2015 is the International Year of Light and Light-based Technologies (IYL2015), which is also the year that the Executive Board of UNESCO signed the decision to set May 16 every year as the "International Day of Light". The reason for choosing May 16 is...
In 2015, the International Year of Light and Light-based Technologies (IYL2015), the Executive Board of UNESCO signed the decision to designate 16 May each year as the International Day of Light.
May 16 was chosen because on May 16, 1960, the American physicist Meyman created the first laser beam in human history.
Meyman and the ruby laser.
So what exactly is a laser? And why is it so important?
To answer these two questions, we need to understand the causes and consequences of Meyman's work.
Why do objects emit light?
Back in 1912, physicists were still obsessed with what the atom, the foundation of the world, looked like.
In this year, three papers by the Danish physicist Bohr were published, in which Bohr applied quantum theory to Rutherford's model of the atom and proposed the famous Bohr model.
Bohr's model was able to explain phenomena that could not be explained by other models at that time, and predicted some results that could be confirmed by experiments later, so it was generally accepted by the scientific community afterwards.
The Bohr model is a planetary model, which means that negatively charged electrons move around a positively charged nucleus like a planet.
The subtlety of the Bohr model is that the orbits of these electrons are not chosen at random, but only to some definite values.
Bohr's model of the hydrogen atom.
The innermost electron orbital is called the ground state, the orbital in the outer layer is called the first excited state, the outer layer is the second excited state, and so on.
We can notice that the electron energies of these different orbitals are different, so we can "flatten" these orbitals, and we get some energy levels. Spontaneous radiation energy levels.
Because of the conservation of energy, electrons want to jump from low energy levels to high energy levels, you have to absorb the corresponding energy from the outside world, this process we call it stimulated absorption. Similarly, the electron from the high energy level to fall to the low energy level, will certainly also release the corresponding energy, it is proved that this process will emit a photon, that is, the electron will be luminous, so this process is called spontaneous radiation.
The principle of luminescence of common light sources in our life is spontaneous radiation.
Fluorescent lamps.
Making light "behave"
There are some problems with light produced by spontaneous radiation: there are many energy levels in atoms, and these photons may be produced by spontaneous radiation at the first energy level, or by spontaneous radiation at the third energy level ......
This leads to different energies of these photons, and the energy of a single photon determines the frequency of light, that is, the frequency of light produced by spontaneous radiation is random.
Another point is that the timing of spontaneous radiation to produce photons, as well as the direction of photon motion is also not under our control, which will lead to spontaneous radiation to produce light, the phase is also random.
The frequency and phase mentioned here are all properties of light as an electromagnetic wave. Frequency can be understood as the speed of the light wave vibration, which also determines the color of the light we see; phase can be understood as the position of the light wave transmission.
Light as an electromagnetic wave.
In short, the light generated by ordinary light sources is like a bunch of people crowding the subway, they are old and young, male and female, wearing different colors to take the subway, and they are not walking as fast, some have already got on the train, while some are still checking the tickets.
This led to ordinary light sources, although in the life of the lighting has been enough to use, but in the field of scientific research, especially the study of the nature of light, the combat power is really general.
Finally, in 1917, another way to light surfaced, that is, Einstein proposed the theory of stimulated radiation.
Stimulated radiation.
Excited radiation theory is to say, now assume that the first excited state on an electron, when a photon hit, and the energy of this photon exactly equal to the first excited state and the gap between the ground state, then this time, the first excited state on the electron will be "tempted" to complete the case of spontaneous radiation, emitting a The "identical" photon is released.
Because of the existence of this "tempted photon", we call this process excited radiation.
If there are enough high-energy electrons, this process will continue, eventually forming a large group of "seduced" photons, we will call this process light amplification process, the most important thing is that the phase and frequency of these photons is exactly the same. Like a neat and tidy army, and the above "squeeze the subway" spontaneous radiation is completely different.
How many steps does it take to build a laser?
The first step is particle number inversion.
With the theory of excited radiation, people wonder how to use this theory to build a light source that can emit a neat and tidy light.
Some readers may say, "Why not just take the light and shine it through? What's so difficult about it?
Readers who have such doubts should pay attention to the word "enough" mentioned earlier, and don't forget our phenomenon of excited absorption.
If there are not enough electrons at high energy levels, the number of excited radiation is less than the number of excited absorption, when a beam of light hit, will not be emitted light amplification, but will be the ground state electron excited absorption, resulting in light loss.
In fact, in the natural case, the number of ground state electrons is much larger than the number of excited electrons, at room temperature, for example, a two-energy system (that is, only the ground state and the first excited state of the energy system) the number of ground state electrons is about 10 of 170 times the number of excited electrons!
So in order to use the principle of excited radiation to create a light source, the first problem to solve is to make the number of particles at higher energy levels greater than the number of particles at lower energy levels, that is, to achieve particle number inversion.
How to achieve particle number reversal?
The basic idea is to pump the particles from the ground state to the high energy state, just like a pump.
This is easier said than done.
Water pumping particles.
The second step is to build a predecessor.
In 1951, the American physicist Towns thought of how to achieve particle number inversion in the ammonia molecule.
The ammonia molecule is a two-energy system, and it is impossible to achieve particle number inversion under normal circumstances, because the probability of excited absorption and excited radiation are the same, and also the presence of spontaneous radiation, which leads to the fact that the number of particles at higher energy levels must be less than the number of particles in the ground state.
Towns' approach was ingenious, as he used a magnetic field to distinguish between the ground state and excited state ammonia molecules, singling out the excited state ammonia molecules to be placed in a microwave resonant cavity, in which the particle number reversal was achieved.
Three years later, using this idea, Towns built the first "MASER". What is MASER?
