Theory of soliton self-frequency shift in silica optical microresonators with a modified Raman response by the Boson peak.

We theoretically study the Raman-induced self-frequency shift of dissipative Kerr soliton in silica optical resonators by taking into consideration the Boson peak. We find that the Boson peak will greatly increase the soliton self-frequency shift and contribute even more than the shift induced by the Lorentzian response for certain pulse durations. We also show that the revised Raman shock time is associated with the pulse width even for a relatively long pulse. Moreover, we demonstrate that the background continuous wave decreases the self-frequency shift of the soliton via the interference with the soliton. Our theoretical and simulated results display excellent agreement with the previous experimental values in the silica-based Kerr-soliton microcomb.

Six-orders-of-magnitude-spanning dispersion measurement via Kalman filtering-aided white-light interferometry.

Dispersion plays a great role in ultrafast laser oscillators, ultrashort pulse amplifiers, and many other nonlinear optical dynamics. Therefore, dispersion measurement is crucial for device characterization, system design and nonlinear dynamics investigation therein. In this work, we demonstrate a versatile approach, i.e., Kalman filtering-aided white-light interferometry, for group delay dispersion (GDD) characterization. Extended Kalman filter is adopted to track the cosine-like interferogram, and to eliminate the unintended bias and the envelope, providing a nearly ideal phase retrieval and GDD estimation. The measurement range could span from tens of fs2 to tens of ps2, with an uncertainty of about 0.1%, enabling precise GDD measurement for diverse optical components, ranging from a millimeter-thick glass slide to highly dispersive chirped fiber Bragg gratings. Benefited by the simplicity, convenient setup, and easy operation as well as relatively low cost, this approach would help photonic device characterization, dispersion management and nonlinear dynamics investigation in the laboratory and work plant.

Highly Correlated Optomechanical Oscillations Manifested by an Anomalous Stabilization.

Driven by a sufficiently powerful pump laser, a cavity optomechanical system will stabilize in coupled oscillations of its cavity field and mechanical resonator. It was assumed that the oscillation will be continuously magnified upon enhancing the driving laser further. However, based on the nonlinear dynamics of the system, we find that the dynamical behaviors of the system are much more complex than this intuitive picture, especially when it is operated near the blue detuning point by the mechanical resonator's intrinsic frequency. There exists an anomalous stabilization: depending on its intrinsic damping rate and the pump power, the mechanical resonator will metastably stay on one orbit of oscillation after another until it completely stabilizes on the final orbit it can reach. These orbits are consistent with the locked ones with almost fixed oscillation amplitudes, which are realized after the pump power becomes still higher. The oscillatory cavity field is seen to adjust its sidebands following the mechanical frequency shift due to optical spring effect, so that it always drives the mechanical resonator to near those locked orbits once the pump power is over a threshold. In the regimes with such correlation between cavity field sidebands and mechanical oscillation, the system's dynamical attractors are confined on the locked orbits and chaotic motion is also excluded.

Enhanced Frequency Conversion in Parity-Time Symmetry Line.

Non-Hermitian degeneracies reveal intriguing and nontrivial behaviors in open physical systems. Examples like parity-time (PT) symmetry breaking, topological encircling chirality, and enhanced sensing near an exceptional point (EP) are often associated with the abrupt nature of the phase transition around these degeneracies. Here we experimentally observe a cavity-enhanced second-harmonic frequency (SHG) conversion on a PT symmetry line, i.e., a set consisting of open-ended isofrequency or isoloss lines, both terminated at EPs on the Riemann surface in parameter space. The enhancement factor can reach as high as 300, depending on the crossing point whether in the symmetry or the broken phase of the PT line. Moreover, such enhancement of SHG enables sensitive distance sensing with a nanometer resolution. Our works may pave the way for practical applications in sensing, frequency conversion, and coherent wave control.

Kalman filtering-enhanced short-delay self-heterodyne interferometry for linewidth measurement.

We demonstrate an extended Kalman filtering-enhanced linewidth measurement in short-delay self-heterodyne interferometry (SDSHI). We found that a modified SDSHI trace closely resembles a biased cosine wave, which would enable convenient linewidth estimation by its uniform envelope contrast without any correction factor. Experimentally, we adopted this approach for kHz laser linewidth measurement, taking advantages of extended Kalman filtering (EKF) to adaptively track the cosine wave. Apart from the measurement noise suppression, this approach could use as many data points as possible in the noisy trace to make a linewidth estimation at each tracked data point, from which we can deduce valuable statistical parameters such as the mean and standard deviation. This approach involves no more equipment than conventional SDSHI and sophisticated EKF so that it can be easily implemented. Therefore, we believe it will find wide applications in ultra-narrow laser linewidth measurement.

Octave-spanning soliton microcomb in silica microdisk resonators.

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.

Self-pulsations in a microcavity Brillouin laser.

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.

High-power, low-noise Brillouin laser on a silicon chip.

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.

Batch Fabrication of High-Quality Infrared Chalcogenide Microsphere Resonators.

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.

Brillouin-Kerr Soliton Frequency Combs in an Optical Microresonator.

