Dynamical Backaction Research Articles

Optical measurement and feedback control of a ferromagnetic mechanical resonator

The measurement and control of mechanical resonators are critical for cavity magnomechanics, which has emerged as an important frontier for hybrid quantum systems based on magnonics. Traditional microwave-based measurements require handling high-frequency signals and cannot achieve field distribution detection. Here, we demonstrate a method for optically measuring and manipulating a ferromagnetic mechanical resonator. This technique allows for direct observation of the response and field distribution of mechanical oscillations during magnomechanically induced transparency/absorption processes, confirming that the mechanical mode S1,2 is coupled with the magnon. The optical measurement not only serves to validate the magnomechanical coupling theory but also reduces the necessity for high-frequency measurements. Furthermore, we utilize the instantaneously measured results to implement feedback control of the self-oscillating mechanical resonator to overcome the dynamical back-action limit, achieving a threefold enhancement of phonon lasing amplitude. This feedback control lays the foundation for the study of quantum cavity magnomechanics, such as the feedback cooling of a magnomechanical resonator.

Photon Pressure with an Effective Negative Mass Microwave Mode.

Harmonic oscillators belong to the most fundamental concepts in physics and are central to many current research fields such as circuit QED, cavity optomechanics, and photon pressure systems. Here, we engineer a microwave mode in a superconducting LC circuit that mimics the dynamics of a negative mass oscillator, and couple it via photon pressure to a second low-frequency circuit. We demonstrate that the effective negative mass dynamics lead to an inversion of dynamical backaction and to sideband cooling of the low-frequency circuit by a blue-detuned pump field, which can be intuitively understood by the inverted energy ladder of a negative mass oscillator.

Hybrid cavity-antenna architecture for strong and tunable sideband-selective molecular Raman scattering enhancement.

Plasmon resonances at the surface of metallic antennas allow for extreme enhancement of Raman scattering. Intrinsic to plasmonics, however, is that extreme field confinement lacks precise spectral control, which would hold great promise in shaping the optomechanical interaction between light and molecular vibrations. We demonstrate an experimental platform composed of a plasmonic nanocube-on-mirror antenna coupled to an open, tunable Fabry-Perot microcavity for selective addressing of individual vibrational lines of molecules with strong Raman scattering enhancement. Multiple narrow and intense optical resonances arising from the hybridization of the cavity modes and the plasmonic broad resonance are used to simultaneously enhance the laser pump and the local density of optical states, and are characterized using rigorous modal analysis. The versatile bottom-up fabrication approach permits quantitative comparison with the bare nanocube-on-mirror system, both theoretically and experimentally. This shows that the hybrid system allows for similar SERS enhancement ratios with narrow optical modes, paving the way for dynamical backaction effects in molecular optomechanics.

Membrane-in-the-middle optomechanics with a soft-clamped membrane at milliKelvin temperatures

Soft-clamped silicon nitride membrane resonators are capable of coherence times τ exceeding 100 ms at millikelvin bath temperatures. However, harnessing strong optomechanical coupling in dry dilution refrigerators remains a challenge due to vibration issues and heating by optical absorption. Here, we address these issues with an actuator-free optical cavity and mechanical resonator design, with the cavity mounted on a simple vibration-isolation platform. We observe dynamical backaction when the cavity is driven with a free-space optical beam stabilized close to the red sideband using a two-beam locking scheme. Finally, we characterize the effect of absorption heating on coherence time, finding it scales with the intracavity power P as τ ∝ P−(0.34±0.04).

Integrated microcavity optomechanics with a suspended photonic crystal mirror above a distributed Bragg reflector.

Increasing the interaction between light and mechanical resonators is an ongoing endeavor in the field of cavity optomechanics. Optical microcavities allow for boosting the interaction strength through their strong spatial confinement of the optical field. In this work, we follow this approach by realizing a sub-wavelength-long, free-space optomechanical microcavity on-chip fabricated from an (Al,Ga)As heterostructure. A suspended GaAs photonic crystal mirror is acting as a highly reflective mechanical resonator, which together with a distributed Bragg (DBR) reflector forms an optomechanical microcavity. We demonstrate precise control over the microcavity resonance by change of the photonic crystal parameters. We find that the microcavity mode can strongly couple to the transmissive modes of the DBR. The interplay between the microcavity mode and a guided resonance of the photonic crystal modifies the cavity response and results in a stronger dynamical backaction on the mechanical resonator compared to conventional optomechanical dynamics.

