Symmetry between absorption and amplification in disordered media

We address the issue of whether amplification, like absorption, suppresses wave transmission at large gain, as has been claimed in previous studies of wave propagation in active random media. A closer examination reveals that the paradoxical symmetry between absorption and amplification is an artifa ct of unphysical solutions from the time-independent wave equation. Solutio ns from the time-dependent equation demonstrate clearly that when gain is a bove the threshold, the amplitude of both the transmitted and the reflected wave actually increases with time, apparently without bound. The implicati ons of the current finding is discussed. [S0163-1829(99)50314-7].