I'd like to discuss another knob that we have in the design of optoelectronic devices, that is the confinement of photons. Just like we confined electrons, we can also confine photons. So, what you're looking at here, is you're looking at what if the photons can operate in three-dimensions, two-dimensions, what happens to the number of states they're allowed to populate in. Here, you can see what happens if we would squeeze things down even further. So, this is another knob, it's actually a very interesting knob and gives rise to some fascinating physics. What you're looking at here is you're looking at what's referred to as the spontaneous emission factor. So, I told you that a laser needs an amplifying medium and it needs a cavity. The amplifying medium, in addition to emitting in the lasing mode, it can just emit spontaneously. So, this spontaneous emission can go into free space, but if it overlaps with the stimulated emission, that's what's referred to as the enhancement factor. By creating spontaneous emission, enhancement factors that are very close to one, we can essentially make what's referred to as a threshold dose laser. So, it looks like the laser turns on immediately. We'll talk about things like the micro-pillar laser, where we do get very, very tight confinement of photons. This is essentially a very small pillar that's etched and what you can see is the various layers of semiconductor. We'll also talk about the whispering gallery mode laser and so here, what you're looking at is, you're looking at St. Paul's Cathedral. If you go to England and you whisper, the whispers travel around the edge of the the structure. Similarly, to this acoustic effect, we can get the same effect with light and use it to make lasers. We'll talk about photonic crystals. So, photonic crystals also give us the option to confine the photons in very small spaces. Remember, they do those by, we have alternating layers of material with different index of refraction. So, just like with the semiconductor band structure, we can now make an optical band structure and use that to get very tight confinement.