In a recent experimental study, scientists have provided evidence for a phenomenon previously studied only theoretically: The similarity in how light behaves when moving through a gravitational field and through special media called hyperbolic metamaterials.
“When light propagates through a conventional […] transparent material, like glass, with refractive index n, it takes longer for a ray to reach its destination, so the ‘optical length’ in this case is defined as conventional length multiplied by the refractive index n,” explained Vera Smolyaninova, a professor of physics at Towson University and one of the authors of the study, in an email.
“In anisotropic materials, like crystals, the refractive index is different in different directions, so that the length in each direction is multiplied by its own factor,” she continued. “In hyperbolic metamaterials, which are extremely anisotropic, some light propagation directions start to behave as if they are additional [time] dimensions.”
Due to their unique optical properties, hyperbolic metamaterials have proven promising in many fields, such as sensing and electromagnetic wave guides used in telecommunications. It could even be possible to create a sort of invisibility cloak using hyperbolic materials, where their distinctive ability to interact with electromagnetic waves causes light to bend around an object, essentially making it invisible.
In order to achieve these applications in practice, scientists are exploring these materials on a theoretical basis and in a recent study published in Annalen der Physik, they have discovered that light traveling through hyperbolic materials behaves in a similar way to light in curved space i.e., under gravity’s influence as described by Einstein’s general theory of relativity.
In previous studies, aspects of light’s behavior in hyperbolic materials, such as reflection and refraction, had been tested experimentally, but in the present study, Smolyaninova and her colleagues were able to test predictions made by the similarity between the influence of hyperbolic materials and gravity on light when applied to the the formation and interaction of filaments in these materials.
“When intense light propagates through a nonlinear optical material, it changes its refractive index,” said Smolyaninova. “As a result, light starts to focus itself. The resulting self-focused light channels are called ‘optical filaments’.”
To study the behavior of the filaments, the physicists first prepared a hyperbolic metamaterial by placing a cuvette filled with kerosene that iron/cobalt alloy nanoparticles suspended inside using a magnetic field. In the absence of magnets, the nanoparticles are distributed within the kerosene randomly, but application of an external magnetic field leads to the formation of metal nanocolumns within the fluid, a hyperbolic metamaterial.
The scientists then passed a laser beam through this cuvette, and using powerful optics observed the optical filaments that formed within the beam. These appeared to be well resolved, were several tens of microns thick, and were relatively stable, lasting about 10 seconds.
The team determined the way in which the filaments developed their shape, collided, and merged with each other, and showed that all these processes occur in the same way as in a gravitational field, confirming theoretical predictions and carrying important implications for both fundamental science and its applications in industry.
“From the fundamental science point of view, our findings are interesting because three-dimensional gravity is an exactly solvable theory, even in the quantum gravity limit, while quantum gravity [in our four-dimensional spacetime] remains an unsolved ‘holy grail’ of theoretical physics,” explained Igor Smolyaninov, research scientist at the University of Maryland and another of the study’s authors. “From an applications point of view, the fact that we can make large three-dimensional metamaterial samples without sophisticated nanofabrication is very useful.”
“Such metamaterials may be used in novel nonlinear optical devices,” he added. “Optical signal processing is much faster compared to its electronic counterpart.”
Quantum effects were not taken into account when studying the interaction of filaments, but addressing them could help scientists better understand quantum gravity, which has resisted any attempts to unravel it using conventional approaches. These experiments may also help uncover the properties of filaments that arise in other physical systems that have a similar mathematical description.
“It would be very interesting to explore the [optical properties] of quantum hyperbolic metamaterials,” concluded Smolyaninov. “It is known that [some superconductors allow the existence of vortices similar to optical filaments] and may exhibit hyperbolic metamaterial behavior, so a superconductor may start to exhibit behaviors similar to 2+1 dimensional quantum gravity. This would be very interesting indeed.”
Reference: Vera N. Smolyaninova, Igor I. Smolyaninov, et al., Study of Effective 2+1 Dimensional Gravity in Ferrofluid-Based Hyperbolic Metamaterials, Annalen der Physik (2023), DOI: 10.1002/andp.202300408
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