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Hyperbolic metamaterials for high-speed optical communications


Hyperbolic metamaterials are a class of artificial optical media characterized by anisotropic light dispersion. Their unprecedented properties, unattainable with natural materials, can be controlled and tuned over the entire optical range with careful design of their subwavelength units. Among the applications enabled by hyperbolic media, spontaneous emission engineering occupies a prominent position for optical communications. Coupling of an incoherent optical source with a hyperbolic nanostructure results in shortened spontaneous emission lifetime, corresponding to increased modulation bandwidth. This thesis explores solutions to enhance the spontaneous emission, the output power, the directionality and the electronic integrability of emitter-hyperbolic medium systems. After a solid compendium of the hyperbolic metamaterials field is provided, a method to enhance the Purcell factor of a multilayer Si/Ag hyperbolic medium is proposed. Moving the emitter location from the outside to the inside of the multilayer results in a 300-fold Purcell enhancement at visible frequencies, and up to 10-fold power enhancement with respect to free space is obtained by means of 1D gratings. The study then focuses on improving the directional control of power outcoupling, either by shaping the hyperbolic medium into a cylindrical nanoantenna, or by introducing a novel paradigm based on dispersion engineering, which lifts the need for gratings or nanostructures. Design guidelines are provided to achieve emission from the top or from the side of an unpatterned hyperbolic block, and a dipole-hyperbolic block system is optimized for lateral outcoupling at λ0 = 530 nm. Finally, the integration of a plasmonic nanostructure (which can be seen as a special case of hyperbolic nanostructure with 100% metallic component) with an InGaN/GaN LED is addressed. The proposed architecture enables simultaneous high-speed modulation and efficient light emission while preserving effective electrical excitation. Different plasmonic LED structures operating at λ0 = 450 nm are fabricated, and characterized with time-resolved photoluminescence. A limit 2 GHz modulation bandwidth is predicted based on lifetime measurements. A subsequent cross-sectional analysis identifies the nanostructure geometry that maximizes light extraction, providing a clear guideline for future fabrication and contact integration.

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