How to cool the laser? Analysis of the principle of laser refrigeration

Everyone knows that laser has the characteristics of good directionality and high brightness. Its beam is concentrated in a very small emission angle (only about one-tenth of a degree) along the axis. In addition, laser Q-switching and other technologies can reduce the laser energy Compressed into a very narrow pulse (such as one trillionth of a second), it can radiate huge energy. The lasers in my impression are all connected with high energy. In fact, lasers with high energy can also be used in refrigeration.

As early as 1985, Chinese American physicist Zhu Diwen successfully frozen atoms with a laser and won the Nobel Prize in Physics in 1997. In fact, the principle of laser cooling is to reduce the thermal motion of molecules in the object. The temperature of an object is related to the thermal motion of molecules. The more intense the molecular motion, the higher the temperature of the object. Conversely, the slower the molecular motion, the lower the temperature of the object. Laser refrigeration requires precise tuning of the laser first. The two beams of light in opposite directions after tuning are used. When a large number of photons enter the interior of the object, the number of laser particles is quite large, making the particles in the object crowded, plus photons. After colliding with an atom, the bomb will take away a part of the energy and cancel the kinetic energy of the molecular atom itself, causing the molecular atom to be unable to "randomly move" as before, thereby reducing the thermal motion of the molecule, thereby lowering the temperature of the object.

The speed of the atom of an object is usually about 500 meters per second. For a long time, scientists have been looking for ways to make the atoms relatively stationary. Zhu Diwen uses three mutually perpendicular lasers to irradiate the atoms from all aspects, so that the atoms are trapped in the ocean of photons, and their movement is constantly hindered and slowed down. This effect of the laser is vividly called "optical glue". In the experiment, the "sticky" atoms can drop to a low temperature almost close to absolute zero (-273.15°C).

Laser cooling can eliminate the first and second Doppler frequency shifts in order to establish a better frequency reference. This is of great significance for timing, precision measurement and navigation. At present, laser refrigeration technology has important applications mainly on the three levels of biological cells, mitochondria and chromosomes. It is also used in condensed matter physics, atomic fountains, atomic clocks, atomic interferometers, and atomic lithography.