World's First Success in Real-Time Optical Control of Synchronization Between Microscopic Oscillators
～Toward next-generation information technology inspired by brain mechanisms～

News Highlights:

Background

When two independent metronomes are placed on the same platform, they interact through the platform, and their initially irregular rhythms eventually align. This phenomenon is called synchronization [Figure 1(a)]. Synchronization appears in many situations, from the collective flashing of fireflies and the chorus of frogs to technologies such as matching the frequencies of two lasers. It spans scales from microscopic to macroscopic and fields from natural science to engineering applications. The ability to freely control synchronization on artificial devices is extremely important for long-term stabilization between different systems. For example, just as with the metronome case, synchronizing the clocks of two devices with high precision makes it possible to realize devices that keep highly stable time [Figure 1(b)].

At the same time, synchronization is believed to be closely related to information processing in neural networks, which are networks of neurons in the brain¹. The way oscillators synchronize to the same rhythm is similar to the way groups of neurons reach the same excited state, and both are thought to be explained by common principles. In other words, if synchronization can be freely controlled using artificial devices, it could lead to the realization of artificial neural networks² with information processing capabilities comparable to those of the human brain.

In neural circuits in the brain, large numbers of neurons continuously change their states over time, thereby performing information processing as a "function"³. However, as in the metronome case, stabilizing into a single synchronization state alone is not sufficient to handle information. To enable functional behavior between synchronized oscillators (metronomes), it is important to generate multiple synchronization states and introduce a method for transitioning between them at desired timings. Conventional synchronization control technologies, however, have been limited because the interactions between oscillators (the "platform") were fixed by the shape of the device. As a result, it was difficult to control the type of interaction needed to generate multiple synchronization states or to adjust the interaction strength to switch between synchronization and desynchronization at desired timings.

In this study, NTT established a new method to create interactions using light between two mechanical oscillators vibrating about 50 million times per second⁴, and successfully realized multiple distinct synchronization states as well as controlled transitions between them at desired timings [Figure 1(c)].

Figure 1
(a) Example of synchronization in metronomes (oscillators).
(b) Oscillation waves that remain synchronized in a single state.
(c) Oscillation waves that transition over time between different synchronization states.
Synchronization A: a state in which oscillations are synchronized in the same direction.
Synchronization B: a state in which oscillations are synchronized in opposite directions.

Key Technical Points

■ Proprietary Fiber-Type Optomechanical Device

Using its expertise in glass processing, NTT fabricated an optomechanical device⁵ with a narrow neck introduced into a glass fiber as thin as a human hair (Figure 2). In the bottle-shaped section sandwiched by the narrow neck, "mechanical vibrations," in which the structure expands and contracts, and "optical resonance," in which light circulates by total internal reflection along the surface, strongly influence each other (Figure 3). By adjusting this bottle-shaped structure, two mechanical vibrations with different frequencies can be used simultaneously. This proprietary design enabled the realization of an optomechanical device that incorporates two mechanical oscillators (metronomes) interacting with light in a single microstructure.

■ Achieving Synchronization Between Mechanical Oscillators Through Optical Intensity Modulation

To realize synchronization between oscillators, coupling (the "platform") that links the oscillators is required. In this study, a new method was established that creates coupling between two mechanical oscillators by using light intensity modulated at the frequency difference between them. Synchronization between the oscillators was achieved. Furthermore, by comparing asynchronous and synchronous states, it was confirmed that this method suppressed frequency variation by more than a factor of 1,000, resulting in highly stabilized frequencies.

■ Precise Control of Phase Slips Through Synthesized Optical Modulation

A phase slip refers to a phenomenon in which the relative relationship between two synchronized mechanical oscillators shifts by one full oscillation or by an integer fraction. Inducing a phase slip allows transitions between different synchronization states. In this study, a new method was established in which special coupling effects that multiplex synchronization states were induced by light. By temporally varying the strength and frequency of these effects, phase slips could be generated at desired timings with high precision.

Figure 2 Schematic of the fiber-type optomechanical device and its optical microscope image. The orange region in the microscope image shows the fiber-type optomechanical device with a neck (red arrow).

Figure 3 Conceptual diagram of the interaction between light and mechanical vibrations in the fiber-type optomechanical device.

