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Topological phonon transport in an optomechanical system

Physics

Topological phonon transport in an optomechanical system

H. Ren, T. Shah, et al.

This groundbreaking research by Hengjiang Ren, Tirth Shah, Hannes Pfeifer, and their colleagues reveals the experimental realization of topological phonon transport in an optomechanical device. With over 800 cavities designed for site-resolved measurements, they significantly advance the miniaturization of mechanical topological systems.

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Playback language: English
Introduction
The field of cavity optomechanics has advanced significantly, allowing light to be used not just for measuring mechanical motion but also for manipulating it at the level of individual phonons. Miniaturization efforts have led to small-scale optomechanical circuits for on-chip manipulation of mechanical and optical signals. Theoretical proposals suggest that larger optomechanical arrays could enable topologically protected phonon transport, a goal pursued to reduce the size of mechanical topological devices to the nanoscale. This approach aims to utilize hypersonic frequency acoustic wave circuits for robust delay lines and non-reciprocal elements, potentially controlling heat-carrying phonon flow. This paper details the experimental observation of topological phonon transport in a device comprising over 800 optomechanical elements on a silicon microchip, achieving unprecedented carrier frequency and bandwidth compared to existing nanoscale implementations.
Literature Review
The authors reference significant prior work in cavity optomechanics, highlighting the development of small-scale circuits and theoretical proposals for topologically protected phonon transport in larger arrays. They cite various platforms for realizing topological mechanical devices, including pendula, sound waves in fluids, and vibrations in solids. The review emphasizes the importance of scaling down these devices to the nanoscale and the potential for controlling heat flow through topological channels. Specific works on optomechanical circuits, non-reciprocity, and topological insulators in different contexts are referenced to establish the existing body of knowledge and the novelty of the current work.
Methodology
The researchers designed and fabricated a multiscale optomechanical crystal (OMC) device. Unlike standard single-scale OMCs, this device uses a superlattice structure with two different lattice spacings: a larger snowflake-shaped pattern (16.02 µm) defining a phononic crystal, and a smaller cylindrical hole pattern (450 nm) within each unit cell, forming a photonic crystal that hosts high-Q optical nanocavities for site-resolved readout. The snowflake pattern, borrowed from existing designs and theoretical proposals for topological phononics, is modified to create topologically distinct mechanical domains using local changes in the phononic crystal lattice. These domains are designed to support helical edge states based on the Valley Hall effect. The photonic crystal pattern is incorporated to enhance optomechanical interaction. Finite-element method (FEM) simulations were used to model the phononic band structures and mechanical modes. The design allows for tuning the mechanical properties by adjusting the size of the holes. Breaking mirror symmetry by altering hole sizes creates a bandgap and hosts helical edge states. The device fabrication is detailed, including electron-beam lithography and etching techniques. The experimental setup uses a tunable laser coupled to an optical fiber taper for site-resolved optical readout of phonons. The thermal mechanical motion is measured by analyzing intensity modulations in the transmitted laser light using a spectrum analyzer. The sensitivity of the cavity-based measurement is high enough to detect thermal vibrations with amplitudes on the order of 10 fm. Scattering matrix calculations, using FEM simulations as input, were employed to model the expected spectra. Different cavity structures, including a triangular cavity and tree-shaped cavities, were fabricated and measured to test the robustness of topological transport.
Key Findings
The experimental results demonstrate topological phonon transport in the 0.3 GHz band, with a bandwidth of 15 MHz. Site-resolved measurements show a dramatic difference between low-frequency modes (321–327 MHz), exhibiting strong modulation indicating reflection and standing waves, and higher-frequency modes (327–337 MHz), exhibiting no fringes and indicating backscattering-immune running waves. This signifies the formation of a topological mechanical cavity. Comparison of measured and theoretical noise power spectral densities (NPSD) shows excellent agreement, confirming the absence of backscattering in the topological bandwidth. Experiments with tree-shaped cavities with varying segment lengths further confirm the robustness of topological transport, with transmission exceeding 95%. In contrast, a trivial cavity shows strong backscattering, highlighting the topological protection of the edge states. The researchers also theoretically investigated the effect of fabrication disorder on the system, finding a round trip reflection probability of about 1% for a standard deviation of 10 nm in geometrical parameters.
Discussion
The experimental demonstration of topological phonon transport in the multiscale optomechanical crystal validates the theoretical proposals and provides a practical platform for on-chip phononic circuits. The achievement of high frequency (0.3 GHz) and bandwidth (15 MHz) surpasses previous nanoscale implementations. The observed robustness against backscattering confirms the topological protection of the edge states. The results open possibilities for actively controlling topological circuits using optomechanics, enabling applications like cooling, mechanical lasing, and generation of nonclassical quantum states. Future directions include exploring higher frequencies (up to 100 GHz) using advanced lithography, implementing thermal diodes and topologically protected phonon amplification/lasing, and using the platform to investigate quantum acoustodynamics for quantum information processing.
Conclusion
This work successfully demonstrates topological phonon transport in a multiscale optomechanical crystal, achieving high frequency and bandwidth. The robustness of the observed transport validates the topological protection of the edge states and opens avenues for realizing on-chip phononic circuits with active optical control and diverse applications in quantum technologies. Future research should focus on extending the frequency range and exploring applications in quantum acoustodynamics.
Limitations
The study focuses on thermal phonons at room temperature. The theoretical model makes some approximations that might not perfectly capture the system's behavior, leading to slight deviations from ideal topological transport. The achievable topological bandwidth is limited by sharp domain walls and imperfections, and this could be improved using smoother walls. The influence of fabrication disorder is considered theoretically, but its impact in the actual experiments needs to be quantified more precisely.
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