We all know that light has wave-particle duality. It can be believed that a beam of light is composed of many photons, and each photon has a certain amount of energy. The greater the frequency of light or the smaller the wavelength of light, the higher the energy of the photon.

Ultraman and monsters are generally:

Ultraman and monsters fight for three minutes first,

The seriously injured monster stands there and waits for death,

Ultraman launches a kill light to kill the monster.

For Ultraman Tiga , his ultimate skill has a professional name:

Zaipelao Light .

But no matter which Ultraman it is,

Their ultimate moves are based on Speshum Light [1].

Today, the editor will teach you how to emit high lethal Spexum light .

Part 1: Generate laser, biubiu~

We all know that light has wave-particle duality . It can be considered that a beam of light is composed of many photons . Each photon has a certain energy. The larger the frequency of light or the smaller the wavelength of light , and the higher the energy of photons .

photon energy E, frequency ν and wavelength, are as follows, where h is the Planck constant and c is the speed of light.

The following figure lists the frequency (wavelength) spectrum of light . The color part in the middle is the visible light visible light that we can see. The two sides of the visible light are ultraviolet light and infrared light . The light with higher frequency is called x ray , γ ray , and the light with lower frequency is called microwave, radio wave , etc.

light frequency spectrum Image source: Baidu Picture

A beam of light The whole Intensity is the power of light, which represents the number of photons in a beam of light .

To make light have strong lethality, the first thing to do is to ensure that the power of light is very strong , which means that the number of photons in a beam of light is large.

This is easy to understand. For example, using a lens to gather sunlight can ignite flammable materials, and the light will be brighter and more harsh .

Secondly, it is necessary to ensure that photon energy is very high .

We often say that UV radiation is too strong and will cause harm to the human body, or see radiation hazard in the CT room of the hospital (the CT room uses x-ray ) , because the photon energy of these lights is too high.

Spaishum light can essentially be regarded as laser with very high energy.

When the laser light irradiates on an object, the atom , ions or electrons inside the object will absorb the energy of photon . Electrons will be ionized by ; energy will propagate between atoms . As a result, the temperature of the object will increase, so that melts, vaporizes and even becomes plasma .

strong laser acts on the solid and suddenly applies a violent energy disturbance to the solid. During the energy propagation inside the solid, it will form stress wave , and stress wave will cause the solid to fragmentation [2].

So, after Ultraman fires a beam of light and hits the monster, the monster will explode into pieces, which is reasonable.

laser is a strong beam with very strong monochromatic (i.e., single frequency) and strong directivity . So how should laser be generated?

At this time, we can ask for help with Einstein . He proposed the concept of stimulated radiation of light, which is the basis for laser generation.

Quantum mechanics tells us that inside matter, electrons are at separate energy level . When electrons are at higher energy levels, they will spontaneously transition to lower energy levels. During the transition process, a photon will radiate outward . The energy of the radiated photon is equal to the energy difference between these two energy levels .

Schematic diagram of spontaneous radiation

This process is called spontaneous radiation . The fluorescent emitted by the phosphor we see in our daily lives is an example of spontaneous radiation. However, spontaneously radiated photons cannot be aggregated into lasers.

When we illuminate a beam of light from the outside, if the energy of the photon is greater than the energy difference between the two energy levels, the electrons will absorb the photon to obtain its energy, and jump from the low energy level to the high energy level. This process is called stimulated absorption .

stimulated absorption diagram

Focus is here. When the energy of the external photon is equal to the energy difference between two energy levels and the electron is at a high energy level, the electron will jump to the low energy level under the disturbance of the external photon and radiate one photon . This process is called stimulated radiation .

stimulated radiation diagram

We can notice that we incident a photon , and now emits two photons , that is, the stimulated radiation allows us to obtain stronger light.

Here everyone should Pay attention to one thing:

According to the previous statement, as long as the photon is at a high energy level, spontaneous radiation will definitely occur, but we do not want spontaneous radiation to . We hope that more electrons will participate in the stimulated radiation to produce the desired laser.

So how does make the stimulated radiation process more advantageous? ? The answer is: Just ensure that the number of electrons at at high energy level is more than the number of electrons at low energy level . This is the inversion of particle count [3].

However, electrons at high energy level will always jump to low energy level. So how can ensure that electrons at high energy level always have more electrons at low energy level ?

You must have thought of it as smart: Since high-energy electrons are constantly being consumed, I would like to think of a way to supplement electrons from another energy level to this energy level , isn’t it okay?

Congratulations on having understood the principles of ruby ​​laser .

Ruby laser uses an three-level energy system . The energy levels can be divided into E1, E2, E3[4].

Under the irradiation of xenon lamp :

. The electrons in the ruby ​​crystal that were originally in the ground state E1 were excited by to the highest energy level E3 after absorbing the photons emitted by the xenon lamp.

