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A compact cantilever-based voice-eavesdropping microresonator is proposed. The dynamic vibration of the cantilever-microresonator system induced by an applied acoustic pressure is converted into a modulation of the cavity resonances. Then, the resonance-shift-induced optical transmission change is utilized to detect the applied acoustic signals. The enhancement of both the vibration and optical response makes the proposed system highly sensitive and have better performance than many other optical acoustic sensors. The noise equivalent pressure of the proposed system at 900 Hz is 52 μPa/Hz1/2. The system has a favorable broad frequency response range covering 0∼6 kHz. Experimental results prove that the system can acquire and reconstruct a voice signal 7 m away with high sensitivity and reliability, which can meet a variety of needs, such as indoor eavesdropping. Furthermore, the system is easy to fabricate, has low power consumption and is fairly tunable in terms of the frequency response range and sensitivity. This microcavity-based novel acoustic sensor is promising for opening up new possibilities in intrusion detection, voice eavesdropping and many other vibro-acoustic analysis applications.

We realize a chip-based Brillouin microlaser with remarkable features of high power and low noise using a microtoroid resonator. Our Brillouin microlaser is able to output a power of up to 126 mW with a fundamental linewidth down to 245 mHz. Additionally, in the course of Brillouin lasing we observe an intriguing power saturation-like effect, which can be attributed to complex thermo-optic-effect-induced mode mismatch between the pump and Brillouin modes. To have a quantitative understanding of this phenomenon, we develop a model by simultaneously considering Brillouin lasing and the thermo-optic effect occurring in the microcavity. Of importance, our theoretical results match well with experimentally measured data.

Light, sound, and microwave are important tools for many interdisciplinary applications in a multi-physical environment, and they usually are inefficient to be detected simultaneously in the same physical platform. However, at the microscopic scale, these waves can unexpectedly interact with the same microstructure through resonant enhancement, making it a unique hybrid micro-system for new applications across multiple physical channels. Here we experimentally demonstrate an optomechanical microdevice based on Brillouin lasing operation in an optical microcavity as a sensitive unit to sense external light, sound, and microwave signals in the same platform. These waves can induce modulations to the microcavity Brillouin laser (MBL) in a resonance-enhanced manner through either the pressure forces including radiation pressure force or thermal absorption, allowing several novel applications such as broadband non-photovoltaic detection of light, sound-light wave mixing, and deep-subwavelength microwave imaging. These results pave the way towards on-chip integrable optomechanical solutions for sensing, free-space secure communication, and microwave imaging.

Fizeau’s experiment, inspiring Einstein’s special theory of relativity, reveals a small dragging effect acting on light inside a moving medium. Dispersion can enhance such light drag according to Lorentz’s predication. Here fast- and slow-light-enhanced light drag is demonstrated experimentally in a moving optical microcavity through stimulated Brillouin scattering induced transparency and absorption. The strong dispersion provides an enhancement factor up to ~104, greatly reducing the system size down to the micrometer range. These results may offer a unique platform for a compact, integrated solution to motion sensing and ultrafast signal processing applications. In accordance with the relativistic addition of velocities, the Fizeau experiment shows the dragging of light inside a moving medium which can be enhanced in dispersive medium. Here a strong dispersion, induced by stimulated Brillouin scattering, is shown to significantly enhance the light drag effect in a moving optical microcavity.

We present temporal quadratic solitons in doubly resonant temporal dispersive optical parametric oscillators without a walk-off in numerical simulations. In frequency domain, these cavity solitons correspond to coherent dual-combs with smooth envelopes in half-harmonic and pump fields. We also investigate the soliton existence region, and the soliton behavior with parameters in this cavity system. We believe our work offers valuable insights into the practical generation of quadratic soliton combs in optical parametric oscillators.

We demonstrate a chip-based octave-spanning soliton microcomb in a whispering gallery mode microresonator platform. By fabricating a silica microdisk resonator and optimizing its dispersion with dry etching, we achieve an octave-spanning single-soliton microcomb with a repetition rate of ∼670 GHz at an optical pump power of 162.6 mW. Also, two dispersive waves at the end of the spectrum are observed to extend the comb spectral range and improve the comb power.

