By David Herres
Electrons travel freely through the comparatively vast spaces between atoms or they are confined to orbits around an atom’s nucleus. The orbits do not lie in a single flat plane like those of planets orbiting our sun, but instead the orbital planes are angled to one another. They are best pictured as concentric shells. When an electron in an outer shell is struck by a photon, it is knocked out of orbit and travels through space until it collides with another body.
Through the centuries we have learned a lot about electrons and how to induce them to perform useful tasks for us. A noteworthy success has been the laser, an acronym for “light amplification by stimulated emission of radiation.” The laser’s defining characteristic is that its light emission is stimulated, in contrast to spontaneous. If the emission of radiation is spontaneous, the light won’t be coherent (the generated light waves are not all in-phase and of the same frequency) and you will not have a laser. (There are other requirements as well.)
When heated above a certain critical temperature, a solid body will emit light as well as other types of radiation. The light is noncoherent, being a mix of wavelengths (colors) that do not propagate within a single plane. You can focus the light to a point using a concave mirror or convex lens. But regardless of the precision of the equipment and tightness of the focused beam, the beam will spread as it progresses farther away from the source. The cone will widen even if it is travelling through a vacuum.
This is not because of any resistance in the medium or inaccuracy in the grinding of the optics. The beam of light spreads because the light waves lie in different planes. They interfere with one another in a process known as diffraction. If, at the source, all the waveforms are of the same frequency, lie in the same plane and are locked in phase, the beam of light will not spread. Its intensity will not diminish. This is an exact description of light generated by a laser.
Such light is monochromatic, coherent and collimated. The laser creates this specialized type of light by using two quantum processes, absorption and stimulated emission. To understand how it works, first consider that orbiting electrons in an atom are at a higher energy state when they occupy a shell that is at a greater distance from the nucleus. They are in a lower energy state when they are in a shell that is nearer to the nucleus. The electrons are at specific discrete distances from the nucleus, not spread along a continuum like the planets that orbit a star. The energy levels are at specific values.
When a photon strikes an electron that is close to the nucleus, i.e. at a lower energy level, the electron migrates to a higher energy level, occupying an orbit farther from the nucleus. After a short interval of time, it drops back to the lower state (closer orbit), simultaneously emitting a photon. The electron continually shifts between the high and low energy states. At each transition a photon is absorbed or emitted. The electron is more comfortable (you might say) in the lower energy state.
In a laser, there must be a condition called population inversion. This means that electrons are acted upon in such a way that they move to a higher energy level, farther from the nucleus. This has to happen if they are to drop back to the lower level, emitting a photon, an event that can be either spontaneous or stimulated.
Population inversion occurs when there is a continuous injection of energy into the system, and the mechanism for this activity is the laser pump.
We have been discussing the underlying requirements for laser action to take place, and in the next article we’ll look at some specific types of lasers and how the pumps operate.