| Περίληψη: | The topic of the current dissertation is the proposal, design, and study of monolithically integrated nanophotonic elements exploiting the extraordinary properties of contemporary 2D materials for switching applications and light sources in the NIR. The nanophotonic structures under study are based on two distinct constituent entities or structural blocks. The first one, is the highly-favorable light-guiding properties and the technological maturity of the silicon-on-insulator (SOI) and silicon-rich-nitride-on-insulator (SRNOI) technological platforms, which enable the realization of monolithically integrated nanophotonic resonance cavities supporting tightly-confined and high quality-factor modes. The main focus has been concentrated on guided-wave whispering-gallery mode (WGM) disk resonators coupled with appropriate access waveguides, facilitating their on-chip integration with other optical components. The second constituent entity is the exceptional nonlinear and luminescence properties of graphene monolayers and TMD hetero-bilayers, respectively, complemented by their compatibility with the SOI and SRNOI platforms. Taking advantage of both the aforementioned structural blocks, we have proposed and assessed the response of small-footprint, all-optical routing elements and lasing sources of both CW and pulsed operation, characterized by highly-favorable performance metrics, such as very low power requirements, high bandwidth, short pulse duration, and increased repetition rates. All the proposed resonance configurations have been meticulously designed utilizing general design directives that have been derived throughout the dissertation, aiming at enhancing the performance metrics as well as tailoring the interplay between various nonlinear effects. Furthermore, the proposed routing and lasing elements have been properly examined accounting for accurate microscopic descriptions of both graphene and TMD hetero-bilayers, while considering all the underlying physical phenomena of both the sheet- and bulk-type materials. A concurrent and complementary target of this dissertation constitutes the development of robust, rigorous, and efficient computational methods for the accurate evaluation of the overall response of the proposed nanophotonic structures, as well as the acquisition of deeper physical insight and the extraction of general design directives. The computational methods developed in this dissertation are based on the temporal coupled-mode theory (CMT) and first-order perturbation theory fed by linear finite element method (FEM) simulations. Initially, the focus of this dissertation has been concentrated on leveraging the low-power and ultrafast saturable absorption (SA) effect of graphene for all-optical control of light in a practical structure based on a SOI disk resonator. The SA effect is described by a phenomenological microscopic model associating the surface conductivity of graphene to the photogenerated electron density in the conduction band of the sheet material. This description enables the examination of both the SA finite relaxation time and carrier diffusion effects on the collective nonlinear response of the material, the influence of which has been found that affect both the power requirements and the temporal response of the overall nanophotonic structure. Next, the research study focuses on integrated nanophotonic lasers utilizing TMD hetero-bilayers as gain media. Starting from the fundamental Maxwell-Bloch equations, an accurate and efficient framework for nanophotonic resonators with gain, based on CMT, is rigorously and meticulously derived using a limited number of essential and widely used approximations of the laser theory. The developed framework is applicable to ``class C'' lasers and can be used for any nanophotonic laser with arbitrary cavity geometry and underlying materials encompassing either conventional bulk-type or contemporary sheet-type gain media. Subsequently, an integrated lasing element based on a SRNOI disk resonator overlaid by the MoS2/WSe2 TMD hetero-bilayer is proposed, designed and numerically analyzed in full. The proposed lasing source is characterized by single-mode operation, low threshold of 1.3 mW, and an appreciable total efficiency of 1.7%. In the last part of the dissertation, the research interest is steered towards the vastly unexplored field of integrated nanophotonic pulsed lasers. Combining the light-emitting properties of MoS2/WSe2 and the SA effect of graphene, a passively Q-switched lasing element in the NIR is proposed, designed, and theoretically studied. The Q-switched operation is attainable for a wide range of pump power levels spanning from just above the lasing threshold (24.2 μW) up to 3.5 mW. The Q-switched pulses are characterized by mW peak power, ps duration, and repetition rates up to tens of GHz, accompanied by very low pump power requirements. |
| Σημειώσεις: | Committee members:
Kriezis Emmanouil, Gioultsis Traianos, Kantartzis Nikolaos, Rekanos Ioannis,
Papadopoulos Ioannis, Tsilipakos Odysseas, Zografopoulos Dimitrios.
Degree Grantor: Aristotle University Of Thessaloniki (AUTH) |
