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Scientists Discover Way to Send Information into Black Holes Without Using Energy

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Researchers Create a “Reverse-Flowing” Electromagnetic Wave for the First Time. Here's Why It Matters

For decades, scientists have been fascinated by unusual electromagnetic (EM) waves that behave in ways ordinary light cannot. One of the most exciting examples is the backward wave — a special type of wave where the energy flow and the direction of the wave’s movement are opposite. This strange phenomenon is closely connected with advanced technologies such as negative refraction, super-resolution imaging, and next-generation electromagnetic devices.

Now, researchers led by Liu and his team have achieved an important breakthrough by experimentally demonstrating backward wave propagation in a specially designed plasmonic metamaterial. Their work shows that engineered metallic structures can guide electromagnetic waves in reverse, opening new possibilities for compact circuits and advanced communication technologies operating at microwave and terahertz frequencies.


Understanding the Mystery of Backward Waves

In normal electromagnetic waves, the phase velocity and group velocity move in the same direction. The phase velocity describes the movement of the wave pattern, while the group velocity represents the direction in which energy and information travel.

However, in a backward wave, these two velocities move in opposite directions. The wave itself appears to travel backward while the energy moves forward. This unusual property is one of the key features behind negative refractive materials, which can bend light in the opposite direction compared with ordinary materials.

Backward waves became a major research topic with the development of metamaterials — artificially engineered materials designed to control electromagnetic waves in unusual ways. These materials can create effects that are impossible with natural substances, including negative refraction and imaging beyond the traditional diffraction limit.

Such properties have inspired ideas like perfect lenses and hyperlenses, which could potentially allow scientists to observe details much smaller than the wavelength of light.

The Role of Surface Plasmon Polaritons

A major area of interest in this field involves surface plasmon polaritons (SPPs). These are special electromagnetic waves that travel along the boundary between a metal and a dielectric material.

Unlike normal light waves that spread freely, SPPs can become tightly confined near a surface. This allows them to concentrate electromagnetic energy into extremely small regions, making them valuable for creating miniaturized optical circuits and nanoscale devices.

However, traditional SPPs have a major limitation. At optical frequencies, metals absorb energy strongly, causing the waves to lose their strength after traveling only a short distance. Additionally, directly using optical SPP concepts at lower frequencies such as microwaves is difficult because metals behave almost like perfect conductors.

To overcome this problem, scientists developed a new type of artificial structure known as spoof surface plasmon polaritons.

Creating Artificial Plasmons with Metamaterials

Spoof SPPs imitate the behavior of natural surface plasmons but work at much lower frequencies, including microwave and terahertz ranges. They are created by modifying metal surfaces with carefully designed patterns, such as tiny grooves, holes, or corrugations.

These artificial structures allow researchers to control how electromagnetic waves move across a surface. By changing the shape and design of the metal pattern, scientists can adjust the wave properties and create effects similar to those found in optical plasmon systems.

Liu’s team developed a specially designed plasmonic metamaterial based on a corrugated metallic strip. The structure was further improved by adding an interdigital pattern inside the grooves, which increased electromagnetic field concentration and strengthened the interaction between the wave and the metal surface.

This design created a unique waveguide capable of supporting backward propagation.

First Clear Experimental Evidence of Backward Spoof SPP Waves

Although theoretical studies had predicted that plasmonic structures could support backward waves, direct experimental proof had remained difficult.

The main challenge was that plasmonic waves usually suffer from high energy losses, making it extremely difficult to observe their movement clearly.

The new plasmonic metamaterial designed by Liu and his team solved this problem. Their structure provided stronger wave confinement while reducing unwanted energy loss. Through electromagnetic simulations and laboratory measurements, the researchers observed a dispersion relationship with a negative slope — a clear signature of backward wave behavior.

The experiments confirmed that the phase velocity and group velocity moved in opposite directions. In other words, the electromagnetic wave was truly propagating as a backward wave.

This achievement represents one of the first direct demonstrations of backward spoof surface plasmon polariton propagation in a practical plasmonic structure.

A New Path Toward Advanced Electromagnetic Circuits

To demonstrate the practical importance of their discovery, the researchers created a special device called a contra-directional coupler.

This device can control the direction of microwave signals depending on their operating frequency. At one frequency, the signal can travel in one direction, while at another frequency it can be redirected toward the opposite side using backward coupling.

Such technology could become useful for future integrated electromagnetic circuits, allowing engineers to design smaller and more flexible systems for microwave and terahertz applications.

Because the proposed structure is thin, planar, and easy to manufacture, it could potentially be adapted for practical devices such as compact communication components, sensors, and advanced signal-processing systems.

Future Possibilities

The discovery of backward wave propagation in plasmonic metamaterials is more than just an experimental achievement. It provides a new way to manipulate electromagnetic energy and opens the door to technologies based on unusual wave behavior.

Backward waves are closely linked with several extraordinary phenomena, including negative refraction and sub-diffraction imaging. By controlling these waves, scientists may develop future devices capable of overcoming the limitations of conventional electromagnetic systems.

The work by Liu and his team demonstrates that carefully engineered metallic surfaces can act as powerful platforms for controlling waves at the microscopic level. As research continues, these plasmonic metamaterials may play an important role in the development of next-generation microwave and terahertz circuits.

The ability to guide electromagnetic waves backward could eventually lead to smaller, faster, and more efficient technologies — bringing futuristic concepts from theoretical physics closer to real-world applications.

Reference: Liu, X., Feng, Y., Zhu, B. et al. Backward spoof surface wave in plasmonic metamaterial of ultrathin metallic structure. Sci Rep 6, 20448 (2016). https://doi.org/10.1038/srep20448

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