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Severing the Link between Modal Order and Group Index Using Hybrid Guided Space-Time Modes
ACS Photonics ( IF 6.5 ) Pub Date : 2022-06-17 , DOI: 10.1021/acsphotonics.1c01574
Abbas Shiri 1 , Ayman F. Abouraddy 1
Affiliation  

The structure of an optical waveguide determines the characteristics of its guided modes, such as their spatial profile and group index. General features are shared by modes regardless of the waveguiding structure; for example, modal dispersion is inevitable in multimode waveguides, every mode experiences group-velocity dispersion, and higher-order modes usually travel at lower group velocities than their lower-order counterparts. We show here that such trends can be fundamentally altered in a multimode planar waveguide by exploiting hybrid guided space-time modes, whereupon dispersion is eliminated and the link between modal order and group index is altogether severed. Hybrid space-time modes are confined in one-dimension by the planar waveguide, and in the other by the underlying spatiotemporal spectral structure of the field itself. Direct measurements of the modal group delays confirm that the group index for low-loss, dispersion-free, hybrid space-time modes can each be tuned away from the group index of the same-order conventional mode, and that the transverse size of these hybrid modes can be varied independently of the modal order and group index. These findings are verified in a few-mode planar waveguide consisting of a 25.5 mm-long, 4-μm-thick silica film deposited on a MgF2 substrate.

中文翻译:

使用混合引导时空模式切断模态顺序和组索引之间的联系

光波导的结构决定了其导模的特性,例如它们的空间分布和群指数。无论波导结构如何,模式都共享一般特征;例如,模态色散在多模波导中是不可避免的,每个模式都会经历群速度色散,并且高阶模式通常以低于低阶对应模式的群速度传播。我们在这里表明,通过利用混合引导的时空模式,可以从根本上改变多模平面波导中的这种趋势,从而消除色散,并完全切断模态顺序和群指数之间的联系。混合时空模式在一维中被平面波导所限制,而在另一维中则被场本身的时空光谱结构所限制。模态群延迟的直接测量证实,低损耗、无色散、混合时空模式的群指数都可以从同阶常规模式的群指数中调整,并且这些模式的横向尺寸混合模式可以独立于模式顺序和组索引而变化。这些发现在少模平面波导中得到验证,该波导由沉积在 MgF2 上的 25.5 毫米长、4 微米厚的二氧化硅膜组成2基材。
更新日期:2022-06-17
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