Strong Interactions Between Light and Sound

Light is injected into an optical microresonator via a tapered optical fiber. The light circulates many thousands of times inside the structure and couples strongly to high-frequency acoustic waves. (Source: Quantum Measurement Lab, ICL)

Light and high-frequency acoustic sound waves in a tiny glass structure can strongly couple to one another and perform a dance in step. A team of researchers from Imperial College London, the Uni­versity of Oxford, and the National Physical Labora­tory have experi­mentally achieved a long-standing goal to demon­strate the strong-coupling regime between light and high-frequency acoustic vibra­tions. The team’s research will have impact for classical- and quantum-infor­mation processing and even testing quantum mechanics at large scales.

Central to the team’s research are whis­pering-gallery-mode reso­nances where light bounces many times around the surface of a tiny round glass structure shown in the figure above. “It is fascinating that these glass ring reso­nators can store exces­sive amounts of light, which can shake the molecules in the material and generate acoustic waves,” said Pascal Del’Haye of the National Physical Labora­tory.

As the light circulates around the circum­ference of the glass structure it interacts with an 11 GHz acoustic vibra­tion that causes light to be scattered in the reverse direction. This inter­action allows energy to be swapped between the light and sound at a certain rate. However, both the light- and the sound-fields will decay due to fric­tion-like processes, preventing the two from dancing in step.

The team overcame this challenge by uti­lizing two such whis­pering-gallery-mode reso­nances and achieved a coupling rate that is larger than these friction-like processes, allowing the signa­tures of the light-sound dance to be observed. Lead scientist of the project, Georg Enzian at the Univer­sity of Oxford, said: “Achieving this strong-coupling regime was a thrilling moment for us.” Ian Walmsley said: “I’m excited about the near- and longer-term-prospects for this new experi­mental platform.”

Looking ahead, the team are now preparing the next gene­ration of these experi­ments that will operate at tempera­tures close to absolute zero. “This will allow highly sensi­tive quantum mechanical behaviour to be explored and utilized for the development of quantum tech­nologies,” said principal inves­tigator of the project, Michael Vanner from the Quantum Measure­ment Lab at Imperial College London. (Source: ICL)

Reference: G. Enzian et al.: Observation of Brillouin optomechanical strong coupling with an 11  GHz mechanical mode, Optica 6, 7 (2019); DOI: 10.1364/OPTICA.6.000007

Link: Quantum Measurement Lab, Imperial College London, London, UK

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