We have observed the near-field emission patterns of a number of GaAs diode lasers. The onset of laser action is accompanied by an abrupt increase in the intensity of light emission at one or several spots along the junction. The adsorption of a dielectric film at a constant rate on one or both surfaces of a laser causes the threshold current to vary periodically with time. The position of the threshold spot did not vary as long as the film was of uniform thickness along the junction; however, the emission pattern could be altered by a suitable nonuniform film. The variation of the threshold current with film thickness can be fitted by a simple model that assumes the gain in the active region is proportional to the current density. Calculated values for the loss and gain factors ranged from 10 to 60 cm-1 and 0.5 to 4×10-2 cm/A, respectively. © 1964 The American Institute of Physics.