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Vibrational spectroscopy is a ubiquitous technology that derives the species, constituents, and morphology of an object from its natural vibrations. However, the vibrational spectra of mesoscopic particles - including most biological cells - have remained hidden from existing technologies. These particles are expected to vibrate faintly at megahertz to gigahertz rates, imposing unpractical sensitivity and resolution for current optical and piezoelectric spectroscopy. Here we demonstrate the real-time measurement of natural vibrations of single mesoscopic particles using an optical microresonator, extending the reach of vibrational spectroscopy to a new spectral window. Conceptually, a spectrum of vibrational modes of the particles is stimulated photoacoustically, and correlated to a high-quality-factor optical resonance for the ultrasensitive readout. Experimentally, this scheme is testified by measuring mesoscopic particles with different constituents, sizes, and internal structures, showing an unprecedented signal-to-noise ratio of 50 dB and detection bandwidth over 1 GHz. This new technology is further applied for the biomechanical fingerprinting of single microbial cells with different species and living states. The present method opens up new avenues to study single-particle mechanical properties in vibrational degrees of freedom, and may find applications in photoacoustic sensing and imaging, cavity optomechanics and biomechanics., Comment: 15 pages, 10 figures

For the applications of the frequency comb in microresonators, it is essential to obtain a fully frequency-stabilized microcomb laser source. Here, we demonstrate an atom-referenced stabilized soliton microcomb generation system based on the integrated microring resonator. The pump light around $1560.48\,\mathrm{nm}$ locked to an ultra-low-expansion (ULE) cavity, is frequency-doubled and referenced to the atomic transition of $^{87}\mathrm{Rb}$. The repetition rate of the soliton microcomb is injection-locked to an atomic-clock-stabilized radio frequency (RF) source, leading to mHz stabilization at $1$ seconds. As a result, all comb lines have been frequency-stabilized based on the atomic reference and could be determined with very high precision reaching $\sim18\,\mathrm{Hz}$ at 1 second, corresponding to the frequency stability of $9.5\times10^{-14}$. Our approach provides an integrated and fully stabilized microcomb experiment scheme with no requirement of $f-2f$ technique, which could be easily implemented and generalized to various photonic platforms, thus paving the way towards the portable and ultraprecise optical sources for high precision spectroscopy., Comment: 7 pages, 8 figures

Liquid-crystal microcavity lasers have attracted considerable attention because of their extraordinary tunability and sensitive response to external stimuli, and they operate generally within a specific phase. Here, we demonstrate a liquid-crystal microcavity laser operated in the phase transition, in which the reorientation of liquid-crystal molecules occurs from aligned to disordered states. A significant wavelength shift of the microlaser is observed, resulting from the dramatic changes in the refractive index of liquid-crystal microdroplets during the phase transition. This phase-transition microcavity laser is then exploited for sensitive thermal sensing, enabling two-order-of-magnitude enhancement in sensitivity compared with the nematic-phase microlaser operated far from the transition point. Experimentally, we demonstrate an exceptional sensitivity of -40 nm/K and an ultrahigh resolution of 320 uK. The phase-transition microcavity laser features compactness, softness, and tunability, showing great potential for high-performance sensors, optical modulators, and soft matter photonics., Comment: 15 pages, 4 figures

The frequency and orbital angular momentum (OAM) are independent physical properties of light that both offer unbounded degrees of freedom. However, creating, processing, and detecting high-dimensional OAM states have been a pivot and long-lasting task, as the complexity of the required optical systems scales up drastically with the OAM dimension. On the other hand, mature toolboxes -- such as optical frequency combs -- have been developed in the frequency domain for parallel measurements with excellent fidelity. Here we correlate the two dimensions into an equidistant comb structure on a photonic chip. Dissipative optical solitons formed in a nonlinear microresonator are emitted through the engraved angular gratings with each comb line carrying distinct OAM. Such one-to-one correspondence between the OAM and frequencies manifests state-of-the-art extinction ratios over 18.5 dB, enabling precision spectroscopy of optical vortices. The demonstrated vortex soliton microcombs provide coherent light sources that are multiplexed in the spatial and frequency domain, having the potential to establish a new modus operandi of high-dimensional structured light.

