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The laser at 50
When people mention the year 1960, what are some of the things that come to mind? The Beatles perhaps? Or may be is it the mini-skirt and hippies? But would the laser come to mind? Well, it certainly should.
Fifty years ago, on 16 May 1960, the first laser was demonstrated at Hughes Research Laboratories by Theodore Maiman. It is often said that the laser was a solution in search of a problem and if we were to believe that, fifty years after its demonstration, it is clear that it has found a great number of problems to solve. We obviously use them quite a bit but what is a laser? I think that the 50th birthday of the laser is a great opportunity to answer that question.
Laser stands for Light Amplification by Stimulated Emission of Radiation. Strictly speaking laser devices not really amplify the light, they are better described as oscillators, however you might understand that the acronym LOSER would not be the best to market around.
Some of the key words in the acronym take us back to 1917, when Einstein, following his successes with relativity and the theory of the photon, established the idea of stimulated emission. In order to understand this, one must understand a bit about the light emission and absorption properties of atoms, as well as their electronic structure. Every atom has a nucleus surrounded by electrons, which are bound to the nucleus by electrical attraction between the positively charged protons in the nucleus and the negatively charged electrons around it.
We can think of the atoms as existing in different energy levels depending on the configuration in which the electrons are arranged. We can visualise these energy levels as steps on a ladder in which the steps are not equally spaced. Each higher step on the energy ladder has higher energy than the one below. The first step on this energy ladder is the ground state of the atom and all the next ones are ‘excited energy’ states. An electron can gain some energy when it absorbs a photon and as a result it moves up to a higher excited state. When an electron jumps from an energy level, to a lower energy level, it loses energy and emits a photon. It is possible therefore that an electron makes a spontaneous transition from a higher to a lower energy state and this is what is known as spontaneous emission.
Let us now consider the case in which we force a number of electrons to be in an excited state by bombarding the atoms with photons, this process is known as pumping. As a result of pumping we end up with more electrons in the higher energy states than when we started, this is what we call population inversion.
Now that we have achieved population inversion, an interesting process can take place. If we bombard the atom with a photon with the right energy, the electron can be stimulated to jump into a lower energy level by emitting a photon identical to the one that was used to start with. This is what is called stimulated emission and a very important feature is that now we have two identical photons at the end of the process. If this is repeated with a bunch of atoms then we have a great number of identical photons. The light output is monochromatic or single wavelength, as all the atoms emit photons of same energy and it’s coherent as the photon emissions occur in unison! That is what we call a laser.
The laser is thus a great application of quantum mechanics and one that has permeated many areas of modern life from telecommunications to medicine and even popular culture: just think of that famous Goldfinger scene where James Bond is expected to die under the power of a laser or the battles in Star Wars. So next time you use a laser later today, do not forget to wish it a very happy 50th birthday.
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