Integrated microcavity electric field sensors using Pound-Drever-Hall detection.

Discerning weak electric fields has important implications for cosmology, quantum technology, and identifying power system failures. Photonic integration of electric field sensors is highly desired for practical considerations and offers opportunities to improve performance by enhancing microwave and lightwave interactions. Here, we demonstrate a high-Q microcavity electric field sensor (MEFS) by leveraging the silicon chip-based thin film lithium niobate photonic integrated circuits. Using the Pound-Drever-Hall detection scheme, our MEFS achieves a detection sensitivity of 5.2 µV/(m[Formula: see text]), which surpasses previous lithium niobate electro-optical electric field sensors by nearly two orders of magnitude, and is comparable to atom-based quantum sensing approaches. Furthermore, our MEFS has a bandwidth that can be up to three orders of magnitude broader than quantum sensing approaches and measures fast electric field amplitude and phase variations in real-time. The ultra-sensitive MEFSs represent a significant step towards building electric field sensing networks and broaden the application spectrum of integrated microcavities.

Theoretical Analysis of Microcavity Simultons Reinforced by χ

High-Q microcavities with quadratic and cubic nonlinearities add lots of versatility in controlling microcombs. Here, we study microcavity simulton and soliton dynamics reinforced by both χ^{(2)} and χ^{(3)} nonlinearities in a continuously pumped microcavity. Theoretical analysis based on the Lagrangian approach reveals the soliton peak power and gain-loss balance are impacted by the flat part of the intracavity pump, while the dark-pulse part of the pump leads to a nearly constant soliton group velocity change. We also derived a soliton conversion efficiency upper limit that is fully determined by the coupling condition and the quantum-limited soliton timing jitter in the χ^{(2,3)} system. Numerical simulations confirm the analytical results. Our theory is particularly useful for investigating AlN microcombs and sheds light on the interplay between χ^{(2)} and χ^{(3)} nonlinearities within microcavity simultons.

Spatiotemporal mode-locking and dissipative solitons in multimode fiber lasers.

Multimode fiber (MMF) lasers are emerging as a remarkable testbed to study nonlinear spatiotemporal physics with potential applications spanning from high energy pulse generation, precision measurement to nonlinear microscopy. The underlying mechanism for the generation of ultrashort pulses, which can be understood as a spatiotempoal dissipative soliton (STDS), in the nonlinear multimode resonators is the spatiotemporal mode-locking (STML) with simultaneous synchronization of temporal and spatial modes. In this review, we first introduce the general principles of STML, with an emphasize on the STML dynamics with large intermode dispersion. Then, we present the recent progress of STML, including measurement techniques for STML, exotic nonlinear dynamics of STDS, and mode field engineering in MMF lasers. We conclude by outlining some perspectives that may advance STML in the near future.

Plug-and-play fiber-optic sensors based on engineered cells for neurochemical monitoring at high specificity in freely moving animals.

In vivo detection of neurochemicals, including neurotransmitters and neuromodulators, is critical for both understanding brain mechanisms and diagnosing brain diseases. However, few sensors are competent in monitoring neurochemical dynamics in vivo at high specificity. Here, we propose the fiber-optic probes based on engineered cells (FOPECs) for plug-and-play, real-time detection of neurochemicals in freely moving animals. Taking advantages of life-evolved neurochemical receptors as key components, the chemical specificity of FOPECs is unprecedented. We demonstrate the applications of FOPECs in real-time monitoring of neurochemical dynamics under various physiology and pathology conditions. With no requirement of viral infection in advance and no dependence on animal species, FOPECs can be widely adopted in vertebrates, such as mice, rats, rabbits, and chickens. Moreover, FOPECs can be used to monitor drug metabolisms in vivo. We demonstrated the neurochemical monitoring in blood circulation systems in vivo. We expect that FOPECs will benefit not only neuroscience study but also drug discovery.

Fundamental linewidth of an AlN microcavity Raman laser.

