Optomechanical Coupling Research Articles

Nonlinearity enhancement and photon blockade in hybrid optomechanical systems.

The nonlinear optomechanical coupling is an attracting characteristic in the field of optomechanics. However, the strength of single photon optomechanical coupling is still within weak coupling regime. Using the optomechanical coupling to achieve strong nonlinear interaction between photons is still a challenge. In this paper, we propose a scheme by employing optomechanical and spin-mechanical interactions to enhance the nonlinearity of photons. An effective Hamiltonian is derived, which shows that the self-Kerr and cross-Kerr nonlinearity strengths can be enhanced by adjusting the classical pumping or enhancing the spin-mechanical coupling strength. In addition, we investigate the potential usage of the nonlinearity in the photon blockade. We demonstrate that the single and two photon blockades can occur in two super modes.

Improving few-photon optomechanical effects with coherent feedback.

Few-photon effects such as photon blockade and tunneling have potential applications in modern quantum technology. To enhance the few-photon effects in an optomechanical system, we introduce a coherent feedback loop to cavity mode theoretically. By studying the second-order correlation function, we show that the photon blockade effect can be improved with feedback. Under appropriate parameters, the photon blockade effect exists even when cavity decay rate is larger than the single-photon optomechanical coupling coefficient, which may reduce the difficulty of realizing single-photon source in experiments. Through further study of the third-order correlation function, we show that the tunneling effect can also be enhanced by feedback. In addition, we discuss the application of feedback on Schrödinger-cat state generation in an optomechanical system. The result shows that the fidelity of cat state generation can be improved in the presence of feedback loop.

Acousto-optic cavity coupling in 2D phoxonic crystal with combined convex and concave holes

A two-dimensional cross-like phoxonic crystal (PxC) model is proposed, which exhibits simultaneously large complete photonic crystal (PtC) and phononic crystal (PnC) bandgaps. The most salient trait of the structure is the wide range of geometrical parameters compatible with large complete bandgaps. After geometrical optimization, photonic and phononic bandgaps with gap-to-midgap ratios of 11.5% and 90.7% are obtained, respectively. These values are close to the best topology-optimized reported values but are obtained with simple shapes compatible with nanoscale fabrication technology. These characteristics make the convex–concave topology a promising candidate for PxC devices. A cavity is then introduced by filling up one cross-like hole in the 7 × 7 super-cell. PtC and PnC bands with defects appear in the respective large complete bandgaps, confining phonons and photons in the same cavity. Acousto-optic (AO) coupling between photonic and phononic defect modes is further investigated by the finite element method, taking both photoelastic and moving interface mechanisms into consideration. The symmetries of both photonic and phononic modes play a dominant role in the coupling strength. Results show that the strongest linear coupling between a photonic transverse magnetic mode and phononic breathing mode is obtained due to the in-phase superposition in the x and y directions. A quadratic nonlinear coupling is observed when photonic modes are coupled with the phononic stretching mode due to the inverse superposition of x and y directions. Finally, the optomechanical coupling rates relative to zero-point motion are estimated.

Strong single-photon optomechanical coupling in a hybrid quantum system.

Engineering strong single-photon optomechanical couplings is crucial for optomechanical systems. Here, we propose a hybrid quantum system consisting of a nanobeam (phonons) coupled to a spin ensemble and a cavity (photons) to overcome it. Utilizing the critical property of the lower-branch polariton (LBP) formed by the ensemble-phonon interaction, the LBP-cavity coupling can be greatly enhanced by three orders magnitude of the original one, while the upper-branch polariton (UBP)-cavity coupling is fully suppressed. Our proposal breaks through the condition of the coupling strength less than the critical value in previous schemes using two harmonic oscillators. Also, strong Kerr effect can be induced in our proposal. This shows our proposed approach can be used to study quantum nonlinear and nonclassical effects in weakly coupled optomechanical systems.

Engineering Long-Lived Vibrational States for an Organic Molecule.

