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Scientists Successfully Reveal Backflow of Optical Light Propagating Forward

For the first time, scientists at Tel Aviv University have revealed the optical backflow of light propagating forward.

Image Credit: Tel Aviv University.

Although quantum physicists had theorized this phenomenon more than five decades ago, it has never been demonstrated effectively in any experiment, until now.

This ‘backflow’ phenomenon is quite delicate and requires exquisite control over the state of a particle, so its demonstration was hindered for half a century. This phenomenon reveals an unintuitive behavior of a system comprised of waves, whether it’s a particle in quantum mechanics or a beam of light.

Dr Alon Bahabad, Department of Physical Electronics, School of Electrical Engineering, Tel Aviv University

Bahabad headed the research for this analysis.

Our demonstration could help scientists probe the atmosphere by emitting a laser beam and inducing a signal propagating backward toward the laser source from a given point in front of the laser source. It’s also relevant for cases in which fine control of light fields is required in small volumes, such as optical microscopy, sensing and optical tweezers for moving small particles,” added Dr Bahabad.

Dr Bahabad’s graduate students, Dr Yaniv Eliezer, currently at Yale University, and Thomas Zacharias, performed the study, which was published in the Optica journal on January 16th, 2020.

In a sense, light shares similar properties with quantum particles—that is, both can be developed from interfering waves. This kind of construction, where many waves are combined together to create a new wave, is called a superposition. If a unique superposition of waves, all propagating forward, is built, then the overall wave can achieve the so-called “optical backflow.”

The researchers performed a holography experiment, in which they divided and restructured a laser beam in the form of light waves. These light waves propagated at positive angles with regard to an axis.

The varying beams of light beams had to be carefully built, with accurate values for their delay and strength. After creating the superposition, a tiny slit was set and moved in a direction perpendicular to the beam to actually quantify the beam’s direction in different places.

The light exiting from the small slit was disclosed in a majority of the locations as traveling at a positive angle. However, in certain locations, the light that escaped the tiny slit propagated at a negative angle, although the light striking the other side of the small slit included a superposition of beams that all propagated at a positive angle.

We used holography to create a clear manifestation of the backflow effect. We realized at some point that we can utilize a previous study of ours, where we discovered the mathematical phenomenon known as suboscillation, to help us design a beam of light with backflow.

Dr Alon Bahabad, Department of Physical Electronics, School of Electrical Engineering, Tel Aviv University

Bahabad continued, “To conclude, if interfering waves, all going in one direction, are constructed in a special manner, and you were to measure the direction of propagation of the overall wave at specific locations and times, you just might find the wave going backward. This wave can describe a particle using quantum mechanics.

This surprising behavior violates any intuition that we gained from our daily experience with the movement of macroscopic objects. Nevertheless, it still obeys the laws of nature.

Dr Alon Bahabad, Department of Physical Electronics, School of Electrical Engineering, Tel Aviv University


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