Temperature is the physical quantity that indicates the degree of heat and cold of an object. Microscopically, it is the severity of thermal motion of an object's molecules. As we all know, all the molecules and atoms around us are carrying out the irregular heat movement that never stops. The essence of our refrigeration is to reduce the intensity of the overall thermal motion of these molecules or atoms, fiber laser marking machine.
1. A very important technology in laser cooling is Doppler cooling technology. The principle of Doppler cooling technology is to block the thermal motion of atoms by emitting photons by laser, and this hindrance process is to reduce the momentum of atoms. Realized. So, how exactly does the laser reduce the momentum of these atoms?
First, quantum mechanics suggests that atoms can only absorb photons of a specific frequency, thereby changing their momentum. The Doppler effect indicates that the frequency becomes higher as the wave source moves toward the observer, and becomes lower as the wave source moves away from the observer. The same conclusion can be obtained when the observer moves.
Similarly, the same is true for atoms. When the direction of motion of the atom is opposite to the motion of the photon, the frequency of the photon will increase, and when the direction of motion of the atom is the same in the direction of motion of the photon, the photon frequency will decrease. Then, another physics principle is that although light has no static mass, it has momentum. Then by combining the above physics characteristics, we can construct a simple model of laser cooling.
2. The frequency of the laser is adjustable within a certain range, and when the frequency of the laser is adjusted to a frequency slightly lower than that of an atom, there is an unexpected result. This happens when such a beam of light illuminates a particular atom. If the atom moves toward the laser beam, the frequency of the photon increases due to the Doppler effect of the light, and the frequency of the original laser photon is just slightly less than the absorbable frequency of the atom, then the Doppler effect is just right. Absorbed by atoms.
And this absorption is manifested by momentum changes. Because the direction of motion of the photon is opposite to the direction of movement of the atom, after the photon collides with the atom, the atom transitions to the excited state, and the momentum decreases, so the kinetic energy also decreases. For atoms in other directions of motion, the frequency of the corresponding photons does not increase, so the photons in the laser beam cannot be absorbed, so there is no such thing as an increase in momentum, which is the same with respect to kinetic energy.
When we use multiple lasers to illuminate atoms from different angles, the momentum of the atoms in different directions of motion decreases and the kinetic energy decreases. Since the laser only reduces the momentum of the atom, after this process continues for a while, the momentum of most atoms will reach a very low level, thus achieving the purpose of refrigeration.
However, the scope of application of this technology is mostly used for atomic cooling, and for molecules, it is difficult to cool it to ultra-low temperature. However, ultracold molecules are more meaningful than ultracold atoms because their properties are more complex. Currently, methods for cooling molecules are to combine ultracold base atoms to produce dibasic molecules. Not long ago, Yale University cooled the strontium fluoride (SrF) to a few hundred micro-opens.
Another type of laser cooling, also known as anti-Stokes fluorescence cooling, is a new concept of refrigeration that is evolving. The basic principle is the anti-Stokes effect, which uses the energy difference between scattering and incident photons to achieve refrigeration. The anti-Stokes effect is a special scattering effect in which the scattered fluorescent photon wavelength is shorter than the incident photon wavelength.
Therefore, the scattering fluorescent photon energy is higher than the incident photon energy, and the process can be simply understood as: the low-energy laser photon is used to excite the luminescent medium, the illuminating medium scatters high-energy photons, and the original energy in the luminescent medium is taken out of the medium to be cooled. . Compared with the traditional cooling method, the laser provides the function of providing refrigeration power, and the scattered anti-Stokes fluorescence is the heat carrier.