Raman lasing can be a promising way to generate highly coherent chip-based lasers, especially in high-quality (high-Q) crystalline microcavities. Here, we measure the fundamental linewidth of a stimulated Raman laser in an aluminum nitride (AlN)-on-sapphire microcavity with a record Q-factor up to 3.7 million. An inverse relationship between fundamental linewidth and emission power is observed. A limit of the fundamental linewidth, independent of Q-factor, due to Raman-pump-induced Kerr parametric oscillation is derived.

Self-starting spatiotemporal mode-locking using Mamyshev regenerators.

Bridging multi-mode fibers and Mamyshev regenerators holds promise for pulse energy scaling in fiber lasers. However, initialization of a multi-mode Mamyshev oscillator remains a practical challenge. Here we report self-starting spatiotemporal mode-locking (STML) in a multi-mode Mamyshev oscillator without active assistance. The first initialized mode-locking is unstable, but stable STML can be attained by increasing the filter separation. Simulations verify the capability of reaching self-starting STML using Mamyshev regenerators and unveil the effect of filter separation on the self-starting ability.

Self-frequency shift of AlN-on-sapphire Kerr solitons.

We study the self-frequency shift of continuously pumped Kerr solitons in AlN-on-sapphire microcavities with Raman gain bandwidths narrower than the cavity free-spectral range. Solitons are generated in ∼230GHz microcavities via high-order mode dispersion engineering. The dependence of the self-frequency shift on soliton pulse width is measured and differs from amorphous material microcavities. Our measurement and simulation reveal the impact of frequency detuning between the cavity resonances and Raman gain peaks, as well as the importance of all three Raman gain peaks. The interplay between the Raman effect and dispersive wave recoil and a potential quiet point are also observed.

A New Optical Fiber Probe-Based Quantum Dots Immunofluorescence Biosensors in the Detection of Staphylococcus aureus.

Staphylococcus aureus (S. aureus) is one of the most common clinical pathogenic bacteria with strong pathogenicity and usually leads to various suppurative infections with high fatality. Traditional bacterial culture for the detection of S. aureus is prone to diagnosis and antimicrobial treatment delays because of its long-time consumption and low sensitivity. In this study, we successfully developed a quantum dots immunofluorescence biosensor for S. aureus detection. The biosensor combined the advantages of biosensors with the high specificity of antigen-antibody immune interactions and the high sensitivity and stability of quantum dots fluorescence. The results demonstrated that the biosensor possessed high specificity and high sensitivity for S. aureus detection. The detection limit of S. aureus reached 1 × 104 CFU/ml or even 1 × 103 CFU/ml, and moreover, the fluorescence intensity had a significant positive linear correlation relationship with the logarithm of the S. aureus concentration in the range of 103-107 CFU/ml (correlation coefficient R2 = 0.9731, P = 0.011). A specificity experiment showed that this biosensor could effectively distinguish S. aureus (1 × 104 CFU/ml and above) from other common pathogenic (non-S. aureus) bacteria in nosocomial infections, such as Klebsiella pneumoniae, Pseudomonas aeruginosa, Acinetobacter baumannii and Escherichia coli. Additionally, the whole detection procedure spent only 2 h. In addition, the biosensor in this study may not be affected by the interference of the biofilm or other secretions since the clinical biological specimens are need to be fully liquefied to digest and dissolve viscous secretions such as biofilms before the detection procedure of the biosensor in this study. In conclusion, the biosensor could meet the need for rapid and accurate S. aureus detection for clinical application.

Pure quartic solitons in dispersion-engineered aluminum nitride micro-cavities.

