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We study amplified spontaneous emission (ASE) from wavelength-scale compositebodies--complicated arrangements of active and passive media--demonstratinghighly directional and tunable radiation patterns, depending strongly on pumpconditions, materials, and object shapes. For instance, we show that underlarge enough gain, $\mathcal{PT}$ symmetric dielectric spheres radiate mostlyalong either active or passive regions, depending on the gain distribution. Ourpredictions are based on a recently proposed fluctuating volume--current (FVC)formulation of electromagnetic radiation that can handle inhomogeneities in thedielectric and fluctuation statistics of active media, e.g. arising from thepresence of non-uniform pump or material properties, which we exploit todemonstrate an approach to modelling ASE in regimes where Purcell effect (PE)has a significant impact on the gain, leading to spatial dispersion and/orchanges in power requirements. The nonlinear feedback of PE on the activemedium, captured by the Maxwell--Bloch equations but often ignored in linearformulations of ASE, is introduced into our linear framework by aself-consistent renormalization of the (dressed) gain parameters, requiring thesolution of a large system of nonlinear equations involving many linearscattering calculations.

Amplified and directional spontaneous emission from arbitrary composite bodies: self-consistent treatment of Purcell effect below threshold