Generalized Coherent Photogalvanic Effect in Coherently Seeded Waveguides

The coherent photogalvanic effect leads to the generation of a current under the absorption interference of coherent beams and allows for the inscription of space-charge gratings leading to a second-order susceptibility (chi((2))). The inscribed grating automatically results in quasi-phase-matching between the interfering beams. Theoretical and experimental studies, considering the degenerate case of second-harmonic generation, show significant conversion efficiency enhancements. However, the link between the theory and experiment is not fully established such that general guidelines and achievable conversion efficiency for a given material platform are still unclear. In this work, the phenomenological model of coherent photogalvanic effect in optical waveguides is theoretically analyzed. This model predicts the existence of non-degenerate sum-frequency generation quasi-phase-matching gratings, which is confirmed experimentally for the first time. Furthermore, the time dynamics of the space-charge grating inscription in coherent photogalvanic process is formulated. Based on the developed theoretical equations, the material parameters governing the process for stoichiometric silicon nitride are extracted. The results obtained provide a basis to compare the performances and potentials of different platforms. This work not only supplements the theory of coherent photogalvanic effect, but also enables us to identify critical parameters and limiting factors for the inscription of chi((2)) gratings.

Formation Rules and Dynamics of Photoinduced χ(2) Gratings in Silicon Nitride Waveguides

Camille Sophie Brès, Davide Grassani, Tobias Kippenberg, Tianyi Liu, Edgars Nitiss, Martin Hubert Peter Pfeiffer

Silicon nitride has emerged as a prominent platform for building photonics integrated circuits. While its nonlinear properties based on third-order effects have been successfully exploited, an efficient second harmonic generation in standard stoichiometric silicon nitride (Si3N4) waveguides can also be achieved after all-optical poling, as was recently shown. The root of such a phenomenon has been attributed to the inscription of a self-organized periodic space-charge grating along the waveguide, allowing an effective χ(2) and automatic quasi-phase-matching of pump and second harmonic. However, the different parameters and their role in increasing the efficiency of the process are still not fully comprehended. In this work, we use optical means to identify the general conditions of mode matching occurring during all-optical poling. The overlap integral between pump and second harmonic optical modes is shown to be the governing parameter in determining the features of the χ(2) gratings. Two-photon microscopy measurements of the χ(2) gratings reveal the presence of a secondary periodicity in some of the waveguides used in the study. According to overlap integral simulations, such an effect can occur due to mode mixing in the waveguide bends. From a study of poling dynamics, we observe that poling efficiency and rate increase as a function of optical pump power and waveguide length. However, in order to initiate poling, a critical pump intensity, which is lower for longer waveguides, must be coupled into a waveguide. Temporal and thermal stability tests reveal the nature of charge traps responsible for grating inscription. After applying thermally activated hopping as a conductivity mechanism in our samples, we show that only shallow traps seem to be activated during the all-optical poling process.

2019

Broadband quasi-phase-matching in dispersion-engineered all-optically poled silicon nitride waveguides

Camille Sophie Brès, Tobias Kippenberg, Junqiu Liu, Edgars Nitiss, Rui Ning Wang, Ozan Yakar, Boris Zabelich

Quasi-phase-matching (QPM) has become one of the most common approaches for increasing the efficiency of nonlinear three-wave mixing processes in integrated photonic circuits. Here, we provide a study of dispersion engineering of QPM second-harmonic (SH) generation in stoichiometric silicon nitride (Si3N4) waveguides. We apply waveguide design and lithographic control in combination with the all-optical poling technique to study the QPM properties and shape the waveguide dispersion for broadband spectral conversion efficiency inside Si3N4 waveguides. By meeting the requirements for maximal bandwidth of the conversion efficiency spectrum, we demonstrate that group-velocity matching of the pump and SH is simultaneously satisfied, resulting in efficient SH generation from ultrashort optical pulses. The latter is employed for retrieving a carrier-envelope-offset frequency of a frequency comb by using an f - 2f interferometric technique, where supercontinuum and SH of a femtosecond pulse are generated in Si3N4 waveguides. Finally, we show that the waveguide dispersion determines the QPM wavelength variation magnitude and sign due to the thermo-optic effect. (C) 2020 Chinese Laser Press

2020

Difference frequency generation in optically poled silicon nitride waveguides

Camille Sophie Brès, Edgars Nitiss, Ezgi Sahin, Ozan Yakar, Boris Zabelich

All-optical poling leads to an effective second-order nonlinearity (chi((2))) in centrosymmetric materials without the need for sophisticated fabrication techniques or material processing, through the periodic self-organization of the charges. The absence of the inherent chi((2)) in prevailing silicon-based platforms can be surmounted through all-optical poling. Using the induced effective chi((2)) in silicon nitride (Si3N4) waveguides, nonlinear frequency up-conversion processes, such as second-harmonic generation, were previously demonstrated on Si3N4. Here, we report near- and non-degenerate difference-frequency generation in all-optically poled Si3N4 waveguides. We show the agreement between the theory and the measurements and optimize achievable QPM bandwidth range, reaching conversion efficiency of 1 %/W.

SPIE-INT SOC OPTICAL ENGINEERING2021

Formation Rules and Dynamics of Photoinduced χ(2) Gratings in Silicon Nitride Waveguides

Camille Sophie Brès, Davide Grassani, Tobias Kippenberg, Tianyi Liu, Edgars Nitiss, Martin Hubert Peter Pfeiffer

2019