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Formation Rules and Dynamics of Photoinduced χ <sup>(2)</sup> Gratings in Silicon Nitride Waveguides
摘要： 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 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 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 waveguide. Temporal and thermal stability tests reveal the nature of charge traps responsible for grating inscription. After applying thermally activated hopping as conductivity mechanism in our samples, we show that only shallow traps seem to be activated during the all-optical poling process.
关键词： stoichiometric silicon nitride,nonlinear optics,all-optical poling,second harmonic generation,quasi-phase-matching
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Optically Engineered Nonlinear Photonic Structures in Ferroelectrics
摘要： Performance of many optical devices based on frequency conversion critically depends on spatial modulation of the nonlinear optical response of the materials. This modulation ensures, via the quasi phase matching (QPM), an efficient energy exchange between optical waves at different frequencies. The QPM structures, also known as the nonlinear photonic crystals, offer a variety of novel properties and functionalities that cannot be obtained in uniform nonlinear crystals. Typically, such nonlinear structures have been fabricated via electric-poling. However, this technique is restricted to particular crystallographic geometries, and does not allow to create isolated structures inside the nonlinear material. This means that one cannot realize fully 3-dimensional nonlinear structures via traditional electrical poling. In this work we discuss our novel technique capable of engineering quadratic nonlinearity, based on all-optical poling of ferroelectrics. This new approach, is similar to traditional laser writing in optical media. It involves a tight focusing of femtosecond infrared beam and subsequent its two photon absorption which heats locally the medium. The large temperature gradient induces thermoelectric field which, if sufficiently high, inverts locally direction of spontaneous polarization, changing the sign of nonlinearity. The method is extremely flexible, can be applied to different crystallographic orientations and enables formation of localised spatially modulated nonlinear response inside optical media. Here, we demonstrate its application to create structures for efficient frequency conversion in Lithium Niobate waveguides and transverse second harmonic generation. Moreover, we provide here the first ever experimental evidence of the 3D Nonlinear Photonic Crystal, fabricated in ferroelectric barium calcium titanate. Finally we show formation of multiple nonlinear photonic crystals in form of fork structures inside the bulk of ferroelectric calcium barium niobate, and demonstrate their application for wave-shaping in frequency doubling process, directly creating second harmonic optical vortices and conical beams.
关键词： frequency conversion,nonlinear photonic structures,ferroelectrics,all-optical poling,second harmonic generation,quasi phase matching
更新于2025-09-12 10:27:22
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All-Optical Poling and Two-Photon Absorption in Heterocyclic Azo Dyes with Different Side Groups
摘要： We investigated the optical properties of heterocyclic azo dyes with different side groups prepared: as powders, in PMMA matrix and side-chain polymers. We characterized the nonlinear optical effects of these compounds by Z-scan and all-optical poling techniques and supported these experiments with theoretical modeling. It was also found that these materials show strong two-photon absorption cross-sections exceeding 20 GM, which is one order of magnitude stronger than that of similar materials (azobenzene and p-aminoazobenzene). Our study reveals the dominant role of the side-chain system over the guest-host one and stress out the importance of the substituent in the efficiency of the poling process.
关键词： All-Optical Poling,Hybrid Materials,Heterocyclic Azo Dyes,Two-Photon Absorption,Plasmonics,Optical,Magnetic
更新于2025-09-04 15:30:14