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 -49 dBc/Hz at 10 Hz, -130 dBc/Hz at 10 kHz, and -149 dBc/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.

Generation of Optical Frequency Comb via Giant Optomechanical Oscillation.

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.

Hyperboloid-Drum Microdisk Laser Biosensors for Ultrasensitive Detection of Human IgG.

Whispering gallery mode (WGM) microresonators have been used as optical sensors in fundamental research and practical applications. The majority of WGM sensors are passive resonators that require complex systems, thereby limiting their practicality. Active resonators enable the remote excitation and collection of WGM-modulated fluorescence spectra, without requiring complex systems, and can be used as alternatives to passive microresonators. This paper demonstrates an active microresonator, which is a microdisk laser in a hyperboloid-drum (HD) shape. The HD microdisk lasers are a combination of a rhodamine B-doped photoresist and a silica microdisk. These HD microdisk lasers can be utilized for the detection of label-free biomolecules. The biomolecule concentration can be as low as 1 ag mL-1 , whereas the theoretical detection limit of the biosensor for human IgG in phosphate buffer saline is 9 ag mL-1 (0.06 aM ). Additionally, the biosensors are able to detect biomolecules in an artificial serum, with a theoretical detection limit of 9 ag mL-1 (0.06 aM ). These results are approximately four orders of magnitude more sensitive than those for the typical active WGM biosensors. The proposed HD microdisk laser biosensors show enormous detection potential for biomarkers in protein secretions or body fluids.

Controllable coupling between an ultra-high-Q microtoroid cavity and a graphene monolayer for optical filtering and switching applications.

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.

Photonic Flywheel in a Monolithic Fiber Resonator.

We demonstrate the first compact photonic flywheel with sub-fs time jitter (averaging times up to 10 µs) 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 generation. Controllable interaction between stimulated Brillouin lasing and Kerr nonlinearity enhances the DKS coherence and mitigates 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.

Transmission Nonreciprocity in a Mutually Coupled Circulating Structure.

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.

Parity-Time symmetry helps breaking a new limit.

Parity-Time (PT) symmetry is an emerging concept in quantum mechanics where non-Hermitian Hamiltonians can exhibit real eigenvalues. Now, PT symmetric optical microresonators have been demonstrated to break the bandwidth-efficiency limit for nonlinear optical signal processing.

Strong interactions between solitons and background light in Brillouin-Kerr microcombs.

Dissipative Kerr-soliton combs are laser pulses regularly sustained by a localized solitary wave on top of a continuous-wave background inside a nonlinear resonator. Usually, the intrinsic interactions between the background light and solitons are weak and localized. Here, we demonstrate a strong interaction between the generated soliton comb and the background light in a Brillouin-Kerr microcomb system. This strong interaction enables the generation of a monostable single-soliton microcomb on a silicon chip. Also, new phenomena related to soliton physics including solitons hopping between different states as well as controlling the formations of the soliton states by the pump power, are observed owing to such strong interaction. Utilizing this monostable single-soliton microcomb, we achieve the 100% deterministic turnkey operation successfully without any feedback controls. Importantly, it allows to output turnkey ultra-low-noise microwave signals using a free-running pump.

Femtosecond-laser-assisted high-aspect-ratio nanolithography in lithium niobate.

We report the successful fabrication of high-aspect-ratio lithium niobate (LN) nanostructures by using femtosecond-laser-assisted chemical etching. In this technique, a 1 kHz femtosecond laser is first used to induce local modifications inside the LN crystal. Then, selective chemical wet etching is conducted using a buffered oxide etch (BOE) solution. The etching rate in the laser-modified area reaches 2 µm h-1, which is enhanced by a factor of ∼660 in comparison to previous reports without laser irradiation. Such high selectivity in chemical etching helps realize high-performance maskless nanolithography in lithium niobate. In the experiment, we have fabricated high-quality LN nanohole arrays. The nanohole size reaches ∼100 nm and its aspect ratio is above 40 : 1. The minimal period of the LN hole array is 300 nm. Our work paves a way to fabricate LN nano-integrated devices for advanced optic and electronic applications.

Microlaser based on a hybrid structure of a semiconductor nanowire and a silica microdisk cavity.

We experimentally demonstrate a hybrid structure microlaser on chip with a single CdSe nanowire attached to a high-Q silica microdisk cavity at room temperature. When pumped by a 532 nm pulse laser, both single-longitudinal mode and multi-longitudinal mode lasers with linewidth of 0.18 nm are obtained from the hybrid structure with a 58-µm-diameter microdisk and a 250-nm diameter nanowire. The measured lasing threshold of the microlaser is as low as 100 µJ/cm².

Controllable optical analog to electromagnetically induced transparency in coupled high-Q microtoroid cavities.

We experimentally demonstrate an all-optical analog to electromagnetically induced transparency (EIT) on chip using coupled high-Q silica microtoroid cavities with Q-factors above 10(6). The transmission spectrum of the all-optical analog to EIT is precisely controlled by tuning the distance between the two microtoroids, as well as the detunings of the resonance frequencies of the two cavities.