Overview of the Experiment

An optomechanical fiber device was fabricated by introducing two narrow necks of about 78 micrometers into a glass fiber with a diameter of 80 micrometers. A tapered optical fiber, thinned to about 1 micrometer, was placed in orthogonal contact with the device to resonate laser light (Figure 3). When the input laser frequency was tuned near the sum of the optical resonance frequency and the mechanical vibration frequency, the optical energy was transferred to mechanical vibrations, enabling the vibrations to be excited by light. Because the output light from the resonator changes in intensity according to the mechanical vibrations, it was possible to achieve both vibration control and vibration measurement using a single laser beam. Utilizing these control and measurement techniques, self-sustained oscillations⁶ of the two mechanical oscillators were observed. This corresponds to the case in the initial example where two independent metronomes were prepared.

Next, coupling between the oscillators was created by modulating the intensity of the incident laser light at the frequency difference of the two mechanical oscillators coexisting in one bottle-shaped structure. In the experiment, synchronization was evaluated by measuring the beat signal of the two vibrations from the output light. When optical intensity modulation was applied, the position of the beat signal (the phase difference) was confirmed to remain constant [Figure 4(a)]. This clearly demonstrated that synchronization of mechanical oscillators, corresponding to the synchronization of metronomes, was achieved through light.

Finally, the intensity modulation was synthesized by combining the frequency difference between the mechanical oscillators with its second or third harmonic [Figure 4(b) and (c)]. By temporally varying the phase of the difference-frequency modulation signal, a special coupling effect was created that enabled both synchronization multiplexing and phase slips. As a result, two or three phase slips occurred within one cycle, successfully causing transitions between multiple different synchronization states. These states correspond to vibration conditions shifted by 180 degrees or 120 degrees in phase, which is equivalent to metronomes moving in the same rhythm while their directions of oscillation or pointer positions change instantaneously.

Figure 4 Synchronization of the beat signal positions (phase differences) generated by the novel optical intensity modulation method. The colors of the optical intensity plots (left) correspond to the shaded regions indicating phase jumps in the central plot.

Outlook

This experiment has established the fundamental technology for controlling synchronization between mechanical oscillators using light. By extending the real-time synchronization control demonstrated between two oscillators to larger numbers of oscillators, this achievement is expected to open the way to new information processing technologies based on microscopic vibration devices. In particular, because synchronization between oscillators can be freely designed with light, the results are anticipated to contribute to the development of bio-inspired technologies that enable advanced information processing, similar to neurons in the brain that are interconnected through diverse and complex interactions.

Acknowledgments

This research and development was supported by the Japan Society for the Promotion of Science (JSPS) through the Grant-in-Aid for Scientific Research (S), "Development of Ultrasonic Topological Phononics for Multifunctional Elastic Wave Devices" (Project No. JP21H05020), and "Ultrahigh-speed magnophononic resonator devices" (Project No. JP23H05463).

Publication Information

Journal: Science Advances
Title: Synthesized Kuramoto potential via optomechanical Floquet engineering
Authors: Motoki Asano, Hajime Okamoto, and Hiroshi Yamaguchi
DOI: 10.1126/sciadv.ady4167
URL: https://doi.org/10.1126/sciadv.ady4167

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[Glossary]

¹Reference on the relationship between synchronization phenomena and neural networks:
J. J. Hopfield, "Neural networks and physical systems with emergent collective computational abilities", Proc. Natl. Acad. Sci. USA 79, 2554-2558 (1982).

²Artificial Neural Networks:
By preparing multiple small laser devices, micro magnets, or electrical circuits, and creating special interactions among carriers of information such as light, magnetic poles, or electric current, various information processing functions that mimic parts of the brain can be realized. This approach is expected to enable new IoT sensors, actuators, and other devices with adaptive and autonomous capabilities.

³Reference on effective information transfer through synchronization in neural circuits:
P. Fries, A mechanism for cognitive dynamics: neuronal communication through neuronal coherence, Trends Cogn. Sci. 9, 474 (2005).

⁴Mechanical Oscillator:
A structure that mechanically vibrates, similar to a guitar string or drumhead, fabricated at micro- or nanoscale. A common example is the quartz oscillator used in wristwatches. In this study, fiber-type devices that expand and contract were used.

⁵Optomechanical Device:
Light exerts a force that can push or pull objects. An optomechanical device couples this optical force with mechanical vibrations. Using such devices allows ultrahigh-sensitivity vibration measurements far beyond conventional laser vibrometers and enables exploration of various light-driven vibration phenomena.

⁶Self-Sustained Oscillation:
A phenomenon in which a system continuously maintains strong and stable oscillations by drawing energy from its surroundings without any externally applied periodic force or signal. Similar to how blowing into a flute produces a steady tone, simply introducing light generates strong oscillations at a stable frequency.

About NTT

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