The optomechanical character of molecules was discovered by Raman about one century ago. Today, molecules are promising contenders for high-performance quantum optomechanical platforms because their small size and large energy-level separations make them intrinsically robust against thermal agitations. Moreover, the precision and throughput of chemical synthesis can ensure a viable route to quantum technological applications. The challenge, however, is that the coupling of molecular vibrations to environmental phonons limits their coherence to picosecond time scales. Here, we improve the optomechanical quality of a molecule by several orders of magnitude through phononic engineering of its surrounding. By dressing a molecule with long-lived high-frequency phonon modes of its nanoscopic environment, we achieve storage and retrieval of photons at millisecond timescales and allow for the emergence of single-photon strong coupling in optomechanics. Our strategy can be extended to the realization of molecular optomechanical networks.

Optomechanical isolation with tunable center frequency

Optomechanical (OM) isolators have the ability to stop the optical field for one direction and to pass the field in the opposite direction. Thus they work as bandstop filter in one direction and bandpass filter in the opposite direction. The maximum isolation happens when the frequency of the field becomes equal to that of the mechanical membrane inside the isolator. This frequency, called the isolation (or center) frequency, however is restricted by the fact that the fundamental frequency of the membrane depends upon its physical parameters (such as mass and length), which are fixed during fabrication of the membrane. We propose a dynamical way of tuning this center frequency, by using a mechanical drive attached to a membrane with quadratic OM coupling in a membrane-in-the-middle setup. This provides a way for future applications where tunable filtering is required.

Strong optomechanical coupling in chain-like waveguides of silicon nanoparticles with quasi-bound states in the continuum

We propose and demonstrate that strong optomechanical coupling can be achieved in a chain-like waveguide consisting of silicon nanorods. By employing quasi-bound states in the continuum and mechanical resonances at a frequency around 10 GHz, the optomechanical coupling rate can be above 2 MHz and surpass most microcavities. We have also studied cases with different optical wave numbers and size parameters of silicon, and a robust coupling rate has been verified, benefiting the experimental measurements and practical applications. The proposed silicon chain-like waveguide of strong optomechanical coupling may pave new ways for research on photon-phonon interaction with microstructures.

Implementation of weak measurement amplification with the weak coherent light and optomechanical system

Most studies for postselected weak measurement in optomechanical system focus on using a single photon as a measured system. However, we find that using weak coherent light instead of a single photon can also amplify the mirror’s position displacement of one photon. In the weak value amplification (WVA) regime, the weak value of one photon can lie outside the eigenvalue spectrum, proportional to the difference of the mirror’s position displacement between the successful and failed postselection and its successful postselection probability is dependent of the mean photon number of the coherent light and improved by adjusting it accordingly. Outside the WVA regime, the amplification limit can reach the level of the vacuum fluctuations, and when the mean photon number and the optomechanical coupling parameter are selected appropriately, its successful postselection probability becomes higher, which is beneficial to observe the maximum amplification value under the current experimental conditions. This result breaks the constraint that it is difficult to detect outside the WVA regime. This opens up a new regime for the study of a single photon nonlinearity in optomechanical system.

Quantum simulation of tunable and ultrastrong mixed-optomechanics.

We propose a reliable scheme to simulate tunable and ultrastrong mixed (first-order and quadratic optomechanical couplings coexisting) optomechanical interactions in a coupled two-mode bosonic system, in which the two modes are coupled by a cross-Kerr interaction and one of the two modes is driven through both the single- and two-excitation processes. We show that the mixed-optomechanical interactions can enter the single-photon strong-coupling and even ultrastrong-coupling regimes. The strengths of both the first-order and quadratic optomechanical couplings can be controlled on demand, and hence first-order, quadratic, and mixed optomechanical models can be realized. In particular, the thermal noise of the driven mode can be suppressed totally by introducing a proper squeezed vacuum bath. We also study how to generate the superposition of coherent squeezed state and vacuum state based on the simulated interactions. The quantum coherence effect in the generated states is characterized by calculating the Wigner function in both the closed- and open-system cases. This work will pave the way to the observation and application of ultrastrong optomechanical effects in quantum simulators.

