Optomechanical Frequency Comb Based on Multiple Nonlinear Dynamics.

Phonon-based frequency combs that can be generated in the optical and microwave frequency domains have attracted much attention due to the small repetition rates and the simple setup. Here, we experimentally demonstrate a new type of phonon-based frequency comb in a silicon optomechanical crystal cavity including both a breathing mechanical mode (∼GHz) and flexural mechanical modes (tens of MHz). We observe strong mode competition between two approximate flexural mechanical modes, i.e., 77.19 and 90.17 MHz, resulting in only one preponderant lasing, while maintaining the lasing of the breathing mechanical mode. These simultaneous observations of two-mode phonon lasing state and significant mode competition are counterintuitive. We have formulated comprehensive theories to elucidate this phenomenon in response to this intriguing outcome. In particular, the self-pulse induced by the free carrier dispersion and thermo-optic effects interacts with two approximate flexural mechanical modes, resulting in the repetition rate of the comb frequency-locked to exact fractions of one of the flexural mechanical modes and the mode hopping between them. This phonon-based frequency comb has at least 260 comblines and a repetition rate as low as a simple fraction of the flexural mechanical frequency. Our demonstration offers an alternative optomechanical frequency comb for sensing, timing, and metrology applications.

Octave soliton microcombs in lithium niobate microresonators.

Soliton microcombs are regarded as an ideal platform for applications such as optical communications, optical sensing, low-noise microwave sources, optical atomic clocks, and frequency synthesizers. Many of these applications require a broad comb spectrum that covers an octave, essential for implementing the f - 2f self-referencing techniques. In this work, we have successfully generated an octave-spanning soliton microcomb based on a z-cut thin-film lithium niobate (TFLN) microresonator. This achievement is realized under on-chip optical pumping at 340 mW and through extensive research into the broadening of dual dispersive waves (DWs). Furthermore, the repetition rate of the octave soliton microcomb is accurately measured using an electro-optic comb generated by an x-cut TFLN racetrack microresonator. Our results represent a crucial step toward the realization of practical, integrated, and fully stabilized soliton microcomb systems based on TFLN.

Photonic time-delayed reservoir computing based on series-coupled microring resonators with high memory capacity.

On-chip microring resonators (MRRs) have been proposed to construct time-delayed reservoir computing (RC) systems, which offer promising configurations available for computation with high scalability, high-density computing, and easy fabrication. A single MRR, however, is inadequate to provide enough memory for the computation task with diverse memory requirements. Large memory requirements are satisfied by the RC system based on the MRR with optical feedback, but at the expense of its ultralong feedback waveguide. In this paper, a time-delayed RC is proposed by utilizing a silicon-based nonlinear MRR in conjunction with an array of linear MRRs. These linear MRRs possess a high quality factor, providing enough memory capacity for the RC system. We quantitatively analyze and assess the proposed RC structure's performance on three classical tasks with diverse memory requirements, i.e., the Narma 10, Mackey-Glass, and Santa Fe chaotic timeseries prediction tasks. The proposed system exhibits comparable performance to the system based on the MRR with optical feedback, when it comes to handling the Narma 10 task, which requires a significant memory capacity. Nevertheless, the dimension of the former is at least 350 times smaller than the latter. The proposed system lays a good foundation for the scalability and seamless integration of photonic RC.

Repetition rate tuning and locking of solitons in a microrod resonator.

Recently, there has been significant interest in the generation of coherent temporal solitons in optical microresonators. In this Letter, we present a demonstration of dissipative Kerr soliton generation in a microrod resonator using an auxiliary-laser-assisted thermal response control method. In addition, we are able to control the repetition rate of the soliton over a range of 200 kHz while maintaining the pump laser frequency, by applying external stress tuning. Through the precise control of the PZT voltage, we achieve a stability level of 3.9 × 10-10 for residual fluctuation of the repetition rate when averaged 1 s. Our platform offers precise tuning and locking capabilities for the repetition frequency of coherent mode-locked combs in microresonators. This advancement holds great potential for applications in spectroscopy and precision measurements.

Magnetic-free polarization rotation in an atomic vapor cell.

Magnetic-free nonreciprocal optical devices have attracted great attention in recent years. Here, we investigated the magnetic-free polarization rotation of light in an atom vapor cell. Two mechanisms of magnetic-free nonreciprocity have been realized in ensembles of hot atoms, including electromagnetically induced transparency and optically-induced magnetization. For a linearly polarized input probe light, a rotation angle up to 86.4° has been realized with external control and pump laser powers of 10 mW and is mainly attributed to the optically-induced magnetization effect. Our demonstration offers a new approach to realize nonreciprocal devices, which can be applied to solid-state atom ensembles and may be useful in photonic integrated circuits.

