+ +

Emission of LASER light

This page is devoted to phenomena that allow the emission of laser light.

To make a laser, a system must be used to create a radiation consisting of identical photons, the electric field oscillating "in phase" in the direction perpendicular to the emitted radiation. This radiation must also be sufficiently large, and we will see that a system of amplification of light is required: this amplification usually has for seat the laser cavity.

If the description of the characteristics of the laser light is mainly based on a wave design, the mechanisms linked to its emission by atoms require a "corpuscular" approach, through the taking into account of the energy transported by the light particles, the photons.

The interaction between light and material involves several actors: among the main ones, besides of course the photons, are certainly the electrons that gravitate (or rather occupy certain positions, or energy levels) around atomic nuclei.

Energy level and light emission

A simplified model is often used that helps to understand the atomic effects needed to generate a laser emission: each atom has particular "discrete" energy levels where electrons are placed, and to move from one level (or "compensate") to another, the amount of energy to be supplied must correspond exactly to the energy difference between these levels. Photons are "packets" (or quanta) of pure energy: they can disappear in energetic reactions where their energy is absorbed and allows an energetic transition.

Conversely, they may be emitted by certain reactions that "unleash" energy. The energy of a photon is proportional to its frequency: it is equal to hν or h is the Planck constant, and ν the frequency (in Herz).

This "quantitative" aspect is a pillar of the quantum world; Taken advantage of, it allows to generate the laser radiation if one keeps in mind that the energy of a photon is proportional to its frequency length (and inversely proportional to its wavelength).

Thus, the energy required to move from an electronic "level" (Call N ° 1, (E1) to another higher (N ° 2, Energy E2) can be brought only by a photon ... provided its wavelength makes that its energy corresponds exactly to the difference between energy levels (i.e.: E2-E1). On the other hand, an electron moving from a high level (n ° 2) to a lower energy level (n ° 1) "disposes" of the difference by emitting a photon whose energy (the wavelength) corresponds to the difference in levels (E2-E1).

Light emission by deexcitation

An electron can pass from a ground state (E1) to an excited state thanks to the energy provided by a photon. Likewise, the disexcitation results in the emission of a photon whose energy corresponds to the difference (E2-E1).

If a set of atoms with two E1 (fundamental) and E2 (excited) levels were considered, and that we sought to know their state at various successive moments, it would be observed that there is a majority of electrons would be in E1 state, the others being of course E2.  Over time, for constant temperature and constant volume, the proportion would be unchanged, but it would be noted that each atom can be successively in the E1 state (after a spontaneous photon emission) or E2 (after spontaneous absorption of a photon).

The laser light emission is based on a particular encounter: that of a Photon and an Atom Already "excited". Occurs then what is called a "Stimulated" program, which is the scenario.

For a photon encountering an atomic medium, there are two possibilities: the first atom encountered by the photon can be in a fundamental state, or an excited state. If the photon meets an atom already "excited", it occurs what is called a stimulated emission : The atom cannot absorb this photon and under the effect of the "collision", it emits a photon whose wavelength is identical to the incident photon, but also the phase (and polarization). These two photons form an "embryo" of laser light!

Atomic energy levels and spontaneous emission

Spontaneous emission: an electron from the upper level spontaneously travels to the lower level by emitting a photon whose energy is equal to that which separates the two levels.

Spontaneous emission is, as its name implies, a spontaneous phenomenon: some levels of the atom have a short duration; Those with a longer "long" duration are called "metastable" levels.

Laser stimulated emission

Stimulated emission: The incident light of frequency υ forces the passage of an electron from the upper level to the lower level and the emission of a photon of energy h υ = (es-EI). This is a coherent phenomenon, and the emitted light radiation is in phase. To obtain a (coherent) stimulated emission that is more important than spontaneous emission, the photon density must be sufficiently high, which necessitates the presence of an amplifier medium and an optical resonator (consisting of two mirrors facing each other).

 

It is intuitively suggested that the phenomenon of stimulated emission should be increased to obtain a laser beam of light.

However, at this stage, the two photons from the stimulated emission process are able to propagate in any direction (even if this direction is identical for each photon). If the medium has a significant proportion of atoms in E1 (ground level), there is a high likelihood that these photons will then be "victims" of spontaneous absorption during the next atomic encounter, ... unless the probability of encountering an atom in E2 (excited level) is significantly higher than that of encountering an E1 atom. However, the laws of physics forbid that in the "natural" state, an atomic mixture has more atoms in E1 state than in E2 state.

Even though this proportion would be identical (number of atoms in e1 = number of atoms in E2), it could be predicted that statistically, each photon introduced into the medium has a chance in two to be absorbed, and a chance in two to elicit a stimulated emission.

In the end, if one "bombards" the medium populated with two-level atoms with an external source of energy photons E2-E1 (or other particles or physical means delivering the required energy), at the end of the day (and admitting that there are no other sources of "photon loss" than absorption) statistically an equal proportion (50%) (50%) of atoms in E1 and E2 ... and this regardless of the initial proportion of atoms in E1 and E2 (remember that the laws of thermodynamics impose only in the "normal" state,) there are more atoms in E1 state – less energy than in E2 – stronger energy.

This is why it is not possible to create a laser light with atoms at only two levels of energy: there is not enough emission stimulated, the phenomena of absorption and spontaneous emission dominate the table.

To create a laser light, the probability of encountering an excited atom (in E2) is significantly greater than the likelihood of encountering an E1 atom; Imagining that this is possible, one could imagine that the number of photons "in phase", stemming from the phenomenon of stimulated emission, gradually increases, by the fact of a reaction in casecade.