Optical frequency combs (OFCs) are essential in precision metrology, spectroscopy, distance measurement, and optical communications. Significant advances have been made recently in achieving micro-OFC devices based on parametric frequency conversion or electro-optic phase modulation. Here, we demonstrate a new kind of microcomb using a cavity optomechanical system with giant oscillation amplitude. We observe both optical and microwave frequency combs in a microtoroid resonator, which feature a flat OFC with 938 comb lines and a repetition rate as low as 50.22 MHz, as well as a flat microwave frequency comb with 867 comb lines. To generate such giant oscillation amplitude, we excite an overcoupled optical mode with a large blue detuning that is assisted with the thermo-optic nonlinearity. A new type of nonlinear oscillation, induced by competition between the optomechanical oscillation and thermo-optic nonlinearity, is also observed.

We demonstrate the first compact photonic flywheel with sub-fs time jitter (averaging times up to 10 {\mu}s) at the quantum-noise limit of a monolithic fiber resonator. Such quantum-limited performance is accessed through novel two-step pumping scheme for dissipative Kerr soliton (DKS) generation. Controllable interaction between stimulated Brillouin lasing and Kerr nonlinearity enhances the DKS coherence and mitigate the thermal instability challenge, achieving a remarkable 22-Hz intrinsic comb linewidth and an unprecedented phase noise of -180 dBc/Hz at 945 MHz carrier at free running. The scheme can be generalized to various device platforms for field-deployable precision metrology.

By generating a Brillouin laser in an optical microresonator, we realize a soliton Kerr microcomb through exciting the Kerr frequency comb using the generated Brillouin laser in the same cavity. The intracavity Brillouin laser pumping scheme enables us to access the soliton states with a blue-detuned input pump. Because of the ultranarrow linewidth and the low-noise properties of the generated Brillouin laser, the observed soliton microcomb exhibits narrow-linewidth comb lines and stable repetition rate. Also, we demonstrate a low-noise microwave signal with phase noise of $\ensuremath{-}49\text{ }\text{ }\mathrm{dBc}/\mathrm{Hz}$ at 10 Hz, $\ensuremath{-}130\text{ }\text{ }\mathrm{dBc}/\mathrm{Hz}$ at 10 kHz, and $\ensuremath{-}149\text{ }\text{ }\mathrm{dBc}/\mathrm{Hz}$ at 1 MHz offsets for a 10.43 GHz carrier with only a free-running input pump. The easy operation of the Brillouin-Kerr soliton microcomb with excellent performance makes our scheme promising for practical applications.

Photonic gauge potentials are crucial for manipulating charge-neutral photons like their counterpart electrons in the electromagnetic field, allowing the analogous Aharonov–Bohm effect in photonics and paving the way for critical applications such as photonic isolation. Normally, a gauge potential exhibits phase inversion along two opposite propagation paths. Here we experimentally demonstrate phonon-induced anomalous gauge potentials with noninverted gauge phases in a spatial-frequency space, where near-phase-matched nonlinear Brillouin scatterings enable such unique direction-dependent gauge phases. Based on this scheme, we construct photonic isolators in the frequency domain permitting nonreciprocal propagation of light along the frequency axis, where coherent phase control in the photonic isolator allows switching completely the directionality through an Aharonov–Bohm interferometer. Moreover, similar coherent controlled unidirectional frequency conversions are also illustrated. These results may offer a unique platform for a compact, integrated solution to implement synthetic-dimension devices for on-chip optical signal processing.

Whispering-gallery-mode optical microresonators have found impactful applications in various areas due to their remarkable properties such as ultra-high quality factor (Q-factor), small mode volume, and strong evanescent field. Among these applications, controllable tuning of the optical Q-factor is vital for on-chip optical modulation and various opto-electronic devices. Here, we report an experimental demonstration with a hybrid structure formed by an ultra-high-Q microtoroid cavity and a graphene monolayer. Thanks to the strong interaction of the evanescent wave with the graphene, the structure allows the Q-factor to be controllably varied in the range of 3.9 × 105 ∼ 6.2 × 107 by engineering optical absorption via changing the gap distance in between. At the same time, a resonant wavelength shift of 32 pm was also observed. Besides, the scheme enables us to approach the critical coupling with a coupling depth of 99.6%. As potential applications in integrated opto-electronic devices, we further use the system to realize a tunable optical filter with tunable bandwidth from 116.5 MHz to 2.2 GHz as well as an optical switch with a maximal extinction ratio of 31 dB and response time of 21 ms.

