Fighting monsters, carving, starting nuclear fusion, why are you everywhere?

When I was a kid, I sat in front of the TV and watched Ultraman fighting monsters. Every time Ultraman was hit by a monster, the energy indicator light on his chest would start flashing (Ultraman’s inner thoughts: time is running out, I can’t act anymore), and then he would use his signature skill - the Spacium Ray!

(Image source: Wikipedia)

When I was still a primary school student, every time I finished watching Ultraman, I would stretch out my hand and gesture, hoping to rub out a Spacium ray with my hand, but unfortunately I was not a citizen of the M78 Nebula.

Later, my brain, armed with knowledge of modern physics, told me: the Specium rays emitted by Ultraman Jack, the Amerium rays emitted by Ultraman Seven, and the Zaperion rays emitted by Tiga - probably they should all be lasers.

In 1916, Einstein first described the relationship between stimulated and spontaneous radiation of atoms. For a long time afterwards, people speculated whether this phenomenon could be used to strengthen the light field. On May 16, 1960, Maiman, a scientist at the Hughes Laboratory in California, USA, announced the successful production of lasers. This was the first laser beam ever obtained by mankind, and Maiman also became the first scientist in the world to introduce lasers into practical fields.

Maiman and his ruby ​​laser

(Image source: Wikipedia)

Before talking about the various magical applications of lasers, let us first look at how lasers are produced.

Making photons "obedient": lasers and stimulated emission of radiation

In Maiman's laser, the glowing substance is a small rod made of ruby. Ruby is made up of many atoms, and atoms are made up of a nucleus and electrons orbiting around the nucleus:

(Image source: self-made by the author)

When an electron jumps from an orbit far from the nucleus to an orbit close to the nucleus, it releases some energy in the form of light. In various luminous objects, atoms are undergoing this process.

(Image source: self-made by the author)

However, in nature, due to the randomness of electron orbital transitions , the energy of photons generated by electron transitions around different atomic nuclei is also different, so the photons emitted are a hodgepodge and there is nothing special about them. We call this light natural light.

The biggest difference between laser and natural light is that the energy of laser photons is the same and their behavior is very consistent. The photons in laser can be so obedient because people use the stimulated radiation phenomenon discovered by Einstein.

The phenomenon of stimulated radiation means that when a photon passes through an area where electron transition has not yet occurred, other electrons are affected by this photon and begin to transition one after another, thereby generating many photons with the same energy and propagation direction.

It’s just like when it’s almost time for lunch at school and everyone is getting ready to go to the cafeteria. As long as one classmate takes the lead, everyone will follow and run to the cafeteria.

(Image source: self-made by the author)

Using the stimulated radiation phenomenon, scientists have successfully developed lasers. Since the photons emitted by lasers have the same energy and are in sync, they are more directional, have a single color, and have concentrated energy than natural light. After all, "unity is strength." Ultraman was able to defeat the monster thanks to the highly concentrated energy of the laser.

Okay, now that the laser has been produced, what else can it do besides defeating monsters?

Laser can also do delicate work - femtosecond laser processing

Femtoseconds are also called femtoseconds. How short is this time? One femtosecond is only one quadrillionth of a second. We know that the fastest substance in nature is light, which can travel 300,000 kilometers per second, but in one femtosecond, light can only travel 300 nanometers.

Femtosecond laser processing refers to the time when we use laser to ablate materials, the time when the laser interacts with the material is in the femtosecond level. Since the laser energy concentration is very high, if the time when the laser interacts with the material is not controlled to be very short, the places that you don’t want to burn will also be burned.

In laser surgery to treat myopia, it is very important to control the laser according to the thickness of the corneal tissue, otherwise... the consequences will be serious.

With femtosecond processing technology, lasers can process very precise structures. It can not only fight monsters, but also do delicate work like Zhang Fei threading a needle - rough but also delicate.

The picture below shows a microlens made on a glass surface using femtosecond laser processing technology. Its size is only 10 microns, while the diameter of a human hair can reach 60-100 microns.