Nanoscale materials usually can trap light and strongly interact with it leading to many photonic device applications. The light-matter interactions are commonly probed by optical spectroscopy, which, however, have some limitations such as diffraction-limited spatial resolution, tiny momentum transfer and non-continuous excitation/detection. In this work, using scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) with ultra-wide energy and momentum match and sub-nanometer spatial resolution, we study the optical microcavity resonant spectroscopy in a single SiC nanowire. The longitudinal Fabry-Perot (FP) resonating modes and the transverse whispering-gallery modes (WGMs) are simultaneously excited and detected, which span from near-infrared (~ 1.2 {\mu}m) to ultraviolet (~ 0.2 {\mu}m) spectral regime and the momentum transfer can be ranging up to 108 cm{^{-1}}. The size effects on the resonant spectra of nanowires are also revealed. Moreover, the nanoscale decay length of resonant EELS is revealed, which is contributed by the strongly localized electron-photon interactions in the SiC nanowire. This work provides a new alternative technique to investigate the optical resonating spectroscopy of a single nanowire structure and to explore the light-matter interactions in dielectric nanostructures, which is also promising for modulating free electrons via photonic structures.

The transportation of photons and phonons typically obeys the principle of reciprocity. Breaking reciprocity of these bosonic excitations will enable the corresponding non-reciprocal devices, such as isolators and circulators. Here, we use two optical modes and two mechanical modes in a microresonator to form a four-mode plaquette via radiation pressure force. The phase-controlled non-reciprocal routing between any two modes with completely different frequencies is demonstrated, including the routing of phonon to phonon (MHz to MHz), photon to phonon (THz to MHz), and especially photon to photon with frequency difference of around 80 THz for the first time. In addition, one more mechanical mode is introduced to this plaquette to realize a phononic circulator in such single microresonator. The non-reciprocity is derived from interference between multi-mode transfer processes involving optomechanical interactions in an optomechanical resonator. It not only demonstrates the non-reciprocal routing of photons and phonons in a single resonator but also realizes the non-reciprocal frequency conversion for photons and circulation for phonons, laying a foundation for studying directional routing and thermal management in an optomechanical hybrid network., Comment: 6 pages,4 figures

We propose a general and experimentally feasible approach to realize simultaneous ground-state cooling of arbitrary number of near-degenerate, or even fully degenerate mechanical modes, overcoming the limit imposed by the formation of mechanical dark modes. Multiple optical modes are employed to provide different dissipation channels that prevent complete destructive interference of the cooling pathway, and thus eliminating the dark modes. The cooling rate and limit are explicitly specified, in which the distinguishability of the optical modes to the mechanical modes is found to be critical for an efficient cooling process. In a realistic multi-mode optomechanical system, ground-state cooling of all mechanical modes is demonstrated by sequentially introducing optical drives, proving the feasibility and scalability of the proposed scheme. The work may provide new insights in preparing and manipulating multiple quantum states in macroscopic systems.

On-chip bright quantum sources with multiplexing ability are extremely high in demand for the integrated quantum networks with unprecedented scalability and complexity. Here, we demonstrate an ultrabright and broadband biphoton quantum source generated in a lithium niobate microresonator system.Without introducing the conventional domain poling, the on-chip microdisk produces entangled photon pairs covering a broad bandwidth promised by natural phase matching in spontaneous parametric down conversion.Experimentally, the multiplexed photon pairs are characterized by $30\ \rm nm$ bandwidth limited by the filtering system, which can be furthered enlarged.Meanwhile, the generation rate reaches $5.13\ {\rm MHz}/\upmu \rm W$ with a coincidence-to-accidental ratio up to $804$.Besides, the quantum source manifests the prominent purity with heralded single photon correlation $g_H^{(2)}(0)=0.0098\pm0.0021$ and energy-time entanglement with excellent interference visibility of $96.5\%\pm1.9\%$. Such quantum sources at the telecommunication band pave the way for high-dimensional entanglement and future integrated quantum information systems., Comment: 8 pages,4 figures

Soliton microcombs based on Kerr nonlinearity in microresonators have been a prominent miniaturized coherent light source. Here, for the first time, we demonstrate the existence of Kerr solitons in an optomechanical microresonator, for which a nonlinear model is built by incorporating a single mechanical mode and multiple optical modes. Interestingly, an exotic vibrational Kerr soliton state is found, which is modulated by a self-sustained mechanical oscillation. Besides, the soliton provides extra mechanical gain through the optical spring effect, and results in phonon lasing with a red-detuned pump. Various nonlinear dynamics is also observed, including limit cycle, higher periodicity, and transient chaos. This work provides a guidance for not only exploring many-body nonlinear interactions, but also promoting precision measurements by featuring superiority of both frequency combs and optomechanics.

