Five Surprising New Discoveries About Light

We might think that after centuries of studying light, we know everything about it.

Indeed, we have made breakthrough after breakthrough in the use of light, from illumination to communication, from studying the micro and macro universes to scanning our own bodies. We know that light is an electromagnetic wave, thanks to James Clerk Maxwell, whose equations established this in 1865, and we know that light is a quantum packet of electromagnetic energy known as a photon, which Albert Einstein recognized in 1905.

But the more we study light, the more we see and the more we learn. The classical view of light as a wave is still generating new science as light waves interact with man-made "metamaterials"; we are still exploring the view of light as a quantum particle. Both approaches offer ways to manipulate light that were once the stuff of science fiction.

Here are the five most recent advances in optics.

The magical invisibility rings and cloaks of fantasy stories reflect the ancient human dream of hiding things and people from view. Cloaking also appears in science fiction, such as in Star Trek, where hostile Romulan ships hide themselves with cloaking devices. This utilizes a concept from the theory of relativity: namely, that strongly distorted space-time causes light to bend around the ship as if it were not there.

Physicists don't yet know how to do this, but the classical optics of light waves and light rays point to another solution. What we see is the interaction of an object with incoming light. In principle, an invisibility cloak could intercept these incoming rays and bend or refract them into the cloak itself so that they propagate within the cloak and emerge along their original path. An observer seeing what appears to be undisturbed light will think there is nothing there, just as flowing water splits smoothly around a rock and then reassembles itself without seeing the rock downstream. But for the light to follow this complex path, the cloak needs to be made of metamaterials.

In 2006, researchers first tested this idea with a rigid metamaterial invisibility cloak. This invisibility cloak is a hollow cylinder with thousands of small structures in its walls that allow microwaves to travel the right path within the walls. Placed around an opaque metal object, the cloak enabled the object to almost completely disappear under the microwave radiation. Since then, researchers have made small inanimate objects, a fish, a cat and a hand disappear under ordinary visible light, but only when seen from a narrow viewpoint. Others have developed a flexible cloak that wraps around small objects and makes them disappear, but only at one wavelength ......

Schematic of an adaptive hypersurface cloak that supports deep learning

A chronological summary of paper works on various optical cloaking methods

Now that research on invisibility is booming, we are approaching Harry Potter's marvelous cloak.

Photons, like thrown stones, transfer momentum to an object upon impact. This radiation pressure is what pushes the comet's tail away from the sun by sunlight, and it's also what allows it to propel spacecraft.

In 2010, the Japan Aerospace Exploration Agency (JAXA) launched IKAROS, an interstellar kite spacecraft accelerated by solar radiation in honor of the mythical Icarus who flew close to the sun. Its thin, tennis-court-sized polymer sail gathered solar photons that together exerted a tiny force that steadily accelerated IKAROS; six months and 300 million miles later, it reached its target near Venus without using any fuel for propulsion. Now, JAXA and other space agencies are considering longer missions with larger, more efficient solar sails.

Remarkably, a light source can also pull an object toward itself, in the opposite direction of light propagation. Physicists have shown that in a specially shaped laser beam, the forward thrust of photons on particles is dominated by the reverse force generated by the electromagnetic response of the particles themselves. This effect is enough to pull microscopic objects like biological cells backwards toward the laser.

However, a related experiment in 2023 showed that a low-power laser could pull a relatively large macroscopic object (0.2 inches x 0.1 inches). This is not the powerful "tractor beam" of science fiction that can sweep away an entire spacecraft, but it could provide a new way to remotely sample the atmospheres of Earth and other planets, as well as phenomena such as comet tails.

Scientists from China use lasers to create macroscopic tractor beams

Suppose you wanted to image objects like living cells that could be altered or harmed by light energy. Ghost imaging techniques utilize the phenomenon of photon entanglement to produce excellent images of objects that are barely illuminated. Entangled photon pairs formed by certain optical processes are quantum correlated, so that measuring the properties of one pair immediately reveals the properties of the other, no matter the distance.

