Recent experiments have revealed a surprising phenomenon in the interaction between light and metal surfaces. Physicists have long believed that light hitting a metal surface at an angle pushes on the free electrons, moving them forward. However, new experiments show the opposite – incident light seems to pull electrons backward.
The experiments were conducted by a team of researchers led by Dr. Michael Strait and Henri Lezec. In their initial experiment, they aimed an infrared laser at a thin gold film held in vacuum, where no stray molecules could affect the metal surface. The team placed an electrode at the far end of the metal sample to detect any voltage created if the light drove the electrons along the metal.
As they varied the angle, the team measured a voltage that largely agreed with theoretical expectations based on the simple light-pushing-electrons picture. However, the voltage they measured was the opposite of that expected, implying that the current flow was in the wrong direction.
“It’s a weird effect,” says Strait. “It’s as if the electrons are somehow managing to flow backward when hit by the light.” The team also conducted experiments with the metal exposed to air, and in striking contrast, the light pushed the electrons forward. These results show that the previous experiments weren’t actually inconsistent, but that light’s interactions with a metal surface are far more complex than was previously believed, says Strait.
“We can no longer ignore this counterintuitive backwards current,” he says. “It is a surprising indication of the complexity of the solid-state system.” “This finding is most unexpected and puzzling,” says quantum optics specialist Stephen Barnett of the University of Glasgow in the UK. “I’m really not quite sure what to make of it. Clearly there remains much to be understood in the interaction between light and surfaces”.
“One very speculative idea, says Lezec, is that the incident photons may act not only on the free electrons in the metal, but also on so-called core electrons, which are not free to move through the material”. In some unknown way, he suggests, these might act to produce a current in the opposite direction.
Source: https://www.nist.gov/programs-projects/photon-drag-metal-films
Keywords: light, electrons, photons, optical force, current
