High‐Order Nonlinear Photonic Crystal with Ordered Structures for Giant Enhancement of Frequency Tripling

Nonlinear optical conversion can generate new laser frequency which is difficult to achieve by direct lasing. Till now, second-order nonlinearity with χ⁽²⁾, e.g. frequency doubling, has been well developed with birefringent phase-matching for the frequency dispersion compensation. Beyond χ⁽²⁾, third-order χ⁽³⁾ nonlinearity can triple the optical frequency directly and allow multi-photon entanglement, which is helpful to reduce the device size and improve the capacity of on-chip photonics. However, triple-photon suffers from the extremely low efficiency (10^-10∼10^-8) due to the difficulty of compensating optical dispersion at large frequency difference, as well as small χ⁽³⁾ coefficients. Here, we propose a high-order nonlinear photonic crystal with artificially ordered structures for regulating phase compensation and boosting the third-order conversion efficiency. Taking Y₃Al₅O₁₂ crystal as an example, we realize the efficient frequency tripling in meta-YAG with a conversion efficiency of 4.5×10^-3, which is six orders of magnitude stronger than that of conventional YAG crystal. Moreover, a wide spectral tunability from 331 to 356 nm is achieved in the meta-YAG, indicating its great potential for broadband triple-photon generation. This study provides valuable insights into regulating all χ⁽³⁾-active nonlinear optical materials and opens the way to new frontiers on multi-photon entanglement in the nonlinear photonic crystal platform.