Photoluminescence and radiative-rate modifications in 1D silicon photonic crystals infiltrated with luminescent conjugated polymers

We report the modification of the photoluminescence (PL) of one-dimensional (1D) porous silicon photonic crystals (PS-PhCs) infiltrated with luminescent conjugated polymers. First, we used an aqueous HF-based electrochemical etch of p++-silicon substrates to prepare rugate filters, a special case of 1D PS-PhCs [1]. These are nanostructured systems featuring quasi-cylindrical vertical cavities whose diameter (a few hundreds nanometres) along the direction perpendicular to the substrate surface (“z-direction”) is modulated by varying the sinusoidal etching current. Structures with sinusoidally-shaped periodic porosity (between 55.3% and 62.3%) and, in turn, with sinusoidally-shaped refractive index (maximum contrast of about 0.25) are produced. We then infiltrated PS-PhCs prepared with different period repetitions (25 to 100), with either F8BT (poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt- (benzo[2,1,3]thiadiazol-4,8-diyl)]) or MDMO-PPV (poly[2-methoxy-5-(3′,7′- dimethyloctyloxy)-1,4-phenylenevinylene]). We measured reflectance, photoluminescence (PL), and time-resolved PL of both bare and infiltrated PS-PhCs. We found that the photonic stop-band is red-shifted upon polymer infiltration, as expected due to reduced refractive index contrast of the polymer-infiltrated nanostructured system [2]. We also observed both suppression (in the stop-band) and enhancement (at the band-edge) of the photoluminescence. Remarkably, time-resolved measurements reveal a modification of the emission lifetime, which is enhanced at the band- edge and suppressed within the stop-band, and clearly point to a variation of the radiative decay rate of excitations in such hybrid organic-inorganic photonic nanostructures. Keywords [1] A. M. Ruminski, et al., Adv. Funct. Mat., 18, 3418-3426 (2008) [2] G. Barillaro, et al., Opt. Lett., 34, 1912-1914 (2009) [3] L. Berti, et al., J. Phys. Chem. C., 114, 2403-2413, (2010)