[论文解读] Highly effective relativistic free carrier plasma mirror confined within a silicon slow light photonic crystal waveguide

Mahmoud Gaafar, Dirk Jalas+5 more|arXiv (Cornell University)|May 13, 2017

Photonic and Optical Devices参考文献 9被引用 3

一句话总结

该论文展示了一种在硅光子晶体波导中实现的高效相对论性自由载流子等离子体镜，仅需1.2×10⁹ W/cm²的泵浦功率，即可在载流子浓度为5×10¹⁷ cm⁻³时实现35%的反射率，远低于传统等离子体镜所需的量级。该效应源于色散工程化的慢光波导中的间接带内跃迁，使探测光与等离子体镜均被限制，从而通过相对论多普勒频移实现强前向反射与频率上移。

ABSTRACT

Reflection at relativistically moving plasma mirrors is a well-known approach for frequency conversion as an alternative to nonlinear techniques. A key issue with plasma mirrors is the need for a high carrier concentration, of order 10^21 cm^-3, to achieve an appreciable reflectivity. To generate such high carrier concentrations, short laser pulses with extreme power densities of the order >10^15 W/cm^2 are required. Here, we introduce a novel waveguide-based method for generating relativistically moving plasma mirrors that requires much lower pump powers and much less carrier concentration. Specifically, we achieve an experimental demonstration of 35% reflection for a carrier concentration of 5*10^17/cm^3 generated by a power density of only 1.2*10^9 W/cm^2. Both the plasma mirror and the signal are confined and propagating within a solid state silicon slow light photonic crystal waveguide. This extraordinary effect only becomes possible because we exploit an indirect intraband optical transition in a dispersion engineered slow light waveguide, where the incident light cannot couple to other states beyond the moving front and has to reflect from it. The moving free carrier (FC) plasma mirror is generated by two photon absorption of 6 ps long pump pulse with a peak power of 6.2 W. The reflection was demonstrated by the interaction of a continuous wave (CW) probe wave co-propagating with the relativistic FC plasma mirror inside a 400 micro-meter long slow light waveguide. Upon interaction with the FC plasma mirror, the probe wave packets, which initially propagate slower than the plasma mirror, are bounced and accelerated, finally escaping from the front in forward direction. The forward reflection of the probe wave packets are accompanied by a frequency upshift. The reflection efficiency is estimated for the part of the CW probe interacting with the pump pulse.

研究动机与目标

• 解决传统相对论性等离子体镜对高泵浦功率和高载流子浓度的依赖问题。
• 在固态波导中利用低功率、低密度自由载流子等离子体实现高效频率上移。
• 利用慢光色散特性与间接带内跃迁，实现光与物质相互作用的增强与限制。
• 在固态波导平台上实验演示连续波探测光的前向反射及可观测的频率上移。

提出的方法

• 采用400 µm长的硅光子晶体波导，其设计用于实现慢光传播。
• 使用6 ps的泵浦脉冲，峰值功率为6.2 W，通过双光子吸收产生自由载流子。
• 将泵浦光与连续波（CW）探测光限制在同一波导模式中。
• 利用色散工程化波导中的间接带内光学跃迁，避免与其它能级耦合。
• 依赖自由载流子等离子体镜的相对论性运动，诱导多普勒频移并实现探测波的反射。
• 通过探测光与运动等离子体前缘相互作用的部分来测量反射效率。

实验结果

研究问题

• RQ1 是否可以在远低于以往要求的载流子浓度和泵浦功率下生成自由载流子等离子体镜？
• RQ2 光子晶体波导中的慢光色散如何增强等离子体镜形成过程中的光与物质相互作用？
• RQ3 间接带内跃迁在限制探测光并实现反射中起到何种作用？
• RQ4 是否可在固态波导平台中实验验证前向反射及频率上移？
• RQ5 在低功率、低密度等离子体条件下，可实现的反射效率是多少？

主要发现

• 实验实现了连续波探测光与相对论性自由载流子等离子体镜相互作用时35%的反射效率。
• 等离子体镜在载流子浓度仅为5×10¹⁷ cm⁻³时即被生成，远低于传统有效反射所需的10²¹ cm⁻³阈值。
• 所需泵浦功率密度仅为1.2×10⁹ W/cm²，显著低于传统方法所需的>10¹⁵ W/cm²。
• 反射后探测光脉冲被向前加速，表明等离子体镜运动成功实现了动量传递。
• 反射探测光的频率上移已得到证实，与相对论多普勒频移物理理论一致。
• 探测光与等离子体在同一波导模式中的限制，使得尽管载流子密度较低，仍能实现强相互作用与高效率。