The emerging microresonator-based frequency combs revolutionize a broad range of applications from optical communications to astronomical calibration. Despite of their significant merits, low energy efficiency and the lack of all-optical dynamical control severely hinder the transfer of microcomb system to real-world applications. Here, by introducing active lasing medium into the soliton microcomb, for the first time, we experimentally achieve the chiral soliton with agile on-off switch and tunable dual-comb generation in a packaged microresonator. It is found that such a microresonator enables a soliton slingshot effect, the rapid soliton formation arising from the extra energy accumulation induced by inter-modal couplings. Moreover, tuning the erbium gain can generate versatile multi-soliton states, and extend the soliton operation window to a remarkable range over 18 GHz detuning. Finally, the gain-assisted chirality of counterpropagating soliton is demonstrated, which enables an unprecedented fast on-off switching of soliton microcombs. The non-trivial chiral soliton formation with active controllability inspires new paradigms of miniature optical frequency combs and brings the fast tunable soliton tools within reach., Comment: 11 pages, 8 figures

A single photon in a strongly nonlinear cavity is able to block the transmission of the second photon, thereby converting incident coherent light into anti-bunched light, which is known as photon blockade effect. On the other hand, photon anti-pairing, where only the entry of two photons is blocked and the emission of bunches of three or more photons is allowed, is based on an unconventional photon blockade mechanism due to destructive interference of two distinct excitation pathways. We propose quantum plexcitonic systems with moderate nonlinearity to generate both anti-bunched and anti-paired photons. The proposed plexitonic systems benefit from subwavelength field localizations that make quantum emitters spatially distinguishable, thus enabling a reconfigurable photon source between anti-bunched and anti-paired states via tailoring the energy bands. For a realistic nanoprism plexitonic system, two schemes of reconfiguration are suggested: (i) the chemical means by partially changing the type of the emitters; or (ii) the optical approach by rotating the polarization angle of the incident light to tune the coupling rate of the emitters. These results pave the way to realize reconfigurable nonclassical photon sources in a simple quantum plexcitonic platform with readily accessible experimental conditions.

In single microdisks, embedded active emitters intrinsically affect the cavity mode of microdisks, which results in a trivial symmetric backscattering and a low controllability. Here we propose a macroscopical control of the backscattering direction by optimizing the cavity size. The signature of positive and negative backscattering directions in each single microdisk is confirmed with two strongly coupled microdisks. Furthermore, the diabolical points are achieved at the resonance of two microdisks, which agrees well with the theoretical calculations considering backscattering directions. The diabolical points in active optical structures pave a way to implement quantum information processing with geometric phase in quantum photonic networks., Comment: 16 pages, 4 figures

Exploiting semiconductor fabrication techniques, natural carriers of quantum information such as atoms, electrons, and photons can be embedded in scalable integrated devices. Integrated optics provides a versatile platform for large-scale quantum information processing and transceiving with photons. Scaling up the integrated devices for quantum applications requires highperformance single-photon generation and photonic qubit-qubit entangling operations. However, previous demonstrations report major challenges in producing multiple bright, pure and identical single-photons, and entangling multiple photonic qubits with high fidelity. Another notable challenge is to noiselessly interface multiphoton sources and multiqubit operators in a single device. Here we demonstrate on-chip genuine multipartite entanglement and quantum teleportation in silicon, by coherently controlling an integrated network of microresonator nonlinear single-photon sources and linear-optic multiqubit entangling circuits. The microresonators are engineered to locally enhance the nonlinearity, producing multiple frequencyuncorrelated and indistinguishable single-photons, without requiring any spectral filtering. The multiqubit states are processed in a programmable linear circuit facilitating Bell-projection and fusion operation in a measurement-based manner. We benchmark key functionalities, such as intra-/inter-chip teleportation of quantum states, and generation of four-photon Greenberger-HorneZeilinger entangled states. The production, control, and transceiving of states are all achieved in micrometer-scale silicon chips, fabricated by complementary metal-oxide-semiconductor processes. Our work lays the groundwork for scalable on-chip multiphoton technologies for quantum computing and communication.

