Lasers

Introduction

The first laser was built by Theodore Maiman and is recorded as having been first displayed on 16 May 1960. This invention is particular, in my opinion, because it is not a naturally occurring phenomenon in the visible spectrum. Unlike lots of other inventions which come simply from us harnessing phenomena that we have discovered, lasing is a step ahead of what nature gives us, a complex application of several principles together to create something new.

Explanation

Laser is really an acronym—Light Amplification by Stimulated Emission of Radiation—which was first postulated by Einstein in 1917. As the name suggests, a laser is really the combination of two separate optical phenomena, stimulated emission and light amplification, which we will explain here.

Stimulated Emission

Emission, as its name connotes, is the term we use for a photon which is created by an atom. To understand this phenomenon, we need to understand atoms a little bit more.

When you picture an atom in your head, you probably imagine a small solar system sort of design with a nucleus of protons and neutrons in the middle and little electrons spinning around it in circles. Sadly, this is not the case, but the model serves well to illustrate emission; so we'll use it with the understanding that it is really not particularly accurate. Electrons in every atom under normal conditions orbit the nucleus in the closest possible orbit (which, for quantum mechanical reasons, is not physically touching the nucleus). Certain molecules (H₂ gas, for example) undergo excitation when they are hit by photons of sufficient energy which means that the electron is temporarily pushed to an orbit further away from the center. However, as things in physics tend towards the lowest and most stable energy state, the electron jumps back down to the ground state. The effect can be imagined as being like marbles in a funnel. The faster you push the marbles, the higher they rise in the funnel as they spin around. But over time, no matter how hard you first pushed them (assuming that they can't leave the funnel) gravity will pull them back to the lowest available spot. And since energy can't just disappear, the energy that the electron lost by jumping back down to a lower orbital is emitted as a photon of light of that exact amount of energy (we'll call this precise value ΔE). This is emission.

Stimulated emission is somewhat more complicated. An excited electron in a higher orbital will, obviously, spend some amount of time (it's really short) in the excited state before jumping back to ground state. If a photon whose energy is exactly ΔE passes very very close by the excited electron, the electron will jump before it normally would. Thus the emission was artificially stimulated.

Light Amplification

Light amplification is a direct result of stimulated emission under correct circumstances. If there is an excited medium (maybe an energetic cloud of H₂ gas), we can imagine that eventually one of the excited atoms will revert to ground state and emit a photon with energy ΔE. That photon will almost definitely pass near enough to another excited atom (if the cloud is big and dense enough) and stimulate the emission of another photon. Luckily for us, when a photon is emitted by stimulation, it is released in phase with and in the same direction as the incident photon. In other words, where there was one photon, now there are two traveling in exactly the same direction at the same time and in basically the same space. The light is now twice as bright. But these two photons will eventually collide with other excited electrons and stimulate more emission in the same direction. A chain reaction causes a short, bright burst of energy as all of the excited electrons in the direction of stimulation are forced to revert to ground state.

Lasing

The problem with the described situation above is that the cloud of gas runs out of excited electrons extremely quickly. To produce a laser, we need a continuous stream of stimulated photons. To produce this effect, we continually excite the gain medium by a very energetic source of light (a flash lamp or another laser) so that every time an electron jumps to ground state, it is quickly re-excited. Then we put the gain medium between two mirrors that face each other. Eventually, stimulated emission happens in the direction of the mirrors and an amplified light source bounces back and forth between the gain medium, becoming even more amplified. If the optical pump is strong enough, the cloud will never run out of electrons to stimulate. The amplification cycle is infinite (not that it increases in brightness forever, only that it will forever produce a continuous beam of light of a certain brightness that is unidirectional and in phase). To release the beam from the mirrors, we make a part of one of the mirrors semi-translucent so that some of the photons escape when the beam hits that mirror. The escaping photons come out in a beam which we call a laser.

Applications

We use lasers more than you might think. The ubiquitous laser pointer is, of course, one use. However, lasers now assist in medical surgeries, read CDs, cut and weld metals, and are used in printers (you know, laser printers) among many other things. They have become widely used and are on the forefront of our active scientific pursuits today.

Photoelectric Effect

Einstein won the Nobel Prize in Physics in 1921. Lots of people assume that he won it either for his work in relativity or for the immensely influential equation E=mc². However, it was for his groundbreaking discoveries in a physical phenomenon known as the photoelectric effect for which he was awarded the Prize. Herein we will discuss the phenomenon and its subsequent applications and implications.

Explanation:

Simply, the photoelectric effect is the emission of electrons from a metal as a result of incident light. In other words, sometimes, when you shine light on a piece of metal, some of the electrons in the metal come unbound and fly freely though space. I guess we need to back up and talk a little about metals.

One of the properties of metals is the configuration of its electrons. When a whole lot of iron atoms (to use one of many metals in the periodic table) get together, they start to share their electrons. However, unlike other solids, metals share their electrons with the entire solid. The top layer of electrons are free to flow anywhere about the surface of the metal, bound to no specific atom. Incidentally, this property is what makes metals such good conductors of electricity; the "fluid" electrons on the surface carry and transport charge very efficiently in much the same way as it is easier to slide over a wet surface than a dry one.

This property of metals is what makes the photoelectric effect possible. When you shine light on metal, the sea of electrons (as it is often called) receives lots of energy, causing some of the electrons to shoot off. However, not just any kind of light can make it happen. Imagine a swimming pool that is only filled up half way with water. If you were to throw a rock into the pool, you could make some of the water splash out, but only if the rock was traveling fast enough. Even a whole bunch of rocks traveling too slow would only make lots of splashes that didn't remove any of the water. In the same way, light needs to be energetic enough to cause the electrons to escape from the sea. The minimum energy that is required for a photon to remove an electron from a metal is called the work function (symbolized by the Greek letter φ).

Implications:

The implications of this discovery were shattering to the world of physics. There was a huge debate at the time concerning the nature of light--whether it was a particle or a wave. Einstein's discovery helped us to understand the truth. As I mentioned before, only light with a certain minimum energy (equal to φ) could make electrons leave the metal. Einstein discovered that this minimum energy could only be achieved by changing the color of light, not the intensity. That means that red light, no matter how bright, will never induce the photoelectric effect, whereas very very weak ultraviolet light will always do so. We learned some great truths through this. First, the frequency of light (its color) is directly related to its energy. In fact, frequency is the only factor that determines photon energy. Intensity (brightness) of light corresponds not to energy, but to the number of photons hitting the area per unit time.In other words, shining really bright, red light on the metal was like throwing lots and lots of rocks really slowly into the pool. But shining really weak UV light was like throwing just a few rocks really really fast, causing a large splash (but only a few times). To induce a large photoelectric current, one needs only to produce an intense UV source.

Applications:

The applications of the photoelectric effect are many and influential. This is the basic idea that makes solar energy possible (taking light and making electrical energy out of it). Also, from this idea came photomultipliers (which created such devices as night-vision goggles) and CCDs (which are the imaging devices in digital cameras and telescopes), to name just a few.