When you hear the word laser, does your mind tend to divert to some futuristic menacing-looking deathstar-type machinery that is intent on destroying the planet? Or perhaps you think of those fragile laser pointers you used to use as a child to toy with your pet cat? Thankfully our timeline does not involve Dark Lords bent on mass destruction, and has progressed significantly beyond the technology of cheap, novelty items. Lasers in the modern world are used in almost all devices and make up a large part of scientific research. We are only just beginning to harness their true power. Actually, now that I think about it, maybe George Lucas has a point…
To start off with, I need to briefly explain how most light works. Atoms contain electrons which live in distinct layers around the nucleus, and orbit it constantly. This atom is in its ground-state energy level when all the electrons occupy their normal layers. Energy can be supplied to this atom via heat, light, or electricity. When this occurs, the electrons gain energy, become excited and jump up to a higher energy level which is further from the nucleus. In this higher state, the electrons become much less stable, so they want to return back down to the ground state. When it does this, the electron releases the energy that it once absorbed in the form of a photon – a particle of light. We call this process spontaneous emission of radiation.
Okay, so what is a laser? What makes a laser beam differ from that of a normal flashlight?
Well, there are three major properties of lasers that make them stand out. Firstly, the light that they release is monochromatic. This means that it only contains one specific wavelength of light, and therefore only one colour. This wavelength is determined by the amount of energy released when the electron falls to a lower orbit. With a normal flashlight, the ‘white’ light is composed of a mixture of many different colours, and their light waves all have different frequencies and wavelengths.
Secondly, the light that lasers release is coherent, which means that the light waves are organised; the crests of every wave are lined up with the crests of every other wave. You could think of the photons in a flashlight beam as a crowd of commuters, pushing and jostling their way through the platform of a busy train station; by comparison, a laser beam is like a parade of soliders marching precisely in step.
Finally, a beam of laser light is very tight and concentrated, and can be maintained over a much longer distance (we say that it is highly collimated). A flashlight beam, on the other hand, spreads out into a cone shape, with light waves leaving in many directions. This causes the light to be significantly weaker and allows it to diffuse much more easily.
The word “laser” itself actually gives quite a lot away about how the device operates. “Laser” is an acronym for light amplification by stimulated emission of radiation. You may remember earlier I was talking about spontaneous emission of radiation which happens for most light sources. Here we can see that lasers make use of a phenomenon called “stimulated emission of radiation”. This is essentially when photons of a certain wavelength collide with atoms which contain orbiting electrons which are in an excited energy level, with energy equal to that of the colliding photon. In order to achieve stability so that the excited electrons do not instantly fall back down to the ground level, we maintain the atoms in a temporarily excited state known as a meta-stable state.
A photon is then expelled from the atom, and has the same wavelength as the incoming one. In effect, we get two photons out from simply putting one in, which doubles and amplifies our light. Buy one, get one free. These two photons then go on to stimulate more photons in other atoms, and we get a cascade of photons in a chain reaction of collisions and stimulations. Mirrors are also used at either end of the lasing medium so that the photons can travel back and forth through the medium, stimulating more and more electrons so that more photons of the same wavelength and frequency are produced. At one end, the mirror is half-silvered so that it reflects some of the light and lets the rest through. From this, we produce a beam of pure, coherent laser light.
The reason I find lasers particularly interesting is due to their vast array of applications, some of which may even come as a surprise. Most modern spectroscopy is performed using lasers, which allows us to observe properties of materials without physically touching them. This is especially useful for measuring distant objects in our solar system and beyond. They seem to be used in all sorts of accurate measuring equipment, such as in the recent LIGO experiments to detect extremely weak gravitational waves. Lasers have also been used for the advancement of nuclear fusion due to their very high and intense power which is capable of squeezing together substances through immense impact forces from all sides. Recently, NASA have even been testing laser technology to propel rockets much more efficiently and accurately than the currently used chemical methods.
And of course, they can be used for building planet-sized weapons of mass destruction, if that’s your type of thing.