The idea that light has momentum is not new, but the exact nature of the interaction between light and matter has been a mystery for the past 150 years. In a paper just published in the journal Nature Communications, researchers may reveal one of the darkest secrets of light.
Study the history of optical radiation pressure
â—‹The trajectory of a comet between planets drawn by Kepler in De cometis in 1619. | Image source: Linda Hall Library
In 1619, the famous German astronomer and mathematician Kepler proposed for the first time that the pressure from sunlight may be the reason why the tail of a comet always points away from the sun. He drew the trajectory of a comet through the planets in the book "De cometis" (pictured above). The three arc-shaped orbits surrounding the sun are Mercury orbit (Sphaera Mercurii), Venus orbit (Sphaera Veneris), and Earth orbit (Sphaera Tellurae). The clusters of short lines in the figure represent the tails of comets, and they always deviate from each other. The direction of the sun.
â—‹ Comet Hale-Bopp (C/1995 O1) is one of the brightest and most spectacular comets discovered outside Jupiter's orbit. Its return period is 2533 years. The solar radiation pressure causes the ice in the comet to evaporate, forming a long gas dust tail, pointing away from the sun. | Image source: E. Kolmhofer, H. Raab; Johannes Kepler Observatory, April 4, 1997
It was not until 1873 that Maxwell predicted that light is a kind of electromagnetic radiation, and this radiation pressure is derived from the momentum that exists in the electromagnetic field of light itself.
When the electromagnetic wave is emitted to the surface of an object, the momentum of the electromagnetic wave and the momentum of the object exchange each other, thereby generating pressure acting on the surface of the object, that is, radiation pressure. If the incident momentum is perpendicular to the plane, then the momentum change is exactly twice that of the incident momentum, and the resulting object will also be twice the incident momentum (as shown in the figure below).
Kenneth Chau, the co-author of this study, said: “Before this, we are still not sure how this momentum is transformed into force or motion. Because the momentum carried by light is very small, we have not yet had enough sensitive equipment to do so. solve this problem."
Experiment: How to convert photon momentum into force
Now that technology has caught up, Chau and international research teams from Slovenia and Brazil are trying to reveal this mystery.
In order to measure this extremely weak interaction between photons and matter, researchers must keep interference and background noise to a minimum. They built a special mirror equipped with an acoustic sensor and a thermal barrier, using a solid dielectric as the material of the mirror, and alternately covering the surface with a thin film with a reflectivity of up to 99.93% consisting of two materials: ZrOâ‚‚ and SiOâ‚‚. , The purpose is to reduce the heat generated by the absorption of light radiation.
â—‹ Experimental setup: The material of the mirror is a solid dielectric (gray), and the ZrOâ‚‚ and SiOâ‚‚ covered on the surface form a thin film (green) with a reflectivity of up to 99.93%. When the laser pulse is irradiated vertically on the mirror surface, elastic waves (red and blue arcs) are generated in the material, which propagate through the material at the speed of sound. | Image source: [1]
Then, when the laser pulse is irradiated perpendicularly to the mirror surface, elastic waves will be generated in the material, which are like the ripples of water in a pond. They will propagate in the material at the speed of sound and form picometers (10^ -12m) spikes of magnitude. Elastic waves cause material displacement, so with the help of sensitive acoustic sensors, researchers can measure the elastic waves passing through the mirror by detecting the displacement of the mirror, thereby recording the process of light interacting with objects.
Chau said: "We cannot directly measure the momentum of the photon, so our method is to detect the impact of the photon momentum by'listening' to the elastic wave passing through the mirror. We can probe the characteristics of these waves along the way, until we trace back to the laser. The momentum of the pulse itself, which opens the door for the final definition and simulation of the photon momentum inside the material."
â—‹The propagation process of the elastic wave in the mirror: the laser pulse is irradiated on the mirror, (g) firstly generate the primary wave (P-wave), (h) then the secondary wave (Secondary wave, S-wave), ( j) Then a Rayleigh wave (R-wave) is generated, and the primary wave reaches the bottom of the mirror and is reflected back, further forming a more complex waveform in (kl). | Image source: [1]
The elastic wave caused by the optical radiation pressure propagates in the material, causing a displacement perpendicular to the surface of the material (z direction). The vertical displacement detected by the sensor in the experiment is consistent with the simulation result that excludes the influence of thermoelastic waves and uses radiation pressure as the only energy source of elastic waves. This shows that radiation pressure is the absolute dominant factor that causes the vertical displacement of the surface, rather than absorbed Heat or other forces.
This discovery is important for advancing our basic understanding of light, and Chau also pointed out the practical application of radiation pressure.
Practical application
"Imagine traveling on an interstellar yacht driven by a solar sail to a distant star. Or on Earth, developing optical tweezers that can assemble microscopic machines."
○(Left) Japan’s IKAROS is the world’s first probe mainly driven by solar sails, which are interstellar kites accelerated by solar radiation. Its English name is abbreviated as IKAROS, or Icarus. It is a character in Greek mythology. His wings made of wax and bird feathers flew towards the sun, flying higher and higher, and eventually the wax wings melted because of being too close to the sun and fell to the sea. . (Right) The crescent-shaped Venus seen from Icarus. | Image source: Andrzej Mirecki & JAXA
"We have not gone that far, but the discovery of this work is an important step, and I am looking forward to where it will take us in the next step."
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