Cavity Optomechanical Bistability with an Ultrahigh Reflectivity Photonic Crystal Membrane

AbstractPhotonic crystal (PhC) membranes patterned with sub‐wavelength periods offer a unique combination of high reflectivity, low mass, and high mechanical quality factor. Using a PhC membrane as one mirror of a Fabry–Perot cavity, a finesse as high as is demonstrated, corresponding to a record high PhC reflectivity of and an optical quality factor of . The fundamental mechanical frequency is 426 kHz, more than twice the optical linewidth, placing it firmly in the resolved‐sideband regime required for ground‐state optical cooling. The mechanical quality factor in vacuum is , allowing values of the single‐photon cooperativity as high as . Optomechanical bistability is easily observed as hysteresis in the cavity transmission. As the input power is raised well beyond the bistability threshold, dynamical backaction induces strong mechanical oscillation above 1 MHz, even in the presence of air damping. This platform will facilitate advances in optomechanics, precision sensing, and applications of optomechanically‐induced bistability.

Coherent optical control of a superconducting microwave cavity via electro-optical dynamical back-action

Recent quantum technologies have established precise quantum control of various microscopic systems using electromagnetic waves. Interfaces based on cryogenic cavity electro-optic systems are particularly promising, due to the direct interaction between microwave and optical fields in the quantum regime. Quantum optical control of superconducting microwave circuits has been precluded so far due to the weak electro-optical coupling as well as quasi-particles induced by the pump laser. Here we report the coherent control of a superconducting microwave cavity using laser pulses in a multimode electro-optical device at millikelvin temperature with near-unity cooperativity. Both the stationary and instantaneous responses of the microwave and optical modes comply with the coherent electro-optical interaction, and reveal only minuscule amount of excess back-action with an unanticipated time delay. Our demonstration enables wide ranges of applications beyond quantum transductions, from squeezing and quantum non-demolition measurements of microwave fields, to entanglement generation and hybrid quantum networks.

Engineered entropic forces allow ultrastrong dynamical backaction.

When confined within an optical cavity light can exert strong radiation pressure forces. Combined with dynamical backaction, this enables important processes, such as laser cooling, and applications ranging from precision sensors to quantum memories and interfaces. However, the magnitude of radiation pressure forces is constrained by the energy mismatch between photons and phonons. Here, we overcome this barrier using entropic forces arising from the absorption of light. We show that entropic forces can exceed the radiation pressure force by eight orders of magnitude and demonstrate this using a superfluid helium third-sound resonator. We develop a framework to engineer the dynamical backaction from entropic forces, applying it to achieve phonon lasing with a threshold three orders of magnitude lower than previous work. Our results present a pathway to exploit entropic forces in quantum devices and to study nonlinear fluid phenomena such as turbulence and solitons.

Molecular Optomechanics Induced Hybrid Properties in Soft Materials Filled Plasmonic Nanocavities.

The optomechanical interaction between nanocavity plasmons and molecular vibrations can result in interfacial phenomena that can be tailored for sensing and photocatalytic applications. Here, we report for the first time that plasmon-vibration interaction can induce laser-plasmon detuning dependent plasmon resonance linewidth broadening, indicating energy transfer from the plasmon field to collective vibrational modes. The linewidth broadening accompanied by the large enhancement of the Raman scattering signal is observed as the laser-plasmon blue-detuning approaches the CH vibrational frequency of the molecular systems integrated in gold nanorod-on-mirror nanocavities. The experimental observations can be explained based on the molecular optomechanics theory that predicts dynamical backaction amplification of the vibrational modes and high sensitivity of Raman scattering when the plasmon resonance overlaps with the Raman emission frequency. The results presented here suggest that molecular optomechanics coupling may be manipulated for creating hybrid properties based on interactions between molecular oscillators and nanocavity electromagnetic optical modes.

Optical Coherent Feedback Control of a Mechanical Oscillator

Feedback is a powerful and ubiquitous technique both in classical and quantum system control. Its standard implementation relies on measuring the state of a system, processing the classical signal, and feeding it back to the system. In quantum physics, however, measurements not only read out the state of the system but also modify it irreversibly. Coherent feedback is a different kind of feedback that coherently processes and feeds back quantum signals without actually measuring the system. Here, we report on the experimental realization and the theoretical analysis of an optical coherent feedback platform to control the motional state of a nanomechanical membrane in an optical cavity. The coherent feedback loop consists of a light field interacting twice with the same mechanical mode through different cavity modes, without {performing any} measurement. Tuning the optical phase and delay of the feedback loop allows us to control the motional state of the mechanical oscillator, its resonance frequency and also its damping rate, which we use to cool the membrane close to the quantum ground state. Our theoretical analysis provides the optimal cooling conditions, showing that this new technique enables ground-state cooling. Experimentally, we show that we can cool the membrane to a state with $\bar{n}_m = 4.89 \pm 0.14 $ phonons (${480}\,{\mu \mathrm{K}}$) in a ${20}\,\mathrm{K}$ environment. This lies below the theoretical limit of cavity dynamical backaction cooling in the unresolved sideband regime and is achieved with only 1$\%$ of the optical power required for cavity cooling. Our feedback scheme is very versatile, offering new opportunities for quantum control in a variety of optomechanical systems.

