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Publication# Photoinduced Second-Order Nonlinearities in Centrosymmetric Media

Abstract

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.

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Second-harmonic generation (SHG, also called frequency doubling) is a nonlinear optical process in which two photons with the same frequency interact with a nonlinear material, are "combined", and generate a new photon with twice the energy of the initial photons (equivalently, twice the frequency and half the wavelength), that conserves the coherence of the excitation. It is a special case of sum-frequency generation (2 photons), and more generally of harmonic generation.

Frequency comb

In optics, a frequency comb is a laser source whose spectrum consists of a series of discrete, equally spaced frequency lines. Frequency combs can be generated by a number of mechanisms, including periodic modulation (in amplitude and/or phase) of a continuous-wave laser, four-wave mixing in nonlinear media, or stabilization of the pulse train generated by a mode-locked laser. Much work has been devoted to this last mechanism, which was developed around the turn of the 21st century and ultimately led to one half of the Nobel Prize in Physics being shared by John L.

Polarization density

In classical electromagnetism, polarization density (or electric polarization, or simply polarization) is the vector field that expresses the density of permanent or induced electric dipole moments in a dielectric material. When a dielectric is placed in an external electric field, its molecules gain electric dipole moment and the dielectric is said to be polarized. The electric dipole moment induced per unit volume of the dielectric material is called the electric polarization of the dielectric.

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