Creation of new lasing modes with spatially nonuniform gain

We report on the creation of new lasing modes with spatially nonuniform profiles of optical gain in a one-dimensional random structure. It is demonstrated numerically that even without gain saturation and mode competition, the spatial nonuniformity of gain can cause dramatic and complicated changes. New lasing modes appear with frequencies between those of the lasing modes with uniform gain. We examine new modes in detail and find that they exhibit high output directionality. Our results show that random lasing properties may be modified significantly without changing the underlying structure.