Dynamical backaction evading magnomechanics

The interaction between magnons and mechanical vibrations dynamically modify the properties of the mechanical oscillator, such as its frequency and decay rate. Known as dynamical backaction, this effect is the basis for many theoretical protocols, such as entanglement generation or mechanical ground-state cooling. However, dynamical backaction is also detrimental for specific applications. Here, we demonstrate the implementation of a cavity magnomechanical measurement that fully evades dynamical backaction effects. Through careful engineering, the magnomechanical scattering rate into the hybrid magnon-photon modes can be precisely matched, eliminating dynamical backaction damping. Backaction evasion is confirmed via the measurement of a drive-power-independent mechanical linewidth.

Magnomechanical backaction corrections due to coupling to higher-order Walker modes and Kerr nonlinearities

The radiation pressurelike coupling between magnons and phonons in magnets can modify the phonon frequency (magnomechanical spring effect) and decay rate (magnomechanical decay) via dynamical backaction. Such effects have been recently observed by coupling the uniform magnon mode of a magnetic sphere (the Kittel mode) to a microwave cavity. In particular, the ability to evade backaction effects was demonstrated [C. A. Potts et al., Phys. Rev. B 107, L140405 (2023)], a requisite for applications such as magnomechanical-based thermometry. However, deviations were observed from the predicted magnomechanical decay rate within the standard theoretical model. In this work, we account for these deviations by considering corrections due to (i) magnetic Kerr nonlinearities and (ii) the coupling of phonons to additional magnon modes. Provided that such additional modes couple weakly to the driven cavity, our model yields a correction proportional to the average Kittel magnon mode occupation. We focus our results on magnetic spheres, where we show that the magnetostatic Walker modes couple to the relevant mechanical modes as efficiently as the Kittel mode. Our model yields excellent agreement with the experimental data.

Nonlocal transport of heat in equilibrium drift-diffusion systems

The amount of heat an integer quantum Hall edge state can carry in equilibrium is quantized in universal units of the heat flux quantum ${J}_{q}=\frac{\ensuremath{\pi}{k}_{B}^{2}}{12\ensuremath{\hbar}}{T}^{2}$ per edge state. We address the question of how heat transport in realistic one-dimensional devices can differ from the usual chiral Luttinger liquid theory. We show that a local measurement can reveal a nonquantized amount of heat carried by the edge states, despite a globally equilibrium situation. More specifically, we report a heat enhancement effect in edge states interacting with Ohmic reservoirs in the presence of nonlocal interactions or chirality-breaking diffusive currents. In contrast to a nonequilibrium, nonlinear drag effect, we report an equilibrium, linear phenomenon. The chirality of the edge states creates additional correlations between the reservoirs, reflected in a higher-than-quantum heat flux in the chiral channel. We show that for different types of coupling the enhancement can be understood as static or dynamical back action of the reservoirs on the chiral channel. We show that our results qualitatively hold by replacing the dissipative Ohmic reservoirs by an energy-conserving mesoscopic capacitor and consider the respective transmission lines for different types of interaction.

Enhanced Cavity Optomechanics with Quantum-Well Exciton Polaritons.

Semiconductor microresonators embedding quantum wells can host tightly confined and mutually interacting excitonic, optical, and mechanical modes at once. We theoretically investigate the case where the system operates in the strong exciton-photon coupling regime, while the optical and excitonic resonances are parametrically modulated by the interaction with a mechanical mode. Owing to the large exciton-phonon coupling at play in semiconductors, we predict an enhancement of polariton-phonon interactions by 2 orders of magnitude with respect to mere optomechanical coupling: a near-unity single-polariton quantum cooperativity is within reach for current semiconductor resonator platforms. We further analyze how polariton nonlinearities affect dynamical backaction, modifying the capability to cool or amplify the mechanical motion.

Phononic Cavity Optomechanics of Atomically Thin Crystal in Plasmonic Nanocavity.

