Experiments and modelling of vertically coupled Microresonators

Microresonators are fundamental building blocks in the growing field of integrated photonics and several resonator-based devices such as filters, switches and routers are currently used in common optical telecommunication networks. In order to exploit the peculiar features offered by integrated resonators, a complete and consistent comprehension of their physics and of the processes they can accommodate is needed. More specifically, coupling of light to and from a resonator represents a crucial point: a correct comprehension of the coupling dynamics, a proper model for the system and its validation through experimental procedure are all essential elements for a fruitful exploitation of the device. Among the different resonator-waveguide coupling schemes, the most widely used is the in-plane coupling and it consists of a waveguide placed near to a microresonator and laying on the same plane. However, an alternative approach is represented by the vertical coupling scheme, where the waveguide lays under the resonator edge. The peculiar position of the waveguide in this last configuration causes the device to show specific properties not present in other common coupling schemes: namely, a working range spanning from almost visible wavelengths (780nm) to the near IR domain (1600nm), the selective excitation of high order resonator radial modes and the possibility to fabricate wedge and free-standing resonators without any detrimental effect on the bus waveguide. In order to fully exploit these and other features of the vertical coupling scheme, a detailed investigation has been carried out throughout this thesis. The waveguide-microresonator system has been studied at different levels, from the general coupling dynamics to more specific and peculiar phenomena. In particular, the basic model proposed for the vertical coupling has been extended to consider wavelength dependences and an experimental validation has been carried out consequently. The reactive coupling model, which describes the internal dynamics of a vertically coupled resonator in the case of multimodal operation, has been experimentally proven. A general model considering the presence counterpropagating modes has been theoretically proposed and experimentally investigated. Finally, the bistable behaviour generated by thermo-optic effect when a large amount of power circulates in the microresonator has been experimentally studied. In order to better characterize the system response, a specific interferometric setup has been implemented. It consists in a Mach-Zehnder computer driven interferometer, whose peculiar characteristic is the ability to perform simultaneous pump and probe transmittance and phase measurements of any integrated photonic device provided with input and output ports. In this way, the information carried by the phase of the propagating optical signal is added to the one provided by its intensity and contributes to produce a more complete picture of the investigated system. In the case of microresonators this phase information becomes even more fundamental. Indeed, the phase response of a resonating structure is highly influenced by variations in the coupling strength, and the phase spectrum of a single resonance allows to clearly identify the resonator coupling regime for that specific resonance. This fact does not hold in the case of transmittance measurements, where single resonance spectrum carries information only on the total losses of the system. Finally, in order to exploit the combined information provided by this measurement procedure, a phasor plot representation is extensively used throughout the thesis work.