,
electrons have a very short lifespan of , about picosecond level, and a large number of particles will spontaneously radiate from E3 level to intermediate level E2.

, electrons are at the E2 energy level with a longer lifespan of , about milliseconds, so the E2 energy level will gradually accumulate a large number of electrons , realizing the inversion of the particle number of E1 energy level and E2 energy level .

ruby ​​laser electronic transition process

so crystal will amplify and output the optical components with frequency satisfying hv=E2-E1. The output laser has a red light with a wavelength of 94nm.

Now we generate a large number of photons through stimulated radiation, so how can turn them into lasers and output them to the outside world? ?

We can install two mirrors parallel to each other at both ends of laser working substance (such as ruby). These two reflecting surfaces form an resonant cavity . At this time, only light parallel to the mirror axis can continuously reflect back and forth inside [3].

laser basic structure

light passes through the working substance and enhances through stimulated radiation, and the final emitted laser has strong unidirectional .

From the above analysis, we can see that the laser has strong unidirectionality that makes the photons bound in a very small space, that is, the number of photons in a unit space is large and , so the laser has strong power.

So, is it enough now? Ordinary lasers may burn to , but compared to Ultraman’s monster that penetrates a hundred meters of a beam of light and is hard in , our strength is not enough for !

Part2: pulse laser, biu~biu~biu~

So please next our protagonist: pulse laser .

The laser we generated earlier is Continuous laser , that is, the energy distribution of the light beam is continuous.

and pulse laser is a beam of light composed of segments of pulses. The duration of a beam of pulses is called pulse width .

pulse light time waveform diagram, Image source: Baidu Picture

We can do this Imagine :

For a continuous beam of light, all photons are evenly distributed in the entire beam of light. For pulse light , photons only exist during this period of pulse duration .

If the total number of photons of these two beams is the same, one pulse of the pulse light will have more photons .

pulse laser has more photons in a single pulse time

That is to say, although the average power of the two is the same, the pulse light will have a stronger instantaneous power than continuous light.

It can be foreseen that the narrower the pulse width of the pulse light , the stronger the instantaneous power of the laser . So generating a pulsed laser with ultra-short pulse width is our next goal.

So how does generate pulsed light? ?

mentioned earlier that the light generated by the stimulated radiation oscillates back and forth in the resonant cavity to achieve amplification of the output laser. However, there must be loss in the resonant cavity. Only when the gain of the light of the medium exceeds the loss can the continuous output laser be achieved [3].

We can adjust the loss of the resonant cavity .

First adjust the of the cavity first loses , and the laser cannot generate laser. In this way, can accumulate a large number of excited state particles on the upper energy level of the laser, and there are many inverted particles.

When the number of inverted particles reaches a threshold of , the loss of the cavity is suddenly reduced, and a large number of excited particles in quickly complete the stimulated radiation .

Then in the next pulse cycle, the same operation is performed again. This generates a very narrow light pulse of .

Adjust the resonant cavity loss to obtain pulsed laser diagram

Generally, the pulse width of the pulsed laser can reach the nanosecond level by changing the resonant cavity loss.

We can change its loss by placing an electro-optical crystal in the resonant cavity [3].

The so-called electro-optical crystal is . When the voltage is applied in two segments of a crystal, light will change the polarization direction through the crystal . At this time, another polarization direction of is added with polarization plate with a polarization direction perpendicular to . The light cannot pass through the polarization plate so that cannot oscillate back and forth in the resonant cavity.

uses electro-optical crystals to change the resonant cavity loss to generate pulsed laser. Image source: References [1]

So how can further generate lasers with shorter pulse width and stronger instantaneous power ?

We know that light is a kind of electromagnetic wave , and the light intensity is mainly the vibration of electric field . The light in the vibration direction parallel to the axis of the resonant cavity is called vertical mode , and the light in the vibration direction perpendicular to the axis is called horizontal mode .

For example, the vibration of the spring is a vertical mode, and the vibration of the water wave is a horizontal mode.

The vibration of the electric field includes amplitude, vibration period, and phase .

For general lasers, the internal light field vibrations are completely different and the phases are random . Therefore, does not interfere with each other .

Now, we can add an modulator in the resonant cavity or add a thing called kil crystal to lock the phase between each vibration mode [5].

Then after a period of time, some vibration modes meet at their respective maximum amplitude , realizing that superimposes , and finally outputs a larger amplitude.

phase locking superposition between different modes to stronger pulse laser diagram

In this way, we realize the laser mode lock amplification . Through this method, we can realize the pulse laser with pulse width of picosecond or even femtosecond level.

But it is still not enough now, we still need a stronger laser.

We have achieved very strong instantaneous power by turning continuous light into femtosecond pulse laser . So can continue to amplify the femtosecond pulse laser directly? ?

