Collaborative Research: NSF/ENG/ECCS-BSF: Complex liquid droplet structures as new optical and optomechanical materials

在高质量的光学微谐振器中，将光限制在数百甚至数十微米的范围内，可以实现显著的电磁能量集中。密闭的光对环境变化更加敏感，产生放大的机械力，即使在很小的光强下也能产生显著的非线性效应。因此，光学微谐振器在光学冷却或机械运动放大、精密计量、激光、超灵敏生物传感等领域得到了积极的研究。光的限制通常是用固体材料来实现的，但这个项目提议用液体微结构来实现。向液滴的过渡带来了重大挑战，但也带来了新的机遇。首先，液滴的机械柔软性使它们比固体材料更容易接受光诱导力，从而产生许多数量级的机械响应，从而提高了光学冷却或加热的效率。其次，液滴允许进入谐振器的内部区域。由于谐振器内部的电磁场比外部大几个数量级，因此可以预期基于液滴谐振器的生物传感器的灵敏度相应提高几个数量级。第三，利用液滴可以实现一种新型的光子分子，这种分子基于液滴中的液滴结构，具有超强的光学键，其中一个或多个较小的液滴被封装在较大的液滴中。总的来说，这个项目的目标是展示液滴谐振器在光学冷却、激光、传感和计量领域的变革潜力。该项目的跨学科性质，包括物理学家，电气和机械工程师，将确保研究生和本科生将接触到不同学科的文化和方法。此外，该项目将在美国和以色列的研究人员和学生之间建立联系，并加强参与该项目的美国大学与以色列理工学院（Technion）之间的合作。该项目由美国国家科学基金会（NSF-BSF）合作项目提供支持，并由BSF资助的以色列团队参与。本项目将微流体学与光学低语通道模式谐振器领域相结合，提出了对新型液滴光子结构的光学和光力学特性的研究。液滴的机械柔软性及其多功能性和可调性将使主要研究人员能够研究新的光学和光力学效应，如毛细波的光学冷却、异常点附近的拓扑能量传递等。为此项目组建的国际多学科团队将利用最先进的微流体技术制造不同结构的液滴，每个液滴都可以作为高质量的光子谐振器。将开发数值模拟和理论模型，以了解与项目中开发的新结构相关的物理。参与该项目的实验人员将对所提出的结构进行光学表征，并深入了解其新的光学和光力学效应。这项研究将通过应用最先进的3D打印技术来制造新型微流体器件和生成复杂结构的微液滴，从而推动光流体领域的发展。对具有独特性质的新型光子结构的研究也将为光低语-廊模谐振器领域开辟新的方向。计算电动力学的一般领域也将受益于这项工作，因为它将t矩阵形式主义远远超出其标称领域，并将其应用于液滴的光学耦合复杂结构的模式。

Confining light to region of hundreds or even tens of micrometers in high-quality optical microresonators, one can achieve a significant concentration of electromagnetic energy. The confined light becomes much more sensitive to environmental changes, exerts an amplified mechanical force, and can generate significant nonlinear effects even at small light intensities. For this reason, optical microresonators are being actively studied in the context of optical cooling or amplification of mechanical motion, for precision metrology, lasing, ultrasensitive biosensing and other areas. Confinement of light is usually achieved using solid materials, but this project proposes to achieve it using liquid microstructures. The transition to liquid droplet creates significant challenges, but also opens up new opportunities. Firstly, mechanical softness of droplets makes them more receptive than solid materials to the light-induced forces resulting in many orders of magnitude larger mechanical responses and hence increased efficiency of optical cooling or heating. Secondly, liquid droplets allow access to the resonator's interior regions. Because electromagnetic field is orders of magnitude larger inside than outside of the resonator, one can expect the corresponding increase in sensitivity of biosensors based on droplet resonators by several orders of magnitude. Thirdly, use of liquid droplets allows realizing a novel class of photonic molecules with extra strong optical bonds based on droplet-in-droplet structures, in which one or more smaller droplets are encapsulated in a larger droplet. Overall, the objective of this project is to demonstrate the transformative potential of liquid droplet resonators in the fields of optical cooling, lasing, sensing and metrology. The interdisciplinary nature of the project, which includes physicists, and electrical and mechanical engineers, will ensure that graduate and undergraduate students will be exposed to the culture and methodology of different disciplines. In addition, the project will build connections between American and Israeli researchers and students and strengthen the collaboration between American universities participating in the project and Technion, Israel's premiere engineering school. The support for this project is provided within the collaborative NSF-BSF (Binational US-IL Science Foundation) program with participation of the Israel team financed by BSF.This project merges the fields of microfluidics and optical whispering-gallery- mode resonators by proposing the study of the optical and optomechanical properties of novel photonic structures composed of fluid droplets. The mechanical softness of liquid droplets combined with their versatility and tunability will allow the principal investigators to study novel optical and optomechanical effects such as optical cooling of capillary waves, topological energy transfer in the vicinity of exceptional points, and others. The international multidisciplinary team formed for this project will exploit state-of-the-art microfluidic technologies to fabricate different structures of droplets, with each droplet serving as a high-quality photonic resonator. Numerical simulation and theoretical models will be developed to understand the physics associated with the novel structures developed in the project. Experimentalists working on the project will carry out optical characterization of the proposed structures and develop in-depth understanding of their novel optical and optomechanical effects. This research will advance the field of optofluidics by applying state-of-the-art 3D printing technologies to the fabrication of novel microfluidic devices and generation of complex structures of microdroplets. Study of novel photonic structures with unique properties will also open new directions in the field of optical whispering-gallery-mode resonators. The general field of computational electrodynamics will also benefit from this work by taking the T-matrix formalism well outside its nominal domain and applying it to the modes of optically coupled complex structures of liquid droplets.