Synchronization is of importance in both fundamental and applied physics, but their demonstration at the micro/nanoscale is mainly limited to low-frequency oscillations like mechanical resonators. Here, we report the synchronization of two coupled optical microresonators, in which the high-frequency resonances in optical domain are aligned with reduced noise. It is found that two types of synchronization emerge with either the first- or second-order transition, both presenting a process of spontaneous symmetry breaking. In the second-order regime, the synchronization happens with an invariant topological character number and a larger detuning than that of the first-order case. Furthermore, an unconventional hysteresis behavior is revealed for a time-dependent coupling strength, breaking the static limitation and the temporal reciprocity. The synchronization of optical microresonators offers great potential in reconfigurable simulations of many-body physics and scalable photonic devices on a chip.

Spontaneous symmetry breaking has revolutionized the understanding in numerous fields of modern physics. Here, we theoretically demonstrate the spontaneous time-reversal symmetry breaking in a cavity quantum electrodynamics system in which an atomic ensemble interacts coherently with a single resonant cavity mode. The interacting system can be effectively described by two coupled oscillators with positive and negative mass when the two-level atoms are prepared in their excited states. The occurrence of symmetry breaking is controlled by the atomic detuning and the coupling to the cavity mode, which naturally divides the parameter space into the symmetry broken and symmetry unbroken phases. The two phases are separated by a spectral singularity, a so-called exceptional point, where the eigenstates of the Hamiltonian coalesce. When encircling the singularity in the parameter space, the quasi-adiabatic dynamics shows chiral mode switching which enables topological manipulation of quantum states.

Quantum manipulation of coupled mechanical resonators has become an important research topic in optomechanics because these systems can be used to study the quantum coherence effects involving multiple mechanical modes. A prerequisite for observing macroscopic mechanical coherence is to cool the mechanical resonators to their ground state. Here we propose a theoretical scheme to cool two coupled mechanical resonators by introducing an optomechanical interface. The final mean phonon numbers in the two mechanical resonators are calculated exactly and the results show that the ground-state cooling is achievable in the resolved-sideband regime and under the optimal driving. By adiabatically eliminating the cavity field in the large-decay regime, we obtain analytical results of the cooling limits, which show the smallest achievable phonon numbers and the parameter conditions under which the optimal cooling is achieved. Finally, the scheme is extended to the cooling of a chain of coupled mechanical resonators., Comment: 14 pages, 8 figures

Localized-surface plasmon resonance is of importance in both fundamental and applied physics for the subwavelength confinement of optical field, but realization of quantum coherent processes is confronted with challenges due to strong dissipation. Here we propose to engineer the electromagnetic environment of metallic nanoparticles (MNPs) using optical microcavities. An analytical quantum model is built to describe the MNP-microcavity interaction, revealing the significantly enhanced dipolar radiation and consequentially reduced Ohmic dissipation of the plasmonic modes. As a result, when interacting with a quantum emitter, the microcavity-engineered MNP enhances the quantum yield over 40 folds and the radiative power over one order of magnitude. Moreover, the system can enter the strong coupling regime of cavity quantum electrodynamics, providing a promising platform for the study of plasmonic quantum electrodynamics, quantum information processing, precise sensing and spectroscopy., Comment: 5 pages, 4 figures

A deformed dielectric microcavity is used as an experimental platform for the analysis of the statistics of chaotic resonances, in the perspective of testing fractal Weyl laws at optical frequencies. In order to surmount the difficulties that arise from reading strongly overlapping spectra, we exploit the mixed nature of the phase space at hand, and only count the high-Q whispering-gallery modes (WGMs) directly. That enables us to draw statistical information on the more lossy chaotic resonances, coupled to the high-Q regular modes via dynamical tunneling. Three different models [classical, Random-Matrix-Theory (RMT) based, semiclassical] to interpret the experimental data are discussed. On the basis of least-squares analysis, theoretical estimates of Ehrenfest time, and independent measurements, we find that a semiclassically modified RMT-based expression best describes the experiment in all its realizations, particularly when the resonator is coupled to visible light, while RMT alone still works quite well in the infrared. In this work we reexamine and substantially extend the results of a short paper published earlier [L. Wang, D. Lippolis, Z.-Y. Li, X.-F. Jiang, Q. Gong, and Y.-F. Xiao, Phys. Rev. E 93, 040201(R) (2016)]., Comment: 14 pages, 11 figures