We demonstrate a Kerr soliton comb generation from a high-quality-factor fiber Fabry- Perotmicroresonator. Single soliton can be achieved, with narrow beatnote linewidth and nearquantum-noise-limited low phase noise.

We demonstrate a new, to the best of our knowledge, kind of self-pulsation in a microcavity Brillouin laser. This specific self-pulsation is generated by the interplay between the Brillouin lasing and the thermo-optic effect in an optical microcavity. Intriguingly, the self-pulsation behaviors are simultaneously present in both forward input pump and backward Brillouin lasing emission. By developing a coupled-mode theory, our numerical simulations display an excellent agreement with the experimental results.

Nanoscale light source is the key element for on-chip integrated optical communication system. As an important property of light source, polarization can be exploited to improve the information capacity of optical communication and the sensitivity of optical sensing. We demonstrate a novel TE-polarized light source based on graphene-nanoribbon (G-NR) hybrid structure. Thanks to the polarizing dependent absorption along graphene layer, the random polarized emission of nanoribbon (NR) can be transferred into the same TE polarization. In addition, lasing action in G-NR hybrid structure is also investigated. We attribute the polarization control to the differential attenuation of electromagnetic modes in graphene. Our simulation revealed electromagnetic field distribution and far field polar images of TE and TM modes in nanoribbon, which is consistent with experimental results. The compact G-NR hybrid structure light source offers a new way to realize the polarization controllable nanoscale light source and facilitate the practical applications of nanowire or nanoribbon light source.

We demonstrate ultralow-threshold neodymium-doped silica microsphere lasers on a silicon chip with lasing wavelengths of ~900 nm and ~1060 nm. Neodymium-doped microsphere cavities are fabricated with a series of doping concentrations using silica sol–gel films. Experimentally, we observe single-mode lasing emissions from the high-Q microsphere cavities with a threshold of as low as 1.2 µW.

Nonlinear non-Hermitian systems with saturable nonlinearities give rise to versatile phenomena that have no counterparts in linear systems. Here, we theoretically and experimentally demonstrate saturable nonlinearity (saturable absorption and gain) of erbium ions doped in silica microcavities at different concentrations. Our results show that when the doped concentration is low, the single isolated model agrees well with the experimental results, and also can be used as a basic description of highly doped samples to analyze the saturable nonlinearity. Yet, for highly doped microcavities we find that the clustering effect shall be also taken into account in order to match experimental data and to achieve a reasonable agreement. To confirm our finding, we experimentally characterize the saturable nonlinearity for ion concentrations varying from $\phantom{\rule{4pt}{0ex}}1.0\phantom{\rule{4pt}{0ex}}$ to $5.0\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. Our experiment shows excellent consistency with the theory, which in turn suggests other potential applications of the theory based on saturable nonlinearity, including optical bistability and nonlinear parity-time symmetry.

Breaking Lorentz reciprocity was believed to be a prerequisite for nonreciprocal transmissions of light fields, so the possibility of nonreciprocity by linear optical systems was mostly ignored. We put forward a structure of three mutually coupled microcavities or optical fiber rings to realize optical nonreciprocity. Although its couplings with the fields from two different input ports are constantly equal, such system transmits them nonreciprocally either under the saturation of an optical gain in one of the cavities or with the asymmetric couplings of the circulating fields in different cavities. The structure made up of optical fiber rings can perform nonreciprocal transmissions as a time-independent linear system without breaking Lorentz reciprocity. Optical isolation for inputs simultaneously from two different ports and even approximate optical isolator operations are implementable with the structure., 6 pages and 4 figures. The resolutions of the figure files are lowered, and the supplementary materials are not included, due to the upload limit imposed by ArXiv

We demonstrate the fabrication of ultrahigh quality (Q) factor silica microdisk resonators on a silicon chip by inductively coupled plasma (ICP) etching. We achieve a dry-etched optical microresonator with an intrinsic Q factor as high as 1.94 × 10 8 from a 1-mm-diameter silica microdisk with a thickness of 4 μm. Our work provides a chip-based microresonator platform operating in the ultrahigh-Q region that will be useful in nonlinear photonics such as Brillouin lasers and Kerr microcombs.

