The checkout scanner at the supermarket, the printer in your office, the pointer you used in yesterday’s meeting: lasers are practically a part of everyday life now. You think very little of them, even when they do amazing things like instantly read barcodes or correct your nearsightedness through LASIK surgery.
But what really is a laser? What makes them so special and so useful? In fact, what differentiates a laser from a simple light bulb? The answers lie in the remarkable weirdness of quantum physics. Lasers are a quintessential quantum phenomenon.
Atomic Energy
The key issue we have to deal with here is the interaction of light and matter. In classical physics, light is made up of waves of electromagnetic energy that travel through space. These waves can be emitted or absorbed by accelerating electrically charged particles of matter. This is what happens in a radio tower: electrical charges are accelerated up and down the tower to create the electromagnetic waves that travel through space to your car and allow you to listen to the station of your choice.
At the turn of the century, scientists wanted to apply this classic idea to create models of atoms. They imagined an atom as a small solar system, with positively charged protons in the center and negatively charged electrons orbiting around them. If an electron were to emit or absorb some light, i.e. electromagnetic energy, it would speed up or slow down. But this model did not hold. For one thing, there is always an acceleration when one thing orbits another, this is called centripetal acceleration. So the electron in this classical model of the atom must always be emitting radiation as it orbits and thus losing energy. That makes the orbit unstable. The electron would quickly fall on the proton.
Niels Bohr solved this problem with a new model of the atom. In the Bohr model, an electron can only occupy a set of discrete orbits around the proton. These orbits were visualized as circular train tracks in which the electrons traveled while circling the proton. The farther an orbit was from the proton, the more “excited” it was and the more energy it had.
In Bohr’s model, the emission and absorption of light consisted of electrons jumping between these orbits. To emit light, an electron jumps from a higher orbit to a lower orbit, emitting a bundle of light energy called a photon. An electron could also jump from a lower orbit to a higher one if it absorbed one of these light packets. The wavelength of light emitted or absorbed was directly related to the difference in energy between the orbits.
There was a lot of quantum weirdness in all of this. If the electron was bound to these orbits, that meant it was never between them. It jumped from one place to another without ever occupying the space in between. Furthermore, light was both a particle, a photon that had a bundle of energy, and a wave that spread out through space. How do you imagine that? While Bohr’s model was only a first step, modern versions of the theory still feature discrete energy levels and duality of particles and photon waves.
Lasers make photons jump
How does this relate to lasers? LASER stands for Light Amplification through Stimulated Emission of Radiation. The ideas of “amplification” and “stimulated emission” in a laser are based on those specific energy levels of electrons in atoms.
To make a laser, you take some material and exploit its quantum energy levels.
The first step is to invert the population of the levels. Typically, most of the electrons will reside in the lower energy levels of the atom, that’s where they like to rest. But lasers rely on driving most of the electrons to a higher excited level, also called an excited state. This is done by using a “pump” that pushes electrons into a specific excited state. Then, when some of these electrons spontaneously start falling again, they emit a specific wavelength of light. These photons travel through the material and tickle other electrons in the excited state, encouraging them to jump down and causing more photons of the same wavelength to be emitted. By placing mirrors at each end of the material, this process builds up until there is a nice, steady beam of photons that have the same wavelength. A fraction of the synchronized photons then escape through a hole in one of the mirrors. That’s the make you see coming from your laser pointer.
This is exactly what doesn’t happen in a light bulb, where the atoms in the heated filament have electrons jumping up and down chaotically between different levels. The photons they emit have a wide range of wavelengths, which makes their light appear white. It is only by exploiting the strange quantum levels of electrons in an atom, the strange quantum jumps between those levels, and finally the strange wave-particle duality of light itself, that these amazing and very useful lasers come about.
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Of course, there is much more to this story. But the basic idea you want to remember the next time you’re at the supermarket checkout is simple. A world beyond your perception, the nanoworld of atoms, is incredibly different from the one you live in. Somehow, we humans have peered into that little realm and come back with an understanding deep enough to reshape the macroworld we inhabit.