I've attached a emission spectrum of my laser, which is GaAs/AlGaAs based with InAs quantum dots embedded in the active region. You can see a sharp peak which represents the lasing mode, but it occurs off center of the spontaneous emission peak (underlying broader curve).
Hi Joe,
The lasing wavelength of semiconductor lasers is primarily determined by the gain peak, which is slightly different than the PL peak. For example when you pump your laser, the lower energy states (longer wavelength) are inverted more easily due to the lower density of states and will thus have more gain at low excitation compared to the higher energy states. But since their density is lower, they also contribute less to the PL signal. The relationship between PL and gain is not very trivial but it is possible to compute it. See for example :
https://doi.org/10.1016/S0038-1101(02)00405-7
I hope this helps,
Cheers.
Hi Alexandre,
Thanks for taking the time to answer. You mention that longer wavelengths, ie lower density states contribute less to the PL signal. However, in my spectrum the lasing mode occurs slightly to the right of the PL peak at a longer wavelength. Does this mean that the gain peak happened to coincide with that location and this overrode the lower contribution to PL from lower density of states? Just want to check if I interpreted your answer correctly.
Yes your lasing wavelength will almost always correspond to the gain peak (as long as the cavity mirrors are not spectrally selective). The fact that the PL was lower in this region is likely due to the lower density of states at longer wavelength. More PL doesn't mean more gain! In other words, only a limited number of carriers can occupy these lower energy states and subsequently emit photons but these states are filled or inverted first when you turn up your pump and have thus the highest gain. At shorter wavelength, the density of state is higher giving you a higher PL signal but the gain is not as high because these states are not necessarily inverted. You don't need inversion to observe PL but you do to have gain. I hope this is clearer.
remark/question: What's about the temperature raising of your device ?
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