Optical microsphere resonators working in the near- and mid-infrared regions are highly required for a variety of applications, such as optical sensors, filters, modulators, and microlasers. Here, a simple and low-cost approach is reported for batch fabrication of high-quality chalcogenide glass (ChG) microsphere resonators by melting high-purity ChG powders in an oil environment. Q factors as high as 1.2 × 106 (7.4 × 105 ) are observed in As2 S3 (As2 Se3 ) microspheres (≈30 µm in diameter) around 1550-nm wavelength. Smaller microspheres with sizes around 10 µm also show excellent resonant responses (Q ≈ 2.5 × 105 ). Based on the mode splitting of an azimuthal mode in a microsphere resonator, eccentricities as low as ≈0.13% (≈0.17%) for As2 S3 (As2 Se3 ) microspheres are measured. Moreover, by coupling ChG microspheres with a biconical As2 S3 fiber taper, Q factors of ≈1.7 × 104 (≈1.6 × 104 ) are obtained in As2 S3 (As2 Se3 ) microspheres in the mid-infrared region (around 4.5 µm). The high-quality ChG microspheres demonstrated here are highly attractive for near- and mid-infrared optics, including optical sensing, optical nonlinearity, cavity quantum electrodynamics, microlasers, nanofocusing, and microscopic imaging.

Despite being fundamentally challenging in integrated (nano)photonics, achieving chip-based light non-reciprocity becomes increasingly urgent in signal processing and optical communications. Because of material incompatibilities in conventional approaches based on the Faraday effect, alternative solutions have resorted to nonlinear processes to obtain one-way transmission. However, dynamic reciprocity in a recent theoretical analysis has pinned down the functionalities of these nonlinear isolators. To bypass such dynamic reciprocity, we here demonstrate an optical isolator on a silicon chip enforced by phase-matched parametric amplification in four-wave mixing. Using a high-Q microtoroid resonator, we realize highly non-reciprocal transport at the 1,550 nm wavelength when waves are injected from both directions in two different operating configurations. Our design, compatible with current complementary metal-oxide-semiconductor (CMOS) techniques, yields convincing isolation performance with sufficiently low insertion loss for a wide range of input power levels. Moreover, our work demonstrates the possibility of designing chip-based magnetic-free optical isolators for information processing and laser protection., Non-reciprocal optical elements usually require the presence of magnetic fields, which makes chip integration difficult. Here, Hua et al. demonstrate a non-magnetic optical isolator with bidirectional injection on a silicon platform utilizing parametric amplification in four-wave mixing.

The device of free-space self-interference microresonator coupled by fiber tapers is proposed. Different from sensors with similar structures on-chip and benefit to the combination between microcavity and optical fiber sensing, the coupling regime is adjustable and a separated sensing area from coupling regions is available. This method makes it feasible to optimize coupling efficiency in detection and broaden the scope of application in dissipative sensing. The transmission spectrum exhibits a distinct phenomenon under a long optical path of sensing arm, including quasi-sinusoidal modulation and profile split. Based on that, transmission characteristics are analyzed by theoretical stimulations and the subsequent experiments are in good agreement with the theory. In addition, we estimate the sensing performance of this device and that the sensitivity can reach −4.76 dB/(10−7 RIU) with a 1 m sensing arm theoretically when applied in monitoring the refractive index change, which is almost an order of magnitude larger than the previously reported data. Besides being adjustable, compact, and efficient, this device shows great potential in the precision measurement and expands the applicable measurement field of similar structures, such as pull pressure that cannot be detected using bus straight waveguides.