Pure quartic soliton (PQS) is a new class of solitons demonstrated in recent years and provides innovations in nonlinear optics and its applications. Generating PQSs in micro-cavities offers a novel way to achieve coherent microcombs, presenting a promising application potential. Here we numerically investigate the PQS generation in a dispersion-engineered aluminum nitride (AlN) micro-cavity. To support PQS, a well-designed shallow-trench waveguide structure is adopted, which is feasible to be fabricated. The structure exhibits a dominant fourth-order dispersion reaching up to -5.35×10-3 ps4/km. PQSs can be generated in this AlN micro-cavity in the presence of all-order-dispersion and stimulated Raman scattering. Spectral recoil and soliton self-frequency shift are observed in the PQS spectrum. Furthermore, we find that due to the narrow Raman gain spectrum of crystalline AlN, the PQS evolves directly to chaos rather than turning into a breather. The threshold pump power with which the PQS turns into chaos is also theoretically calculated, which squares with the simulation results.

Spatiotemporal Mode-Locking in Lasers with Large Modal Dispersion.

Dissipative nonlinear wave dynamics have been investigated extensively in mode-locked lasers with single transverse mode, whereas there are few studies related to three-dimensional nonlinear dynamics within lasers. Recently, spatiotemporal mode locking (STML) was proposed in lasers with small modal (i.e., transverse-mode) dispersion, which has been considered to be critical for achieving STML in those cavities because the small dispersion can be easily balanced. Here, we demonstrate that STML can also be achieved in multimode lasers with much larger modal dispersion, where we find that the intracavity saturable absorber plays an important role for counteracting the large modal dispersion. Furthermore, we observe a new STML phenomenon of passive nonlinear autoselection of single-mode mode locking, resulting from the interaction between spatiotemporal saturable absorption and spatial gain competition. Our work significantly broadens the design possibilities for useful STML lasers thus making them much more accessible for applications, and extends the explorable parameter space of the novel dissipative spatiotemporal nonlinear dynamics that can be achieved in these lasers.

Raman pure quartic solitons in Kerr microresonators.

Pure quartic solitons (PQSs) are a unique class of solitons advantageous for developing promising applications due to their broad and flat-top spectrum, as well as the distinctive energy scaling law. Here we investigate the characteristics and dynamics of the PQS in the presence of the Raman effect based on Kerr microresonators. Stimulated Raman scattering leads to reduced pulse peak power, self-frequency shifts, and distortion of the symmetry pulse shape of the PQS. Besides, dynamical evolution of the Raman PQS, especially the breathing state, is investigated. We discover the appearance of an intermediate stable state between the existence region of breathers and chaos in the parameter space, which has not been discovered yet in other soliton regimes. A stability analysis is performed to investigate the spatial dynamics in the context of the Raman PQS.

Vector quartic solitons in birefringent fibers.

We theoretically investigated the vector properties of quartic solitons in a pure fourth-order-dispersion birefringent fiber and a quartic-dispersion-dominant mode-locked fiber laser. We found that, compared with scalar pure quartic solitons, a vector quartic soliton (VQS) in the birefringent fiber still preserves the Gaussian shape, except for the distinctions of reduced peak power, central frequency offset, slight frequency chirp, and mitigated oscillatory tails. We also demonstrated that pulse shaping in the mode-locked laser cavity could explicitly facilitate the formation of Kelly sidebands and distortion of oscillatory tails. Furthermore, dynamical evolutions of quartic group-velocity-locked and polarization-rotating vector solitons were obtained to enrich the nonlinear community of VQSs. We believe that our elaborate findings will bring insights into both the fundamental understanding and potential applications of VQSs.

Buildup dynamics of asynchronous vector solitons in a polarization-multiplexed dual-comb fiber laser.

Polarization-multiplexed dual-comb fiber lasers enable significant applications in dual-comb spectroscopy and optical sensing. However, the complexity of the underlying formation dynamics of dual-comb solitons has not been unveiled so far. Here, we capture the real-time spectral evolutions of both vector solitons from the initial fluctuations, with the help of the time-stretch dispersive Fourier transform technique. Both vector solitons experience the relaxation oscillation, quasi-mode-locking, beating dynamics, and mode locking, accompanying central wavelength shifts in opposite directions, which might be induced by the gain saturation during their buildup processes. Moreover, polarization-dependent gain in the gain fiber leads to the different buildup time of both vector solitons. Our findings open new perspectives for dual-comb buildup dynamics and might impact laser design for applications.

