Tunable Edge States and Defect States in Metal-Dielectric Photonic Crystals with Organic Semiconductor Dielectric Layers

We investigate the band structure of metal-dielectric photonic crystals comprising stacked organic semiconductor microcavities with silver metal mirrors. Employing organic semiconductor dielectric layers allows the unit cells in the crystal to function as optoelectronic devices including OLEDs and photodetectors, whether addressed individually or collectively. Two geometric variables are presented to tune the band structure and corresponding photonic wave functions within the crystal. In the first case, single defects are introduced into crystals with uniform cavity size. Such defects constitute individual unit cells with aperiodic dimensionality of the organic dielectric layer. The resulting mid-gap defect states are shown to hybridize with a photonic band at certain resonant dimensions. The resonance of the defect cavity affects the transmittance of light through the device, disrupting or enhancing the coupling between otherwise resonant cavities. If the defect is periodic throughout the crystal, the effects on band structure are very different. We introduce a periodic defect to every other metal layer, which doubles the size of the unit cell and has previously been shown to induce a Peierls bandgap. Defining a ratio R of the thickness of the even numbered and odd numbered metal layers, the condition R=1 results in no Peierls gap as the cavities are uniform. Here we demonstrate that by varying that ratio from R<1 to R>1, there is a point where the band collapses resulting in photonic edge states, heavily localized in the two outer-most cavities. The background theory, computational simulation, and experimental verification of these phenomena are presented.