Plasmon Enhanced Optical Magnetism in Metal Nanostructures

本研究探讨了用光产生和控制磁场的策略。磁性是光与金金属相互作用的结果。虽然金金属在黑暗中没有磁性行为，而光通常也不会产生稳定的磁场，但该系统的两个特点使光感应磁性成为可能。首先，金以非常小的粒子的形式被制备出来，这种粒子被称为纳米粒子，比光的波长小得多。由于其体积小，金纳米颗粒具有极大浓缩和增强光吸收的独特能力。其次，光是圆偏振的，这意味着光束也会引起金属纳米粒子中的电子的旋转运动。这些综合效应在纳米颗粒中产生巨大的循环电流，进而产生磁场。每个纳米粒子就像一块小而强的磁铁。磁场只有在光线照射时才会出现，所以磁场的开启和关闭就像光线的开启和关闭一样快。有了更好的理解，这种行为可以允许更小更快的磁开关，而不是目前的计算机存储系统，将信息存储在可切换的磁铁中。这项工作还支持学生在向非技术受众进行科学传播方面的培训。本研究分析了在圆偏振光激励下等离子体金属纳米结构中相干、旋转电荷位移的共振增强所导致的光学、磁和热行为。由于从圆偏振光场中直接传递角动量而在物质中产生的磁场被称为反法拉第效应，是这些实验研究的中心焦点。利用时间分辨泵浦探针光谱和连续波光谱分析了金纳米颗粒胶体悬浮液的反法拉第效应，该胶体悬浮液被设计为提供入射光场的等离子体增强。实验的重点是区分光感应磁与其他非线性光学现象，如光学克尔效应，或非相干热化过程。实验还量化了光诱导磁性的幅度、频率依赖性、时间响应和微观机制，以阐明通过超快光调制磁场产生实现纳米结构中光电和光磁功能的新途径。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This research examines strategies for generating and controlling magnetic fields with light. The magnetism is the result of an interaction of light with gold metal. Although gold metal has no magnetic behavior in the dark, and light does not usually generate stable magnetic fields, two features of this system enable light-induced magnetism. First, the gold is prepared in the form of very small particles, called nanoparticles, that are much smaller than the wavelength of light. Because of their small size, gold nanoparticles have a unique ability to greatly concentrate and enhance the absorption of light. Second, the light is circularly polarized, meaning that the beam of light also causes rotational motion of electrons in the metal nanoparticles. These combined effects produce large circulating electric currents in the nanoparticles that, in turn, give rise to magnetic fields. Each nanoparticle behaves like a small, very strong magnet. The magnetic field is only present when the light is shining, so the magnetism can be switched on and off as quickly as the light can be turned on and off. With better understanding, this behavior can allow for even smaller and faster magnetic switching than current computer memory systems that store information in switchable magnets. This work also supports student training in scientific communication to non-technical audiences.This research analyzes the optical, magnetic, and thermal behavior that results from resonant enhancement of coherent, rotating charge displacement in plasmonic metal nanostructures during excitation with circularly polarized light. The generation of magnetic fields in matter due to the direct transfer of angular momentum from circularly polarized optical fields is termed the inverse Faraday effect, and is the central focus of study in these experiments. The inverse Faraday effect is analyzed using both time-resolved pump-probe spectroscopy and continuous wave spectroscopy on colloidal suspensions of gold nanoparticles that are designed to provide plasmonic enhancement of the incident optical field. Experiments are focused on distinguishing optically-induced magnetism from other non-linear optical phenomena, such as the optical Kerr effect, or non-coherent thermalization processes. The experiments also quantify the magnitude, frequency dependence, time response, and microscopic mechanism of the optically-induced magnetism, in order to elucidate new avenues of optoelectronic and opto-magnetic functionality in nanostructures enabled by ultrafast, optically modulated magnetic field generation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.