Novel Electro-Optic Modulator for Silicon Nitride Waveguides

Under the guidance of Moore’s Law, the number of transistors on a central processing unit has doubled every two years for over 50 years, but the ever-growing demand for data processing, computational performance, and the general advancement of the capabilities of technology has pushed conventional computing systems, which are limited by the von Neumann bottleneck, to a digital efficiency wall of performance. Optical signal processing and neural network architectures present a possible solution to overcoming the limitations of conventional computing systems, given that computationally heavy Fourier-space computations are trivial in the optical domain. Spatial light modulators play a key role in an all-optical system and are required to perform to a highly-efficient level with the desire to push the dimensions close to that of microelectronic systems. Electro-optic modulators, a branch of spatial light modulators, operate on the principle that the optical property of a material may be controlled with electrical gating. Indium tin oxide has been widely identified as a suitable material, due to the plasma dispersion effect exhibited in the material. Typically, electro-optic modulators utilise a silicon platform to operate in this wavelength range, but in this work, I propose to use a silicon nitride platform and take advantage of the low-loss operation and high-power throughput of the platform. In this work, I show that a competitive phase modulator can be realised using a metal-oxide-semiconductor capacitor to modulate the local permittivity in an indium tin oxide layer, the change in permittivity is overlapped with a guided mode in a silicon nitride waveguide platform and Mach-Zehnder Interferometer architecture to produce a phase change. I have demonstrated a half-wave voltage-length between 0.5-1.0Vmm in the visible spectrum, achieved with a dual-mode interaction within the modulator. The simulations and experiments in this work demonstrate the capabilities of indium tin oxide as a material for electro-optic modulation, and the suitability of silicon nitride as a platform in the wider field of spatial light modulators. I also demonstrate that the electrical thickness of the insulating material in the capacitor may be reduced without affecting the optical behaviour of the modulator, as has been hinted to in the literature.