(Image source: References)

These delicate patterns are all carved using lasers with very low energy. If you use lasers with extremely high energy to do delicate work, it would be like Ultraman threading a needle - rough and rough, rough and rough, and fine. Usually, we use a magnifying glass to focus sunlight into a small spot, which can burn wood or even ignite paper. So what will happen if we focus several laser beams with high energy and high concentration together? This is the "method of creating the sun" that we are going to talk about next - laser nuclear fusion.

Laser-induced nuclear fusion: great power brings about miracles

The development of mankind is accompanied by the continuous progress of energy development and utilization technology. Clean and safe controlled nuclear fusion power generation technology is the ideal power generation technology that mankind has been pursuing. The controlled nuclear fusion technology routes currently being studied include magnetic confinement nuclear fusion, laser confinement nuclear fusion and Z pulse power facilities. The National Ignition Facility NIF in the United States, the Shenguang 1 and Shenguang 2 in Jiading, Shanghai, and the Shenguang 3 in Sichuan are all large-scale experimental facilities used to study laser nuclear fusion.

The basic principle of laser nuclear fusion is to use laser to heat solid spherical nuclear materials (small balls with a diameter of about two millimeters) to a temperature that can produce nuclear fusion (about 100 million degrees Celsius). Ideally, the energy released after nuclear fusion of the nuclear materials is greater than the energy consumed in heating the nuclear materials, and the extra energy can be used by humans.

At present, there is still a long way to go before nuclear fusion can be put into practical use. Taking the National Ignition Facility in the United States as an example, in order to make the nuclear fuel reach the temperature that can produce nuclear fusion, 192 laser beams are needed to hit the nuclear material with a diameter of about 3 mm from all directions at the same time. The NIF equipment is as large as three football fields, and it is very difficult to achieve such high control accuracy.

Inside the National Ignition Facility

(Image source: Wikipedia)

Nuclear material pellets

(Image source: Wikipedia)

The scientists responsible for operating the National Ignition Facility tried every possible way to make the 192 laser beams thin and straight so that they could be accurately aimed at the ball. **Some scientists are racking their brains to make the lasers bend. **Is this after watching Ultraman Zeta's Zeta bending light and wanting to replicate it in reality? This kind of light that propagates along a curved path was introduced in 2007 and is called an Airy beam.

Light that turns on its own - Airy beam

This magical beam is named after the English astronomer and mathematician Sir Airy. In 1838, he discovered the Airy equation when studying the question "Why is the rainbow curved?" If it is a seed, then the Airy beam is a tree that has grown over a hundred years. However, because the mechanism of the Airy beam is too complicated, I will not explain how it is produced in detail in this article.

Airy beam propagation path measured in the laboratory

(Image source: References)

Taking advantage of the self-bending properties of Airy beams, scientists have created optical tweezers that use Airy beams to transport tiny particles:

(Image source: References)

If the energy of the Airy beam is large enough, the laser weapons made with it will be able to deal with enemies hiding behind cover; if the propagation distance of the Airy beam is long enough, we can illuminate places that ordinary beams cannot reach; if the Airy beam is fine enough, we can create tiny structures with more complex shapes.

The emergence of the steam engine triggered the Industrial Revolution, and the discovery of electromagnetic induction brought mankind into the electrical age. Where will the laser, which is known as the "fastest knife", "most accurate ruler" and "brightest light", take us in the future? Let's wait and see.

References:

[1] Cao Xiaowen, Zhang Lei, Yu Yongsen, et al. Femtosecond laser preparation of micro-optical components and their applications[J]. Chinese Journal of Lasers, 2017, 44(1):13.

[2] Siviloglou GA, Christodoulides DN. Accelerating finite energy Airy beams[J]. Optics Letters, 2007, 32(8):979-81.

[3] Lin Zunqi. The development of laser nuclear fusion (invited paper) [J]. Chinese Laser, 2010(9):2202-2207.

[4]https://zhuanlan.zhihu.com/p/26697972

Produced by: Science Popularization China

Author: Salted Fish in the Sea

Producer: China Science Expo