  A large body of work has accumulated over the years in the study of the
optical properties of single and coupled microcavities for a variety of
applications, ranging from filters to sensors and lasers. The focus has been
mostly on the geometry of individual resonators and/or on their combination in
arrangements often referred to as photonic molecules (PMs).
  Our primary concern will be the lasing properties of PMs as ideal candidates
for the fabrication of integrated microlasers, photonic molecule lasers.
Whereas most calculations on PM lasers have been based on cold-cavity (passive)
modes, i.e. quasi-bound states, a recently formulated steady-state ab initio
laser theory (SALT) offers the possibility to take into account the spectral
properties of the underlying gain transition, its position and linewidth, as
well as incorporating an arbitrary pump profile. We will combine two
theoretical approaches to characterize the lasing properties of PM lasers: for
two-dimensional systems, the generalized Lorenz-Mie theory will obtain the
resonant modes of the coupled molecules in an active medium described by SALT.
Not only is then the theoretical description more complete, the use of an
active medium provides additional parameters to control, engineer and harness
the lasing properties of PM lasers for ultra-low threshold and directional
single-mode emission.
