In this video, we will discuss laser diode. Laser diode is essentially the same as light emitting diode, in that you have a p-n junction, and you apply forward bias and drive a current, and these injected electrons and holes recombine near the junction and produce light. Now, when the current is very high, then the injected electron and hole concentration can become so high that you reach the population inversion condition and as a result, you start having stimulated emission of photons. These stimulated emitted photons then find some reflecting surfaces and bounce back and forth within these semiconductor region, then you have a laser. So, as shown in the third bullet here, you reach a very high injection level by driving a very high current, and then you reach population inversion that results in stimulated emission, and that stimulated emitted photon bounce back and forth, and you get laser oscillation. So, there are two types of architectures of laser diode that are commonly used. One is edge emitting laser diode, and that is you have a p-n junction in the lateral direction as shown in the figure here in the left. Then, the facet, you cleave your semiconductor using a diamond scribe or something, and you create a very clean facet. Because semiconductor has a relatively high refractive index, most of them do, so the reflection coefficient at this semiconductor facet tends to be pretty high. So, a lot of the light will be reflected back at the facet and reflected back into the semiconductor region, and that leads to a photon oscillation. When the laser threshold is reached, then you get the laser emission out of this facet. So, this edge emitting laser is commonly used in, for example, laser printer. Then, the other type is VCSEL, vertical cavity surface emitting laser. In this case, as the name says, your cavity is in the vertical direction. So, the p-n junction is along this vertical direction. Below and above p-n junction, you have a multilayer structure. This multilayer structure creates something called the Bragg reflection. So, it creates a very high reflection band for a certain wavelength range, and you can change, you can design at what wavelengths they have a high reflectivity. So, you have this very high reflector at the bottom, as shown here, and then higher reflector at the top. The top reflector is thinner, so the reflectivity is not as high as the bottom one, and in between, you have a p-n junction. So, the stimulated emission occurs within the p-n junction in the middle, and the light bounces back and forth between these two reflectors. When you reach the laser threshold, then the laser light is emitted to the top surface. So, this is vertical cavity surface emitting laser. This is a laser that's commonly used for optical communications. Semiconductor laser share the common characteristics as any other lasers. It's a laser. So, it's a highly monochromatic, and you get the coherent light emission, meaning that all of the light that comes out have the same phase, and also it is highly directional. However, semiconductor laser exhibit some distinct characteristics that make it quite different and quite attractive for many applications. One is that the semiconductor exhibits high gain. So, in conventional laser, like gas laser or solid state lasers, the quantum transition responsible for this laser light emission is typically between two discrete energy levels of gas atoms or ions doped in a solid. So, the density of those ions or atoms tend to be relatively small. However, in semiconductors, you have a transition between two bands: conduction band and valence band. These continuum states, continuum energy band tends to give you a much greater density of states compared to these atomic densities in the conventional gas or solid state laser. So, for a typical semiconductor, the density of states near the band edge is of the order of 10 to the 19th to 10 to the 20th per cubic centimeters, and typical atomic density in a gas laser, for example, helium, neon, or argon-ion laser, then your density of those atoms is basically the density of states, and that density tends to be only 10 to the 10th. So, you can see that the density of the states that can contribute to light emission is far, far higher in semiconductor than any other medium that can be used for laser. Also, the laser transition tends to be very fast. So, you can turn on and off these lasers very, very quickly, you can modulate the laser signal very quickly, and that is another very advantageous characteristic for optical communication. Now, like any other gain medium, semiconductors can exhibit spontaneous emission, stimulated emission, and absorption. These three processes are in play at any given time, and the interplay between these three processes determine the actual light emission characteristics of any material. The factors that determine these transition rates are four. So, first, occupancy of electronic levels in conduction and valence band. You have to have electrons in the conduction band, holes in the valence band in order to have a light emission. Then, density of incident photons, because the stimulated emission is induced by an incident photon. So, you have to have an incident photon in order to have a stimulated emission, and the density of possible optical transitions. The optical transitions is not allowed between any two arbitrary state. You have to have a certain conditions met. For example, you have to have the momentum conservation condition met in order to have an optical transition. So, there is a certain pair of states, the pair of conduction band state and valence band state, between which you can have optical transition. So, that's the third component, density of possible optical transitions. Then, finally, even if you have everything met, you don't automatically have, you don't always have optical transition. This is a stochastic process. There is a certain probability associated with this transition, and that transition probability is given by these coefficients called the Einstein's coefficients. So, A_21, B_12, and B_21 are these Einstein's coefficients for spontaneous emission and the stimulated emission and absorption processes. Without deriving the condition for net stimulated emission is when the difference between the two quasi-Fermi levels is greater than the light emission energy. So, think of these laser operating conditions in terms of quasi-Fermi level. So, you drive your p-n junction very hard. So, you pass a very high current. So, electron concentration goes up very high, and the hole concentrations goes up very high, high above the equilibrium carrier concentrations. So, when your semiconductor is driven like this, then the Fermi level that was originally given by your equilibrium carrier concentration is going to split into two: quasi-Fermi level for electrons and quasi-Fermi level for holes. The quasi-Fermi level for electrons will go up in energy towards the conduction band as the electron concentration is increased, and quasi-Fermi level for holes will go down towards the valence band as hole concentration increase. So, as you inject more and more carriers, the separation between the quasi-Fermi levels will increase. When that separation, that difference between the two quasi-Fermi levels exceeds the light energy, then you have a condition for net stimulated emission for that light energy, for that light frequency. Now, the minimum energy that a semiconductor can emit is the bandgap energy because there are no states within the bandgap, and so there are no transitions possible for energies less than the bandgap. So, the minimum condition for net stimulated emission is when your quasi-Fermi levels split or the difference between the two quasi-Fermi levels is greater than the bandgap. When that condition is reached, then you have a net stimulated emission, and then photons emitted by the stimulated emission then bounce back and forth in the cavity, and then you get laser emission. So, the typical, the output characteristics, if you look at the output power, photon power, light power versus the current, in LED, they are just linearly proportional to each other. However, in the laser diodes, you have a very slow increase. This is where they just behave like an LED. So, light emission, below threshold, is dominated by the stimulated emission. But when you reach the threshold, that's when the net stimulated emission condition is met, then you have a rapid increase in the light power. That threshold behavior is a very characteristic behavior for any kind of laser, and semiconductor laser exhibit that too. At the onset of the laser emission, also this originally broadband emission of the spontaneous emission collapses into a very narrow line width which is determined by the cavity formed by the two reflectors in your semiconductor laser. For the edge emitting laser, that will be the cavity defined by the two facets, front and back facets. In the vertical cavity surface emitting laser, then the cavity is determined by the two multilayer reflectors, as shown in the very first slide.
