Laser en
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| AI-translated from Italian. Original page: Laser |
The word LASER is an acronym that stands for "Light Amplification by Stimulated Emission of Radiation"
To illustrate how the LASER works, let us examine these words one by one
Stimulated emission of radiation
The electrons in an atom in its stationary state cannot have just any energy, but are arranged on well-defined energy levels. The following diagram shows the first levels of an atom with a single electron (red dot)
These levels are easy to observe experimentally, thanks to the fact that when an electron jumps from one level to another it emits electromagnetic energy, typically in the ultraviolet region but also in the visible. The "atomic spectrum", that is, the set of all the electromagnetic energy emitted, is therefore not a continuum but a set of well-defined lines. The following diagram shows the first possible energy jumps of the electron, and the corresponding spectral lines.
But how does an electron jump from one level to another?
The two main mechanisms, spontaneous emission and stimulated emission, are illustrated in the following table.
| spontaneous emission | stimulated emission. |
|---|---|
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In spontaneous emission an electron loses energy by going from a higher-energy level to a lower-energy one. The energy lost leaves the atom in the form of a photon, that is, a quantum of light (electromagnetic radiation). |
The emission process can be stimulated by means of another quantum of light whose energy is exactly equal to the energy difference between the two electronic levels involved. The process is the following: the atom is hit by a photon; in the collision an electron drops to a lower level and produces another photon. |
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Balance: at the beginning there is an atom with an electron (red dot) in an excited state; at the end there is the atom in a non-excited state, and a photon (light blue arrow). |
Balance: at the beginning there is an atom with an electron (red dot) in an excited state and a photon (light blue arrow); at the end there is the atom in a non-excited state, and two photons. |
Light amplification
The most remarkable feature of stimulated emission is that the two photons in the final state are completely indistinguishable: they have the same energy and the same phase, and they are emitted in the same direction.
We therefore see that this process can be exploited to amplify light. Of course this amplification does not create new energy out of nothing: remember that the system of atoms has to be "pumped" into an excited state before being stimulated. The stimulation merely extracts the energy that we have supplied to the system.
The advantage is that the energy is extracted while reinforcing the coherence and the monochromaticity of the incident light.
Where does resonance come into this process?
- A first resonance phenomenon (of quantum type) appears in the emission process itself (whether spontaneous or stimulated). It is described above: the "stimulating" photon must be in resonance with the two levels from which another photon is to be extracted for the process to take place. In other words, the incident photon must have an energy equal to the energy difference between the initial and final levels between which the electron moves.
- Then there is a classical resonance in the construction of the device. The laser is in fact itself a resonant cavity, as illustrated in the next section.
How a laser is made
Going schematically through the operating principle illustrated above, we see that to build a laser we need the following components.
- An active medium, that is, a set of atoms in which the electrons can be excited and de-excited. It can be a noble gas, a metal vapour, a semiconductor crystal, an organic compound (even a liquid one), and, in a special case, a beam of electrons (see FEL)
- An energy source that "pumps" the medium into an excited state (a condition called "population inversion"). It can be an electric discharge, but also a flash of light, or another laser.
- An optical resonant cavity, whose purpose is to reflect the photons back and forth so that they pass through the medium again and again and sustain the light amplification against the energy losses of the system. The optical resonant cavity is essentially a region of space of well-defined length, enclosed between two mirrors. The electromagnetic waves travel back and forth between the mirrors, forming standing waves.
