Contents I. Introduction
Organic planar microcavities (MCs), [1], [2] consist of an active organic layer with a thickness d and a refractive index embedded between two dielectric reflectors with reflectivities . They have attracted considerable attention for device applications because of their high quality factors and small modal volumes V [3] . Organic MCs [4], [5] are especially interesting [6] due to their inhomogeneously broadened emission and gain spectra, thus allowing the design of lasers [7]– [9] tunable over tens of nanometers [10]–[12]. The transmission spectrum of a MC, measured using a collimated beam propagating within the cavity layer along the angle , has spectrally narrow resonances at wavelengths (with the mode index ). The resonances show a symmetric Lorentzian shape as a function of the vacuum wavelength λ and the outcoupling angle described by the Airy formula [13]: \begin{equation} T(\lambda, \theta _c) = \frac{{(1 - R)^2 }}{{(1 - R)^2 + 4R\,\,{\rm sin}^2 (2\pi n_c d\,\,{\rm cos}\,\,\theta _c /\lambda)}} \end{equation} with the half-width and phase \begin{equation} {\rm tan}\,\,\delta _t = \frac{{1 - R}}{{1 + R}}{\rm tan}\left({\frac{{2\pi n_c d{\rm cos}\theta _c }}{\lambda }} \right). \end{equation}
