Threshold dependency on photon energy in GaAs laser diodes

Abstract

This paper shows the relationship between threshold current density and photon energy for GaAs laser diodes at a given temperature. In all measurements taken, the laser photon energy was obtained at the leading edge of the pulse of a time-resolved spectrum in order to eliminate heating effects. When the spectrum is measured in this way, the photon energy values differ only slightly for laser diodes fabricated from the same region of the GaAs wafer. This difference arises from a second-order spectroscopic effect. This spectroscopic effect is manifested as a first-order effect in the laser characteristics, because the absorption curves change rapidly in the spectral region in which the laser radiation falls. It is shown that the threshold increases with increasing laser photon energy at 77°K. Similar data are presented for a temperature of 300°K. A model is presented to explain these data. This model is based upon the Heisenberg uncertainty principle in which the laser photon oscillation spreads out into the non-inverted p- and n-type regions surrounding the inverted population region of the f- n junction. Thus, lasers exhibiting higher photon energy at a given temperature show greater cavity losses. Because the p-type absorption coefficient is usually considerably greater than that of the n-type, the latter is neglected. The increased absorption in the p-region is considered the dominant factor causing lower cavity Q.