  Context: The Monte Carlo method is probably the most widely used approach to
solve the radiative transfer problem, especially in a general 3D geometry. The
physical processes of emission, absorption, and scattering are easily
incorporated in the Monte Carlo framework. Net stimulated emission, or
absorption with a negative cross section, does not fit this method, however.
Aims: We explore alterations to the standard photon packet life cycle in Monte
Carlo radiative transfer that allow the treatment of net stimulated emission
without loss of generality or efficiency. Methods: We present the explicit
absorption technique that allows net stimulated emission to be handled
efficiently. It uses the scattering rather than the extinction optical depth
along a photon packet's path to randomly select the next interaction location,
and offers a separate, deterministic treatment of absorption. We implemented
the technique in a special-purpose Monte Carlo code for a two-stream 1D
radiative transfer problem and in the fully featured 3D code SKIRT, and we
studied its overall performance using quantitative statistical tests. Results:
Our special-purpose code is capable of recovering the analytical solutions to
the two-stream problem in all regimes, including the one of strong net
stimulated emission. The implementation in SKIRT is straightforward, as the
explicit absorption technique easily combines with the variance reduction and
acceleration techniques already incorporated. In general, explicit absorption
tends to improve the efficiency of the Monte Carlo routine in the regime of net
absorption. Conclusions: Explicit absorption allows the treatment of net
stimulated emission in Monte Carlo radiative transfer, it interfaces smoothly
with... (abridged)
