Optical Bound States and Non-linearity in Geometrically-Modulated Dielectric Nanowires

非技术描述光与微观大小的球形或圆柱形粒子的相互作用经常被用来理解光如何与不同大小和形状的物质相互作用的一般原理。例如，与光的波长相当或小于光的波长的微观粒子可以产生惊人的强光散射和吸收。最近，已经有可能制造出形状超越简单圆柱体的微观粒子，而不是在它们的大小上进行周期性调制。在适当的条件下，这些几何调制使粒子以一种新的方式与光相互作用，从而产生所谓的“连续介质中的光学束缚态”（BIC）。该项目采用理论、计算、合成和测量相结合的方法来了解微观粒子的几何形状如何用于控制bic的性质。bic是令人兴奋的，因为它们极大地增加了光与粒子相互作用的程度。在某些情况下，相互作用足够强，导致粒子将红光或红外光转化为蓝光或紫外线，这是一种“非线性”效应。该项目的结果为控制几何调制粒子中的bic提供了一般原理，从而为在微观尺度上控制大范围颜色的光提供了基本原理。这项研究工作涉及本科生、研究生和博士后，他们参与了一个连接化学、物理和工程的项目，为纳米材料合成、微加工、光谱学、显微镜和建模提供了广泛的经验。在圆柱形介电粒子中，米氏共振可以引起惊人的强光散射和光吸收，并且米氏共振，漏模共振和导模在这些结构中的重要性和相互作用已经被研究过。一种相对较新的共振，BIC，已经被确定，理论上可以在理想系统中无限时间地捕获光。本课题研究了纳米级横向尺寸介质圆柱体中bic的基本光散射、光吸收和非线性特性。硅结构是通过自下而上的气-液-固工艺合成的，使用原位掺杂调制结合湿化学蚀刻来创建精确定义的形态。介质圆柱体直径的周期性调制可以引入一系列不同阶数、极化和对称型的bic。对控制几何形状的单个NWs的光谱测量与理论模型进行了比较，为理论预测提供了验证。通过将入射光的频率增加一倍或三倍，还评估了硅NWs中的bic实现非线性效应的能力。总的来说，本研究揭示了介质圆柱体中BIC模式的基本特征，并强调了这些BIC模式的重要实际应用。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。

Nontechnical descriptionThe interaction of light with spherical or cylindrical particles of microscopic size has frequently been used to understand the general principles of how light interacts with matter of different sizes and shapes. For example, microscopic particles that are comparable to or smaller than the wavelength of light can give rise to surprisingly strong light scattering and absorption. Recently, it has become possible to make microscopic particles with shapes that go beyond simple cylinders to instead have periodic modulations in their size. Under the right conditions, these geometric modulations cause the particles to interact with light in a new way, giving rise to what is termed an “optical bound state in the continuum,” or BIC. This project uses a combination of theory, computation, synthesis, and measurement to understand how the geometry of microscopic particles can be used to control the properties of BICs. BICs are exciting because they dramatically increase the extent to which light interacts with particles. In some cases, the interaction is sufficiently strong to cause the particles to convert red or infrared light into blue or ultraviolet light, a “non-linear” effect. The results of this project provide the general principles for controlling BICs in geometrically-modulated particles, thus providing the fundamental principles for controlling light over a broad range of colors at a microscopic scale. The research effort involves undergraduate, graduate, and postdoctoral students in a project that bridges the interface between chemistry, physics, and engineering—providing breadth of experience in nanomaterial synthesis, microfabrication, spectroscopy, microscopy, and modeling.Technical DescriptionIn cylindrical dielectric particles, Mie resonances can cause surprisingly strong light scattering and light absorption, and the importance and interplay of Mie resonances, leaky-mode resonances, and guided modes in these structures has previously been studied. A relatively new class of resonance, the BIC, has been identified and can theoretically trap light for an infinite time in an ideal system. This project studies the fundamental light scattering, light absorption, and nonlinear properties of BICs in dielectric cylinders with nanoscale lateral size. The silicon structures are synthesized by a bottom-up vapor-liquid-solid process using in situ dopant modulation combined with wet-chemical etching to create precisely-defined morphology. Periodic modulation of the diameter of a dielectric cylinder is shown to introduce a range of BICs of different order, polarization, and symmetry type. Spectroscopic measurements on single NWs of controlled geometry are compared to theoretical models, providing validation of theoretical predictions. The capacity of BICs in silicon NWs to enable nonlinear effects by doubling or tripling the frequency of incoming light is also evaluated. Overall, the proposed study reveals the fundamental characteristics of BICs in dielectric cylinders and highlights important practical applications of these BIC modes.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.