During the past few years, the optics and photonics communities have renewed their attention toward transparent conducting oxides (TCOs), which for over two decades have been broadly employed for the fabrication of transparent electrodes in photovoltaic and communication technologies. This reinvigorated research curiosity is twofold: on the one hand, TCOs, with their metal-like properties, low optical absorption, and fabrication flexibility, represent an appealing alternative to noble metals for designing ultra-compact plasmonic devices. On the other hand, this class of hybrid compounds has been proved to possess exceptionally high optical nonlinearities when operating on a frequency window centered around their crossover point, the wavelength point at which the real part of the dielectric permittivity switches sign. Because TCOs are wide-bandgap materials with the Fermi level located in the conduction band, they are hybrid in nature, thus presenting both interband and intraband nonlinearities. This is the cause of a very rich nonlinear physics that is yet to be fully understood and explored. In addition to this, TCOs are epsilon-near-zero (ENZ) materials within a broad near-infrared spectral range, including the entire telecom bandwidth. In this operational window a myriad of novel electromagnetic phenomena have been demonstrated experimentally such as supercoupling, wavefront freezing, and photon doping. Furthermore, TCOs stand out among all other ENZ systems due to one fundamental characteristic, which is hardly attainable even by using structured materials. In fact, around their ENZ wavelength and for a quite generous operational range, these materials can be engineered to have an extremely small real index. This peculiarity leads to a slow-light effect that is ultimately responsible for a significant enhancement of the material nonlinear properties and is the cornerstone of the emerging field of near-zero-index photonics. In this regard, the recent history of nonlinear optics in conductive oxides is growing extremely fast due to a great number of experiments reporting unprecedentedly remarkable effects, including unitary index change, bandwidth-large frequency shift, efficient ultra-low-power frequency conversion, and many others. This review is meant to guide the reader through the exciting journey of TCOs, starting as an industrial material for transparent electrodes, then becoming a new alternative for low-loss plasmonics, and recently opening up new frontiers in integrated nonlinear optics. The present review is mainly focused on experimental observations.

中文翻译：

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透明导电氧化物：从全电介质等离子体到集成光子学的新范式

在过去几年中，光学和光子学界重新关注透明导电氧化物 (TCO)，二十多年来，透明导电氧化物已广泛用于制造光伏和通信技术中的透明电极。这种重新激发的研究好奇心是双重的：一方面，TCOs 具有类金属特性、低光吸收和制造灵活性，代表了用于设计超紧凑等离子体器件的贵金属的有吸引力的替代品。另一方面，已证明这类杂化化合物在以交叉点为中心的频率窗口上运行时具有异常高的光学非线性，交叉点是介电常数实部切换符号的波长点。由于 TCO 是费米能级位于导带中的宽带隙材料，因此它们本质上是混合的，因此呈现带间和带内非线性。这是尚未完全理解和探索的非常丰富的非线性物理学的原因。除此之外，TCO 是广泛的近红外光谱范围内的ε-近零 (ENZ) 材料，包括整个电信带宽。在这个操作窗口中，已经通过实验证明了无数新的电磁现象，例如超耦合、波前冻结和光子掺杂。此外，由于一个基本特性，TCO 在所有其他 ENZ 系统中脱颖而出，即使使用结构化材料也很难实现这一点。事实上，在它们的 ENZ 波长附近和相当大的工作范围内，这些材料可以设计成具有极小的实际指数。这种特性导致慢光效应最终导致材料非线性特性的显着增强，并且是近零折射率光子学新兴领域的基石。在这方面，由于大量实验报告了前所未有的显着效果，包括单一折射率变化、带宽大频移、高效超低功率频率转换和好多其它的。这篇评论旨在引导读者完成 TCO 的激动人心的旅程，从作为透明电极的工业材料开始，然后成为低损耗等离子体的新替代品，最近开辟了集成非线性光学的新领域。本综述主要集中在实验观察上。