RESUMO
The optical properties of photonic crystals (PhCs) are strongly affected by their spatial symmetry characteristics. We observe anticrossing phenomena for large-area slab-type silicon PhCs sandwiched between a glass substrate and air. If a glass superstrate plus an index-matching fluid is added, thus establishing a mirror symmetry in z-direction, the anticrossing disappears. These characteristics are analyzed numerically using a finite-element Maxwell solver, and experimentally using large area samples made by nanoimprint lithography. We further discuss the findings by symmetry considerations.
RESUMO
We numerically study coupling of light into silicon (Si) on glass using different square and hexagonal sinusoidal nanotextures. After describing sinusoidal nanotextures mathematically, we investigate how their design affects coupling of light into Si using a rigorous solver of Maxwell's equations. We discuss nanotextures with periods between 350 nm and 1050 nm and aspect ratios up to 0.5. The maximally observed gain in the maximal achievable photocurrent density coupled into the Si absorber is 7.0 mA/cm2 and 3.6 mA/cm2 for a layer stack without and with additional antireflective silicon nitride layers, respectively. A promising application is the use as smooth anti-reflective coatings in liquid-phase crystallized Si thin-film solar cells.
RESUMO
We report on an experimental and theoretical investigation of an integrated Bragg-like grating coupler for near-vertical scattering of light from photonic crystal waveguides with an ultra-small footprint of a few lattice constants only. Using frequency-resolved measurements, we find the directional properties of the scattered radiation and prove that the coupler shows a good performance over the complete photonic bandgap. The results compare well to analytical considerations regarding 1d-scattering phenomena as well as to FDTD simulations.