 All right, so the next topic we're going to talk about is lasers. So these days lasers are pretty common. You've probably used or seen a laser relatively recently or you've used a laser certainly maybe in a form of an actual laser pointer where you can pick up a laser and point it at things but you've probably likely used lasers in other formats as well like in a CD player perhaps although CDs aren't the most common way to listen to music these days anymore. Lots of consumer electronics use lasers. So the question though for us is what is a laser exactly? Clearly it's a light source. It's a way of generating light of a particular type but how is that different from any other light source like a light bulb, like a flashlight, incandescent bulbs versus fluorescent bulbs versus LED bulbs? Are those lasers or not and how do lasers differ from them? So the characteristics we need to consider of a laser and certainly these will not be, at least two out of these three will not be surprising to you if you've played with a laser pointer before. The first characteristic of a laser is that it's going to be a monochromatic light source. Monochromatic just meaning single color. The light that comes out of a laser is of a single frequency or single wavelength or at least very tightly bunched around a single wavelength. So I'll draw some illustrations to show you what I mean. So here's a laser, cylindrical laser like a laser pointer. The light that comes out of that laser. So there's a beam of light, electromagnetic radiation coming out of that laser. All the photons that come out of that laser are the same wavelength if it's a red laser, in this case this pink laser. All the photons, there's a bunch of photons coming out at the same time but they're all the same frequency as each other. That's completely different than a light source like a light bulb. So here's a sketch of a light bulb. Certainly photons of that frequency might come out of the light bulb as well. But in addition to those red photons, you're also going to, if it's a white light bulb, you're going to get blue photons with a shorter wavelength, higher frequency. You're going to get photons of all sorts of different colors coming out of that light bulb. So it's not a monochromatic light source, it's a polychromatic light source that gives off lots of different colors or wavelengths of light. So lasers are going to be monochromatic. Another feature that they share is that the beam of photons that emerges from a laser is collimated. What that word means is the photons are all pointing in the same direction. The dispersion of this beam doesn't spread out very much, it stays tightly collimated. We don't use the word focus because you can take a beam that's quite dispersed and then focus it and beyond the focus point it won't remain focused. This beam remains tightly bunched over a long distance. So I've also drawn this picture of the light bulb so that this is certainly not a collimated beam. The photons come out from a light bulb and scatter in all directions. They don't all go in the same direction as each other. So this picture of the photons coming out of the laser pointer, those are all pointing in the same direction. If I go a long distance they might begin to diverge a little bit very far from the light source. But again, as you know, if you've ever played with a laser pointer you can point it something a long distance away, a quarter mile, a half mile away and you can still see a tight laser beam spot from that laser if it's a decent laser. The last of these features that laser light shares is in addition to being monochromatic and collimated, the light from a laser is coherent. So that one's less familiar, it's not visible to your naked eye when you look at a laser beam unless you do something like pass it through a diffraction grating. But the beams of light from this laser are coherent, the photons are coherent, and again I've included that in the drawing here by drawing these photons so that their peaks are all in the same place as each other, their troughs are all in the same places as each other. So this is a coherent beam, another way of saying the beam is coherent would be to say that the photons are in phase with one another, again peaks and troughs appearing in the same place, a light bulb, an incoherent or non-coherent source of light, if I could draw two photons coincidentally of the same wavelength coincidentally heading in the same direction, if I draw them like this so that the peaks and troughs are in different places or like this, those are all photons that are not coherent with one another, their peaks and troughs occur in different places. So we only call them coherent if they are in phase, if they line up with one another, the peaks and troughs are in the same places. So those are the three characteristics of a laser, monochromatic, collimated and coherent. The next question becomes how do they get that way? What is special about the construction of a laser that produces this monochromatic and collimated and coherent beam of light that a light bulb does not? So we'll take those features of the laser one at a time. Let's start in fact with coherence, actually no, let's start with monochromatic. That's probably the easiest one to explain. So why do we get these features? As we know from studying lots of quantum mechanical systems, there are a lot of materials with quantized energy levels. So in particular, the stuff that's inside this laser that we'll talk more about soon that is giving off the light, that involves energy levels that are quantized, meaning there's some lower, so these are energy levels, there's a ground state and there's an excited state of this material, whatever the material is. And whenever there's a molecule up in the excited state of the system, when it falls down to the ground state, it reduces its energy by some amount delta E. The energy of the photon that gets generated is exactly that energy difference between these two states. So the photon that gets generated, every time a molecule falls from the upper state to the lower state or the material changes from the upper state to the lower state, gives off a photon of exactly the same wavelength. So energy levels remain the same, so the fact that those energy levels are quantized, I can only have this energy and this energy and nothing in between, means that that transition always has the same energy, always gives off a photon of the same wavelength. So that's relatively easy way to make a monochromatic light source is to