A novel type of mid-IR microresonator, the chalcogenide glass (ChG) microfiber knot resonator (MKR), is demonstrated, showing easy fabrication, fiber-compatible features, resonance tunability, and high robustness. ChG microfibers with typical diameters around 3 μm are taper-drawn from As 2 S 3 glass fibers and assembled into MKRs in liquid without surface damage. The measured Q factor of a typical 824 μm diameter ChG MKR is about 2.84 × 10 4 at the wavelength of 4469.14 nm. The free spectral range (FSR) of the MKR can be tuned from 2.0 nm (28.4 GHz) to 9.6 nm (135.9 GHz) by tightening the knot structure in liquid. Benefitting from the high thermal expansion coefficient of As 2 S 3 glass, the MKR exhibits a thermal tuning rate of 110 pm · ° C − 1 at the resonance peak. When embedded in polymethyl methacrylate (PMMA) film, a 551 μm diameter MKR retains a Q factor of 1.1 × 10 4 . The ChG MKRs demonstrated here are highly promising for resonator-based optical technologies and applications in the mid-IR spectral range.

High quality (Q) factor microdisks are fundamental building blocks of on-chip integrated photonic circuits and biological sensors. The resonant modes in microdisks circulate near their boundaries, making their performances strongly dependent upon surface roughness. Surface-tension-induced microspheres and microtoroids are superior to other dielectric microdisks when comparing Q factors. However, most photonic materials such as silicon and negative photoresists are hard to be reflowed and thus the realizations of high-Q microdisks are strongly dependent on electron-beam lithography. Herein, we demonstrate a robust, cost-effective, and highly reproducible technique to fabricate ultrahigh-Q microdisks. By using silica microtoroids as masks, we have successfully replicated their ultrasmooth boundaries in a photoresist via anisotropic dry etching. The experimentally recorded Q factors of passive microdisks can be as large as 1.5 × 106. Similarly, ultrahigh Q microdisk lasers have also been replicated in dye-doped polymeric films. The laser linewidth is only 8 pm, which is limited by the spectrometer and is much narrower than that in previous reports. Meanwhile, high-Q deformed microdisks have also been fabricated by controlling the shape of microtoroids, making the internal ray dynamics and external directional laser emissions controllable. Interestingly, this technique also applies to other materials. Silicon microdisks with Q > 106 have been experimentally demonstrated with a similar process. We believe this research will be important for the advances of high-Q micro-resonators and their applications.

We have demonstrated a chip-based Raman laser by using a silica microdisk resonator with an intrinsic optical Q-factor of 1.5⨯107. The achieved lasing threshold is as low as 3.9 mw with a conversion efficiency of 7.5%.

On-chip nonreciprocal light transport holds a great impact on optical information processing and communications based upon integrated photonic devices. By harvesting gain-saturation nonlinearity, we recently demonstrated on-chip optical asymmetric transmission at telecommunication bands with superior nonreciprocal performances using only one active whispering-gallery-mode microtoroid resonator, beyond the commonly adopted magneto-optical (Faraday) effect. Here, detailed theoretical analysis is presented with respect to the reported scheme. Despite the fact that our model is simply the standard coupled-mode theory, it agrees well with the experiment and describes the essential one-way light transport in this nonreciprocal device. Further discussions, including the connection with the second law of thermodynamics and Fano resonance, are also briefly made in the end.

We demonstrate radiation-pressure-driven mechanical oscillations from high optical quality factor silica microdisk resonators on chip. Mechanical quality factors of 3520 in air and 12540 in vacuum for the fundamental radial breathing modes are obtained from 73 μm-diameter silica microdisks with mechanical frequencies of ∼51 MHz. The measured mechanical oscillation threshold powers for the input light are determined to be 62.5 μW in air and down to 26.6 μW in vacuum.

Construction of a chip-based optical microcavity from multi-component glass has long been a significant fundamental challenge in the cross field of materials science and photonics. Here we introduced a scalable non-hydrolytic sol–gel method for deposition of multi-component glass film with high thickness and superior homogeneity. Prototypically, we demonstrated success in the fabrication of multi-component tellurite thick film, and construction of the tellurite microcavity on a silicon chip through a combined etching technology for the first time. The collaborative studies by using the steady-state spectrum, whisper gallery mode (WGM) resonance spectrum and electric field distribution firmly indicate that the obtained thick film and microcavity present excellent properties, pointing to their promising applications in integrated photonics.

