Development in CMOS-compatible materials for on-chip nonlinear optical devices
Silicon photonics is a system that utilizes silicon as the medium and photons as carriers for communication applications. The operating wavelength of silicon photonic system are typically at 1.55 micormeter which has been used by most fiber telecommunication systems. The components on the silicon photonic are usually patterned in sub-micron precision using technologies that are available from current electronic industry. With the functionalities from variant components including source, coupler, amplifier, modulator and detector, the silicon photonic chip has the capability to carry, modulate, deliver specific optical information efficiently.
Optical modulator is a device that could be used for modulating the shape and amplitude of the light. Modulators can be categorized into phase modulators, polarization modulators and amplitude modulators dependent on the changed parameters, and the modulators can also be divided into absorptive and refractive modulators based on the properties of material that is manipulated. Refractive modulators has the benefits in low loss compared the absorption type modulators, and for silicon photonic field, electro-optic effect is mostly.
Current technique for efficient electro-optic modulator includes the use of Pockels and Kerr effects, which utilizes materials' second- and third-order optical nonlinear properties, respectively. Both effect produces changes of refractive in an optical medium induced by an electric field, while the change from Pockels effect is proportional to the electric field and Kerr effect is proportional to the square of the field. The Pockels effect occurs only in crystals that lack of inversion symmetry while all materials show Kerr effect. Crystal materials like lithium niobate and gallium arsenide exhibit large second-order nonlineaity for Pockels effect, however, these materials are not CMOS compatible and have challenges to be integrated on the current silicon photonic platforms.
Even though silicon and most dielectric materials used in electronic industry inherently has zero second-order nonlinearity for performing Poeckels effect, there are still alternative ways to engineer nonlinearities in these CMOS compatible materials, such as strained technique, electric field induced second-harmonic generation, asymmetry from the interfaces and introduction of silicon nanoclusters. These ways have been already investigated and proved as methods to change and synthesize material's both optical linear and nonlinear properties.
Overall, this thesis presents the contributions to systematical study in the engineered optical nonlinear properties of proposed metamaterials, which utilize different mechanisms. The large and tunable optical nonlinearities of these materials have already shown great potential for efficient nonlinear devices. I believe that with further optimization on the material selection and fabrication, the proposed metamaterials and silicon nitride based materials can be widely used in the fields of on-chip light modulation, switching and light conversion.