[论文解读] Resonant phase-matching between a light wave and a free-electron wavefunction

Saar Nehemia, Raphael Dahan+9 more|arXiv (Cornell University)|Oct 12, 2020

Laser-Matter Interactions and Applications参考文献 77被引用 25

一句话总结

该论文通过超快透射电子显微镜中相对论电子波函数与光波之间的相位匹配相互作用，首次实验观测到量子切伦科夫辐射。通过在数百微米范围内保持相位匹配，电子相干地发射和吸收数百个光子，形成类似于频率梳的量子化能量平台，证明了切伦科夫效应的量子本质，并为基于量子电动力学的新应用铺平了道路。

ABSTRACT

As charged particles surpass the speed of light in an optical medium they produce radiation - analogously to the way jet planes surpass the speed of sound and produce a sonic boom. This radiation emission, known as the Cherenkov effect, is among the most fundamental processes in electrodynamics. As such, it is used in numerous applications of particle detectors, particle accelerators, light sources, and medical imaging. Surprisingly, all Cherenkov-based applications and experiments thus far were fully described by classical electrodynamics even though theoretical work predicts new Cherenkov phenomena coming from quantum electrodynamics. The quantum description could provide new possibilities for the design of highly controllable light sources and more efficient accelerators and detectors. Here, we provide a direct evidence of the quantum nature of the Cherenkov effect and reveal its intrinsic quantum features. By satisfying the Cherenkov condition for relativistic electron wavefunctions and maintaining it over hundreds of microns, each electron simultaneously accelerates and decelerates by absorbing and emitting hundreds of photons in a coherent manner. We observe this strong interaction in an ultrafast transmission electron microscope, achieving for the first time a phase-matching between a relativistic electron wavefunction and a propagating light wave. Consequently, the quantum wavefunction of each electron evolves into a coherent plateau, analogous to a frequency comb in ultrashort laser pulses, containing hundreds of quantized energy peaks. Our findings prove that the delocalized wave nature of electrons can become dominant in stimulated interactions. In addition to prospects for known applications of the Cherenkov effect, our work provides a platform for utilizing quantum electrodynamics for applications in electron microscopy and in free-electron pump-probe spectroscopy.

研究动机与目标

• 证明切伦科夫效应的量子本质，超越经典电动力学的范畴。
• 在长距离上实现相对论电子波函数与传播光波之间的相位匹配。
• 在量子 regime 下观测电子对光子的相干、受激发射和吸收。
• 为基于量子电动力学的电子显微镜和泵浦-探测光谱学应用建立平台。

提出的方法

• 利用超快透射电子显微镜，成像相对论电子与光子纳米结构的相互作用。
• 通过调控电子束和光子模式，使系统在数百微米范围内满足切伦科夫相位匹配条件。
• 采用时间分辨电子衍射和能量分析，测量电子波函数能量态的演化过程。
• 通过测量电子能量谱中相干平台的形成，表明光子发射与吸收的同步性。
• 通过高分辨率能量谱仪确认了量子化能量峰的存在。
• 通过调节电子速度和光子模式的相速度，使两者匹配于切伦科夫条件，从而维持相位匹配。

实验结果

研究问题

• RQ1 能否通过电子-光波相互作用，实验观测到切伦科夫效应的量子本质？
• RQ2 在相位匹配的电子-光波系统中，光子的相干、受激发射和吸收是否可能实现？
• RQ3 在量子电动力学条件下，电子波函数是否能演化为类似频率梳的量子化能量平台？
• RQ4 电子的非定域波特性在与光强耦合、相干相互作用中起到何种作用？
• RQ5 相位匹配的电子-光波相互作用能否在电子显微镜和光谱学中催生新型量子基应用？

主要发现

• 在超快透射电子显微镜中，成功实现并维持了相对论电子波函数与光波在数百微米范围内的相位匹配。
• 每个电子通过相干地吸收和发射数百个光子，同时经历加速和减速。
• 电子波函数演化为具有数百个量子化能量峰的相干能量平台，类似于频率梳。
• 观测到的平台结构为切伦科夫效应的量子本质提供了直接证据。
• 相互作用强度和相干性足够强，表明电子的非定域波特性在受激电子-光相互作用中起主导作用。
• 该结果为基于量子电动力学的光源设计以及自由电子泵浦-探测光谱学建立了新平台。