强相位失配级联相互作用产生的超快和倍频程光学非线性
Ultrafast and octave-spanning optical nonlinearities from strongly phase-mismatched cascaded interactions
摘要：
级联非线性引起了人们极大的兴趣，但超快应用受到了同时要求接近相位匹配和具有超快飞秒响应时间的严重阻碍。在这里，我们表明，在强相位失配的非线性频率转换晶体的泵浦脉冲可以经历一个大的和极宽的自散焦级联克尔类非线性。通过与晶体中最大的二次张量元素的相互作用来确保大的级联非线性，并且强相位失配确保具有倍频程跨越带宽的超快非线性响应。我们通过实验验证了这一点，显示几个周期的孤子压缩与非临界级联二次谐波产生：高能47 fs的红外脉冲压缩在一个只有1毫米长的散装锂晶体的17 fs（下4个光学周期）与80%的效率，并在进一步传播的octave跨越超连续谱观察。这种超快级联预计将发生在使用标准非线性晶体跨越近红外和中红外的宽范围的泵浦波长。
英文摘要：
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.
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