Data augmentation using continuous conditional generative adversarial networks for regression and its application to improved spectral sensing.

Machine learning-assisted spectroscopy analysis faces a prominent constraint in the form of insufficient spectral samples, which hinders its effectiveness. Meanwhile, there is a lack of effective algorithms to simulate synthetic spectra from limited samples of real spectra for regression models in continuous scenarios. In this study, we introduced a continuous conditional generative adversarial network (CcGAN) to autonomously generate synthetic spectra. The labels employed for generating the spectral data can be arbitrarily selected from within the range of labels associated with the real spectral data. Our approach effectively produced spectra using a small spectral dataset obtained from a self-interference microring resonator (SIMRR)-based sensor. The generated synthetic spectra were subjected to evaluation using principal component analysis, revealing an inability to discern them from the real spectra. Finally, to enhance the DNN regression model, these synthetic spectra are incorporated into the original training dataset as an augmentation technique. The results demonstrate that the synthetic spectra generated by CcGAN exhibit exceptional quality and significantly enhance the predictive performance of the DNN model. In conclusion, CcGAN exhibits promising potential in generating high-quality synthetic spectra and delivers a superior data augmentation effect for regression tasks.

Machine learning-assisted high-accuracy and large dynamic range thermometer in high-Q microbubble resonators.

Whispering gallery mode (WGM) resonators provide an important platform for fine measurement thanks to their small size, high sensitivity, and fast response time. Nevertheless, traditional methods focus on tracking single-mode changes for measurement, and a great deal of information from other resonances is ignored and wasted. Here, we demonstrate that the proposed multimode sensing contains more Fisher information than single mode tracking and has great potential to achieve better performance. Based on a microbubble resonator, a temperature detection system has been built to systematically investigate the proposed multimode sensing method. After the multimode spectral signals are collected by the automated experimental setup, a machine learning algorithm is used to predict the unknown temperature by taking full advantage of multiple resonances. The results show the average error of 3.8 × 10-3°C within the range from 25.00°C to 40.00°C by employing a generalized regression neural network (GRNN). In addition, we have also discussed the influence of the consumed data resource on its predicted performance, such as the amount of training data and the case of different temperate ranges between the training and test data. With high accuracy and large dynamic range, this work paves the way for WGM resonator-based intelligent optical sensing.

Thermal oscillation in the hybrid Si3N4 - TiO2 microring.

The hybrid microcavity composed of different materials shows unique thermal-optical properties such as resonance frequency shift and small thermal noise fluctuations with the temperature variation. Here, we have fabricated the hybrid Si3N4 - TiO2 microring, which decreases the effective thermo-optical coefficients (TOC) from 23.2pm/K to 11.05pm/K due to the opposite TOC of these two materials. In this hybrid microring, we experimentally study the thermal dynamic with different input powers and scanning speeds. The distorted transmission and thermal oscillation are observed, which results from the non-uniform scanning speed and the different thermal relaxation times of the Si3N4 and the TiO2. We calibrate the distorted transmission spectrum for the resonance measurement at the reverse scanning direction and explain the thermal oscillation with a thermal-optical coupled model. Finally, we analyse the thermal oscillation condition and give the diagram about the oscillation region, which has significant guidance for the occurrence and avoidance of the thermal oscillation in practical applications.

Controllable atomic collision in a tight optical dipole trap.

Single atoms are interesting candidates for studying quantum optics and quantum information processing. Recently, trapping and manipulation of single atoms using tight optical dipole traps has generated considerable interest. Here we report an experimental investigation of the dynamics of atoms in a modified optical dipole trap with a backward propagating dipole trap beam, where a change in the two-atom collision rate by six times has been achieved. The theoretical model presented gives a prediction of high probabilities of few-atom loading rates under proper experimental conditions. This work provides an alternative approach to the control of the few-atom dynamics in a dipole trap and the study of the collective quantum optical effects of a few atoms.

kHz-precision wavemeter based on reconfigurable microsoliton.

