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Optical near-field characterization of Slow-Bloch Mode based photonic crystal devices

Abstract : 2D-Photonic crystal (PC) structures have enabled the fabrication of a wide variety of nanophotonic components. In perfect PCs, the exploitation of the enhanced local density of states at critical points of the band diagram has attracted considerable attention. Near these points, where the group velocity vanished, low curvature flat bands give rise to delocalized and stationary optical slow Bloch modes (or slow light modes). Properties of slow light make them good candidates to enhance Purcell or various non-linear effects or to design low-threshold lasers. Among these modes, slow Bloch modes (SBMs) emitting in the vertical direction, i.e. located at the Γ- point of the Brillouin zone are particularly interesting for integrating 2D PC architectures with free space optics. In particular, some SBMs proved to be suitable for achieving strong vertical emission with peculiar polarization properties. Other promising applications concern disorder: by introducing a controlled randomness into the PC structure, it is possible to induce a transition from slow Bloch mode (in ordered PC) to Anderson’s localization (in disordered PC) as a function of disorder degree. In this PhD dissertation, Slow Bloch modes have been studied and characterized by the means of Near-field Scanning Optical microscopy (NSOM). We particularly focused on Slow Bloch laser mode at Γ- point of a honeycomb 2DPC. This NSOM technique enables to visualize the evanescent component of the mode with a spatial resolution below the diffraction limit. In this work, we showed that the far-field and the near-field image of the mode at the 2D-PC surface are different and that near-field results yield a better insight in the real mode structure inside the PC slab in agreement with theoretical prediction. The importance of the probe selection (bare silica, metallized tip and bow-tie aperture nanoantenna) for studying III-V photonic crystal structures was also demonstrated. Besides intensity measurement of the electromagnetic field, the polarization of the electric field has been measured at the nanoscale for the first time by using a bow-tie nano-antenna probe. These results enable the unambiguous identification of the modes with the 3D-FDTD simulations.In this work is also reported the first observation of two-dimensional localization of light in two types of 2D random photonic crystal lasers, where Slow Bloch Mode (SBM) is scattered by artificial structural randomness in triangular PCs. The structural randomness is introduced whether by nanometer displacements in the positions of lattice elements (air holes), whether by variation of the hole diameters. The direct near-field imaging of the lasing mode by use of NSOM for the first time, allowed us to observe the transition of the extended planar SBM to be Anderson localized.
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Thanh Phong Vo. Optical near-field characterization of Slow-Bloch Mode based photonic crystal devices. Other. Ecole Centrale de Lyon, 2011. English. ⟨NNT : 2011ECDL0024⟩. ⟨tel-00758323⟩

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