Do you believe in lasers?

Produced by: Science Popularization China

Author: Xiaoxiao Changguang (Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences)

Producer: China Science Expo

2015 is the International Year of Light and Light-based Technologies (IYL2015). It was also in this year that the Executive Board of UNESCO signed and approved the decision to designate May 16 of each year as the International Day of Light. The reason for choosing May 16 is that on May 16, 1960, American physicist Maiman created the first laser in human history.

Maiman and ruby ​​lasers

(Image source: Wikipedia)

So what exactly is a laser? And why is it so important? To answer these two questions, we need to understand the context of Maiman's work.

Why do objects glow?

In 1912, physicists were still obsessed with what the atom, the foundation of the world, looked like. In that year, three papers by Danish physicist Bohr were published. In these three papers, Bohr applied quantum theory to Rutherford's atomic model and proposed the famous Bohr model. The Bohr model could explain phenomena that other models at the time could not explain, and predicted some results that could be confirmed by experiments later, so it was widely accepted by the scientific community.

Let's take a look at the Bohr model . The Bohr model is a planetary model, that is, negatively charged electrons move around positively charged nuclei like planets. The subtlety of the Bohr model is that the orbits of these electrons are not chosen randomly, but can only choose certain values.

Bohr model of the hydrogen atom

(Image source: Wikipedia)

The innermost electron orbit is called the ground state, the outermost orbit is called the first excited state, the outermost is the second excited state, and so on. The Bohr model can well explain why objects emit light. We can notice that the energy of electrons in these different orbits is different. We might as well "flatten" these orbits, so that we can get some energy levels.

Spontaneous emission energy levels

(Image source: Wikipedia)

Due to the law of conservation of energy, if an electron wants to jump from a low energy level to a high energy level, it must absorb the corresponding energy from the outside world. We call this process stimulated absorption. Similarly, if an electron drops from a high energy level to a low energy level, it will definitely release the corresponding energy. It turns out that this process will emit a photon, that is, the electron will emit light, so this process is called spontaneous radiation. The principle of light emission of common light sources in our lives is spontaneous radiation.

fluorescent lamp

(Image source: Wikipedia)

Make light "obedient"

There are some problems with the light generated by spontaneous radiation: there are many energy levels in atoms, and these photons may be generated by spontaneous radiation of the first energy level, or by spontaneous radiation of the third energy level... This will result in different energies of these photons, and the energy of a single photon determines the frequency of the light, that is, the frequency of the light generated by spontaneous radiation is random . Another point is that the timing of the generation of photons by spontaneous radiation and the direction of the photon movement are not controlled by us, which will cause the phase of the light generated by spontaneous radiation to be random.

The frequency and phase mentioned here are both properties of light as an electromagnetic wave. Frequency can be understood as the speed of light wave vibration, which also determines the color of light we see; phase can be understood as the position where the light wave is transmitted.

Light as an electromagnetic wave

(Image source: Wikipedia)

In short, the light produced by ordinary light sources is like a group of people on the subway. They are old and young, male and female, wearing clothes of different colors, and walking at different speeds. Some have already boarded the train, while others are still checking their tickets. This results in ordinary light sources being sufficient for daily lighting, but in the field of scientific research, especially in studying the properties of light, their effectiveness is really average.

Finally, in 1917, another way of emitting light surfaced: the theory of stimulated radiation proposed by Einstein.

Stimulated emission of radiation

(Image source: Wikipedia)

The theory of stimulated radiation says that, now suppose there is an electron in the first excited state, and a photon hits it at this time, and the energy of this photon is exactly equal to the difference between the first excited state and the ground state, then at this time, the electron in the first excited state will complete spontaneous radiation under the condition of "being tempted" and emit a "hair-like" photon.

Because of the existence of this "tempted photon", we call this process stimulated radiation. If there are enough high-energy electrons, this process will continue and eventually form a large group of "tempted" photons. We call this process light amplification. The most important thing is that the phase and frequency of these photons are exactly the same. It is like a well-organized army, which is completely different from the spontaneous radiation of "crowding in the subway" mentioned above.

How many steps are there to build a laser?

The first step is to invert the particle number.

