有机介线亚波长晶面控制的定向二次谐波产生
Directional Second Harmonic Generation Controlled by Sub-wavelength Facets of an Organic Mesowire
摘要：
定向谐波产生是表征非线性光学天线在空间明确定义的区域中扩散信号的能力的重要性质。在这里，我们展示了如何可以利用有机分子介线晶体的亚波长小平面来系统地改变前向散射几何结构中的二次谐波产生（SHG）的方向性。我们证明了这种能力的结晶diamonoanthraquinone（DAAQ）介线与亚波长方面。我们观察到，径向角的SHG发射可以在130度的范围内进行调谐。这种角度变化是由于激发的焦点体积中的空间分布的非线性偶极子以及中间线的几何横截面和小平面取向而产生的。近场激发分布的数值模拟证实了介线几何形状在定位电场中的作用。除了介线的定向二次谐波外，我们还在实验上观察到了非线性双光子激发荧光（TPEF）的光波导。有趣的是，我们观察到，对于一个给定的泵浦激发，TPEF信号是各向同性和离域的，而SHG发射是定向的，并在激发的位置本地化。所有观察到的效应不仅对有源非线性光学天线，而且对非线性信号处理都有直接的影响。
英文摘要：
Directional harmonic generation is an important property characterizing the ability of nonlinear optical antennas to diffuse the signal in well-defined region of space. Herein, we show how sub-wavelength facets of an organic molecular mesowire crystal can be utilized to systematically vary the directionality of second harmonic generation (SHG) in the forward scattering geometry. We demonstrate this capability on crystalline diamonoanthraquinone (DAAQ) mesowires with subwavelength facets. We observed that the radial angles of the SHG emission can be tuned over a range of 130 degrees. This angular variation arises due to spatially distributed nonlinear dipoles in the focal volume of the excitation as well as the geometrical cross-section and facet orientation of the mesowire. Numerical simulations of the near-field excitation profile corroborate the role of the mesowire geometry in localizing the electric field. In addition to directional SHG from the mesowire, we experimentally observe optical waveguiding of the nonlinear two-photon excited fluorescence (TPEF). Interestingly, we observed that for a given pump excitation, the TPEF signal is isotropic and delocalized, whereas the SHG emission is directional and localized at the location of excitation. All the observed effects have direct implications not only in active nonlinear optical antennas, but also in nonlinear signal processing.
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