Photoinduced Second-Order Nonlinearities in Centrosymmetric Media

The interaction of light and matter enables nonlinear frequency conversion and the creation of coherent currents. The optical control of electric currents is of fundamental relevance and prominent research focus in the last decades. These photocurrents enable efficient light frequency doubling in a centrosymmetric medium, where two photons are combined to create a photon of doubled frequency interacting with electric dipoles inside a non-centrosymmetric medium. Recently, this has been demonstrated in integrated photonics platforms comprising amorphous materials where the coherent photogalvanic effect generates photocurrent raised by quantum interference of multiphoton absorption. While this has attracted considerable attention due to fundamental interest as well as potential applications in communications, spectroscopy, and quantum optics, the underlying physics and fundamental bounds remained unexplored. This thesis investigates the coherent photogalvanic effect in optical waveguides theoretically and experimentally shows generalized sum-frequency generation, and backward second-harmonic generation, as well as analyzes the dynamics of frequency doubling. The developed model of photoinduced second-harmonic generation establishes a general framework to compare different material platforms for frequency doubling which is tested in the different silicon concentrations of silicon nitride waveguides.

In three-wave mixing processes, such as frequency doubling or sum-frequency generation, efficient energy transfer requires the momenta of the involving photons to be matched, else, the dipoles should be organized in a sign-alternating manner to compensate for the momentum mismatch. We predict that injecting phase-locked waves satisfying energy conservation of the targeted three-wave mixing process in the waveguide, results in a periodic charge separation providing the necessary symmetry breaking in centrosymmetric media. We experimentally demonstrate it in silicon nitride by measuring the time-growth of sum-frequency light and imaging the dipoles in the optically inscribed structure. Meanwhile, absorption of light increases the conductivity which can limit the process, providing a complex interplay. We theoretically solve the dynamics of frequency doubling and, therefore, acquire the coefficients governing photoconductivity and photocurrent by fitting the experimental data. Hence, we establish fundamental bounds of light conversion efficiency in optical waveguides.

This thesis brings a new understanding of photo-induced second-order nonlinear effects in optical integrated waveguides and microresonators and provides new grounds for the development of versatile classical and quantum light sources.