Nonlinear response in a material increases with its index of refraction as n⁴. Commonly, n ~ 1 so that diffraction, dispersion, and chromatic walk-off limit nonlinear scattering. Ferroelectric crystals with a periodic 3D polarization structure overcome some of these constraints through versatile Cherenkov and quasi-phase-matching mechanisms. Three-dimensional self-structuring can also lead to a giant optical refraction. Here, we perform second-harmonic-generation experiments in KTN:Li in conditions of giant broadband refraction. Enhanced response causes wavelength conversion to occur in the form of bulk Cherenkov radiation without diffraction and chromatic walk-off, even in the presence of strong wave-vector mismatch and highly focused beams. The process occurs with a wide spectral acceptance of more than 100 nm in the near infrared spectrum, an ultra-wide angular acceptance of up to ±40^∘, with no polarization selectivity, and can be tuned to allow bulk supercontinuum generation. Results pave the way to highly efficient and adaptable nonlinear optical devices with the promise of single-photon-to-single-photon nonlinear optics.

材料的非线性响应随其n ⁴的折射率而增加。通常，n〜1使衍射，色散和色散信号限制非线性散射。具有周期性3D极化结构的铁电晶体通过通用的Cherenkov和准相位匹配机制克服了其中一些限制。三维自结构化还可能导致巨大的光学折射。在这里，我们在宽宽带折射条件下，在KTN：Li中进行二次谐波产生实验。增强的响应使波长转换以整体Cherenkov辐射的形式发生，而没有衍射和色散现象，即使存在强波矢量失配和高度聚焦的光束也是如此。该过程的发生是在近红外光谱中接受超过100 nm的宽光谱，超宽角度接受高达± ^40∘，没有极化选择性，并且可以进行调整以生成大量超连续谱。结果为单光子到单光子非线性光学的前景铺平了向高效和适应性强的非线性光学器件的道路。