Ultra-smooth lithium niobate photonic micro-structures by surface tension reshaping.

Annealing of micro-structured lithium niobate substrates at temperatures close to, but below the melting point, allows surface tension to reshape preferentially melted surface zones of the crystal. The reshaped surface re-crystallizes upon cooling to form a single crystal again as it is seeded by the bulk which remains solid throughout the process. This procedure yields ultra-smooth single crystal superstructures suitable for the fabrication of photonic micro-components with low scattering loss.

Time delay distribution in Bragg gratings.

A layer-by-layer analysis of the time delay of both reflected and transmitted light in one-dimensional photonic band-gap structures is developed and applied to uniform Bragg gratings. An effective Fabry-Pérot cavity is associated with every layer along the Bragg grating, multiple paths with a well defined layer traversal time are identified, and the average time is computed, introducing an appropriate weighting factor that accounts for interference between different paths. The analysis presented leads directly to a complex-valued time delay whose real part is shown to be equivalent to the classic phase time delay. Physical meaning is also given to the imaginary part. The local dwell time, interpreted as the average time spent by light in the layer independently of the final (transmitted or reflected) state, is proved analytically to be related to the energy density distribution when small index change gratings are considered. The time delay evolution is derived at different wavelengths and the nonuniform distribution along the grating is discussed. Nonintuitive features such as superluminal transmission time delay for propagation inside the band gap and negative reflection time delay close to transmission resonances are addressed. Finally, the effect of introducing a small perturbation in the structure is shown to be directly related to the local time delay and is proposed as a possible experimental measurement scheme for both its real and imaginary parts.

Effect of periodic background loss on grating spectra.

The effect of periodic loss on the performance of refractive-index gratings has been studied in detail. It is shown that the loss periodicity and relative phase strongly affects the symmetry of the reflection, transmission, and loss spectra. This asymmetry is explained successfully through consideration of the overlap between the standing-wave intensity distribution and the periodic loss pattern.