After the theory of stimulated radiation was discovered, people began to wonder how to use this theory to create a light source that emits uniform light. Some readers may ask, isn't it enough to just shine a light over it? What's so difficult about that?

Readers who have such questions should pay attention to the three words "enough" mentioned above, and don't forget our stimulated absorption phenomenon. If there are not enough high-energy level electrons, the number of stimulated radiation is less than the number of stimulated absorption. At this time, a beam of light will not emit light amplification, but will be stimulated by ground state electrons, resulting in light loss.

In fact, under natural conditions, the number of ground state electrons is much larger than the number of excited state electrons. Taking room temperature as an example, the number of ground state electrons in a two-level system (that is, an energy level system with only the ground state and the first excited state) is approximately 10 to the 170th power times the number of excited state electrons!

Therefore, if we want to use the principle of stimulated radiation to create a light source, the first problem we need to solve is to make the number of high-energy level particles greater than the number of low-energy level particles, that is, to achieve particle number inversion.

How to achieve particle number inversion? The basic idea is to pump, just like a water pump, to pump the ground state particles to the high energy state. It is easier said than done.

Water pumping particles

(Image source: Wikipedia)

The second step is to build a predecessor.

In 1951, American physicist Townes figured out how to achieve particle number inversion in ammonia molecules.

The ammonia molecule is a two-level system. Under normal circumstances, it is impossible to achieve particle number inversion because the probabilities of stimulated absorption and stimulated radiation are the same. At the same time, there is spontaneous radiation, which means that the number of particles in the high energy level must be less than the number of particles in the ground state.

Townes' method was very clever. He used a magnetic field to distinguish between the ground state and excited state ammonia molecules, and then picked out the excited state ammonia molecules and put them into the microwave resonant cavity, achieving a population inversion in this resonant cavity. Three years later, using this idea, Townes built the first "MASER". What is MASER?

The full name of MASER is Microwave Amplification by Stimulated Emission of Radiation, which means “amplifying microwaves by stimulated radiation”. The full name of LASER is light Amplification by Stimulated Emission of Radiation, which means “amplifying light by stimulated radiation”.

We mentioned above that light is an electromagnetic wave, and microwaves are another kind of electromagnetic wave. Electromagnetic waves can be classified according to the size of the frequency. The frequency of microwaves is between 300 MHz and 300 GHz, while the frequency of visible light is between 3.9 and 7.5 times 10 to the 14th power Hz. From the names we can see the difference between MASER and LAZER, which mainly lies in the different working bands. MASER is only one step away from LASER.

Townes and the first MASER

(Image source: Wikipedia)

The third step is to complete the three major laser components.

The advent of MASER solved the problem of particle number inversion. In just three years, this technology has made great progress. At this time, everyone hopes to go one step further and turn this microwave amplifier into an optical amplifier to create the dream light source, that is, laser.

At this point we can vaguely summarize the three major components of the laser: **The first is the need for a substance that can achieve particle number inversion, such as ammonia molecules, which we call a gain medium; the second is a suitable pumping method, which we call a pump; the third is the resonant cavity used by Townes mentioned above, **As for the role of the resonant cavity, we will talk about it later.

In 1958, Townes and Schawlow co-wrote a theoretical article that theoretically predicted the feasibility of lasers for the first time. At this time, Townes was ready except for the east wind!

As a result, we all know that Towns thought he was Zhou Yu who borrowed the wind, but he turned out to be Cao Cao who was deceived by the wind. On May 16, 1960, Maiman took a different approach and was the first to create the first laser in human history. If you are interested, you can learn about the story of how Maiman got there first. It is full of twists and turns and is very exciting. However, we will focus on his ruby ​​laser here.

Ruby laser schematic

(Image source: Wikipedia)

This laser very clearly demonstrates the three main components of a laser, so let's take a look at each one in turn.

Gain medium: The gain medium chosen by Maiman was ruby, which is aluminum oxide doped with chromium.

Schematic diagram of three-level system

(Image source: self-made by the author)

This gain medium is a three-level system, and the method of achieving population inversion in this three-level system 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 population inversion through its pumping process.