Carbon nanotube-synchronized dual-color fiber laser for coherent anti-Stokes Raman scattering microscopy.

In this Letter, we present a passively synchronized dual-color fiber laser at 1.03 and 1.53 µm for coherent anti-Stokes Raman scattering (CARS) microscopy. This fiber laser consists of both Yb- and Er-doped laser cavities, combined by an ∼1.2m common branch with a single-walled carbon-nanotube-based saturable absorber. Stable passively synchronized mode-locked state is obtained within a total cavity length mismatch of 50 µm. We demonstrate the capability of the synchronized dual-color fiber laser for CARS spectroscopy in both low- and high-wavenumber regions, with a resolution of 6.6cm-1. Furthermore, a CARS image in the field of view 150×60µm2 of polystyrene at the Raman shift of 3041cm-1 has also been achieved. The results show the feasibility of our passively synchronized dual-color fiber laser to be employed as a stable and low-cost ultrafast laser source for CARS.

Nonlinear multimode interference-based dual-color mode-locked fiber laser.

We present a dual-color-soliton fiber laser at two different wavebands by nonlinear multimode interference. A saturable absorber (SA) with single-mode-multimode (MMF)-single-mode fiber structure is placed in the common branch shared by two sub-cavities. Saturable absorption effects can be simultaneously satisfied at 1.5 and 2 µm at a proper length of the MMF. Dual-color solitons can still remain, even by slightly tuning the length of the MMF. The periodical characteristic of this SA provides a flexible choice of MMF length, making it simple for simultaneous mode locking (SML) at two separate wavebands in practice. Our approach not only paves the way for SML at two or more wavebands by the MMF but also could lead to significant applications in pump-probe spectroscopy.

Stretchable and upconversion-luminescent polymeric optical sensor for wearable multifunctional sensing.

Stretchable sensors with multiple sensory functions are in high demand for healthcare monitoring and artificial intelligence. Despite recent advances in wearable electronic sensors, it remains a significant challenge to achieve simultaneous sensing of both thermal and mechanical stimuli with a single sensor while integrating high stretchability. Herein, a stretchable and multifunctional optical sensor (SMOS) with simultaneous readout of temperature and strain is developed for wearable physiological monitoring of the human body. The SMOS primarily consists of a stretchable optical sensing fiber made from polymer nanocomposites containing lanthanide-based upconversion nanoparticles (UCNPs). Temperature measurements are achieved by ratiometric intensity measurements of the dual-emission UCNPs upon near-infrared excitation. By virtue of the ratiometric detection, the temperature readout of the SMOS is independent of strain deformations, enabling stable and continuous measurements of skin temperature during body motions. Furthermore, deformation of the SMOS by stretching leads to detectable and reversible changes in its light transmission, allowing tensile strains to be simultaneously measured. As a proof of concept, we demonstrate the capabilities of the SMOS in real time and simultaneous detection of both skin temperature and motion activities of the human body.

Soft and Stretchable Polymeric Optical Waveguide-Based Sensors for Wearable and Biomedical Applications.

The past decades have witnessed the rapid development in soft, stretchable, and biocompatible devices for applications in biomedical monitoring, personal healthcare, and human-machine interfaces. In particular, the design of soft devices in optics has attracted tremendous interests attributed to their distinct advantages such as inherent electrical safety, high stability in long-term operation, potential to be miniaturized, and free of electromagnetic interferences. As the alternatives to conventional rigid optical waveguides, considerable efforts have been made to develop light-guiding devices by using various transparent and elastic polymers, which offer desired physiomechanical properties and enable wearable/implantable applications in optical sensing, diagnostics, and therapy. Here, we review recent progress in soft and stretchable optical waveguides and sensors, including advanced structural design, fabrication strategies, and functionalities. Furthermore, the potential applications of those optical devices for various wearable and biomedical applications are discussed. It is expected that the newly emerged soft and stretchable optical technologies will provide a safe and reliable alternative to next-generation, smart wearables and healthcare devices.

