Glass Mediates Interaction Between Photons

Microscope image of the laser written waveguide array fabricated in glass. Laser light traveling through the ornately microfabricated glass has been shown to interact with itself. (Source: Rechtsman Lab, Penn State)

Laser light traveling through ornately micro­fabricated glass has been shown to interact with itself to form self-sustaining wave patterns – solitons. The intricate design fabricated in the glass is a type of photonic topological insulator, a device that could potentially be used to make photonic technologies like lasers and medical imaging more efficient. Topo­logical materials have the ability to protect the flow of waves through them against unwanted disorder and defects. Until now, our under­standing of topological protection of light has been mostly limited to particles of light acting inde­pendently, but now, researchers at Penn State report that they have used the glass to mediate inter­action between photons, directly observing the funda­mental wave patterns of these intricate devices.

“People are perhaps more familiar with electronics, but there is a whole parallel world of photonics, where we are concerned with the pro­perties of light instead of electrons,” said Mikael Rechtsman, Downsbrough Early Career Development Professor of Physics at Penn State. “There are myriad applications of photonics, including in solar energy, fiber optics for tele­communication, manu­facturing using laser cutting, and lidar, which is used, for example, to help control autonomous vehicles. Topo­logical protection offers the promise to make photonic devices more energy efficient, lighter, and more compact.”

The concept of topo­logical protection can be applied in electronic, photonic, atomic, and mechanical systems. In electronics, for example, topo­logical protec­tion can improve efficiency by getting electrons to flow reliably through a material without scattering. For electrons this protection requires extremely cold tempera­tures, nearing absolute zero, and very often a strong external magnetic field, but with photons all of the experiments can be performed at room tempera­ture, and because photons do not have a charge, without a magnetic field.

To perform their experiments, the researchers shine a laser through a piece of glass that has a series of extremely precise tunnels carved through it, each with a diameter of about one-tenth that of a human hair. These wave­guides act like wires, concentrating the flow of light through them. The wave­guides in the piece of glass are arranged in a grid, forming an array, but the path of each waveguide through the glass is not straight – it is perhaps better described as serpentine, with twists and turns designed by the researchers with a geometry that leads to the topological protection of light.

“We had to build the fabri­cation facility in our lab to precisely carve the three-dimen­sional wave­guides through the glass with femto­second laser writing,” said Sebabrata Mukherjee, a postdoctoral researcher at Penn State. “The ability to write three-dimen­sional waveguides is crucial to making the device topological, a property that is confirmed experi­mentally by observing the ‘protected’ one-way flow of light along the edge of the device.” Through the Kerr effect, the properties of the glass are changed due to the presence of the intense laser light. This change in the glass mediates an inter­action between the many photons, which usually do not interact, propa­gating through the array. As the power was increased, the light collapsed into a beam that didn’t spread out, but rather rotated in spirals. The spiral rotation of the solitons is a signature of the specific shape of the wave­guides designed by the researchers and an indicator that the device is, indeed, topo­logical.

“Under normal circum­stances, photons are oblivious to one another,” said Rechtsman. “You can cross two laser beams and neither will be changed by the other. In our system, we were able to get photons to interact and form solitons because the intensity of the laser altered the pro­perties of the glass. The photons became aware of each other through the change in their environment.” Solitons are known to be the most funda­mental waveforms in many systems where interaction is mediated by the sur­rounding environment. “Theo­retically under­standing and experi­mentally probing solitons in topo­logical systems like our waveguide arrays will be a key ingredient in applying topo­logical protection for practical use in photonic devices, especially those that require high optical power,” said Rechtsman. (Source: PSU)

Reference: S. Mukherjee & M. C. Rechtsman: Observation of Floquet solitons in a topological bandgap, Science 368, 856 (2020); DOI: 10.1126/science.aba8725

Link: Laboratory for emergent phenomena and technology in the optical sciences, Dept. of Physics, Pennsylvania State University, University Park, USA, 

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