Chirality is an important concept that describes the asymmetry property of a system, which usually emerges spontaneously due to mirror symmetry breaking. Such spontaneous chirality manifests predominantly as parity breaking in modern physics, which has been studied extensively, for instance, in Higgs physics, double-well Bose-Einstein condensates, topological insulators and superconductors. In the optical domain, spontaneous chiral symmetry breaking has been elusive experimentally, especially for micro- and nano-photonics which demands multiple identical subsystems, such as photonic nanocavities, meta-molecules and other dual-core settings. Here, for the first time, we observe spontaneous emergence of a chiral field in a single ultrahigh-Q whispering- gallery microresonator. This counter-intuitive effect arises due to the inherent Kerr nonlinearity-modulated coupling between clockwise (CW) and counterclockwise (CCW) propagating waves. At an ultra-weak input threshold of a few hundred microwatts, the initial chiral symmetry is broken spontaneously, and the CW-to-CCW output ratio reaches 20:1. The spontaneous chirality in microresonator holds great potential in studies of fundamental physics and applied photonic devices., Comment: 6 pages, 4 figures

Ultrasensitive optical detection of nanometer-scaled particles is highly desirable for applications in early-stage diagnosis of human diseases, environmental monitoring, and homeland security, but remains extremely difficult due to ultralow polarizabilities of small-sized, low-index particles. Optical whispering-gallery-mode microcavities, which can enhance significantly the light-matter interaction, have emerged as promising platforms for label-free detection of nanoscale objects. Different from the conventional whispering-gallery-mode sensing relying on the reactive (i.e., dispersive) interaction, here we propose and demonstrate to detect single lossy nanoparticles using the dissipative interaction in a high-$Q$ toroidal microcavity. In the experiment, detection of single gold nanorods in an aqueous environment is realized by monitoring simultaneously the linewidth change and shift of the cavity mode. The experimental result falls within the theoretical prediction. Remarkably, the reactive and dissipative sensing methods are evaluated by setting the probe wavelength on and off the surface plasmon resonance to tune the absorption of nanorods, which demonstrates clearly the great potential of the dissipative sensing method to detect lossy nanoparticles. Future applications could also combine the dissipative and reactive sensing methods, which may provide better characterizations of nanoparticles.

We theoretically propose a scheme for the generation of a non-classical single-mode motional state of a mechanical resonator (MR) in the three-mode optomechanical systems, in which two optical modes of the cavities are linearly coupled to each other and one mechanical mode of the MR is optomechanically coupled to the two optical modes with the same coupling strength simultaneously. One cavity is driven by a coherent laser light. By properly tuning the frequency of the weak driving field, we obtain engineered Liouvillian superoperator via engineering the selective interaction Hamiltonian confined to the Fock subspaces. In this case, the motional state of the MR can be prepared into a non-Gaussian state, which possesses the sub-Poisson statistics although its Wigner function is positive., Comment: 6 pages, 5 figures

Artificial metasurfaces are capable of completely manipulating the phase, amplitude, and polarization of light with high spatial resolutions. The emerging design based on high-index and low-loss dielectrics has led to the realization of novel metasurfaces with high transmissions, but these devices usually operate at the limited bandwidth, and are sensitive to the incident polarization. Here, for the first time we report experimentally the polarization-independent and high-efficiency dielectric metasurfaces spanning the visible wavelengths about 200 nm, which are of importance for novel flat optical devices operating over a broad spectrum. The diffraction efficiencies of the gradient metasurfaces consisting of the multi-fold symmetric nano-crystalline silicon nanopillars are up to 93% at 670 nm, and exceed 75% at the wavelengths from 600 to 800 nm for the two orthogonally polarized incidences. These dielectric metasurfaces hold great potential to replace prisms, lenses and other conventional optical elements.

Ground state cooling of massive mechanical objects remains a difficult task restricted by the unresolved mechanical sidebands. We propose an optomechanically-induced-transparency cooling scheme to achieve ground state cooling of mechanical motion without the resolved sideband condition in a pure optomechanical system with two mechanical modes coupled to the same optical cavity mode. We show that ground state cooling is achievable for sideband resolution $\omega_{m}/\kappa$ as low as 0.003. This provides a new route for quantum manipulation of massive macroscopic devices and high-precision measurements., Comment: 6 pages, 5 figures

Distributions of eigenmodes are widely concerned in both bounded and open systems. In the realm of chaos, counting resonances can characterize the underlying dynamics (regular vs. chaotic), and is often instrumental to identify classical-to-quantum correspondence. Here, we study, both theoretically and experimentally, the statistics of chaotic resonances in an optical microcavity with a mixed phase space of both regular and chaotic dynamics. Information on the number of chaotic modes is extracted by counting regular modes, which couple to the former via dynamical tunneling. The experimental data are in agreement with a known semiclassical prediction for the dependence of the number of chaotic resonances on the number of open channels, while they deviate significantly from a purely random-matrix-theory-based treatment, in general. We ascribe this result to the ballistic decay of the rays, which occurs within Ehrenfest time, and importantly, within the timescale of transient chaos. The present approach may provide a general tool for the statistical analysis of chaotic resonances in open systems., Comment: 5 pages, 5 figures, and a supplemental information

