Optical Coherence of Planar Microcavity Emission

An analytical expression for the self coherence function of a microcavity and partially coherent source is derived from first principles in terms of the component self coherence functions. Excellent agreement between the model and experimental measurements of two Resonant Cavity LEDs (RCLEDs) is evident. The variation of coherence length as a function of numerical aperture is also described by the model. This is explained by a microcavity's angular sensitivity in filtering out statistical fluctuations of the underlying light source. It is further demonstrated that the variable coherence properties of planar microcavities can be designed by controlling the underlying coherences of microcavity and emitter whereby coherence lengths ranging over nearly an order of magnitude could be achieved. The last two decades have seen widespread use of optical microcavities, both for experimental physics and commercial applications. Microcavities redistribute emission from an underlying source and depending on the ensuing radiation pattern allow light collection for use elsewhere. Commercially available microcavity devices such as Resonant Cavity Light Emitting Diodes (RCLEDs) use the planar microcavity geometry to increase the extraction efficiency of spontaneous emission from materials with high dielectric constants. 1 More recently microcavities have been used to spectrally and spatially isolate quantum dot emitters to increase the efficiency of single photon production. Recent work on planar microcavities has identified the dependence of Numerical Aperture (NA) on emission properties such as spectral linewidth 4,5,6 and coherence length, 7 the latter of which is the focus of the following paper. Note that, migration of these results to the spectral domain are trivial due to the implicit Fourier relationship with the coherence domain. In addition to the choice of domain, the name coherence has been used, instead of time. Coherence highlights the statistical properties of light and not necessarily to the time dependence of the emission process that produces the light. This distinction between the light and the emission event that produced it is necessary for the descriptions presented in this letter. The coherence length is a relevant attribute of any optical device as it defines the length scales over which mutual interference occurs. Applications such as Low Coherence Interferometry, for non invasive medical imaging 8 (also known as Optical Coherence Tomography) and Optical Time Domain Reflectometry (OTDR), for ranging measurements in optical components 9 and surface mapping in integrated circuits 10 , rely on the coherence of source being both small enough to eliminate coherent reflections …