Novel materials and optical waveguide systems for silicon photonics

Guider, Romain (2009) Novel materials and optical waveguide systems for silicon photonics. PhD thesis, University of Trento.

PDF (Novel materials and optical waveguide systems for silicon photonics) - Doctoral Thesis


My research in these three years has been mainly focused on the characterization of structural and optical properties of three kinds of Si-based materials and devices. The first one is the study of SiOC thin films, carried within the TMR-POLYCERNET project financed by the European community. The scientific objective of this project is to create molecularly-tailored, nanostructured SiOC ceramics with unusual multifunctional properties, including photoluminescence. In addition these novel, polymer-derived ceramics, or PDCs, will have high resistance to oxidation, degradation, corrosion and deformation at temperatures above 1400°C. The aim of our work is focused on the optical characterization of PDCs. The PDCs are constituted from polymeric precursors which can be tailored and designed at the molecular level. These multifunctional properties are then carried over into the ceramic phase by self-assembly and controlled pyrolysis. Thus, these novel materials are apparently polymer-like in their structure (for example, they are seemingly amorphous but contain nanodomains) but have the chemical, mechanical and functional properties of high temperature ceramics. The photoluminescence will be the most important property that will be studied and optimized. The specific objective of this work is to optimize chemistry and processing to achieve bright emission and high external quantum efficiency in high-quality thin film. In the second chapter of the thesis, starting from a brief description of SiOC glasses and the sol-gel process to introduce our work, we explained how the thin films were prepared and characterised. A study on the absorption coefficient of the films is reported, in order to compare it with results in literature of similar samples. The major part of this chapter was focalized on our work on the photoluminescence of the films. At high annealing temperature, we observed a very high yellow luminescence from the films, most notably due to the presence of SiC nanoclusters and C clusters in our samples. A well detailed discussion on the origin of the strong emission is reported. A study of the effects of Boron addition on the photoluminescence of our thin films was also effectuated and by comparing the evolution of the B-free (SiOC) and B-containing samples (SiBOC), the important role of boron in promoting the evolution of nanostructure in our thin films is described. Finally, to have an idea of the potential of our films, their external quantum efficiency (E.Q.E.) was measured. A detailed description of the measurement technique is reported and the results are compared with Si-nc samples whose EQE is well-known. Very high EQE were found for TH films pyrolysed at 1200 °C (11.5 %) and THDH2 films pyrolysed at 1200 °C (5%). These external quantum efficiency values are very promising and make SiOC a very interesting material for LED applications. Another part of the work was devoted to the study of Si-based waveguides, and more particularly Silicon on Insulator (SOI) waveguides and Slot SOI waveguides. This work was carried out within the European project PHOLOGIC. The general objective of the PHOLOGIC project is to explore the mass-manufacturing feasibility of Silicon Nanocrystals inside SiO2 matrix in terms of CMOS technology compatibility for a highly scalable photonic logic gate structure. A XOR gate was chosen as functional validation device. The third and fourth chapters of this thesis are dedicated to this work. In the third chapter, we characterized various building blocks like splitters, MMI and bends made in Silicon on Insulator technology. The loss figures found for these building blocks were useful as a benchmark for further development of silicon microphotonics components and circuits on SOI platform like photonic crystals and ring resonators. In effect, the results of this chapter are basic to the development of the SCISSOR structures based on SOI technology, described in chapter five. In the fourth chapter, we studied nano-Si slot waveguides. Horizontal slot waveguides filled with Si-nc have been realized and characterized in terms of propagation losses as a function of the layer deposition conditions (i.e. Si excess and annealing temperature). We were able to reach propagation losses as low as 3 dB/cm which is the best result reported so far for slot waveguides of very small width (50 nm). We presented also experimental results of resonant optical cavities such single and double ring resonators coupled to the horizontal slot waveguides with very high quality factors. The importance of this works relies on the fact that by optimizing the annealed SRSO (i.e. Si-nc) in the slot, we have significantly reduced the propagation losses and at the same time we can add new functionalities related to the Si-nc optical properties (i.e. light emission and/or non-linear optical effects). Finally, a one-dimensional photonic crystal structure based on horizontal slot waveguide with a photonic band gap around 1.55 μm has also been designed and optically characterized. Finally, the last part of this thesis will be devoted to the characterization of Silicon on Insulators Multi-Resonators. This work is a continuation of the study of SOI building blocks described above, and was carried within the European project WADIMOS. The main goal of the WADIMOS project is to build a complex photonic interconnect layer incorporating multi-channel microsources, microdetectors and different advanced wavelength routing functions directly integrated with electronic driver circuits. Our work in this project is to test innovative optical waveguide division multiplexing circuits based on coupled ring resonators. In the last part of this thesis, we will characterize various coupled ring/disks resonators structures, from simple double coupled rings until eight coupled ring SCISSOR. In the last chapter, we measured and compared the characteristic of the light propagation of different connection geometries for sequences of microrings (or microdisks) resonators. In this work, we studied various configurations of coupled disk/ring resonators. With these various structures, we observed the differences in the transmission spectrum between rings and disks resonators, we noticed the whispering gallery modes and the effect of the gap in the CRIT effect for a serially double-disks resonator. On a first time, we studied the serially coupled configuration CROW where each ring resonator is coupled to one another. For this structure, we restrict our attention to two ring/disk based units. We observed the differences in the transmission spectrum between rings and disks resonators and we noticed the whispering gallery modes and the effect of the gap in the electromagnetically induced transparency effect for a serially double-disks resonator. The second configuration that was studied is the SCISSOR configuration. In this case, all resonators are coupled to both the input and drop port waveguides. We characterized the behaviour of complex eight-resonators SCISSOR devices in the case of microdisks and microrings resonators. In order to facilitate the characterization of these complex structures, a new set-up was also build up, which allowed us to study the scattered light of the resonators from the top as a function of the wavelength. Finally, with this technique, for the first time, we demonstrated the presence of EIT-like band even in complex structures. Extremely small differences between adjacent rings can give rise to the appearance of EIT states, delocalized over only few rings and with a great Q-factor and strong out-of-plane scattering.

Item Type:Doctoral Thesis (PhD)
Doctoral School:Physics
PhD Cycle:XXII
Subjects:Area 02 - Scienze fisiche > FIS/01 FISICA SPERIMENTALE
Repository Staff approval on:11 Jan 2010 13:48

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