Near-field characterization of Bloch surface waves based 2D optical components
Richa Dubey
2017
Bloch surface waves (BSWs) are surface electromagnetic modes that propagate at the interface between a multilayer substrate and a homogeneous external medium. The optical field of the surface mode is confined near the surface of the multilayer. This vertical confinement as well as the low absorption inherent to the dielectric materials make the BSWs an interesting candidate for the development of 2D optical systems and sensors. Such a periodic multilayer structure is introduced as a platform on
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... which many optical functions can therefore be created. In this thesis, two-dimensional optical components based on the Bloch wave platform are studied, in particular: a disk resonator, a Bessel-like beam generator and a waveguide grating as a Bragg mirror engraved in a waveguide. The optical properties of the components such as the resonance inside the disc, the "quasi non-diffracting" behavior and the reflection properties are presented. The 2D optical components are designed from a commercial FDTD program (CST Microwave studio). They are then characterized by a near-field scanning microscope with multiheterodyne detection (MH-SNOM). Thanks to the MH-SNOM, it is possible to map the field distribution locally at the surface of the structures with a resolution lower than the wavelength. Simultaneous measurement of amplitude and phase allows a detailed reconstruction of the complex amplitude of the electric field. In a first part, the influence of a device layer of material with a high refractive index (TiO2) is studied. The impact of the thickness of the TiO2 layer on the propagation properties of the BSWs is presented. It is demonstrated that by adapting the thickness of the device layer, the BSW dispersion curve position can be moved within the photonic band gap and consequently the BSW mode propagation properties can be adapted. The propagation properties of the BSWs include, for example, the propagation length and the effective refractive index. Thanks to the low losses and the design of our multilayer platform, propagation lengths in the range of millimeter are obtained. In a second part, 2D optical components fabricated in a 60 nm (λ/25) device layer of TiO2 are presented, and initially, disk resonators. The latter are key elements in integrated optics systems. For a disk with a radius of 100 μm, an experimental quality factor of the order of 10 3 is obtained.
doi:10.5075/epfl-thesis-7870
fatcat:oxspmz3tj5dd5oogbt7x5hhtvu