First, the ground state particles are transported directly to the E3 energy level through appropriate excitation. There is a radiationless transition process between the E3 energy level and the E2 energy level, that is, the particles on E3 will quickly run to E2 through collision, and the reduced energy will be converted into thermal motion energy instead of luminescence.

In addition, the E2 state is a metastable state, which means that particles that fall from the E3 energy level can remain at the E2 energy level for a long time. This is equivalent to using the E3 energy level as a transition to transport the ground state particles to E2. If this process continues, the number of particles in E2 will exceed the number of particles in the ground state, achieving a particle population inversion.

In fact, the efficiency of ruby ​​laser is very low, only 0.1%, which is limited by the gain medium, because the three-level system requires very high energy to pump the ground state particles to the high energy state. In addition, the wavelength of this laser is 694.3nm, which is also determined by this gain medium.

With the development of lasers, the types of gain media have gradually increased, including gas, solid, liquid, optical fiber, semiconductor, etc. For example, the laser pointer commonly used in the classroom is a semiconductor laser. In short, no matter what kind of gain medium, it must have a method to achieve population inversion.

Pump:

The first pump lamp for ruby ​​lasers

(Image source: Wikipedia)

The most obvious feature of Maiman's laser is that its pump light source is a spiral xenon lamp. The spiral shape ensures that the ruby ​​rod can be placed between the lamp tubes. In addition, this lamp uses pulsed light for pumping, that is, the light it emits is not continuous, but in bursts. This is Maiman's most important design, which avoids continuous high-energy pumping light from damaging the crystal.

Resonant cavity:

Schematic diagram of the resonant cavity

(Image source: Wikipedia)

Maiman placed two mirrors at both ends of the ruby ​​rod and dug a small hole on the right side so that the light emitted by stimulated radiation could shuttle back and forth in the gain medium, "luring" more photons. When a certain intensity was reached, the laser would be emitted from the small hole.

What is the use of laser?

After Maiman invented the laser, he held a press conference. At that press conference, a reporter asked this question. Maiman gave five suggestions:

1. Used to amplify light. For example, when making high-power lasers, optical amplifiers are used to amplify weaker light.

2. Lasers can be used to study matter;

3. Use high-power laser beams for space communications;

4. Used to increase the number of communication channels (this is the fiber optic communication that appeared later);

5. Focusing the light beam to produce ultra-high light intensity is used for cutting or welding materials in industry, or performing surgery in medicine, etc.

We have to admire Maiman's keen scientific research sense. All the suggestions he made came true one by one in the future.

Do you remember the characteristics of photons generated by stimulated emission of radiation? Their frequency and phase are consistent, and laser is essentially the amplification of stimulated emission of light, so the two most important characteristics of laser are good monochromaticity and high energy . These two characteristics determine the use of lasers, and are also the two directions of laser development.

Good monochromaticity means that the laser spectrum is very narrow and it is easy to show the characteristics of light as a wave, so we can use it to record phase information. For example, the holographic photography technology invented by British physicist Dennis Gabor in 1947 essentially uses the phase of light to record the omnidirectional information of an object, creating a stereoscopic photography effect.

Holographic photos can not only record front information but also side information.

(Image source: Wikipedia)

It was not until the invention of the laser that this technology became feasible, and it won the Nobel Prize in Physics in 1971.

High energy is easy to understand. We can use lasers to burn CDs, promote nuclear fusion, cut materials, etc. We can not only produce continuous high-energy lasers, but also use film locking technology and chirp amplification technology to obtain lasers with high energy but very short pulse duration.

Schematic diagram of pulse generation using membrane locking technology

(Image source: Wikipedia)

Gif

Femtosecond lasers are now very popular. The duration of a single pulse of this laser is only on the order of femtoseconds (10 to the negative 15th power of seconds). Using this laser, we can accurately strike materials without causing great damage, such as repairing myopia, changing the surface of materials, and enhancing their anti-corrosion properties.

Conclusion

In 2018, the inventor of chirped amplification technology also won the Nobel Prize in Physics. Currently, there are more than a dozen Nobel Prizes in Physics related to lasers. It can be said that laser is one of the most important inventions of mankind since the 20th century. On the International Day of Light, if someone asks you: Do you believe in light? You can ask him back: Do you believe in lasers?