The dynamics of electron‐beam‐pumped semiconductor lasers are strongly affected by transient waveguiding. We have investigated the impact of such effects on the time delay from the start of the pumping pulse to the onset of laser emission in GaAs at liquid‐nitrogen temperatures. Time delays were measured as a function of pump current density for three values of the electron‐beam energy (20, 30, and 40 keV). Delays up to 200 nsec were observed which are approximately two orders of magnitude larger than the minority‐carrier lifetime. A theoretical model is presented for the time‐dependent variations of the optical confinement to account for such long delays. The time delay to threshold is determined not by the gain buildup but by the time required to sufficiently decrease the laser‐mode diffraction loss in the active layer. Initially, the presence of gain results in a negative change of the refractive index with respect to the passive material. This is opposed by a growing positive index contribution due to the rise in temperature of the pumped layer. The latter eventually dominates, thereby confining the cavity modes. Under appropriate conditions, the ensuing reduction in loss determines the onset of lasing. The importance of the waveguide effect was confirmed by measurements on devices incorporating a permanent index step at the active‐passive region interface.
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