In ghost imaging, each beam in an entangled photon pair interacts with an object and encounters a detector that simply records the arrival of the photon pair. The second beam of the corresponding entangled photon pair never touches the object, but goes directly to a sensitive multi-pixel detector. A computer analyzes the correlation between the results of the two detectors, and a high-quality image of the object can be generated, even if the light is weak.

This approach can be used to convert images taken covertly with invisible infrared light into visible light images detected by a high-resolution camera; or to obtain high-quality X-ray images from patients exposed to low doses of relatively safe X-rays.

In the X-ray ghost imaging method, an object is imaged using a correlation (computationally analyzed) between the intensities of two beams: an "object beam" that hits the object and reaches a single-pixel detector, and a "reference beam" that does not hit the object and reaches a multi-pixel detector. ".

In the famous double-slit experiment, first performed in 1801, a beam of light is split as it passes through two narrow slits in an opaque barrier. At the far end, the beams spread out and overlapped, creating a pattern of bright and dark areas on the screen, demonstrating that light is made up of waves that can interfere with each other. However, in the modern version of the experiment, aiming only one photon at a time at the slit still produces a wave-like interference pattern. According to Richard Feynman, this amazing and still unexplained example of wave-particle duality "lies at the very heart of quantum mechanics ...... It contains the only mystery".

Now, physicists have reproduced the experiment using a time slit rather than a space slit. They used a thin film of indium tin oxide (ITO), which is transparent to infrared light but rapidly becomes reflective in 10-15 seconds when excited by a laser. In the experiment, the researchers directed infrared light at the ITO, and when the ITO was turned into a mirror for a short period of time, the reflected infrared light remained in its original form. However, when the ITO mirror opened and closed very briefly twice in a row, the reflected infrared light clearly showed that it had disturbed itself by passing through not one but two time portals or slits.

One observer commented that this work could become a classic like the original double-slit experiment. By extending it to time rather than space, the work also shows that it is feasible to use metamaterials such as ITO to control light at ultrafast speeds in optical systems and quantum computers.

If there is one physical fact that is well known, it is that light is the fastest thing in the universe, traveling at 186,000 miles per second in a vacuum. When light interacts with ordinary matter, the speed is reduced, for example, to 124,000 miles per second in optical fiber and ordinary glass. This is still fast enough to circle the Earth in a fraction of a second; so it made big news in 1999 when Harvard researcher Lene Hau dramatically reduced the speed of light to the 38 miles per hour that humans can achieve.

This was achieved in a peculiar medium, a dense gas composed of sodium atoms cooled to near absolute zero. The result was a quantum medium known as Bose-Einstein condensed matter. Light interacts more strongly with this medium than with any ordinary medium, so it slows down considerably. Later, scientists added to this achievement by bringing light to a screeching halt, and then later restoring it and continuing on its way.

These results were a breakthrough in fundamental physics, and could have been very useful, except for the need to work at temperatures close to absolute zero. But since that initial work, other researchers have slowed the speed of light in gases and solids at room temperature, making it possible to use slowed and stopped light in practical devices. These devices are currently being developed, for example, to synchronize signals in fiber-optic networks and to store digital data in computers.

Both applications are important steps towards the development of advanced telecommunication networks and quantum computers based entirely on light rather than conventional electronic chips.

Reference links:

[1]https://pubs.aip.org/aip/jap/article/129/23/231101/286411/Optical-cloaking-and-invisibility-From-fiction

[2]https://opg.optica.org/oe/fulltext.cfm?uri=oe-31-2-2665&id=525052

[3]https://physics.aps.org/articles/v9/103

[4]https://www.nature.com/articles/d41586-023-00968-4

[5]https://www.nytimes.com/1999/02/18/us/researchers-slow-speed-of-light-to-the-pace-of-a-sunday-driver.html?searchResultPosition= 6

[6]https://bigthink.com/the-future/discoveries-about-light/