Creation of New Lasing Modes with Spatially Nonuniform Gain

We report on the creation of new lasing modes with spatially nonuniform distributions 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 of lasing modes. New modes appear with frequencies in between those of the lasing modes with uniform gain. We examine some new lasing modes in detail and find they exhibit high output directionality. Our results show that the random lasing properties may be modified significantly without changing the underlying structures.