Light and Sound Signals Now Flow in One Direction Only, Researchers Find
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A new approach to controlling the flow of both light and sound within integrated systems has been achieved. Divya Mishra and Parvendra Kumar at Indian Institute of Technology Delhi theoretically reveal a method for achieving nonreciprocal photon-phonon conversion in coupled optomechanical cavities, enabling signals to travel more effectively in one direction than another. The method distinguishes itself from previous research by showing that this nonreciprocal conversion can occur without violating time reversal symmetry, and importantly, demonstrates a pathway to sharply enhance isolation levels, reaching up to 40 dB through precise control of laser phase. These findings could be key for the development of advanced technologies including optical isolators and highly sensitive sensors. High-efficiency phonon isolation and directional photon-phonon conversion in optomechanical cavities Phonon isolation reached a new high of 60 dB, a substantial improvement over previous methods which struggled to exceed 40 dB. Sharply enhanced control over acoustic wave directionality is now possible, previously unattainable with conventional techniques reliant on magnetic fields. Researchers at the Indian Institute of Technology Delhi demonstrated that this isolation, alongside 40 dB photon-phonon conversion isolation, is achieved by carefully optimising synthetic flux and coupling rates within coupled optomechanical cavities. Their theoretical work reveals a pathway to nonreciprocal photon-phonon conversion, where signals favour one direction, without violating the principle of time reversal symmetry, a key distinction from existing nonreciprocal phonon transport methods. These levels of isolation, measured in decibels, represent the degree to which sound waves can be directed.
The team at the Indian Institute of Technology Delhi utilised coupled optomechanical cavities, structures that link light and mechanical vibrations, to manipulate these waves. Optimisation of ‘synthetic flux’, a technique mimicking magnetic fields without needing actual magnetic materials, and precise tuning of the interaction rates between light and mechanical vibrations within the cavities achieve this control. Controlling the directional transmission of photonic and phononic signals in integrated platforms is fundamentally and practically important in contemporary photonics and cavity optomechanics. Path-dependent asymmetry enables time-reversal-symmetric nonreciprocal photon-phonon conversion Nonreciprocity in photon-phonon conversion can occur without violating time reversal symmetry, arising from path-dependent asymmetry in the conversion process. Modifying the phase difference of the driving lasers can further enhance this nonreciprocity, achieving isolation levels of up to 40 dB. Non-reciprocal devices, such as isolators and circulators, are essential for signal processing and quantum technologies, but traditional approaches rely on magneto-optical materials that disrupt time-reversal symmetry through external magnetic fields. These methods face limitations due to the weak response of such materials at optical frequencies and their limited compatibility with integrated systems. Recent experimental demonstrations show that phase-controlled optical drives can induce complex hopping amplitudes in coupled optomechanical cavities, resulting in synthetic magnetic flux and enabling directional transport of photons. Quantum transduction of microwave photons to optical photons and qubit read out have also been demonstrated using hybrid optomechanical systems. This letter presents a theoretical investigation of non-reciprocal phonon transport and the conversion between photon and phonon signals in coupled optomechanical cavities, involving coupling between two optical modes and two mechanical modes. The research found that, in the case of photon-phonon conversion, breaking time-reversal symmetry via synthetic magnetic flux is not essential. Nonreciprocal photon-phonon conversion arises from interference between nonidentical paths during forward and backward conversion, and this interference is robust against variations in system parameters. Appropriately choosing the value of synthetic flux can sharply enhance phonon isolation, defined as the ratio of forward and backward transmission of phonons. Synthetic flux, controlled through the phases of the driving lasers, can tune both phonon isolation and photon-phonon conversion isolation. With optimised synthetic flux and coupling rates between the optical and mechanical modes, phonon isolation of 60 dB and photon-phonon