Suppressing laser phase noise in an optomechanical system

We propose a scheme to suppress the laser phase noise without increasing the optomechanical single-photon coupling strength. In the scheme, the parametric amplification terms, created by Kerr and Duffing nonlinearities, can restrain laser phase noise and strengthen the effective optomechanical coupling, respectively. Interestingly, decreasing laser phase noise leads to increasing thermal noise, which is inhibited by bringing in a broadband-squeezed vacuum environment. To reflect the superiority of the scheme, we simulate quantum memory and stationary optomechanical entanglement as examples, and the corresponding numerical results demonstrate that the laser phase noise is extremely suppressed. Our method can pave the way for studying other quantum phenomena.

Resonant Raman scattering of single molecules under simultaneous strong cavity coupling and ultrastrong optomechanical coupling in plasmonic resonators: Phonon-dressed polaritons

Plasmonic dimer cavities can induce extreme electric-field hot spots that allow one to access ultrastrong coupling regimes using Raman-type spectroscopy on single vibrating molecules. Using a generalized master equation, we study resonant Raman scattering in the strong coupling regime of cavity quantum electrodynamics, when also in the vibrational ultrastrong coupling regime, leading to ``phonon-dressed polaritons.'' The master equation rigorously includes spectral baths for the cavity and vibrational degrees of freedom, as well as a pure dephasing bath for the resonant two-level system, which play a significant role. Employing realistic parameters for gold dimer cavity modes, we investigate the emission spectra in several characteristic strong coupling regimes, leading to extremely rich spectral resonances due to an interplay of phonon-modified polariton states and bath-induced resonances. We also show explicitly the failure of the standard master equation in these quantum nonlinear regimes.

Strong Coupling Optomechanics Mediated by a Qubit in the Dispersive Regime.

Cavity optomechanics represents a flexible platform for the implementation of quantum technologies, useful in particular for the realization of quantum interfaces, quantum sensors and quantum information processing. However, the dispersive, radiation–pressure interaction between the mechanical and the electromagnetic modes is typically very weak, harnessing up to now the demonstration of interesting nonlinear dynamics and quantum control at the single photon level. It has already been shown both theoretically and experimentally that if the interaction is mediated by a Josephson circuit, one can have an effective dynamics corresponding to a huge enhancement of the single-photon optomechanical coupling. Here we analyze in detail this phenomenon in the general case when the cavity mode and the mechanical mode interact via an off-resonant qubit. Using a Schrieffer–Wolff approximation treatment, we determine the regime where this tripartite hybrid system behaves as an effective cavity optomechanical system in the strong coupling regime.

Polariton multistability in a nonlinear optomechanical cavity

We theoretically study the polariton multistability in a solid state based optomechanical resonator embedded with a quantum well and a χ (2) second order nonlinear medium. The excitonic transition inside the quantum well is strongly coupled to the optical cavity mode. The polariton formed due to the mixing of cavity photons and exciton states are coupled to the mechanical mode which gives rise to the bistable behavior. A transition from bistability to tristability occurs in the presence of a strong χ (2) nonlinearity. Switching between bistability and tristability can also be controlled using exciton-cavity and optomechanical coupling making the system highly tunable. Tristability appears at low input power making it a suitable candidate for polaritonic devices which requires low input power.

Optomechanical controlling of intermolecular interaction and the application in molecular self-assembly.

In this paper, we combined cavity optomechanics and quantum mechanical mechanism of van der Waals force to study the dynamic behavior of interacting bimolecules in the plasmonic localized field, and extend it to the interacting multi-molecular system. We explored how plasmonic optomechanical coupling affects the strength of intermolecular interactions. Based on our results, we propose to use optical field to modulate the intermolecular interaction potential in plasmonic cavity, which can be utilized in the enhancement of the efficiency of the molecular self-assembly process and controlling the yield of the reaction in an optical environment. This research extends molecular optomechanics from intramolecular interactions to intermolecular interactions and may has high application potential in some nanostructure synthesis.

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 $\mathcal{F}=165\phantom{\rule{0.1em}{0ex}}000$.

Optomechanic Coupling in Ag Polymer Nanocomposite Films.

