Spontaneous emission extraction and Purcell enhancement from thin-film 2-D photonic crystals

Electromagnetic band structure can produce either an enhancement or a suppression of spontaneous emission from two-dimensional (2-D) photonic crystal thin films. We believe that such effects might be important for light emitting diodes. Our experiments were based on thin-film InGaAs-InP 2-D photonic crystals at ambient temperature, but the concepts would apply equally to InGaN thin films, for example. We show that the magnitude of Purcell enhancement factor, F_p~2, for spatially extended band modes, is similar to that for a tiny mode in a three dimensional (3-D) nanocavity. Nonetheless, light extraction enhancement that arises from Zone folding or Bragg scattering of the photonic bands is probably the more important effect, and an external quantum efficiency >50% is possible. Angle resolved photoluminescence from inside the photonic crystal gives a direct spectral readout of the internal 2-D photonic band dispersion. The tradeoffs for employing various photonic crystal structures in high efficiency light-emitting diodes are analyzed