Stretchable and Highly Sensitive Optical Strain Sensors for Human-Activity Monitoring and Healthcare.

Flexible and stretchable strain sensors are essential to developing smart wearable devices for monitoring human activities. Such sensors have been extensively exploited with various conductive materials and structures, which, however, are normally in need of complex manufacturing processes and confronted with the challenge to achieve both large stretchability and high sensitivity. Here, we report a simple and low-cost optical strategy for the design of stretchable strain sensors which are capable of measuring large strains of 100% with a low detection limit (±0.09%), a fast responsivity (<12 ms), and high reproducibility (over 6000 cycles). The optical strain sensor (OS2) is fabricated by assembling plasmonic gold nanoparticles (GNPs) in stretchable elastomer-based optical fibers, where a core/cladding structure with step-index configuration is adopted for light confinement. The stretchable, GNP-incorporated optical fiber shows strong localized surface plasmon resonance effects that enable sensitive and reversible detection of strain deformations with high linearity and negligible hysteresis. The unique mechanical and sensing properties of the OS2 enable its assembling into clothing or mounting on skin surfaces for monitoring various human activities from physiological signals as subtle as wrist pulses to large motions of joint bending and hand gestures. We further apply the OS2 for quantitative analysis of motor disorders such as Parkinson's disease and demonstrate its compatibility in strong electromagnetic interference environments during functional magnetic resonance imaging, showing great promises for diagnostics and assessments of motor neuron diseases in clinics.

Free-running dual-comb fiber laser mode-locked by nonlinear multimode interference.

We present a simple and low-cost approach to generate dual combs from a single fiber-ring cavity based on nonlinear multimode interference. A single-mode fiber-graded-index multimode-single-mode fiber structure serves as an all-fiber saturable absorber for mode-locking. A pair of frequency combs with different comb spacing are generated by dual-wavelength and polarization-multiplexed mechanisms. Stable and simplified dual-comb spectroscopy is achieved using the proposed free-running fiber laser, with no need for complex phase-locking configurations. As a principle of concept, we demonstrate the dual-comb fiber laser for spectral determination of fiber Bragg grating (FBG)-based strain sensors, providing a low-cost alternative for interrogation of FBG-based sensing networks.

Effect of oral mucosal transplantation on the expression of EGF and VEGF-C during skin wound repair.

This study evaluated the effects of oral mucosal transplantation on epidermal growth factor (EGF) and vascular endothelial growth factor C (VEGF-C) in skin wound repair. Sixty-four rats were randomly separated into group A, B, C and D (16 rats in each group). The right abdomen skin was excised 1, 3, 5 and 7 days after injury, respectively. Oral mucosa of the rat tongue was transplanted to the right abdomen skin. Fourteen days after the healing of the oral mucosa graft, the rat skin full-thickness model was prepared at the transplant site (the study group) and the contralateral site (the control group). Rats in each group were anesthetized and sacrificed at 1, 3, 5 and 7 days after injury. Expression of EGF and VEGF-C in skin tissue was detected by RT-qPCR and ELISA. At 3 days, expression levels of EGF and VEGF-C mRNA and protein in skin tissue were significantly higher than those at 1 day (P<0.05). At 5 days, expression levels of EGF and VEGF-C mRNA and protein in skin tissue were significantly higher than those at 3 days (P<0.05). At 7 days, expression levels of EGF mRNA and protein in skin tissue were significantly lower than those at 5 days (P<0.05), while VEGF-C levels were significantly increased (P<0.05). Expression levels of EGF and VEGF-C mRNA and protein in the skin tissue of the study group were significantly lower than those in the control group at all days (P<0.05). EGF and VEGF-C may be involved in scar formation, and play an important role in the process of skin wound repair.