We review the field of cavity optomechanics, which explores the interaction between electromagnetic radiation and nano- or micromechanical motion This review covers the basics of optical cavities and mechanical resonators, their mutual optomechanical interaction mediated by the radiation pressure force, the large variety of experimental systems which exhibit this interaction, optical measurements of mechanical motion, dynamical backaction amplification and cooling, nonlinear dynamics, multimode optomechanics, and proposals for future cavity quantum optomechanics experiments In addition, we describe the perspectives for fundamental quantum physics and for possible applications of optomechanical devices

Cavity Optomechanics

Metasurfaces are artificially structured thin films with unusual properties on demand. Different from metamaterials, the metasurfaces change the electromagnetic waves mainly by exploiting the boundary conditions, rather than the constitutive pa-rameters in three dimensional (3D) spaces. Despite the intrinsic similarities in the operational principles, there is not a universal theory available for the understanding and design of metasurface-based devices. In this article, we propose the concept of metasurface waves (M-waves) and provide a general theory to describe the principles of them. Most importantly, it is shown that the M-waves share some fundamental properties such as extremely short wavelength, abrupt phase change and strong chromatic dispersion, which make them different from traditional bulk waves. It is shown that these properties can enable many important applications such as subwavelength imaging and lithography, planar optical devices, broadband anti-reflection, absorption and polarization conversion. Our results demonstrated unambiguously that traditional laws of diffraction, refraction, reflection and absorption should be revised by using the novel properties of M-waves. The theory provided here may pave the way for the design of new electromagnetic devices and further improvement of metasurfaces. The exotic properties of metasurfaces may also form the foundations for two new sub-disciplines called “subwavelength surface electromagnetics” and “subwavelength electromagnetics”.

Principles of electromagnetic waves in metasurfaces

Nanomechanical oscillators are at the heart of ultrasensitive detectors of force, mass and motion. As these detectors progress to even better sensitivity, they will encounter measurement limits imposed by the laws of quantum mechanics. If the imprecision of a measurement of the displacement of an oscillator is pushed below a scale set by the standard quantum limit, the measurement must perturb the motion of the oscillator by an amount larger than that scale. Here we show a displacement measurement with an imprecision below the standard quantum limit scale. We achieve this imprecision by measuring the motion of a nanomechanical oscillator with a nearly shot-noise limited microwave interferometer. As the interferometer is naturally operated at cryogenic temperatures, the thermal motion of the oscillator is minimized, yielding an excellent force detector with a sensitivity of 0.51 aN Hz(-1/2). This measurement is a critical step towards observing quantum behaviour in a mechanical object.

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Nanomechanical motion measured with an imprecision below the standard quantum limit

We demonstrate $\mathcal{P}\mathcal{T}$-symmetry-breaking chaos in an optomechanical system, which features an ultralow driving threshold. In principle, this chaos will emerge once a driving laser is applied to the cavity mode and lasts for a period of time. The driving strength is inversely proportional to the starting time of chaos. This originally comes from the dynamical enhancement of nonlinearity by field localization in the $\mathcal{P}\mathcal{T}$-symmetry-breaking phase. Moreover, this chaos is switchable by tuning the system parameters so that a $\mathcal{P}\mathcal{T}$-symmetry phase transition occurs. This work may fundamentally broaden the regimes of cavity optomechanics and nonlinear optics. It offers the prospect of exploring ultralow-power-laser-triggered chaos and its potential applications in secret communication.

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PT-Symmetry-Breaking Chaos in Optomechanics.

We investigate the nonlinear interaction between a squeezed cavity mode and a mechanical mode in an optomechanical system (OMS) that allows us to selectively obtain either a radiation-pressure coupling or a parametric-amplification process. The squeezing of the cavity mode can enhance the interaction strength into the single-photon strong-coupling regime, even when the OMS is originally in the weak-coupling regime. Moreover, the noise of the squeezed mode can be suppressed completely by introducing a broadband-squeezed vacuum environment that is phase matched with the parametric amplification that squeezes the cavity mode. This proposal offers an alternative approach to control the OMS using a squeezed cavity mode, which should allow single-photon quantum processes to be implemented with currently available optomechanical technology. Potential applications range from engineering single-photon sources to nonclassical phonon states.

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Squeezed optomechanics with phase-matched amplification and dissipation.

