (the spontaneous emission ) goes off at a random direction, so the net effect is that more often than not, the atom slows down along it's direction of motion.
It's also important that the atom divest itself of the energy it gained by stimulated absorption, so it doesn't gain net energy. Thus it's important that the atom have specific energy states that can allow it to undergo stimulated absorption and then, almost immediately, to under spontaneous emission. This is, in effect, the opposite of the laser mechanism. In a laser, you want to supress the spontaneous emission rate, while in laser cooling, you want it to undergo spontaneous emission to rid itself of the excess energy
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holorefugee
Laser cooling!!!!! Please share your knowledge?
I was thinking this thread was about saving water while cooling a laser. That I know something about. 
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Dinesh
Laser cooling!!!!! Please share your knowledge?
Well, that's a heckuva lot simpler! Have a closed loop system for the laser and refrain from either drinking anything or going to the bathroom while the laser is on.