We propose and analyze a hybrid device by integrating a microscale diamond beam with a single built-in nitrogen-vacancy (NV) center spin to a superconducting coplanar waveguide (CPW) cavity. We find that under an ac electric field the quantized motion of the diamond beam can strongly couple to the single cavity photons via dielectric interaction. Together with the strong spin-motion interaction via a large magnetic field gradient, it provides a hybrid quantum device where the dia- mond resonator can strongly couple both to the single microwave cavity photons and to the single NV center spin. This enables coherent information transfer and effective coupling between the NV spin and the CPW cavity via mechanically dark polaritons. This hybrid spin-electromechanical de- vice, with tunable couplings by external fields, offers a realistic platform for implementing quantum information with single NV spins, diamond mechanical resonators, and single microwave photons., Comment: Accepted by Phys. Rev. Applied

Quantum manipulation of macroscopic mechanical systems is of great interest in both fundamental physics and applications ranging from high-precision metrology to quantum information processing. A crucial goal is to cool the mechanical system to its quantum ground state. In this review, we focus on the cavity optomechanical cooling, which exploits the cavity enhanced interaction between optical field and mechanical motion to reduce the thermal noise. Recent remarkable theoretical and experimental efforts in this field have taken a major step forward in preparing the motional quantum ground state of mesoscopic mechanical systems. This review first describes the quantum theory of cavity optomechanical cooling, including quantum noise approach and covariance approach; then the up-to-date experimental progresses are introduced. Finally, new cooling approaches are discussed along the directions of cooling in the strong coupling regime and cooling beyond the resolved sideband limit., Comment: Invited review paper, 32 pages, 10 figures

Laser cooling of mesoscopic mechanical resonators is of great interest for both fundamental studies and practical applications. We provide a general framework to describe the cavity-assisted backaction cooling in the strong coupling regime. By studying the cooling dynamics, we find that the temporal evolution of mean phonon number oscillates as a function of the optomechanical coupling strength depending on frequency mixing. The further analytical result reveals that the optimal cooling limit is obtained when the system eigenmodes satisfy the frequency matching condition. The reduced instantaneous-state cooling limits with dynamic dissipative cooling approach are also presented. Our study provides a guideline for optimizing the backaction cooling of mesoscopic mechanical resonators in the strong coupling regime., Comment: 8 pages, 6 figures

We study the optomechanical coupling of a single-layer graphene with a high-Q Fabry-Perot microcavity in the membrane-in-the-middle configuration. In ordinary dissipative coupling systems, mechanical oscillators modulate the loss associated with the input coupling of the cavity mode; while in our system, the graphene oscillator couples dissipatively with the cavity mode through modulating its absorption loss. By analyzing the effects of the interband transition of a graphene suspended near the node of the cavity field, we obtain strong and tunable dissipative coupling without excessively reducing the optical quality factor. Finally, it is found that the flexural mode of the graphene could be cooled down to its ground state in the present coupling system. This study provides new insights for graphene optomechanics in the visible range., Comment: 13 pages, 5 figures

Ground-state cooling of mesoscopic mechanical resonators is a fundamental requirement for test of quantum theory and for implementation of quantum information. We analyze the cavity optomechanical cooling limits in the intermediate coupling regime, where the light-enhanced optomechanical coupling strength is comparable with the cavity decay rate. It is found that in this regime the cooling breaks through the limits in both the strong and weak coupling regimes. The lowest cooling limit is derived analytically at the optimal conditions of cavity decay rate and coupling strength. In essence, cooling to the quantum ground state requires $Q_{\mathrm{m}}>2.4n_{\mathrm{th}% }$, with $Q_{\mathrm{m}}$ being the mechanical quality factor and $n_{\mathrm{th}}$ being the thermal phonon number. Remarkably, ground-state cooling is achievable starting from room temperature, when mechanical $Q$-frequency product $Q_{\mathrm{m}}{\nu>1.5}\times10^{13}$, and both of the cavity decay rate and the coupling strength exceed the thermal decoherence rate. Our study provides a general framework for optimizing the backaction cooling of mesoscopic mechanical resonators.