High-harmonic generation in the ultraviolet region is promising for wireless technology used for communications and sensing. However, small high-order nonlinear coefficients prevent us from obtaining high conversion efficiency and functional photonic devices. Here, we show highly efficient ultraviolet harmonic generation extending to the fifth order directly from an epsilon-near-zero indium tin oxide (ITO) film. The real part of the annealed ITO films was designed to reach zero around 1050 nm, matching with the central wavelength of an Yb-based fiber laser, and the internal driving electric field was extremely enhanced. A high energy conversion efficiency of ${10}^{-4}$ and ${10}^{-6}$ for 257.5 nm (fourth-order) and 206 nm (fifth-order) ultraviolet harmonic generation was obtained, which is at least 2 orders of magnitude higher than early reports. Our results demonstrate a new route for overcoming the inefficiency problem and open up the possibilities of compact solid-state high-harmonic generation sources at nanoscale.

中文翻译：

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ε-近零铟锡氧化物薄膜的高效紫外高谐波产生

紫外线区域的高谐波产生对于用于通信和传感的无线技术很有希望。但是，小的高阶非线性系数会阻止我们获得高转换效率和功能性光子器件。在这里，我们展示了一种高效的紫外线谐波产生，其直接从ε接近零的铟锡氧化物（ITO）薄膜扩展到了五阶。ITO退火薄膜的实际部分设计为在1050 nm附近达到零，与基于Yb的光纤激光器的中心波长匹配，并且内部驱动电场得到极大增强。能量转换效率高${10}^{-4}$ 和 ${10}^{-6}$获得了257.5 nm（四阶）和206 nm（五阶）的紫外线谐波，比早期报道至少高2个数量级。我们的结果证明了一种解决效率低下问题的新途径，并为纳米级紧凑型固态高谐波产生源开辟了可能性。