Impact of planar microcavity effects on light extraction - Part I: Basic concepts and analytical trends

We address the long-standing issue of extracting light as efficiently as possible from a high-index material, n greater than or equal to 2, where as little as 2%-10% of light not suffering total internal reflection is extracted at standard plane faces due to the small critical angle similar to 1/n. Using a planar microcavity to redirect spontaneous emission toward the surface constructive interferences can bring 15%-50% of the light out, enhancing brightness and efficiency. In this first of two papers, an approximate approach is used showing the importance of small cavity order m(e) and of the m(c)/n(2) ratio. We define a condition for microcavity regime as m(c)<2n(2). It is shown that most of light extraction is usually attained for moderate mirror reflectivities similar to 1 - m(c)/n(2) typically below 90%, and without strong directionality. Balance between emission directionality, radiance (brightness), and spectral narrowing is discussed. We define a brightness enhancement factor B given by Bm(c)Delta Omega = 4 pi where Delta Omega is the largest internal solid angle of either the cavity mode or that deduced from the material emission linewidth. Design rules are applied to distributed dielectric mirrors yielding an optimal number of periods. the underlying physical competition between emission into guided modes, Fabry-Perot modes and the so-called "leaky modes" is analyzed.