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Nonlinear and quantum optical devices based on periodically-poled thin film lithium niobate (PP-TFLN) have gained considerable interest lately, due to their significantly improved performance as compared to their bulk counterparts. Nevertheless, performance parameters such as conversion efficiency, minimum pump power, and spectral bandwidth strongly depend on the quality of the domain structure in these PP-TFLN samples, e.g., their homogeneity and duty cycle, as well as on the overlap and penetration depth of domains with the waveguide mode. Hence, in order to propose improved fabrication protocols, a profound quality control of domain structures is needed that allows quantifying and thoroughly analyzing these parameters. In this paper, we propose to combine a set of nanometer-to-micrometer-scale imaging techniques, i.e., piezoresponse force microscopy (PFM), second-harmonic generation (SHG), and Raman spectroscopy (RS), to access the relevant and crucial sample properties through cross-correlating these methods. Based on our findings, we designate SHG to be the best-suited standard imaging technique for this purpose, in particular when investigating the domain poling process in x-cut TFLNs. While PFM is excellently recommended for near-surface high-resolution imaging, RS provides thorough insights into stress and/or defect distributions, as associated with these domain structures. In this context, our work here indicates unexpectedly large signs for internal fields occurring in x-cut PP-TFLNs that are substantially larger as compared to previous observations in bulk LN. 

Silicon microring resonators are being recently used for high-brightness and efficient photon-pair generation at telecommunication wavelengths. Here, based on detailed theoretical and numerical modeling, we study the impact on pair generation of increasing the optical pump power, which generally causes nonlinear impairments such as free-carrier and two-photon absorption in silicon micro-resonators. Contrary to expectation, the pair generation properties of such devices may seem to be preserved at increasing pump powers, although not better than at a moderate pump power. These results suggest that silicon microrings can be used for pair generation over a wide range of pump powers, which may benefit applications in remotely pumped architectures, where the pump level might not be known a priori. 

Photon-pair generation at telecommunication wavelengths using high-quality silicon microring resonators is an active area of research. Here, we report on significant progress towards the ultimate goal of an integrated silicon microchip for bright generation of photon pairs with multiple stages of tunable optical filtering on the same chip. A high pair generation brightness of 6.5×10¹⁰pairs/s/mW²/nm is achieved. The resonance of the high-Q silicon microring resonator can be monitored using a high dynamic range readout of a photocurrent in an all-silicon p-i-n diode fabricated across the waveguide cross-section, which is used to align the ring resonance to the passbands or stopbands of the filters. 

We demonstrate a technique for measuring the full-speed performance of integrated modulators from ultraweak surface-coupled and scattered light. This can enable rapid characterization of unpackaged, high-speed wafer-scale integrated photonics without test ports or special fabrication. 

Photon-pair generation is shown using periodically-poled thin-film lithium niobate waveguides, with coincidences-to-accidentals ratio CAR>67,000 at 41kHz pairs rate, and heralded single-photon generation with g(2)(0)<0.05 at 860kHz herald rate. 

High-fidelity periodic poling over long lengths is required for robust, quasi-phase-matched second-harmonic generation using the fundamental, quasi-TE polarized waveguide modes in a thin-film lithium niobate (TFLN) waveguide. Here, a shallow-etched ridge waveguide is fabricated in x-cut magnesium oxide doped TFLN and is poled accurately over 5 mm. The high fidelity of the poling is demonstrated over long lengths using a non-destructive technique of confocal scanning second-harmonic microscopy. We report a second-harmonic conversion efficiency of up to 939 %.W⁻¹(length-normalized conversion efficiency 3757 %.W⁻¹.cm⁻²), measured at telecommunications wavelengths. The device demonstrates a narrow spectral linewidth (1 nm) and can be tuned precisely with a tuning characteristic of 0.1 nm/°C, over at least 40 °C without measurable loss of efficiency.