Higher-order sidebands in optomechanically induced transparency are discussed in a generic optomechanical system. We take account of nonlinear terms and give an effective method to deal with such problems. It is shown that, if a strong control field with frequency ${\ensuremath{\omega}}_{1}$ and a weak probe field with frequency ${\ensuremath{\omega}}_{p}$ are incident upon the optomechanical system, then there are output fields with frequencies ${\ensuremath{\omega}}_{1}\ifmmode\pm\else\textpm\fi{}2\ensuremath{\Omega}$ generated, where $\ensuremath{\Omega}={\ensuremath{\omega}}_{p}\ensuremath{-}{\ensuremath{\omega}}_{1}$. We analyze the amplitude of the output field ${\ensuremath{\omega}}_{1}+2\ensuremath{\Omega}$ and look at how it varies with the control field and show that the amplitude of the second-order sideband can be controlled by the strong control field.

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Higher-order sidebands in optomechanically induced transparency

Recently, cavity optomechanics has become a rapidly developing research field exploring the coupling between the optical field and mechanical oscillation. Cavity optomechanical systems were predicted to exhibit rich and nontrivial effects due to the nonlinear optomechanical interaction. However, most progress during the past years have focused on the linearization of the optomechanical interaction, which ignored the intrinsic nonlinear nature of the optomechanical coupling. Exploring nonlinear optomechanical interaction is of growing interest in both classical and quantum mechanisms, and nonlinear optomechanical interaction has emerged as an important new frontier in cavity optomechanics. It enables many applications ranging from single-photon sources to generation of nonclassical states. Here, we give a brief review of these developments and discuss some of the current challenges in this field.

Review of cavity optomechanics in the weak-coupling regime: from linearization to intrinsic nonlinear interactions

We analyze the features of the output field of a generic optomechanical system that is driven by a control field and a nanosecond driven pulse, and find a robust high-order sideband generation in optomechanical systems. The typical spectral structure, plateau and cutoff, confirms the nonperturbative nature of the effect, which is similar to high-order harmonic generation in atoms or molecules. Based on the phenomenon, we show that the carrier-envelope phase of laser pulses that contain huge numbers of cycles can cause profound effects.

Carrier-envelope phase-dependent effect of high-order sideband generation in ultrafast driven optomechanical system.

In this article, a theoretical scheme is proposed to investigate the formation and propagation of three-wave coupled vector optical solitons with ultraslow group velocities in a lifetime-broadened seven-state triple-$\ensuremath{\Lambda}$ atomic system under Raman excitation. We show that in the presence of a weak applied magnetic field that removes the degeneracy of the corresponding sublevels of the atomic medium, three continuous-wave control fields with circularly left or right polarized fields induce a quantum interference effect which can largely suppress the absorption of the three low-intensity pulsed fields, that is, the circularly ${\ensuremath{\sigma}}^{\ensuremath{-}}$ (right), the linearly $\ensuremath{\pi}$, and the circularly ${\ensuremath{\sigma}}^{+}$ (left) polarized fields converted from one weak linear-polarized probe field. By means of the standard method of multiple scales, we solve the equations of motion of atomic response and probe-control electromagnetic fields and derive three-coupled nonlinear Schr\"odinger equations that govern the nonlinear evolution of the envelopes of the probe fields in this scheme. We then demonstrate that because of the nonlinear coupling to one another, the three probe fields can evolve into three-wave temporal, group velocity, and amplitude-matched optical solitons under appropriate conditions, which are produced from the delicate balance of the dispersion effects and the self- and cross-phase modulation effects. This scheme may thus pave the way to generate ultraslow vector optical solitons composed of three field components in a highly resonant atomic medium and result in a substantial impact on this field of nonlinear optics.

Formation and propagation of ultraslow three-wave-vector optical solitons in a cold seven-level triple- Λ atomic system under Raman excitation

Nonlinear interactions between cavity fields and mechanical oscillation in an optomechanical system coupled to a charged object are treated analytically, and the features of second-order sideband generation are discussed, which is beyond the conventional linearized description of optomechanical interactions. We show that resonantly enhanced feedback-backaction arising from radiation pressure can be substantively modified in the presence of electric interactions, which results in tunable optical nonlinearity and convenient optomechanical control. Especially, the system exhibits a remarkable electrical-charge dependent generation of the frequency component at the second-order sideband, which enables a potentially practical scheme for precision measurement of charges.

Liu-Gang Si

Papers

Higher-order sidebands in optomechanically induced transparency

Review of cavity optomechanics in the weak-coupling regime: from linearization to intrinsic nonlinear interactions

Carrier-envelope phase-dependent effect of high-order sideband generation in ultrafast driven optomechanical system.

Formation and propagation of ultraslow three-wave-vector optical solitons in a cold seven-level triple- Λ atomic system under Raman excitation

Precision measurement of electrical charges in an optomechanical system beyond linearized dynamics