Densely integrated photonic structures for on-chip signal processing
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Microelectronics has enjoyed great success in the past century. As the technology node progresses, the complementary metal-oxide-semiconductor scaling has already reached a wall, and serious challenges in high-bandwidth interconnects and fast-speed signal processing arise. The incorporation of photonics to microelectronics provides potential solutions. The theme of this thesis is focused on the novel applications of travelling-wave microresonators such as microdisks and microrings for the on-chip optical interconnects and signal processing. Challenges arising from these applications including theoretical and experimental ones are addressed. On the theoretical aspect, a modified version of coupled mode theory is offered for the TM-polarization in high index contrast material systems. Through numerical comparisons, it is shown that our modified coupled mode theory is more accurate than all the existing ones. The coupling-induced phase responses are also studied, which is of critical importance to coupled-resonator structures. Different coupling structures are studied by a customized numerical code, revealing that the phase response of symmetric couplers with the symmetry about the wave propagating direction can be simply estimated while the one of asymmetric couplers is more complicated. Mode splitting and scattering loss, which are two important features commonly observed in the spectrum of high-Q microresonators, are also investigated. Our review of the existing analytical approaches shows that they have only achieved partial success. Especially, different models have been proposed for several distinct regimes and cannot be reconciled. In this thesis, a unified approach is developed for the general case to achieve a complete understanding of these two effects. On the experimental aspect, we first develop a new fabrication recipe with a focus on the accurate dimensional control and low-loss performance. HSQ is employed as the electron-beam resist, and the lithography and plasma etching steps are both optimized to achieve vertical and smooth sidewalls. A third-order temperature-insensitive coupled-resonator filter is designed and demonstrated in the silicon-on-insulator (SOI) platform, which serves as a critical building block element in terabit/s on-chip networks. Two design challenges, i.e., a broadband flat-band response and a temperature-insensitive design, are coherently addressed by employing the redundant bandwidth of the filter channel caused by the dispersion as thermal guard band. As a result, the filter can accommodate 21 WDM channels with a data rate up to 100 gigabit/s per wavelength channel, while providing a sufficient thermal guard band to tolerate more than ±15°C temperature fluctuations in the on-chip environment. In this thesis, high-Q microdisk resonators are also proposed to be used as low-loss delay lines for narrowband filters. Pulley coupling scheme is used to selectively couple to one of the radial modes of the microdisk and also to achieve a strong coupling. A first-order tunable narrowband filter based on the microdisk-based delay line is experimentally demonstrated in an SOI platform, which shows a tunable bandwidth from 4.1 GHz to 0.47 GHz with an overall size of 0.05 mm². Finally, to address the challenges for the resonator-based delay lines encountered in the SOI platform, we propose to vertically integrate silicon nitride to the SOI platform, which can potentially have significantly lower propagation loss and higher power handling capability. High-Q silicon nitride microresonators are demonstrated; especially, microresonators with a 16 million intrinsic Q and a moderate size of 240 µm radius are realized, which is one order of magnitude improvement compared to what can be achieved in the SOI platform using the same fabrication technology. We have also successfully grown silicon nitride on top of SOI and a good coupling has been achieved between the silicon nitride and the silicon layers.