Plasmonic mid-infrared third harmonic generation in germanium nanoantennas
Plasmonic mid-infrared third harmonic generation in germanium nanoantennas
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DOI：
10.1038/s41377-018-0108-8
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发表时间：
2018-12
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影响因子：
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通讯作者：
M. Fischer;A. Riede;K. Gallacher;J. Frigerio;G. Pellegrini;M. Ortolani;D. Paul;G. Isella;A. Leitenstorfer;P. Biagioni;D. Brida
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文献类型：
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作者：
M. Fischer;A. Riede;K. Gallacher;J. Frigerio;G. Pellegrini;M. Ortolani;D. Paul;G. Isella;A. Leitenstorfer;P. Biagioni;D. Brida
我们展示了由生长在硅衬底上的高掺杂锗组成的等离子体天线中的三次谐波产生，该天线被设计为在中红外频率范围内谐振，这是传统的非线性等离子体材料所无法达到的。由于近场增强，结果是得到了波长在3到5 微米之间可调的超快亚衍射相干光源，并与分子振动的指纹区域理想地重叠。为了观察这一极具挑战性的光谱窗口中的非线性，采用了一种配备了参数频率转换和全反射共焦显微镜装置的高功率飞秒激光系统。我们展示了单个孤立天线结构的线性散射截面和非线性辐射的空间分辨映射。清晰的三阶功率依赖关系和中红外发射光谱证明了光发射的非线性性质。仿真结果支持观测到的双杆天线的谐振长度，并证明了天线材料内部的场强增强是造成非线性混频的原因。
We demonstrate third harmonic generation in plasmonic antennas consisting of highly doped germanium grown on silicon substrates and designed to be resonant in the mid-infrared frequency range that is inaccessible with conventional nonlinear plasmonic materials. Owing to the near-field enhancement, the result is an ultrafast, subdiffraction, coherent light source with a wavelength tunable between 3 and 5 µm, and ideally overlapping with the fingerprint region of molecular vibrations. To observe the nonlinearity in this challenging spectral window, a high-power femtosecond laser system equipped with parametric frequency conversion in combination with an all-reflective confocal microscope setup is employed. We demonstrate spatially resolved maps of the linear scattering cross section and the nonlinear emission of single isolated antenna structures. A clear third-order power dependence as well as mid-infrared emission spectra prove the nonlinear nature of the light emission. Simulations support the observed resonance length of the double-rod antenna and demonstrate that the field enhancement inside the antenna material is responsible for the nonlinear frequency mixing.