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Second harmonic generation from an individual amorphous selenium nanosphere

机译：从单个非晶硒纳米球产生二次谐波

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Among the numerous nonlinear optics effects, second harmonic generation (SHG) is always a hotspot and it is extensively used for optical frequency conversion, biomedical imaging, etc. However, SHG is forbidden in a medium with inversion symmetry under the electric-dipole approximation. Here, we demonstrated SHG from a single amorphous selenium (a-Se) nanosphere under near-infrared femtosecond pulse excitation. It was found that SH spectra are tunable with the size of a-Se nanospheres and the SHG efficiency of a single a-Se sphere with a diameter over 300 nm is estimated at 10(-8). We also established two physical mechanisms of SHG from the amorphous nanospheres. There is an electric-dipole contribution to the second-order nonlinearity in view of the inevitable structural discontinuity at the surface. The discontinuity of the normal component of the electric field strength leads to the quadrupole-type contributions arising from the large electric field gradient. The SHG process can be enhanced by resonance near the fundamental wavelength, giving rise to the detectable second harmonic (SH) spectra of a single a-Se nanosphere (d > 300 nm) or two small a-Se nanospheres (d = 200 nm) aggregated into a dimer, while the single nanosphere with smaller size (d > 300 nm) is undetectable. As an essential trace element for animals, a-Se features unique biological compatibility and has specific properties of optical nonlinearity within the optical window in biological tissue. This discovery makes a-Se nanospheres promising both in nonlinear optics and biomedicine.
机译：在众多的非线性光学效应中，二次谐波（SHG）一直是热点，并且广泛用于光频率转换，生物医学成像等。但是，在电偶极子近似下具有反向对称性的介质中禁止使用SHG。在这里，我们展示了在近红外飞秒脉冲激发下来自单个非晶硒（a-Se）纳米球的SHG。结果发现，SH光谱可随a-Se纳米球的大小而调节，并且直径大于300 nm的单个a-Se球的SHG效率估计为10（-8）。我们还从无定形纳米球建立了SHG的两种物理机制。鉴于表面不可避免的结构不连续性，电偶极子对二阶非线性有贡献。电场强度的法线分量的不连续导致大电场梯度引起的四极型贡献。可以通过在基本波长附近共振来增强SHG过程，从而产生单个a-Se纳米球（d> 300 nm）或两个小的a-Se纳米球（d = 200 nm）的可检测二次谐波（SH）光谱。聚集成二聚体，而单个纳米球的尺寸较小（d> 300 nm）则无法检测到。作为动物必不可少的微量元素，a-Se具有独特的生物相容性，并在生物组织的光学窗口内具有光学非线性的特定特性。这项发现使a-Se纳米球在非线性光学和生物医学领域都有希望。

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