Let's take an extreme case where 100% of the atoms would be in E2 at a given moment: one can imagine that a first photon from a spontaneous first emission will trigger by encountering an excited atom a stimulated emission, generating a photon of the same phase and capable of composing a laser light train embryo. The two photons will then most likely encounter other atoms in the excited state (it is very unlikely that they are again absorbed by one of the first unexcited atoms), and induirons the appearance of two new photons, and so on: each "collision", the number of double photons. It is enough to continue to bring energy "from outside" to generate a chain reaction, leading to the appearance of a high number of "identical" photons.

In order to increase the probability of meeting with excited atoms, this is to be achieved for this so-called "population reversal", i.e. the passage of a higher proportion of excited atoms. How do you make this reversal?

Population reversal

We have seen that the supply of adequate energy photons (E2-E1) tends to equalize the proportion of atoms at two levels (in E1 or E2 state), but cannot reverse it.

Now consider an atom with three levels of energy E1, E2, and E3: In the equilibrium state, the number of atoms in (N-E1) is greater than that of the atoms in E2 (N-E2), and the latter is itself higher than that of the atoms in E3 (N-E3).  By providing the necessary energy requirements (exactly equal to E3-E1), a rebalancing between the number of atoms in E1 and E3 is caused, and this increases the proportion of atoms in the E3 level.

This energy contribution can be made in the form of heat, light, electricity, etc.: it is called "pumping", since everything is going as if one pumping one of the atoms in E1 state to bring them into E3 state. At some point, the proportion of atoms in E3, which will tend to equalize (without being able to exceed) that of the atoms in E1 Can be higher than that of the number of atoms in E2 !! A population reversal was then made between E3 and E2. The return to equilibrium, to restore the equilibrium proportion (n-E1 > n-E2, N-E3) induced a significant transition between E3 and E2, to induce the desired casecade reaction and the creation of laser light.

Pumping and emission stimulated at three levels

(a) An atom has three energy levels (E1: Fundamental, E2 and E3). The probability for the atom to be in E1 is 75%, in E2 by 20% and in E3 by 5%.
(b) To achieve a population inversion, energy is provided corresponding to the difference between E1 and E3 (0.8 ev) levels, thanks to photons that possess this energy. This "pumping" causes a change in probabilities; The probability of being in E3 increases, that of being in E1 decreases. To the maximum, one can hope to obtain an equal probability (40% vs. 40%).
(c) As soon as the probability of being in E3 exceeds 20%, a population reversal occurs between the E3 and E2 levels. If a deexcitation occurs between E3 and E2, energy photons E3-E2 can be produced. These photons have a high probability of meeting other excited atoms;

 

So it takes at least three levels of excitation so that an atom can generate laser light. Most lasers actually operate on 4 levels, with the transitions generating laser light between Level 3 and 2. Level 3 is ideally "metastable", i.e. an atom with an energy level of E3 can remain there "for a while" and does not get too excited: This is to ensure a required proportion of atoms in E3 over time, able to receive photons created by and perpetuate the stimulated emission. Nevertheless, nothing prevents certain excited atoms from spontaneously disengaging, ... making them able to absorb the energy of some of the photons produced from the stimulated emission ... and therefore able to participate again in a stimulated emission.. Provided you meet enough photons etc etc. It is therefore necessary not only to maintain high the proportion of excited atoms, but also to ensure that the photon gain produced is greater than the losses (related to the spontaneous absorption, but also because it is the goal to the production of laser light that "leaves the middle", and other phenomena likely to intervene in the medium of production of laser light like diffraction , etc.). It is not enough to excite the atoms of the medium, it is necessary to "maintain" a sufficient number of photons in this environment so that they can continue to trigger stimulated emissions.

It is the role of amplification and resonance cavity, which are the subject of a specific page.

 

Additional Notes

(1) The energy "bearings" model is seductive by its simplicity but simplistic. Rather than representing the electrons as orbiting beads around the nucleus, which each correspond to a particular energy level, it is more accurate to consider that they form an electronic "cloud". Depending on the energy acquired by the atom, this cloud takes on a different form. Under strange laws of quantum physics, the atom occupies all these energy levels at once ... it is the measure of the energy of the atom that "forces" it to adopt a discrete value (value corresponding to a particular number specific to each atom). This measure is analogous to an event, such as the encounter between a photon and an atom. Depending on the probability that the atom is in a certain excited state, the interaction will produce a particular effect (absorption or spontaneous emission), but again under certain conditions: that the energy brought by the photon corresponds exactly to the net difference between two possible levels of energy of the atom.

(2) Some media are not made up of atoms but molecules: in addition to the levels of energy specific to the atoms that make up them, there are other levels related to the "geometry" of the molecule in question: depending on the energy absorbed or rendered, the molecule can "vibrate", or adopt a particular configuration. In general, the energy differences between these configurations are less than those that separate the electronic layers. Excimer lasers use gaseous molecules to generate ultraviolet radiation: the medium molecules are peculiar because they are formed by the Assembly of two rare gas atoms (eg:) (argon and fluorine) and exist only in the excited state (one speaks of "excited dimer", excited dimer in English, whose contraction has given the term "excimer"). The presence of a significant proportion of these molecules of dimers excited in the cavity is in itself the equivalent of a population reversal. Specific physical conditions condition the appearance of these molecules and make the specificity of the excimer lasers.

 

 

Leave a comment

You can ask questions or comment on this content: for this, use the "comments" form located below. The questions and comments of a general interest will be processed and published, and the information provided on the relevant pages should be clarified or supplemented.

Your e-mail address will not be published. Required fields are indicated with *