The answer is OK!

Next we will use chirped pulse amplification technology Continue to enhance our Spixum light.

We first pass a laser through a set of prisms and then expand its pulse width , so that its peak power will become very low.

Then we let it gain medium amplification . Since we reduced its peak power in advance, will not damage during the laser enhancement process in the gain medium.

laser is amplified by the gain medium and then compresses its pulse width through a set of prisms , which means that more light in is compressed into a very small area , thus greatly increasing the intensity of femtosecond laser pulse.

Image source: Baidu Picture

It is worth mentioning that this method also won the 018 Nobel Prize in Physics . So Ultraman can win the Nobel Prize with this technique!

3

Part 3, High-order Harmonic , biuuu~biuuu~biuuu~

We have now achieved very strong optical power, so how can we continue to improve our strength next?

Back to our initial thoughts, is it the next thing that increases the energy of photons ? That is to increase the frequency of light .

961, Franken and others used the red light produced by a ruby ​​laser through an quartz crystal , and found that the emitted light became Unisex [6].

From the spectrum , it can be found that the frequency of purple light is twice that of red light , which means that they have achieved double frequency of light. The phenomenon they discovered is called second harmonic generation .

OK We seem to have achieved the requirement: increases the energy of photons .

This article is finished. . . .

Just a joke, let's take a look at how the second harmonics of are generated by .

mentioned earlier, Light is the electromagnetic field .

In an object, atomic nucleus is positively charged and electrons are negatively charged. electric field acts on the atom atomic nucleus and electrons will move in two opposite directions .

In this way, a dipole is formed. The dipole will generate another electric field superimposed onto the external applied electric field, thereby changing the magnitude and direction of the external electric field. This process is the polarization process of the electric field .

For the entire solid, the contribution brought by the dipole is called polarization intensity . Through some complex solutions, we can know that the expression of polarization intensity P can be written as the power summation of plus electric field E , power exponent corresponds to several orders nonlinear effect , and the previous coefficient is called nonlinear coefficient [7].

The relationship between polarization intensity and the applied electric field

Second harmonic generation is a second order nonlinear effect . In addition, there are three-time harmonic production, which is a third-order nonlinear effect , which means that the frequency of the emitted light is three times that of the incident light frequency .

In the solid, the nonlinear effect of can generate photons with several times the frequency of , which can basically reach the level of ultraviolet light .

But it is still not enough. Can we continue to improve our strength?

OK!

is to use femtosecond laser ionization gas to generate high-order harmonic . Through this method, the enhancement of frequency can be achieved dozens of times the frequency , and even pulsed lasers in the x-ray band [8] can be generated.

This process is as follows:

Use femtosecond pulse laser to act on gas atom to ionize the electrons inside .

ionized electrons are accelerated by femtosecond laser electric field to obtain very high energy.

electrons are then recombined with the atom . During the recombination process, the sum of the kinetic energy obtained by the electrons in the light field and the transition energy from the continuous state to the ground state (equal to ionization energy) is radiated in the form of higher harmonic photon . The three-step diagram of the generation of the higher harmonics of

. The two figures on the rightmost front are the collision process of the accelerated electrons with other electrons or atoms, and the bottommost figure represents the composite radiation of the accelerated electrons with atoms. Image source: References [6].

In addition, people found that the pulse width of the extreme ultraviolet pulse light of at the can reach the at the level.

In this way, we not only greatly enhance the energy of Spixum's photons , but also obtain , an asecond laser with a shorter pulse width than femtosecond laser, and further enhance the instantaneous power of the light .

In this way, such Spaishum light can already achieve the super destructive power of .

Today's "Basics of Ultra's Achievement-Speshum Light" ends here.

Have you learned it?

References:

[1] Teach you the kill light that fires Ultraman - Zhihu (zhihu.com).

[2] Research progress on strong laser destruction mechanism, Zhou Yichun et al., Mechanics and Practice, Vol. 17, Issue 1, 1995.

[3] Principles of Lasers, Orazio Svelto, 1998 4th edition, Springer.

[4] Ruby Laser, Northwest Institute of Telecommunications Engineering, Laser and Infrared, 1978, (04)

[5] Laser Principles, Zhou Bingkun et al., 6th edition, 2009, National Defense Industry Press,

[6] Generation of optical harmonics, P. A. Franken et al., Phys. Rev. Lett. 7, 118-119 (1961).

[7] Nonlinear Optics, Shi Shunxiang et al., 2012, 2nd edition, Xi'an University of Electronic Science and Technology Press,

[8] Technical principles and progress of a second pulse generation, Wei Zhiyi et al., Science Bulletin, Vol. 66, 2021, Vol. 66, Issue 8: 889 ~ 901

Editor: Garret