The mode-locked microcomb offers a unique and compact solution for photonics applications, ranging from the optical communications, the optical clock, optical ranging, the precision spectroscopy, novel quantum light source, to photonic artificial intelligence. However, the photonic micro-structures are suffering from the perturbations arising from environment thermal noises and also laser-induced nonlinear effects, leading to the frequency instability of the generated comb. Here, a universal mechanism for fully stabilizing the microcomb is proposed and experimentally verified. By incorporating two global tuning approaches and the autonomous thermal locking mechanism, the pump laser frequency and repetition rate of the microcomb can be controlled independently in real-time without interrupting the microcomb generation. The high stability and controllability of the microcomb frequency enables its application in wavelength measurement with a precision of about 1 kHz. The approach for the full control of comb frequency could be applied in various microcomb platforms, and improve their performances in timing, spectroscopy, and sensing.

Nonreciprocal Frequency Conversion and Mode Routing in a Microresonator.

The transportation of photons and phonons typically obeys the principle of reciprocity. Breaking reciprocity of these bosonic excitations will enable the corresponding nonreciprocal 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 nonreciprocal routing between any two modes with completely different frequencies is demonstrated, including the routing of phonon to phonon (megahertz to megahertz), photon to phonon (terahertz to megahertz), 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 nonreciprocity is derived from interference between multimode transfer processes involving optomechanical interactions in an optomechanical resonator. It not only demonstrates the nonreciprocal routing of photons and phonons in a single resonator but also realizes the nonreciprocal frequency conversion for photons and circulation for phonons, laying a foundation for studying directional routing and thermal management in an optomechanical hybrid network.

Magnonic Frequency Comb in the Magnomechanical Resonator.

An optical frequency comb is a spectrum of optical radiation which consists of evenly spaced and phase-coherent narrow spectral lines and is initially invented in a laser for frequency metrology purposes. A direct analog of frequency combs in the magnonic systems has not been demonstrated to date. In our experiment, we generate a new magnonic frequency comb in the resonator with giant mechanical oscillations through the magnomechanical interaction. We observe the magnonic frequency comb contains up to 20 comb lines, which are separated by the mechanical frequency of 10.08 MHz. The thermal effect based on the strong pump power induces the cyclic oscillation of the magnon frequency shift, which leads to a periodic oscillation of the magnonic frequency comb. Moreover, we demonstrate the stabilization and control of the frequency spacing of the magnonic frequency comb via injection locking. Our Letter lays the groundwork for magnonic frequency combs in the fields of sensing and metrology.

Coherent Coupling between Phonons, Magnons, and Photons.

Mechanical degrees of freedom, which have often been overlooked in various quantum systems, have been studied for applications ranging from quantum information processing to sensing. Here, we develop a hybrid platform consisting of a magnomechanical cavity and an optomechanical cavity, which are coherently coupled by the straightway physical contact. The phonons in the system can be manipulated either with the magnetostrictive interaction or optically through the radiation pressure. Together with mechanical state preparation and sensitive readout, we demonstrate the microwave-to-optical conversion with an ultrawide tuning range up to 3 GHz. In addition, we observe a mechanical motion interference effect, in which the optically driven mechanical motion is canceled by the microwave-driven coherent motion. Manipulating mechanical oscillators with equal facility through both magnonic and photonic channels enables new architectures for signal transduction between the optical, microwave, mechanical, and magnetic fields.

Soliton formation and spectral translation into visible on CMOS-compatible 4H-silicon-carbide-on-insulator platform.

Recent advancements in integrated soliton microcombs open the route to a wide range of chip-based communication, sensing, and metrology applications. The technology translation from laboratory demonstrations to real-world applications requires the fabrication process of photonics chips to be fully CMOS-compatible, such that the manufacturing can take advantage of the ongoing evolution of semiconductor technology at reduced cost and with high volume. Silicon nitride has become the leading CMOS platform for integrated soliton devices, however, it is an insulator and lacks intrinsic second-order nonlinearity for electro-optic modulation. Other materials have emerged such as AlN, LiNbO3, AlGaAs and GaP that exhibit simultaneous second- and third-order nonlinearities. Here, we show that silicon carbide (SiC) -- already commercially deployed in nearly ubiquitous electrical power devices such as RF electronics, MOSFET, and MEMS due to its wide bandgap properties, excellent mechanical properties, piezoelectricity and chemical inertia -- is a new competitive CMOS-compatible platform for nonlinear photonics. High-quality-factor microresonators (Q = 4 × 106) are fabricated on 4H-SiC-on-insulator thin films, where a single soliton microcomb is generated. In addition, we observe wide spectral translation of chaotic microcombs from near-infrared to visible due to the second-order nonlinearity of SiC. Our work highlights the prospects of SiC for future low-loss integrated nonlinear and quantum photonics that could harness electro-opto-mechanical interactions on a monolithic platform.

Synthetic five-wave mixing in an integrated microcavity for visible-telecom entanglement generation.

