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Extremely efficient terahertz high-harmonic generation in graphene by hot Dirac fermions

机译：热狄拉克费米子在石墨烯中极高效地产生太赫兹高谐波

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Multiple optical harmonic generation-the multiplication of photon energy as a result of nonlinear interaction between light and matter-is a key technology in modern electronics and optoelectronics, because it allows the conversion of optical or electronic signals into signals with much higher frequency, and the generation of frequency combs. Owing to the unique electronic band structure of graphene, which features massless Dirac fermions(1-3), it has been repeatedly predicted that optical harmonic generation in graphene should be particularly efficient at the technologically important terahertz frequencies(4-6). However, these predictions have yet to be confirmed experimentally under technologically relevant operation conditions. Here we report the generation of terahertz harmonics up to the seventh order in single-layer graphene at room temperature and under ambient conditions, driven by terahertz fields of only tens of kilovolts per centimetre, and with field conversion efficiencies in excess of 10(-3), 10(-4) and 10(-5) for the third, fifth and seventh terahertz harmonics, respectively. These conversion efficiencies are remarkably high, given that the electromagnetic interaction occurs in a single atomic layer. The key to such extremely efficient generation of terahertz high harmonics in graphene is the collective thermal response of its background Dirac electrons to the driving terahertz fields. The terahertz harmonics, generated via hot Dirac fermion dynamics, were observed directly in the time domain as electromagnetic field oscillations at these newly synthesized higher frequencies. The effective nonlinear optical coefficients of graphene for the third, fifth and seventh harmonics exceed the respective nonlinear coefficients of typical solids by 7-18 orders of magnitude(7-9). Our results provide a direct pathway to highly efficient terahertz frequency synthesis using the present generation of graphene electronics, which operate at much lower fundamental frequencies of only a few hundreds of gigahertz.
机译：多光谐波的产生是光和物质之间非线性相互作用的结果，是光子能量的倍增，是现代电子学和光电子学中的一项关键技术，因为它可以将光或电子信号转换为频率更高的信号，并且频率梳的产生。由于石墨烯具有独特的电子能带结构，该结构具有无质量的狄拉克费米子（1-3），因此反复被预测到，在技术上重要的太赫兹频率（4-6）下，石墨烯中的光谐波产生应特别有效。但是，这些预测尚未在技术上相关的操作条件下通过实验得到证实。在这里，我们报告了在室温和环境条件下，单层石墨烯中太赫兹谐波的产生高达七阶的情况，太赫兹场仅由数十千瓦/厘米的太赫兹场驱动，场转换效率超过10（-3） ），10（-4）和10（-5）分别表示三次，第五和第七太赫兹谐波。考虑到电磁相互作用发生在单个原子层中，这些转换效率非常高。在石墨烯中如此高效地生成太赫兹高次谐波的关键是其背景狄拉克电子对驱动太赫兹场的集体热响应。通过热狄拉克费米子动力学产生的太赫兹谐波直接在时域中被观测为电磁场在这些新合成的较高频率处的振荡。三次，五次和七次谐波的石墨烯有效非线性光学系数超过典型固体的相应非线性系数7-18个数量级（7-9）。我们的结果提供了使用新一代石墨烯电子设备进行高效太赫兹频率合成的直接途径，该石墨烯电子设备仅以几百吉赫兹的低得多的基频工作。

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