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  1. Free, publicly-accessible full text available October 20, 2023
  2. We examine the sensitivity of Λ-type optical quantum memories to experimental fluctuations using a variance-based analysis. The results agree with physical interpretations of quantum memory protocols, and are important for practical implementations.

  3. We present our experimental results on generating photon pairs entangled in a transverse-mode Bell state in few-mode optical fiber by controlling the transverse mode of the pump to selectively excite spontaneous four-wave mixing processes.

  4. 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 largemore »signs for internal fields occurring in x-cut PP-TFLNs that are substantially larger as compared to previous observations in bulk LN.« less
  5. Ultra-short poling periods of 1 μm and below in lithium niobate will allow nonlinear optical devices with operation to the UV regime or narrow-band counter-propagating single-photon generation. However, fabrication of such periods in bulk Lithium Niobate penetrating the complete modal areas of waveguides has been challenging. In this work, we demonstrate the fabrication of periodic domain grids with submicrometer periodicity in 300 nm x-cut thin-film lithium niobate. The poling was achieved through application of a single, shaped electrical pulse and electrodes fabricated with a combination of electron-and direct laser-writing lithography. The poling results were investigated with piezo-response force microscopy and second-harmonic microcopy and indicate the poled domains to penetrate fully across the complete film thickness. This will enable the fabrication of novel nonlinear optical devices combining the high efficiency of thin films with ultra-short domain periods.
  6. 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−−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.