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Transcript
Semiconductor Laser Diode Outline This student project deals with the exam question Semiconductor laser diode and covers the following questions: Describe how a semiconductor laser diode works What determines the frequency that is emitted? Why is there a threshold current? How is it different form an ordinary diode? Spontaneous Emission Spontaneous emission describes the process where an electron in an excited state falls back to the ground state. The energy of the photon emitted by this process is given by the energy difference between the excited state E2 and the ground state E1. The recombination rate τ is given by where W is the recombination time Stimulated Emission Stimulated Emission in contrast is a process where the relaxation of the electron from the excited state E2 to E1 is stimulated by and incident photon. Now, the special thing about this process is that the photon emitted by this stimulated emission has the same energy, direction and phase as the incident photon. They are in the same quantum state and thus one speaks of coherent light. The emission rate now is governed by the following law where is the rate for spontaneous emission and is the number of photons already in the state. This means, that the probability for stimulated emission to the state grows with the number of photons already present in the state Describe how a semiconductor laser diode works A semiconductor laser diode consists of several parts: Metal contact P-type material Active region (n-type material) N-type material Metal contact From the picture one can see, that in principle you have the same structure like a diode where you have recombination of charge carriers in the active region. When current starts to flow spontaneous emission kicks in. Now, one actually doesn’t want to have spontaneous emission but stimulated emission which means, that trapping emitted photons is required, since the emission rate for stimulated emission also depends on the amount of photons already in the state . This can be achieved by putting mirrors on the sides of the diode and let the amount of photons inside the active region increase until the emission rate of stimulated emission becomes higher than the rate of spontaneous emission. In order to get laser light out, one mirrors has to be either semi-transparent or have a hole in it. One final note: The active region needs to have a high concentration of charge carriers in the active region which causes the gap of the semiconductor to shrink. One way around this is to use a heterostructure in which the surrounding n- and p-type semiconductors already have a higher bandgap and cause the carriers to be confined to this region What determines the frequency that is emitted? The frequency of the emitted light depends on the length of the optical cavity along the optical axis. This can be understood by looking at the following picture In the first row, we see the carriers when a low current is applied. In this regime we are dealing with spontaneous emission for the most part. By looking at the photon densities of the emitted light for this regime, one will notice, that the energy range of the emitted light is in distributed over a width of around the band gap. This follows from the fact that the electrons occupy states in the range of from the bottom of the conduction band and the holes do the same for the valence band. The absence of any gain is also noteworthy for this regime. In the next row we are in the regime, where the current is turned up a bit more. Now stimulated emission sets in and we can see in the photon intensity that several peaks have formed. This has to do with the resonance constraint of the cavity depicted in the following figure Only the modes labeled can form according to the equation Where L is the distance of the optical resonator along the optical axis, m is the resonator modes and is the wavelength of the mode. Now, for the last row the current is again increased so that the stimulated emission becomes dominant. In the last row we saw, that for the photon intensity several peaks formed with the centered one being the highest. Since stimulated emission depends on the amount of photons present in a certain state, the state in the middle receives the largest gain and as a consequence steals all (not completely since there are 2 more peaks belonging to the second resonator modes which are still occupied to a certain level) the photons from the other states. To summarize, the frequency of the emitted light depends not only on the band gap but can also be adjusted by the cavity length as long as the desired frequency is within Why is there a threshold current? The existence of a threshold current can be explained by the existence of spontaneous and stimulated emission. Below the threshold current spontaneous emission is dominant, while above the threshold current stimulated emission becomes the dominant type of emission How is it different from an ordinary diode? Light emitting diode vs. laser diode The key difference between these two diodes is that while an ordinary LED uses spontaneous emission to generate light, the laser diode uses stimulated emission to generate coherent light. For the LED it is beneficial to couple as much light out as possible, while in a laser diode it is necessary to build up a high number of photons in order to get stimulated emission.