We have demonstrated controllable coupling between an ultra-high-Q microcavity and monolayer graphene with tunability of the Q-factor from 1.59×107 to 1.20×105. The Q-factor has been finely tuned by adjusting the gap between them.

On-chip parity–time-symmetric optics is experimentally demonstrated at a wavelength of 1,550 nm in two directly coupled, high-Q silica microtoroid resonators with balanced effective gain and loss. Switchable optical isolation with a nonreciprocal isolation ratio between −8 dB and +8 dB is also shown. The findings will be useful for potential applications in optical isolators, on-chip light control and optical communications. Compound-photonic structures with gain and loss1 provide a powerful platform for testing various theoretical proposals on non-Hermitian parity–time-symmetric quantum mechanics2,3,4,5 and initiate new possibilities for shaping optical beams and pulses beyond conservative structures. Such structures can be designed as optical analogues of complex parity–time-symmetric potentials with real spectra. However, the beam dynamics can exhibit unique features distinct from conservative systems due to non-trivial wave interference and phase-transition effects. Here, we experimentally realize parity–time-symmetric optics on a chip at the 1,550 nm wavelength in two directly coupled high-Q silica-microtoroid resonators with balanced effective gain and loss. With this composite system, we further implement switchable optical isolation with a non-reciprocal isolation ratio from −8 dB to +8 dB, by breaking time-reversal symmetry with gain-saturated nonlinearity in a large parameter-tunable space. Of importance, our scheme opens a door towards synthesizing novel microscale photonic structures for potential applications in optical isolators, on-chip light control and optical communications.

This paper describes the specially designed geometry of a dry-etched large-wedge-angle silica microdisk resonator that enables anomalous dispersion in the 780 nm wavelength regime. This anomalous dispersion occurs naturally without the use of a mode-hybridization technique to control the geometrical dispersion. By fabricating a 1-μm-thick silica microdisk with a wedge angle as large as 56° and an optical Q-factor larger than 107, we achieve a visible Kerr comb that covers the wavelength interval of 700–897 nm. The wide optical frequency range and the closeness to the clock transition at 698 nm of Sr87 atoms make our visible comb a potentially useful tool in optical atomic clock applications.

Quantum entanglement among multiple spatially separated particles is of fundamental interest, and can serve as central resources for studies in quantum nonlocality, quantum-to-classical transition, quantum error correction, and quantum simulation. The ability of generating an increasing number of entangled particles is an important benchmark for quantum information processing. The largest entangled states were previously created with fourteen trapped ions, eight photons, and five superconducting qubits. Here, based on spontaneous parametric down-converted two-photon entanglement source with simultaneously a high brightness of ~12 MHz/W, a collection efficiency of ~70% and an indistinguishability of ~91% between independent photons, we demonstrate, for the first time, genuine and distillable entanglement of ten single photons under different pump power. Our work creates a state-of-the-art platform for multi-photon experiments, and provide enabling technologies for challenging optical quantum information tasks such as high-efficiency scattershot boson sampling with many photons., Comment: 65 pages, supplementary information included, with all raw data. to appear in Physical Review Letters

In this paper, we analyze a chain-linked triple-cavity photonic molecule (TCPM) with controllable coupling strengths between the cavities on their spectral properties and field (energy) distributions by solving eigenvalues and eigenvectors of the Hamiltonian matrix based on coupled-mode theory. Phase transition is extended from double-cavity photonic molecules (DCPMs) to TCPMs, and evolutions of the supermode frequencies and linewidths are analyzed, which have synchronous relations with the degree of coherence between adjacent optical microcavities and energy distributions in the three cavities, respectively. We develop a superposition picture for the three supermodes of the TCPM, as interferences between supermodes of sub-DCPMs. In particular, we demonstrate the abnormal properties of the central supermode in TCPMs, such as the dark state in the middle cavity and phase shift when energy is flowing between the side cavities; these are promising for information processing and remote control of energy. General properties of TCPMs are summarized and limitations on linewidths are given. Finally, we make an interesting analog to intracavity electromagnetically induced transparency in multilevel atomic systems using the flexible TCPM platform under appropriate conditions.