have only one type of transition generating light. So those quantized energy levels give rise to, let's do that down here, give rise to the monochromaticity of the beam, the single color of the light source. Now let's tackle the coherence. So I've got a collection of molecules or a material with this gap in the energy levels, it can give off, if I have excited state molecules, it can give off lots of photons of exactly the same wavelength as one another. Where I get the coherent beam of light is from a process called stimulated emission. So what stimulated emission is, it's exactly what the words say once you know how to parse them, we know about absorption of light. If I have a molecule in the ground state of this molecule, if a photon comes along of exactly the right wavelength to excite this transition, then this molecule can absorb the light and go up into the upper state. The inverse of that process can happen if the molecule is already in the upper state. If a molecule, if a photon comes along with exactly the right wavelength, again this molecule is intended to be the same as this molecule, so this difference in energy matches, this difference in energy matches Hc over the wavelength of this particular photon that got emitted. When this exact photon with that particular wavelength encounters another molecule in the excited state, if it encounters a molecule in the ground state, the molecule could be absorbed. If it encounters a molecule in the excited state, what can happen is it doesn't get absorbed, that photon will continue moving and pass through that molecule, but it can stimulate the emission of an additional photon. As it passes that molecule, this molecule will fall down to the ground state, emitting a photon of exactly the same wavelength of light, but what happens is that photon gets emitted coherently with the same phase as the photon that stimulated the emission. When we have stimulated emission, some excited state molecules giving off photons that will stimulate emission of more photons of exactly the same frequency, and those photons will be coherent with one another, they'll have the same phase. So that's the method that we use to generate coherent light in a laser. Last thing to consider is this idea of collimation. How do we generate a tight beam of photons that doesn't disperse, doesn't spread out as it goes along, and that could be as simple as just using a cylindrical material. So if I have some material inside this long cylinder that is going to generate coherent monochromatic light, let's say I make that cylinder solid except I only allow light to emerge from a tiny little pinhole exit over here. So if I have a molecule in the center, if it gives off a photon, maybe it gives off a photon, and it shines in this direction, that'll just hit the wall, be absorbed by the wall, and we'll never see that photon. It's only a few of those photons that happen to me heading exactly toward the exit. If they're inside the cylinder heading toward the exit, they're going to tend to be moving in this direction when they leave. They won't be moving in exactly that direction because maybe the molecule is down toward the bottom of the cylinder or up toward the top of the cylinder, so it might be pointed a little downwards or a little bit upwards when it leaves the cylinder. But we'll get a relatively tight beam of light. That's essentially how flashlights work. You can focus the beam from a flashlight a little bit by just shaping the cavity in which the light bulb sits. We can do much better than that. In fact, if we not only have a long, skinny cylinder, let's say we put a mirror on the back end of this cylinder. So now photons, if they're emitted exactly backwards, they'll reflect off this mirror and then proceed out the front side. So essentially, I've doubled the length of my cylinder by putting a mirror on the back side. That's one way to get a little extra collimation. We can do much better than that even if I also put a mirror on the front side. So this mirror is not 100% reflective. So the mirror on the back side, I'll make 100% reflective. All the photons that hit this mirror reflect and bounce toward the front. The mirror on the front side, I won't make perfect. I'll make that only a 90 or 95% or 99% reflective mirror. So 95% of the time when a photon hits this mirror, it'll reflect and go back to the back side. Now if I'm off by just a couple of degrees, it'll reflect off the front, reflect off the back. After a couple of reflections, it'll hit a wall and be absorbed unless the photon is directed exactly parallel to the axis of the cylinder. Then it can bounce back and forth all day as many times as it wants. And then one time out of 20, when it hits the front mirror, it'll escape because the mirror is only 95% reflective and it'll pass straight through. So that is essentially not just double the length of my cylinder, but it's actually now multiplied by another factor of 20 because on average a photon will make 20 round trips back and forth through the cylinder before it escapes out the front. So it's only a small number of the photons that happen to be directed exactly or very close to parallel to the axis of the cylinder that will escape. And those are the ones that escape tightly collimated with one another when they escape the mirror at the front. So this particular construction of this cavity in which the material resides, this is called the optical cavity. And if we put a mirror on the front and the back, we call that an optical resonator. And in particular, if the length of this cavity, if we make that length of the mirror equal to some integer number of wavelengths of the light, actually a half integer number of wavelengths of the light is fine, if we do that, then that will also ensure that the light that comes out of the cavity is also coherent. So by constructing an optical resonator with the characteristics that we're interested in, we can get not only a tightly collimated beam, but also a coherent beam. So the optical cavity helps with both of those features of the laser. So that's a pretty top-level summary. I haven't told you anything about what's going on inside of this cavity other than that photons get generated. They bounce back and forth, and then sometimes they can escape as a monochromatic, collimated, and coherent beam of light. The next step will be to understand a little more detail about what this material is inside the laser and how we can use it to generate photons by falling from the upper state down to the lower state.