MASER is called Microwave Amplification by Stimulated Emission of Radiation, which translates to "amplification of microwaves by stimulated radiation". Laser LASER is called light Amplification by Stimulated Emission of Radiation, which translates to "amplification of light by stimulated radiation".
We mentioned above that light is an electromagnetic wave, and microwave is another electromagnetic wave.
Electromagnetic waves can be classified according to their frequency, with microwaves ranging from 300 MHz to 300 GHz, and visible light ranging from 3.9 to 7.5 times 10 to the 14th power Hz.
From the name we can see the difference between MASER and LAZER, mainly in the difference in operating bands, MASER is only one step away from LASER.
Towns and the first MASER.
The third step is to complete the three major components of the laser.
The introduction of MASER solved the particle number inversion problem. In just three years, this technology has progressed by leaps and bounds, and at this point everyone wants to hurry up and take it a step further by turning this microwave amplifier into an optical amplifier and creating that dream light source, the laser.
So far we have been able to vaguely summarize the composition of the laser three major components:
The first is the need to achieve particle number inversion of the substance, like ammonia molecules, we call the gain medium; the second is the appropriate pumping method, we call it pumping; third is the above mentioned Towns with resonant cavity, as for the role of resonant cavity we will talk later.
In 1958, Towns and Shorro collaborated on a theoretical paper that predicted the feasibility of lasers for the first time from a theoretical point of view. At this point, everything was ready for Towns, except for the wind!
On May 16, 1960, Meyman took a different path and was first to build the first laser in human history.
The story of how Meyman got there first is a fascinating story with many twists and turns. But let's focus here on his ruby laser.
The schematic diagram of the ruby laser.
This laser very clearly shows the three major components of the laser, we may as well introduce them in turn.
Gain medium:
The gain medium chosen by Meyman is ruby, which is chromium-doped aluminum trioxide.
Schematic of the three-energy system.
This gain medium is a three-energy system, and this three-energy system to achieve particle number inversion is much simpler than the previous two-level system. There are some special features of the ruby three-level system, and we can understand how it achieves the particle number inversion by its pumping process.
Firstly, the ground state particles are transported directly to the E3 energy level by a suitable excitation, and there is a radiation-free leap process between the E3 and E2 energy levels, which means that the particles on E3 will quickly run to E2 by collisions, and the reduced energy becomes thermal motion energy instead of luminescence.
In addition, the E2 state is sub-stable, that is, the particles falling on the E3 energy level can remain on the E2 energy level for a long time. This is equivalent to using the E3 energy level as a transition to transport the particles from the ground state to E2, and let the process go on, the number of particles in E2 will exceed the number of particles in the ground state, achieving particle number inversion.
In fact, the efficiency of the ruby laser is very low, only 0.1%, which is limited by the gain medium, because the three-energy system requires very high energy to pump the ground-state particles to the high-energy state. In addition, the wavelength of this laser is 694.3 nm, which is also determined by the gain medium.
With the development of laser, the types of gain medium gradually increased, including gas, solid, liquid, fiber, semiconductor, etc., such as the laser pointer commonly used in the classroom is a semiconductor laser.
In short, no matter which gain medium, it has to have a method that can achieve particle number inversion.
Pumping:
The pump lamp of the first ruby laser.
The most obvious feature of Meynman's laser is that its pump source is a spiral xenon lamp, the spiral shape ensures that the ruby bar is placed between the lamps. In addition this lamp still uses pulsed light for pumping, which means that the light it emits is not continuous, but in bursts. This is the most important design of Meynman, so that the continuous high energy pumping light does not damage the crystal.
Resonant cavity:
Schematic diagram of the resonant cavity.
At the two ends of the ruby bar, Meyman placed two mirrors and dug a small hole in the right side so that the light from the excited radiation could travel back and forth through the gain medium to "lure" more photons, and after reaching a certain intensity, the laser light would be emitted through the small hole.
What is the use of laser?
Mayman held a press conference after the invention of the laser, in which a reporter asked this question, Mayman gave 5 suggestions: 1:
1. it is used to amplify light, for example, when making high power lasers, they use optical amplifiers to amplify the weaker light;
2. can use lasers to study matter;
3. to use high power laser beams for space communications;
4. used to increase the number of channels for communication (this is what later emerged as fiber optic communication);
5. to focus the beam to produce ultra-high light intensity for cutting or welding materials in industry, or for performing surgery in medicine, etc.
We have to admire Mehman's keen scientific sense, and all these suggestions he made were later fulfilled.
Remember the characteristics of photons produced by excited radiation?
They have the same frequency and phase, and the laser is essentially an amplification of the light from the excited radiation, so the two most important characteristics of the laser are good monochromaticity and high energy. These two characteristics determine the uses of lasers, and these are the two directions of laser development.
Good monochromaticity means that the laser spectrum is very narrow and can easily show the characteristics of light as a wave, and we can then use it to record phase information.
For example, the holographic photo technology invented by British physicist Dennis Gerber in 1947 is essentially the use of the phase of light to record the full range of information about the object, so as to produce the effect of three-dimensional photography.
Holographic photographs can record not only frontal information but also side information.
It was only after the invention of the laser that this technology became available and was awarded the Nobel Prize in Physics in 1971.
The high energy is well understood, we can use lasers to burn CDs, to enable nuclear fusion, to cut materials, etc. Not only can we generate continuous high-energy lasers, but we can also obtain high-energy lasers with very short pulse durations by means of the locked-film technique and chirp amplification.
Diagram of pulse generation with film locking technology.
Femtosecond lasers are now widely available, and the duration of a single pulse is only on the order of femtoseconds (minus 15 seconds of 10).
With this laser, we can deliver precise blows to a substance without causing much damage, such as myopia repair surgery, altering the surface of a substance, enhancing its antiseptic properties, etc.
