|Vanadium dioxide (VO2) exhibits an insulator-to-metal phase transition and an associated change from a monoclinic (M1) to a rutile (R) crystal structure. Despite fifty years of research, there are important unresolved questions about its insulator-metal transition. However, given the possibility to reversibly modulate physical properties on the picosecond timescale, VO2 might be useful in switching applications and this continues to drive research about this complex material.
This dissertation begins with descriptions of two collaborative projects that contribute to a richer understanding of VO2 thin films. First, unlike previous computational studies, a single set of density-functional theory parameters were found that reproduced the electronic, structural, and magnetic characteristics of the M1 phase, R state, and a third monoclinic insulating phase denoted M2. Second, holographic x-ray imaging with 50-nm resolution provided the first nanoscale insights into the role of defects and the M2 phase in the phase transition in a VO2 thin film.
These studies provide a backdrop for exploring the feasibility of ultrafast all-optical modulators by incorporating VO2 into silicon photonic structures. Excitation of the devices by nanosecond laser pulses demonstrated modulated transmission of a continuous-wave signal, modulation of 4 dB/µm for an absorption modulator, a 3-nm resonance shift of a hybrid VO2-silicon ring resonator along with 7 dB modulation at the resonance wavelength, and a temporal response limited by laser pulse duration (25 ns). Femtosecond laser pulses yielded a detector-limited response of 250 ps from the VO2 device that was not observed in the silicon-only device. An ultrafast pump-probe experimental configuration, with the probe beam in the silicon waveguide, measured the response time of VO2 films embedded in a silicon waveguide. The measured modulation speed of less than 1 ps breaks the current record by a factor of about three. Unlike that device, these VO2-silicon devices are compatible with CMOS fabrication protocols.
In this dissertation, the advantages and drawbacks to a VO2-based modulation platform are discussed in light of these results and some outstanding issues with incorporating VO2-silicon hybrid devices into current optical communications technology are outlined.