Optical spring enables programmable defect mode in new mechanical crystal

Mechanical crystals, also known as phononic crystals, are materials that can control the propagation of vibrations or sound waves, just like photonic crystals control the flow of light. The introduction of defects in these crystals (i.e., intentional disruptions in their periodic structure) can give rise to mechanical modes within the band gap, enabling the confinement of mechanical waves to smaller regions or the materials—a feature that could be leveraged to create new technologies.

Researchers at McGill University recently realized a new mechanical crystal with an optically programmable defect mode. Their paper, published in Physical Review Letters, introduces a new approach to dynamically reprogram mechanical systems, which entails the use of an optical spring to transfer a mechanical mode into a crystal’s band gap.

“Some time ago, our group was thinking a lot about using an optical spring to partially levitate structures and improve their performance,” Jack C. Sankey, principal investigator and co-author of the paper, told Phys.org. “At the same time, we were watching the amazing breakthroughs in our field with mechanical devices that used the band gap of a phononic crystal to insulate mechanical systems from the noisy environment.”

After witnessing recent breakthroughs in the development of mechanical devices, Sankey and his colleagues started exploring the possibility of optically springing up the drumhead-like resonance of a membrane with a periodic array of holes punched in it. They predicted that this would allow them to drag the frequency into a band gap, drawing the vibrational energy inward like a tractor beam and significantly reducing the resonance’s inertial mass.

“We figured this weird situation in which the number of photons present affects how heavy a mechanical system ‘feels’ would present a lot of new opportunities,” said Sankey. “We did some promising calculations, notably finding that larger structures respond more to each photon, and that an average of a single photon in the apparatus could in principle have a measurable effect on the motion of a very feasible, centimeter-scale device.”

To demonstrate their approach, the team, led by Ph.D. student Tommy Clark, first patterned and released a membrane using standard photolithography techniques. They then aligned the fiber cavity near this membrane’s center, using tight-tolerance guide ferrules.

“We mounted the whole thing on a vibration-isolating stage in ultrahigh vacuum and used additional active feedback to stabilize the cavity mirrors to within the ~10s of picometers required for the laser light to enter the cavity near its natural resonance frequency,” explained Sankey. “Once the system is assembled and stabilized, we used the cavity’s resonant enhancement to create an intense optical field that applies a spring-like pressure to a small section of the membrane.”

Using this optical spring, the researchers deliberately disrupted their membrane’s periodic pattern, generating a defect. By adjusting the laser’s intensity, they could then dynamically and reversibly modify the properties of the defect they introduced.

“I have always loved the idea of coupling light to the shape and mass of a mechanical resonance, but there are a host of interesting applications as well, from new studies of mechanical dissipation to simulations of condensed matter systems,” said Sankey.

“There is also currently a great deal of interest in employing mechanical systems to store and transport quantum information on chip, and to connect nominally disparate quantum systems to each other. Mechanical systems are versatile tools, and Tommy’s (incredible) work demonstrates a qualitatively new way to manipulate motion with light.”

The team’s new approach for the in situ reconfiguration of mechanical defects could open new interesting possibilities for the creation of reprogrammable mechanical systems. For instance, arrays of such defects they generated could be used to program waveguides or other structures designed to route and reroute the flow of mechanical information.

“In the near future, we are most looking forward to exploring the idea that each photon interacts with many similar mechanical resonances simultaneously, while also connecting them all to each other through the same radiation force,” added Sankey. “This creates a dense ‘web’ of interactions that enhances the influence of each photon, and I am interested in leveraging this to generate increasingly macroscopic quantum states of motion.”

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