Nonlinear optics processes lie at the heart of photonics and quantum optics for their indispensable role in light sources and information processing. During the past decades, the three- and four-wave mixing (χ(2) and χ(3)) effects have been extensively studied, especially in the micro-/nano-structures by which the photon-photon interaction strength is greatly enhanced. So far, the high-order nonlinearity beyond the χ(3) has rarely been studied in dielectric materials due to their weak intrinsic nonlinear susceptibility, even in high-quality microcavities. Here, an effective five-wave mixing process (χ(4)) is synthesized by incorporating χ(2) and χ(3) processes in a single microcavity. The coherence of the synthetic χ(4) is verified by generating time-energy entangled visible-telecom photon pairs, which requires only one drive laser at the telecom waveband. The photon-pair generation rate from the synthetic process shows an estimated enhancement factor over 500 times upon intrinsic five-wave mixing. Our work demonstrates a universal approach of nonlinear synthesis via photonic structure engineering at the mesoscopic scale rather than material engineering, and thus opens a new avenue for realizing high-order optical nonlinearities and exploring functional photonic devices.

Numerical characterization of soliton microcomb in an athermal hybrid Si3N4-TiO2 microring.

We theoretically investigate the athermal constructions to cancel the thermorefractive effect of a hybrid Si3N4-TiO2 microring, which merges two materials with opposite thermo-optical coefficients (TOCs). The analytical and numerical results predict that the thermorefractive effect can be reduced under the appropriate parameters. In addition, the soliton state is easily accessed under the athermal condition. The thermorefractive noise due to the fluctuation of the microresonator temperature caused by the heat exchange between the microresonator and the surrounding environment is also suppressed by one order of magnitude, which is critical for the potential applications of soliton microcombs, such as spectroscopy, optical clocks and microwave generation.

Single-Mode Photon Blockade Enhanced by Bi-Tone Drive.

A scheme for observing photon blockade in a single bosonic mode with weak nonlinearity is proposed and numerically verified. Using a simple bi-tone drive, sub- and super-Poissonian light can be generated with high fidelity. With a periodically poled lithium niobate microcavity, a sub-Poissonian photon source with kHz count rate can be realized. Our proposed scheme is robust against parameter variations of the cavity and extendable to any bosonic system with anharmonic energy levels.

All-Optical Synchronization of Remote Optomechanical Systems.

Synchronization and frequency locking between remote mechanical oscillators are of scientific and technological importance. The key challenges are to align the oscillation frequencies and realize strong nonlinear interaction of both oscillators to a common carrier capable of long-distance transmission. Here, we experimentally realize the all-optical synchronization between two different optomechanical systems, a microsphere and a microdisk. The mechanical oscillation of the microsphere induced by the radiation pressure is loaded onto the pump laser via the optomechanical interaction, which is directly transmitted through a 5-km-long single-mode fiber to excite the mechanical oscillation of the microdisk. By finely tuning both the optical and mechanical frequencies of the two microresonators, the oscillation of the microdisk is injection locked to the microsphere, resulting in a synchronized phase relation of the two systems. Our results push a step forward the long-distance synchronization network using optomechanical microresonators.

Thermal tuning of mode crossing and the perfect soliton crystal in a Si3N4 microresonator.

Dissipative Kerr solitons in high quality microresonators have attracted much attention in the past few years. They provide ideal platforms for a number of applications. Here, we fabricate the Si3N4 microring resonator with anomalous dispersion for the generation of single soliton and soliton crystal. Based on the strong thermal effect in the high-Q microresonator, the location and strength of the avoided mode crossing in the device can be changed by the intracavity power. Because the existence of the avoided mode crossing can induce the perfect soliton crystal with specific soliton number, we could choose the appropriate pumped resonance mode and appropriate pump power to obtain the perfect soliton crystals on demand.

Dissipatively Controlled Optomechanical Interaction via Cascaded Photon-Phonon Coupling.

In an optomechanical system, we experimentally engineer the optical density of state to reduce or broaden the effective linewidth of the optical mode by introducing an ancillary mechanical mode, which has a large decay rate, i.e., stimulated backward Brillouin scattering. Based on this dissipation engineering, we could engineer the optical mode linewidth by one order of magnitude. In addition, we can either enhance or suppress the optomechanical cooling and amplification of the target mechanical oscillations. Our scheme demonstrates the cascaded photon-phonon coupling to control the mechanical interactions, and also presents a novel approach for engineering coherent light-matter interaction in hybrid systems, which consist of different types of nonlinear interactions and multiple modes, and promote the performance of quantum devices.