In the picture of molecular cavity optomechanics, surface-enhanced Raman scattering (SERS) can be understood as molecular oscillators parametrically coupled to plasmonic nanocavities supporting an extremely localized optical field. This enables SERS from conventional fingerprint detection toward quantum nanotechnologies associated with, e.g., frequency upconversion and optomechanically induced transparency. Here, we study a phononic cavity optomechanical system consisting of a monolayer MoS2 placed inside a plasmonic nanogap, where the coherent phonon-plasmon interaction involves the collective oscillation from tens of thousands of unit cells of the MoS2 crystal. We observe the selective nonlinear SERS enhancement of the system as determined by the laser-plasmon detuning, suggesting the dynamic backaction modification of the phonon populations. Anomalous superlinear power dependence of a second-order Raman-inactive phonon mode with respect to the first-order phonons is also observed, indicating the distinctive properties of the phononic nanodevice compared with the molecular system. Our results promote the development of robust phononic optomechanical nanocavities to further explore the related quantum correlation and nonlinear effects including parametric instabilities.

Dynamical Backaction Magnomechanics

Dynamical backaction resulting from radiation pressure forces in optomechanical systems has proven to be a versatile tool for manipulating mechanical vibrations. Notably, dynamical backaction has resulted in the cooling of a mechanical resonator to its ground-state, driving phonon lasing, the generation of entangled states, and observation of the optical-spring effect. In certain magnetic materials, mechanical vibrations can interact with magnetic excitations (magnons) via the magnetostrictive interaction, resulting in an analogous magnon-induced dynamical backaction. In this article, we directly observe the impact of magnon-induced dynamical backaction on a spherical magnetic sample's mechanical vibrations. Moreover, dynamical backaction effects play a crucial role in many recent theoretical proposals; thus, our work provides the foundation for future experimental work pursuing many of these theoretical proposals.

Dynamical Backaction in an Ultrahigh-Finesse Fiber-Based Microcavity

The use of low-dimensional objects in the field of cavity optomechanics is limited by their low scattering cross section compared with the size of the optical cavity mode. Fiber-based Fabry-Perot microcavities can feature tiny mode cross sections and still maintain a high finesse, boosting the light-matter interaction and thus enabling the sensitive detection of the displacement of minute objects. Here we present such an ultrasensitive microcavity setup with the highest finesse reported so far in loaded fiber cavities, $\mathcal{F}=195\phantom{\rule{0.1em}{0ex}}000$. We are able to position-tune the static optomechanical coupling to a silicon nitride membrane stripe, reaching frequency pull parameters of up to $\ensuremath{\mid}G/2\ensuremath{\pi}\ensuremath{\mid}=1\phantom{\rule{0.2em}{0ex}}\mathrm{GHz}\phantom{\rule{0.1em}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}$. We also demonstrate radiation pressure backaction in the regime of an ultrahigh finesse up to $\mathcal{F}=165\phantom{\rule{0.1em}{0ex}}000$.

Room temperature cavity electromechanics in the sideband-resolved regime

We demonstrate a sideband-resolved cavity electromechanical system operating at room temperature. It consists of a nanomechanical resonator, a strongly pre-stressed silicon nitride string, dielectrically coupled to a three-dimensional microwave cavity made of copper. The electromechanical coupling is characterized by two measurements, the cavity-induced eigenfrequency shift of the mechanical resonator and the optomechanically induced transparency. While the former is dominated by dielectric effects, the latter reveals a clear signature of the dynamical backaction of the cavity field on the resonator. This unlocks the field of cavity electromechanics for room temperature applications.

Dynamical Backaction in an Ultrahigh-Finesse Fiber-Based Microcavity

Dynamical Backaction in an Ultrahigh-Finesse Fiber-Based Microcavity

Comparing nonlinear optomechanical coupling in membrane-in-the-middle and single-cavity systems

In cavity optomechanics, nonlinear interactions between an optical field and a mechanical resonator mode enable a variety of unique effects in classical and quantum measurement and information processing. Here, we describe nonlinear optomechanical coupling in the membrane-in-the-middle (MIM) system in a way that allows direct comparison to the intrinsic optomechanical nonlinearity in a standard, single-cavity optomechanical system. We find that the enhancement of nonlinear optomechanical coupling in the MIM system as predicted by Ludwig et al (2012 Phys. Rev. Lett. 109 063601) is limited to the degree of sideband resolution of the system. Moreover, we show that the selectivity of the MIM system of nonlinear over linear transduction has the same limit as in a single cavity system. These findings put constraints on the experiments in which it is advantageous to use an MIM system. We discuss dynamical backaction effects in this system and find that these effects per cavity photon are exactly as strong as in a single cavity system, while allowing for reduction of the required input power. We propose using the nonlinear enhancement and reduced input power in realistic MIM systems towards parametric squeezing and heralding of phonon pairs, and evaluate the limits to the magnitude of both effects.