Abstract
Second-harmonic generation (SHG) is widely used for wavelength conversions in various applications including multi-wavelength lasers, imaging, and ultrafast optics. However, it remains a long-standing challenge to achieve both a broad SHG bandwidth and high efficiency due to the medium dispersion. Here, we propose an ultrabroadband efficient SHG on a lithium niobate on insulator (LNOI) platform by dispersion engineering. The waveguide cross section with two V-shaped grooves is carefully designed to minimize first- and second-order dispersion, which is feasible for current fabrication techniques. Due to the flattened dispersion, a record-high, to our knowledge, bandwidth-length product ($\Delta \lambda \cdot {\rm L}$) of ${2444}\;{{\unicode{x00B5}{\rm m}}^2}$ is achieved in a periodically poled 1.64-cm-long LNOI waveguide, which is four times the previously reported result. Such a scheme can also be generalized to SHG waveguides on other material platforms. Our results provide a new strategy of dispersion engineering for integrated photonics, and may be helpful for applications including frequency doubling, supercontinuum generation, and optical frequency combs.
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