Brillouin scattering – the phenomenon named after French physicist Léon Brillouin (1889-1969) – occurs when light waves and mechanical vibrations interact with a material. It can be observed when photons – the elementary particles of electromagnetic force (or simply, light particles) – come into contact with a material medium, emitting or absorbing phonons, which is vibrational energy that arises from the collective oscillation of atoms.
In fiber optic telecommunications, Brillouin scattering is one of the factors that restricts the information transmitted, especially along long-distance communication lines, where just a few milliwatts of power are enough to cause the photons to return to the source of the emission rather than proceed on to the receiver.
Researchers at the Gleb Wataghin Physics Institute (IFGW) of the University of Campinas (UNICAMP) have studied this and other optomechanical effects – resulting from the interaction of light with mechanical movements – in order to be able to manipulate them.
Thiago Pedro Mayer Alegre, a professor in the Department of Applied Physics, is one of the principal investigators of the group, and he talked about the interaction between light and sound on photonic structures at FAPESP Week New York, organized jointly by the City University of New York (CUNY) and the Wilson Center November 26-28, 2018, at the Graduate Center of CUNY.
“Tight confinement provided by physical structures can be used to tune or enhance the dynamic coupling between photons, electrons and phonons. In photonic structures, this enhancement enables a range of novel functionalities, such as changing the color of light in non-linear effects, generating radio-frequency signals, suppressing stimulated light-scattering and manipulating the mesoscopic phonon modes,” said Alegre. In the physics of condensed matter, mesoscopic physics describes phenomena that occur on the scale between macroscopic and microscopic.
According to Alegre, any of these functionalities requires a fine control over the design and fabrication of the microstructure that shapes the optical and acoustic phonon spectra, as well as their interaction.
“We have obtained important findings as a result of the study, such as the design and fabrication of nano-wave guides [structures that guide waves, such as electromagnetic or other sound waves] and optomechanical cavities that can enhance or suppress these interactions,” he said.
Photonics has applications in a wide variety of fields, such as energy, manufacturing, robotics, displays (for smartphones, for example), health and communications. The earliest devices developed on the basis of photonic principles were light-emitting diode semiconductors in the 1960s, and low-loss optical fibers in the decade that followed.
With funding from FAPESP, Alegre and his colleagues developed a new type of optomechanical device that uses a microscopic silicon disk to confine optical and mechanical waves. The new device is compatible with commercial fabrication processes and can be a way to improve sensors that detect force and movement. The device was described in an article published in the journal Optics Express.
“We designed the device to allow an increase in the levels of interaction between the light waves and the mechanical waves that move through it. That way, the device will have practical applications as well as support our basic research, helping to answer some of the questions that arise with regard to the transition between the quantum microscopic world and the classic macroscopic world,” Alegre told Agência FAPESP.
The device created by the researchers, based on a 24 µm silicon disk on a pedestal of silicon dioxide to enable disc vibration, is shaped like a bullseye with nanometric concentric circular grooves. This bullseye design enables it to confine the light and mechanical waves in the device using separate mechanisms (read more about it at: agencia.fapesp.br/24820).
The researchers at IFGW-Unicamp also theoretically developed a silicon photonic device that could enable the interaction between optical and mechanical waves that vibrate on the range of several gigahertz (GHz).
As a result of the projects “Nanophotonics in Group IV and III-V semiconductors” and “Optomechanics in photonic and phononic crystals”, both funded by FAPESP, the device was described in an article published in the journal Scientific Reports.
The researchers proposed, by means of computational simulations, a device to explore Brillouin scattering that could be transposed onto photonic microchips (read more at: agencia.fapesp.br/25137).
Andrea Alu, director of the Advanced Science Research Center (ASRC) of the CUNY Graduate Center, spoke at FAPESP Week New York about research studies his group is conducting to control light in metamaterials. These are artificial materials modified so as to acquire desired properties that do not exist naturally.
“We have a full and ambitious program of basic research directed towards introducing and developing new ideas and revolutionary concepts that will enable us to model, design, analyze, fabricate and characterize metamaterials for the next generation of electromagnetic systems and integrated photonics,” he said.
Researchers at the ASRC are using new theoretical tools (including analytical and numerical methods), techniques for fabricating nanometer-scale objects, two-dimensional materials, and advances in fundamental physics regarding the interaction of light and matter in metamaterials and optomechanics.
“The study of light on the nanoscale has become a vibrant field of research, in that scientists have now mastered the flow of light at length scales well below that of optical waves, exceeding the classic limits imposed by diffraction,” said Alu.
For more information about FAPESP Week New York, visit: www.fapesp.br/week2018/newyork.
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