DTU Fotonik, Department of Photonics Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
Phys Rev Lett. 2012 Jul 27;109(4):043902. doi: 10.1103/PhysRevLett.109.043902. Epub 2012 Jul 26.
Cascaded nonlinearities have attracted much interest, but ultrafast applications have been seriously hampered by the simultaneous requirements of being near phase matching and having ultrafast femtosecond response times. Here we show that in strongly phase-mismatched nonlinear frequency conversion crystals the pump pulse can experience a large and extremely broadband self-defocusing cascaded Kerr-like nonlinearity. The large cascaded nonlinearity is ensured through interaction with the largest quadratic tensor element in the crystal, and the strong phase mismatch ensures an ultrafast nonlinear response with an octave-spanning bandwidth. We verify this experimentally by showing few-cycle soliton compression with noncritical cascaded second-harmonic generation: Energetic 47 fs infrared pulses are compressed in a just 1-mm long bulk lithium niobate crystal to 17 fs (under 4 optical cycles) with 80% efficiency, and upon further propagation an octave-spanning supercontinuum is observed. Such ultrafast cascading is expected to occur for a broad range of pump wavelengths spanning the near- and mid-IR using standard nonlinear crystals.
级联非线性吸引了很多关注，但超快应用受到同时接近相位匹配和具有超快飞秒响应时间的要求的严重阻碍。在这里，我们表明在强相位失配的非线性频率转换晶体中，泵浦脉冲可以经历大的且极其宽带的自聚焦级联类 Kerr 非线性。大的级联非线性通过与晶体中的最大二次张量元素相互作用来保证，而强的相位失配则确保具有覆盖一个倍频程带宽的超快非线性响应。我们通过展示非临界级联二次谐波产生的少周期孤子压缩实验验证了这一点：在仅 1 毫米长的块状铌酸锂晶体中，能量为 47fs 的红外脉冲被压缩到 17fs（在 4 个光学周期内），效率为 80%，并且在进一步传播时，观察到一个覆盖一个倍频程的超连续谱。预计这种超快级联将在使用标准非线性晶体的近红外和中红外的宽泵浦波长范围内发生。
