This is a schematic representation of a ruby laser. At this point virtually all of the atoms of the ruby gain medium (or lasing medium) are in their ground state, which is energetically the most stable state for them.
Stimulated emission is therefore an amplification process for photons (or light). The physical manifestation of this process depends on the wavelength of the photons. If the wavelength is within the optical spectrum of light, we speak of laser light. LASER is an acronym, which describes the above referenced process: Light Amplification by Stimulated Emission of Radiation. If the wavelength of the emitted product falls into the microwave range, we speak of a MASER (Microwave Amplification by Stimulated Emission of Radiation), if it falls into the gamma ray spectrum, we refer to it as a GRASER (Gamma Ray Amplification by Stimulated Emission of Radiation).
In 1913, the Danish physicist Niels Bohr developed an atomic model, which departed slightly from the Rutherford model. It in fact adds consideration of quantum physical calculations to the Rutherford model. Many sources actually combine the two, referring to the Rutherford-Bohr model. The Bohr model is essentially a primitive model of the hydrogen atom, and by today’s standards an obsolete scientific theory, which has been largely replaced by the more complex valence shell atom model. However, due to its simplicity and its correct mathematical calculations for selected systems, it lends itself as an ideal model to explain the generation of laser light.
In his quantum mechanical description of the motions of electrons around the nucleus, Bohr postulated the following with respect to the orbiting electrons:
- Electrons occupy certain fixed orbits around the nucleus, which are identified by their principal quantum number.
- Electrons do not loose energy as they travel around the nucleus. They can only gain or lose energy by jumping from one allowed orbit to another.
- All electrons have certain quantized momenta, which allow them to only jump to specific orbits at certain specific distances from the nucleus.
- An electron can jump from one orbit to another. The energy difference between these orbital jumps is either carried away or supplied by a single quantum of light (called a photon), which has an energy equal to the difference in energy between the two orbits.
The last postulate is the foundation of light generation. An atom can move from the “ground state”, where all electrons are in energy-neutral orbits, to an “excited state”, where one or more electrons have jumped into a higher orbit (further away from the nucleus). This can only happen by absorption of energy, such as heat, electricity or light. When an electron jumps from a “higher energy” orbit (an orbit further away from the nucleus) to a “lower energy” (closer) orbit, it does so by emitting a photon of a specific energy and wavelength. Moreover, when an atom in the exited state is struck by a photon of a specific energy, it can trigger the release of another photon with exactly the same energy and wavelength, by having one of its electrons jump form a higher to a lower orbit. With this type of interaction we end up with two photons of exactly the same energy and wavelength. These two photons can now each trigger the release of another photon of an atom in the exited state. This process is called stimulated emission and forms the basis of laser light. Stimulated emission is very unique to laser light. A flashlight, for instance, generates light by random emission of photons, not by stimulation through other photons.
Now we are at the stage where the pumping source (in this case a flash bulb) is activated, sending photons into the ruby crystal. This starts to "lift" some of the atoms into an excited state (their electrons assume a more distant orbit from the nucleus).
Once the energy has been transferred to the gain medium, spontaneous emission is taking place, where more and more atoms are transformed into the excited state. Once the population of the "excited atoms" is bigger than the "gound-state atoms", we speak of a population inversion. It is precisely at this point, where enough photons are emitted from the excited atoms to generate laser light.
The Generation of Laser Light