conversion isolation of 40 dB can be achieved. Analysis focused on a system composed of two coupled cavities, each supporting a localized optical mode and a mechanical mode, coupled through vacuum optomechanical interaction. Additionally, the system incorporates coupling between the optical and mechanical modes, achieved by physically connecting the two optomechanical cavities using waveguides, and is coupled to external waveguides to explore the coupling and transmission of photon and phonon signals. The linearized Hamiltonian describing the system, including the coupling with input photon and phonon signals, is given as [Appendix A] Hlin = −∑ j=L,R ∆j a†j aj + J a†LaR + a†RaL + ∑ j=L,R ωmj b†j bj + V b†LbR + b†RbL + ∑ j=L,R Gj eiφjaj + e−iφja†j bj + b†j + i ∑ j=L,R pκej ηj ajei(ω−ωd)t −a†j e−i(ω−ωd)t + i ∑ j=L,R pγej ζj bjeiωt −b†j e−iω. The detuning of the optical mode with respect to the driving laser frequency is denoted by ∆j = ωdj − ωcj, where ωdj is the frequency of the laser driving cavity j ∈ {L, R} and ωcj is the corresponding cavity resonance frequency. The mechanical resonance frequencies of the left and right cavities are given by ωmL and ωmR, respectively. Optical and mechanical operators are denoted by a and b, respectively. Careful inspection of the Hamiltonian reveals the parameters governing the system’s dynamics. The study investigated nonreciprocal phonon transport and photon-to-phonon conversion. Optimising the magnitude and phase of the synthetic flux maximizes nonreciprocal phonon isolation. The isolation reaches a maximum value of approximately 40 dB around 5.9GHz for φ = 1.42π. For the opposite synthetic flux, φ = −1.42π, the isolation becomes negative, indicating enhanced backward photon-to-phonon conversion and demonstrating the tunability of isolation via phase control. At the Indian Institute of Technology Delhi, a theoretical demonstration of nonreciprocal phonon transport and the conversion between photon and acoustic phonon signals in coupled optomechanical cavities was performed via phase-dependent driving. Unlike nonreciprocal phonon transport, which requires both dissipation and phase-induced violation of time reversal symmetry, nonreciprocity in photon-phonon conversion can occur without violating time reversal symmetry. This arises from path-dependent asymmetry in photon-phonon conversion. Furthermore, modifying the phase difference of the driving lasers can enhance the nonreciprocity of photon-phonon conversion, achieving isolation levels of up to 40 dB. Demonstrating unidirectional light-sound interaction within integrated microchips Controlling the flow of both light and sound is critical for building the next generation of compact optical devices and highly sensitive sensors. This work demonstrates a new route to achieving this control, manipulating signals at the interface between light and mechanical vibrations within microchips.
The team acknowledges a significant hurdle: their current theoretical model relies on precise optimisation of several system parameters to achieve substantial isolation. Maintaining this level of precision in a real-world device, susceptible to manufacturing imperfections and external disturbances, presents a considerable engineering challenge. Even acknowledging the need for precise calibration, this demonstration of non-reciprocal photon-phonon conversion represents a valuable advance. Non-reciprocity, where signals travel more easily in one direction, is key for building advanced components like optical isolators, preventing unwanted reflections that can disrupt sensitive measurements. This work reveals a pathway to achieve this control without necessarily needing complex system symmetry, potentially simplifying future device designs. This theoretical work establishes that directing the flow of light and sound within integrated systems needn’t always require breaking time-reversal symmetry, a principle governing how physical processes unfold in time. The research successfully demonstrated nonreciprocal photon-phonon conversion within coupled optomechanical cavities, achieving isolation levels of up to 40 dB. This means signals converting between light and mechanical vibrations can be directed more easily in one direction than another. Importantly, this nonreciprocity occurs without violating time reversal symmetry, potentially simplifying the design of future devices. The authors note that achieving these results requires precise optimisation of system parameters, which presents an engineering challenge for practical application. 👉 More information 🗞 Phase-enhanced nonreciprocal photon-phonon conversion via coupled optomechanical cavities 🧠 ArXiv: https://arxiv.org/abs/2604.01879 Tags: