[论文解读] Microspherical photonics: Giant resonant light forces, spectrally resolved optical manipulation, and coupled modes of microcavity arrays

Yangcheng Li|arXiv (Cornell University)|Sep 3, 2015

Orbital Angular Momentum in Optics参考文献 169被引用 5

一句话总结

该论文通过光谱调控的激光镊子，在介电微球中实现了巨大的共振光学力，其力的大小接近动量守恒极限。通过精确调节激光波长至导波模（WGM）共振，实现了光谱分辨的微球操控与分选，且分选均匀性极高；同时，耦合微腔阵列表现出稳定且可分辨的模式分裂图案，可用于光子分子的识别。

ABSTRACT

In this dissertation novel resonant propulsion of dielectric microspheres is studied with the goal of sorting spheres with identical resonances, which are critical for developing microspherical photonics. First, evanescent field couplers were developed by fixing tapered microfibers in mechanically robust platforms. The tapers were obtained by chemical etching techniques. Using these platforms, WGMs modal numbers, coupling regimes and quality factors were determined for various spheres and compared with theory. Second, the spectroscopic properties of photonic molecules formed by spheres with better than 0.05% uniformity of WGM resonances were studied. It was shown that various spatial configurations of coupled-cavities present relatively stable mode splitting patterns in the fiber transmission spectra which can be used as spectral signatures to distinguish such photonic molecules. The third part is the study of giant resonant propulsion forces exerted on microspheres. This effect was observed in suspensions of polystyrene spheres with sufficiently large diameters. By integrating optical tweezers for individual sphere manipulation, the wavelength detuning between a tunable laser and WGMs in each of the spheres was precisely controlled. Resonant enhancement of optical forces was directly demonstrated in experiments. The spectral shape, position and magnitude of the observed propulsion force peaks were explained by efficient transfer of light momentum to microspheres under resonant conditions. The peak magnitude of the resonant force is shown to approach total absorption limit imposed by the conservation of momentum. The transverse movement of the spheres during the propulsion process was studied and the existence of a stable radial trap was demonstrated. Giant resonant propulsion forces can be used for large-scale sorting of microspheres with ultrahigh uniform resonant properties.

研究动机与目标

• 开发一种利用锥形光纤实现对介电微球精确、光谱分辨光学操控的平台。
• 研究由微球构成的光子分子的光谱特征，其WGM共振均匀性优于0.05%。
• 实验演示并量化微球在WGM共振附近产生的巨大共振光学推进力。
• 利用共振力增强实现基于其共振光学响应的大规模微球分选。
• 在共振推进过程中识别稳定的径向捕获条件，以实现受控操控。

提出的方法

• 通过化学腐蚀法制备锥形光纤，并将其集成于机械坚固的平台中，实现稳定的倏逝场耦合。
• 实验测量了导波模（WGM）特性——模数、耦合状态和品质因数——并与理论模型进行比较。
• 通过高共振均匀性的微球组装形成光子分子，并分析其光纤透射光谱中的模式分裂图案。
• 使用可调谐激光精确控制悬浮微球中激光波长与单个WGM之间的失谐。
• 通过追踪共振激发下微球的横向运动测量光学力，力的大小通过动量传递推断得出。
• 基于动量守恒的理论建模，解释了共振力的光谱形状、位置及峰值大小。

实验结果

研究问题

• RQ1 在共振激发下，能否在介电微球中实验观测并量化巨大的共振光学力？
• RQ2 对WGM共振进行光谱调谐如何影响微球上光学力的大小与方向？
• RQ3 耦合微腔阵列（光子分子）在光纤透射光谱中的稳定光谱与空间特征是什么？
• RQ4 共振力在多大程度上可接近微球中的理论动量传递极限？
• RQ5 在共振推进过程中能否实现稳定的径向捕获，以实现受控操控与分选？

主要发现

• 实验验证了巨大的共振光学力，其峰值大小接近由动量守恒所决定的总动量吸收极限。
• 观察到推进力的光谱整形，其峰值大小与位置与WGM共振条件直接相关。
• 在共振力作用下微球的横向运动表明存在稳定的径向陷阱，从而实现受控操控。
• 由WGM共振均匀性优于0.05%的微球构成的光子分子在光纤透射光谱中表现出明显且稳定的模式分裂图案。
• 测量得到的微球品质因数与耦合状态与基于Mie理论和波导耦合理论模型的理论预测高度一致。
• 由共振力增强实现的光谱分辨操控，使得具有极高共振均匀性的微球可实现高精度分选。