38: LASERs
All of us have seen lasers in one way or another. In the classic action movie Mission: Impossible, there is a scene where Ethan Hunt (played by Tom Cruise) ropes down onto the highly secured computer, the vent through which he climbs, being secured by these light sources. In Star Wars, Storm Troopers, rebels, the Death Star, and the Death Star 2.0 all seem to feature lasers as being deadly. By the time these films came out, lasers were still a niche technology but were well-known among filmgoers at the time.
Already five years after the invention of the first laser in 1960, Goldfinger, a James Bond film (arguably one of the best), brilliantly demonstrated what this tool could do in a short scene where Bond is nearly sliced in half. But this wasn’t the first scene to feature one: In The Day the Earth Stood Still, 9 years before its invention, the supposedly evil robot featured a light source that was visually akin to the modern laser but instead of illuminating something, it disintegrated weapons.
Laser technology “is not so high tech” as one might think. They’re certainly easier to understand than modern computers, even if it takes an enormous amount of math and physics education to truly understand what is happening on the quantum mechanical level. But I will continue to avoid the technicality of quantum physics and try to break down the basics of lasers in the most simple-to-understand, intuitive way.
We start as simply as we can. A laser is a light source of a singular wavelength. How one achieves this, is hidden within the etymology of the word: LASER used to be an acronym that stands for light amplification by stimulated emission of radiation. Since its use has skyrocked, it has now been included in dictionaries as its own word, ranking alongside radar and scuba (suit) as words that used to be initialisms, whose meanings have mostly been forgotten.
Let’s break the LASER acronym down into its constituent parts.
Light: Electrons that are in excited states can “fall” to lower states, thus releasing energy in the form of a photon. These excited states are always predefined states, as given by the valence shell rules. Any one drop of electrons, based on the loss of energy they experience (which is proportional to the number of levels they fall), can release different wavelengths of photons. As the wavelength of photons is proportional to the energy they contain, the higher the energy drop, the shorter the wavelengths of the photon.
Usually, a material consisting of billions of atoms releases many numbers of wavelengths of light. All atoms and molecules do this via blackbody radiation. Now, because every atom and every molecule has different valence shell rules, you have different energy levels for each molecule, which means that for some heterogenous substance, i.e., non-pure substance, one can get a large number of different colored photons.
Amplification: The lasers that one can buy in the store tend to be weak enough that one can see the red dot but not enough to hurt one’s eyes. For cheap lasers, there isn’t just one single atom releasing all the same colored photons, but a multitude of them. Even so, with a multitude of atoms, there usually aren’t enough for us to see them very clearly, hence why we need to amplify the intensity of the light. This can be done by releasing more photons by a process known as
Stimulated Emission: Strange things happen in the quantum world. Suppose a war party gets lost in a forest on their way to attack a castle, and they are cluelessly looking around to figure out where to go. If some random forester walks up to them and says, “Hey, the castle is this way!” pointing in a direction, the party members aren’t likely to listen to him. After all, who is this guy? Maybe he belongs to the enemy and is leading them into a trap.
But if the right person shows up, for example, the leader of the war party, and utters the words “Hey, the castle is this way!” pointing in the same direction and going in the same direction, the rest of the party is more likely to follow: the characteristics of the person appearing out of the woods affects how the rest of the people behave.
In the quantum realm, when a photon with a specific (discrete) amount of energy passes by an excited atom that can, in theory, lose that exact amount of energy, then the atom usually does. In other words, in a collection of the same atoms, if in one atom an electron drops energy states, whatever number of states that electron has dropped, the released photon will “inspire” the electrons in other atoms to drop the same amount of levels as well, causing the same wavelength of light to be emitted. Like in our new analogy, these new atoms release the photon in the same direction.
These atoms can also absorb the released photons to excite electrons back to these levels. But: If there are enough photons, then stimulated emission exceeds that of absorption, which means a cascade of more and more photons of the same color light, moving along the same vector/in the same direction.
Radiation: When talking about light, we usually talk about light in the visible spectrum, i.e. purple, blue, green, yellow, red, and all colors in between. When talking about light in general, instead of only humanely invisible light, we tend to talk about radiation. Radiation and light are, in fact, synonymous.
A LASER, then, is any technology that takes some emitted light, allows it to pass by atoms that are the same in terms of atomic number and electron levels, and uses the rules of quantum mechanics (stimulated emission) to create more photons traveling in the same direction, with the same wavelength.
Cool. While all this sounds good and fancy, we still need to create a thing that does this. The central idea is not too far-fetched: Suppose we take two mirrors, where one of them is 100% reflective while the other is only 95% reflective, where the light will exit the laser.
Next, we place these mirrors on opposite ends of a reflective tube, in the middle of which we place the special atoms that allow for visible light stimulated emission, such as ruby (aluminium (2) oxide (3) chromium (1) in a crystal structure) which releases a red light. Now all we need to do is heat the ruby, such as with electrical conduction. Soon, the atom spontaneously releases photons that we direct via mirrors to bounce in the direction of one of the mirrors.
Once at the mirrors, they bounce back, past the ruby in question, and generate more photons of the same light. After that, photons quickly ramp up their production until the ruby atoms are barely ever in an excited state so that enough of the light released from the tube passes through the 95% reflective mirror into the outside world, that us humans can see it. Et voilà: A laser.
From here, we can switch out the ruby atoms for other materials, such as sapphires doped with titanium, which can release various colors depending on the amount of “dopage”. We can also vary the electricity that goes to the atoms, thus increasing or decreasing light intensity.
And though we’ve only given the barest bone overview of lasers, so much more can be done with them, and many more technical details are left in the dust. Chemical lasers, gas lasers, and other types of lasers can be manufactured, all with different specifications on color, intensity, efficiency, etc. Naturally occurring masers (microwave lasers) also exist, which may be water droplets in gas-rich regions within galaxies, though they are useless for the engineering of computers.
Even then, on-earth lasers find only a handful of applications within computers. Where they are needed, however, is when there is no alternative but to use them, placing them among a handful of technologies that initially were a solution to problems no one ever had, to being vital to the operation of sensors, computers, and scientific devices.