Canonical quantum mechanics postulates Hermitian Hamiltonians to ensure real eigenvalues. Counterintuitively, a non-Hermitian Hamiltonian, satisfying combined parity-time (PT) symmetry, could display entirely real spectra above some phase-transition threshold. Such a counterintuitive discovery has aroused extensive theoretical interest in extending canonical quantum theory by including non-Hermitian but PT-symmetric operators in the last two decades. Despite much fundamental theoretical success in the development of PT-symmetric quantum mechanics, an experimental observation of pseudo-Hermiticity remains elusive as these systems with a complex potential seem absent in Nature. But nevertheless, the notion of PT symmetry has highly survived in many other branches of physics including optics, photonics, AMO physics, acoustics, electronic circuits, material science over the past ten years, and others, where a judicious balance of gain and loss constitutes a PT-symmetric system. Here, although we concentrate upon reviewing recent progress on PT symmetry in optical microcavity systems, we also wish to present some new results that may help to accelerate the research in the area. Such compound photonic structures with gain and loss provide a powerful platform for testing various theoretical proposals on PT symmetry, and initiate new possibilities for shaping optical beams and pulses beyond conservative structures. Throughout this article there is an effort to clearly present the physical aspects of PT-symmetry in optical microcavity systems, but mathematical formulations are reduced to the indispensable ones. Readers who prefer strict mathematical treatments should resort to the extensive list of references. Despite the rapid progress on the subject, new ideas and applications of PT symmetry using optical microcavities are still expected in the future.

We theoretically describe and study coupling between two whispering-gallery-mode microcavities through a side coupled fiber taper. The two microcavities can couple to each other bidirectionally with only one fiber taper waveguide due to the modal coupling between a pair of whispering-gallery modes traveling in opposite directions. By choosing appropriate physical parameters, some interesting spectra, such as electromagnetically-induced transparency-like coherent transmission and band-like reflection, can be obtained in such a system. Slow light is discussed in this paper as a direct application of the coupled microresonator structure. This structure also has potential application in coupled cavity quantum electrodynamics.

We realize parity-time symmetry in a coupled active-passive microcavity system with balanced gain and loss. On-chip nonreciprocal light transmission and circulation due to gain-saturation nonlinearity are experimentally demonstrated with superior performance in high-Q microcavity systems.

We demonstrate ultralow-threshold thulium-doped, as well as thulium-holmium-codoped, microtoroid lasers on silicon chips, operating at the wavelength of around 2 μm. High quality factor whispering gallery mode (WGM) microtoroid cavities with proper thulium and holmium concentrations are fabricated from the silica sol-gel films. The highly confined WGMs make the microcavity lasers operate with ultralow thresholds, approximately 2.8 μW and 2.7 μW for the thulium-doped and the thulium-holmium-codoped microlasers, respectively.

By overcoming fabrication limitations, we have successfully fabricated silica toroid microcavities with both large diameter (of 1.88 mm) and ultra-high-Q factor (of 3.3×108) for the first time, to the best of our knowledge. By employing these resonators, we have further demonstrated low-threshold Kerr frequency combs on a silicon chip, which allow us to obtain a repetition rate as low as 36 GHz. Such a low repetition rate frequency comb can now be directly measured through a commercialized optical-electronic detector.

We demonstrate an ultra-low-threshold phonon laser using a coupled-microtoroid-cavity system by introducing a novel coupling approach. The scheme exhibits both high optical quality factors and high mechanical quality factors. We have experimentally obtained the mechanical quality factor up to 18,000 in vacuum for a radial-breathing mode of 59.2 MHz. The measured phonon lasing threshold is as low as 1.2 μW, which is ∼5 times lower than the previous result.

We experimentally demonstrate the electromagnetically induced transparency (EIT)-like effect using a double-wheel microcavity coupled with a silica microtoroid cavity. To match the resonant modes of the two microcavities, we shift the resonant wavelength of the double-wheel microcavity by 1.5 nm via the optical gradient force. The EIT-like transmission spectrum is precisely controlled by changing the distance and the frequency detuning between the two coupled microcavities.