An 1851 experiment in which the French physicist Hippolyte Fizeau showed that light gets “dragged along” when it travels through a moving medium has been given a 21st-century update by researchers in the US, China and Japan.
在1851年的一項實驗中，法國物理學家Hippolyte Fizeau證實，光穿過移動之介質時，被“拖曳”。這已經由美國、中國及日本的研究人員們，進行了21世紀的最新版實驗。
Working in two independent teams, the researchers observed an analogous effect whereby plasmon-polaritons – hybrid quasiparticles made of photons and oscillating electrons – get dragged by drifting electrons within graphene (a one-atom-thick sheet of carbon).
在兩支獨立團隊的研究中，此些研究人員觀察到一種，類似(由光子及振盪之電子組成的混合準粒子)電漿子(等離子體激元)與極化子，被石墨烯(一個原子厚的碳片)內，漂移之電子拖曳的效應。
The new effect, which is much more pronounced than the one Fizeau found for light, provides an additional tool with which to study nonequilibrium effects in electron fluids and could also lead to improvements in photonics devices.
這種比Fizeau發現之光效應更為明顯的新效應提供了一種，用來研究於電子流體中非平衡效應，及也可能在光子學裝置中，導致諸多改善的額外工具。
Any moving medium can drag a wave that passes through it. If the medium is travelling in the same direction as the wave, the wave’s speed will increase; if the medium’s motion is in the opposite direction, the wave’s speed will decrease.
任何移動的介質皆會拖曳一種，通過它的波動。倘若介質以，與波動相同方向行進，則波動速度會加快。倘若介質以反方向移動，則波動速度會減慢。
In 1818, Augustin-Jean Fresnel predicted that light waves would experience the same effect. However, since light normally travels so quickly, the magnitude of the drag effect is extremely small and it can only be detected using highly sensitive techniques.
於1818年，法國物理學家Augustin-Jean Fresnel預測，光波會經歷同樣的效應。不過，由於光通常很快速行進，拖曳效應的幅度非常小。因此，這只能使用高度敏感的技術，被偵測出。
In the new studies, two teams of researchers – one led by Dmitri Basov of Columbia University and the other by Feng Wang of the University of California at Berkeley – chose to study Fizeau drag in graphene because the electrons and plasmon-polaritons in this material travel at similar speeds: the electrons drift at reasonably large velocities and the long-lifetime plasmon-polaritons propagate much slower than light.
在上述新研究中，兩支研究人員團隊(一支由哥倫比亞大學Dmitri Basov領導，另一支由加州大學柏克萊分校Feng Wang領導)選擇研究石墨烯中的Fizeau拖曳。因為，在這種材料中，電子及電漿子與極化子，以相似的速度行進：電子以相當高的速度漂移，而壽命長之電漿子與極化子的傳播比光慢得多。
The fast-moving electrons therefore make a more efficient drag medium for the plasmon-polaritons than the moving medium in Fizeau’s experiments did for light.
因此，對電漿子與極化子而言，快速移動的電子成為比Fizeau實驗中，移動之介質對光所形成，更為有效的拖曳介質。
Both teams of researchers began their experiments by firing a beam of infrared light at a gold nanobar to “launch” plasmon-polaritons in a two-terminal device made of graphene. The researchers then used a unique near-field nanoscopy technique to take a snapshot of these quasiparticles as they propagated along and against the flow of electrons.
上述兩支研究團隊都藉由，向一根金奈米棒發射一束紅外線光，來“激發”一個由石墨烯製成之雙終端裝置中的電漿子與極化子。之後，研究人員們使用一種獨特的近場奈米鏡觀察技術，來拍攝這些順著及逆著電子流傳播的準粒子快照。
Both groups found that plasmons travelling in the direction opposite to that of the electrons’ flow have a shorter wavelength (and thus lower speed) than those travelling in the same direction. The difference between the two speeds is between 3 and 4%.