Particle vibrational spectroscopy has emerged as a new tool for the measurement of elasticity, glass transition, and interactions at a nanoscale. For colloid-based materials, however, the weakly localized particle resonances in a fluid or solid medium renders their detection difficult. The strong amplification of the inelastic light scattering near surface plasmon resonance of metallic nanoparticles (NPs) allowed not only the detection of single NP eigenvibrations but also the interparticle interaction effects on the acoustic vibrations of NPs mediated by strong optomechanical coupling. The “rattling” and quadrupolar modes of Ag/polymer and polymer-grafted Ag NPs with different diameters in their assemblies are probed by Brillouin light spectroscopy (BLS). We present thorough theoretical 3D calculations for anisotropic Ag elasticity to quantify the frequency and intensity of the “rattling” mode and hence its BLS activity for different interparticle separations and matrix rigidity. Theoretically, a liquidlike environment, e.g., poly(isobutylene) (PIB) does not support rattling vibration of Ag dimers but unexpectedly hardening of the extremely confined graft melt renders both activation of the former and a frequency blue shift of the fundamental quadrupolar mode in the grafted nanoparticle Ag@PIB film.

Heat transfer mediated by the dynamical Casimir effect in an optomechanical system

Heat transfer in an optomechanical system consisting of one optical cavity and two movable mirrors with or without a laser field is studied by using the quantum master-equation method. The radiation-pressure interaction between the cavity and the mirrors contains the single-photon optomechanical coupling (SPOC) and the dynamical Casimir effect (DCE). When the laser is not applied to the cavity, the heat transfer is induced by a nonlocal interaction between mirrors that builds up only due to the presence of the DCE. Once the monochromatic field is turned on, both the SPOC and the DCE can be used to modify the energy transfer process. In both cases, the heat transfer can be drastically enhanced at resonant conditions manifested by the DCE. Our results can motivate further studies to actively control heat transfer mediated by the DCE in optomechanical devices.

Force sensing in a dual-mode optomechanical system with linear-quadratic coupling and modulated photon hopping.

A weak force sensor scheme is presented in an optomechanical system, in which the two cavity modes couple to a mechanical mode with linear and quadratic coupling. Due to introducing time-dependent hopping, the linear and quadratic coupling terms coexist under the rotating-wave approximation in the interaction picture. Compared with the quantum non-demolition measurement (ignoring the quadratic optomechanical coupling), the current scheme can decrease the additional noise to a lower level. Our proposal provides a promising platform for improving the detection of a weak force.

Simultaneous Cooling of Two Mechanical Resonators with Intracavity Squeezed Light

AbstractA scheme is proposed to cool two mechanical resonators simultaneously to the quantum ground state, whatever sideband resolution, in an optomechanical system with χ(2) nonlinear medium, where the cavity light is squeezed. By selecting the system parameters appropriately, the influence from cavity dissipation can be completely removed and the quantum backaction heating is suppressed at the same time. It is found that the cooling performance is determined by the coupling strength and frequency detuning between two mechanical resonators. The numerical results indicate that the final phonon occupancy of the two mechanical resonators is no longer strictly limited by the sideband resolution and the cooling scheme can work well no matter in weak or strong effective optomechanical coupling regime.

Dissipative optomechanical coupling with a membrane outside of an optical cavity

We theoretically study an optomechanical system, which consists of a two-sided cavity and a mechanical membrane that is placed outside of it. The membrane is positioned close to one of its mirrors, and the cavity is coupled to the external light field through the other mirror. Our study is focused on the regime where the dispersive optomechanical coupling in the system vanishes. Such a regime is found to be possible if the membrane is less reflecting than the adjacent mirror, yielding a potentially very strong dissipative optomechanical coupling. Specifically, if the absolute values of amplitude transmission coefficients of the membrane and the mirror, $t$ and $t_m$ respectively, obey the condition $ t_m^2< t\ll t_m\ll 1$, the dissipative coupling constant of the setup exceeds the dispersive coupling constant for an optomechanical cavity of the same length. The dissipative coupling constant and the corresponding optomechanical cooperativity of the proposed system are also compared with those of the Michelson-Sagnac interferometer and the so-called "membrane-at-the-edge" system, which are known for a strong optomechanical dissipative interaction. It is shown that under the above condition, the system proposed here is advantageous in both aspects. It also enables an efficient realization of the two-port configuration, which was recently proposed as a promising optomechanical system, providing, among other benefits, a possibility of quantum limited optomechanical measurements in a system, which does not suffer from any optomechanical instability.