Theoretical and experimental evaluation of multilayer porous silicon   structures for enhanced erbium up-conversion luminescence
  The enhancement of Er$^{3+}$-based up-conversion for photovoltaics in multilayer porous silicon photonic structures is considered theoretically and experimentally. Transfer matrix simulations are used to assess the increased photonic density of states that results from the slowing of energy propagation at the short-wavelength edge of one-dimensional photonic band gaps. An indirect calculation of Er$^{3+}$ absorption enhancement within slow-light modes is then used to illustrate an increase in absorption over the bulk value: the effective absorption coefficient is shown to increase by more than 22% over a broad spectral region and by more than 400% over a narrow region. Erbium-doped porous silicon photonic crystals are fabricated with the photonic band edge coincident with the $^{4}I_{15/2} \rightarrow^{4}I_{13/2}$ Er$^{3+}$ transition. Challenges in fabrication and the results of compositional analysis are discussed. An angular-dependent photoluminescence measurement demonstrates emission intensity that varies non-monotonically with the position of the photonic band edge. A maximum of 26.6$\times$ enhancement of Er$^{3+}$ emission intensity is observed for the 550-nm transition, with lower enhancement factors seen for longer wavelengths. 
Craig M. Johnson, Peter J. Reece and Gavin J. Conibeer
physics.optics