這兩團隊發現，與電子流反方向行進的等離子體激元(電漿子)，比那些沿著同方向行進的等離子體激元，具有較短的波長(因此速度較慢)。這兩速度之間的差異，介於3到4%。
Denis Bandurin, a member of Basov’s group, explains that the Fizeau drag effect in graphene can be described by laws similar to those of special relativity. This fact sets graphene’s plasmon-polaritons apart from plasmons in conventional materials, which obey the rules of classical physics.
Basov的團隊成員，Denis Bandurin解釋，石墨烯中的Fizeau拖曳效應，能以類似狹義相對論的那些定律來描述。此事實使石墨烯的電漿子與極化子，不同於傳統材料中，遵循古典物理學法則的電漿子。
In conventional materials, for example, a plasmon’s final velocity is simply the sum of its initial velocity and the electron drift velocity. In graphene, however, electrons behave like massless “Dirac” particles, and must therefore be treated using a quasi-relativistic approach.
譬如，在傳統材料中，電漿子的最終速度，只是其初始速度與電子漂移速度的總和。不過，在石墨烯中，電子表現如同無質量的“狄拉克”粒子。因此，必須使用一種準相對論的方法，來進行處理。
By demonstrating that it is possible to carry out relativistic experiments in a simple tabletop setting, the two teams’ results should open the door to further studies of non-equilibrium light-matter interaction effects at the nanoscale. Their findings may also have applications for photonics devices thanks to a nonlinear property known as non-reciprocity.
藉由證實，可能在簡單的桌面裝置中，進行相對論的實驗。這兩支團隊的研究結果將會，為在此奈米級非平衡之光與物質交互作用效應的進一步研究，開啟門路。由於，被通稱為非相互依存的非線性屬性，他們的研究發現也可能具有，光子學裝置的諸多用途。
This property is normally very difficult to achieve in optical experiments, but it is induced in graphene by the drag effect, and its presence means that graphene’s physical properties change if the direction of time is reversed.
在光學實驗中，這種屬性通常很難實現。不過，在石墨烯中，這是藉由拖曳效應被誘導。因此，其存在意味著，倘若時間方向遭逆轉，則石墨烯的諸多物理屬性發生變化。
Usually, a strong external magnetic field or chiral optical pumping (which requires intense laser light) is needed to break this so-called time-reversal symmetry. Such strong fields and intense light cannot be applied to a real-world device because they would affect all the individual components in it.
通常，需要強大的外部磁場或手徵性的光泵激(這需要強烈的雷射光)，來打破這種所謂的時間逆轉對稱性。如此強大的磁場及強烈的光，無法被應用於現實世界的裝置。因為，它們會影響於其中的所有各個組件。
However, both teams have shown that the flow of drifting electrons might offer an alternative way to break time-reversal symmetry in a graphene-based device. Being able to do this could bring improved control of photonics devices, and perhaps new functionalities as well.
不過，這兩支團隊已經證實，漂移的電子流動可能提供一種，替代方法來打破，於以石墨烯為基礎之裝置中的時間逆轉對稱性。能夠做到這一點，可以導致光子學裝置經改善的控制，或許也會帶來諸多新的功能。
Bandurin and his colleague Yinan Dong are now searching for ways to accelerate the electrons in graphene to even higher velocities so that their flow matches the speed of the plasmon-polaritons in the material much more closely.
目前，Bandurin及其同僚Yinan Dong正在尋找，加速石墨烯中，電子達更高速度的方法，以便它們的流動與此材料中之電漿子與極化子的速度，更密切匹配。
“We will also be looking into Fizeau drag at the technologically important terahertz frequencies, where the effect is expected to be even stronger,” Bandurin tells Physics World. The two groups describe their experiments in back-to-back papers in Nature.
Bandurin 告訴Physics World：「他們也將以技術上重要的兆赫茲頻率(在此頻率，效應被預期會更強大)，來探究Fizeau拖曳。」這兩團隊相繼於《自然》期刊中，記述了他們的實驗。
網址：https://physicsworld.com/a/electrons-in-graphene-drag-light-in-their-wake/
翻譯: 許東榮
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