Ghost hyperbolic surface polaritons in bulk anisotropic crystals.

Polaritons in anisotropic materials result in exotic optical features, which can provide opportunities to control light at the nanoscale1-10. So far these polaritons have been limited to two classes: bulk polaritons, which propagate inside a material, and surface polaritons, which decay exponentially away from an interface. Here we report a near-field observation of ghost phonon polaritons, which propagate with in-plane hyperbolic dispersion on the surface of a polar uniaxial crystal and, at the same time, exhibit oblique wavefronts in the bulk. Ghost polaritons are an atypical non-uniform surface wave solution of Maxwell's equations, arising at the surface of uniaxial materials in which the optic axis is slanted with respect to the interface. They exhibit an unusual bi-state nature, being both propagating (phase-progressing) and evanescent (decaying) within the crystal bulk, in contrast to conventional surface waves that are purely evanescent away from the interface. Our real-space near-field imaging experiments reveal long-distance (over 20 micrometres), ray-like propagation of deeply subwavelength ghost polaritons across the surface, verifying long-range, directional and diffraction-less polariton propagation. At the same time, we show that control of the out-of-plane angle of the optic axis enables hyperbolic-to-elliptic topological transitions at fixed frequency, providing a route to tailor the band diagram topology of surface polariton waves. Our results demonstrate a polaritonic wave phenomenon with unique opportunities to tailor nanoscale light in natural anisotropic crystals.

Spatiotemporal beating and vortices of van der Waals hyperbolic polaritons.

In conventional thin materials, the diffraction limit of light constrains the number of waveguide modes that can exist at a given frequency. However, layered van der Waals (vdW) materials, such as hexagonal boron nitride (hBN), can surpass this limitation due to their dielectric anisotropy, exhibiting positive permittivity along one optic axis and negativity along the other. This enables the propagation of hyperbolic rays within the material bulk and an unlimited number of subdiffractional modes characterized by hyperbolic dispersion. By employing time-domain near-field interferometry to analyze ultrafast hyperbolic ray pulses in thin hBN, we showed that their zigzag reflection trajectories bound within the hBN layer create an illusion of backward-moving and leaping behavior of pulse fringes. These rays result from the coherent beating of hyperbolic waveguide modes but could be mistakenly interpreted as negative group velocities and backward energy flow. Moreover, the zigzag reflections produce nanoscale (60 nm) and ultrafast (40 fs) spatiotemporal optical vortices along the trajectory, presenting opportunities to chiral spatiotemporal control of light-matter interactions. Supported by experimental evidence, our simulations highlight the potential of hyperbolic ray reflections for molecular vibrational absorption nanospectroscopy. The results pave the way for miniaturized, on-chip optical spectrometers, and ultrafast optical manipulation.

Hybrid Ghost Phonon Polaritons in Thin-Film Heterostructure.

Ghost phonon polaritons (g-PhPs), a unique class of phonon polaritons in the infrared, feature ultralong diffractionless propagation (>20 µm) across the surface and tilted wavefronts in the bulk. Here, we study hybrid g-PhPs in a heterostructure of calcite and an ultrathin film of the phase change material (PCM) In3SbTe2, where the optical field is bound in the PCM film with enhanced confinement compared with conventional g-PhPs. Near-field optical images for hybrid g-PhPs reveal a lemniscate pattern in the momentum distribution. We fabricated In3SbTe2 gratings and investigated how different orientations and periodicities of gratings impact the propagation of hybrid g-PhPs. As the grating period decreases to zero, the wavefront of hybrid g-PhPs can be dynamically steered by varying the grating orientation. Our results highlight the promise of hybrid g-PhPs with tunable functionalities for nanophotonic studies.

Efficient second harmonic generation in a high-Q Fabry-Perot microresonator on x-cut thin film lithium niobate.

Microresonators facilitate enhanced light-matter interactions within a limited space, showing great promise for nonlinear optics. Here, we demonstrate a high-quality (Q) factor Fabry-Perot microresonator (FPR) for second harmonic generation (SHG) on an x-cut thin film lithium niobate (TFLN) platform. The FPR exhibits Q factors of Qpump = 1.09 × 105 and QSH = 1.15 × 104 at the 1560 nm pump wavelength and 780 nm second harmonic wavelength, respectively. Under low pump power, a normalized SHG efficiency of 158.5 ± 18.5%/W is attained. We experimentally verify that increased temperatures mitigate photorefractive effects that degrade SHG performance. This work highlights the immense capabilities of one-dimensional planar optical waveguide resonators for efficient on-chip nonlinear wavelength conversion.

Integrated waveguide coupled ultralow-loss multimode waveguides based on silicon nitride resonators.

On-chip micro-ring resonators (MRRs) with low loss and large free spectral ranges (FSRs) are important for photonic devices. So far, ultra-low-loss silicon-nitride (Si3N4) waveguides are primarily fabricated in laboratories, as they often demand special processes to reduce transmission losses. While, Si3N4 waveguides fabricated by the standard multi-project wafer (MPW)-based processes often suffer from significant sidewall scattering, resulting in high scattering losses. Here, we present an innovative approach to photonics by introducing a compact and multi-mode structure. This approach significantly reduces the contact between the optical field and the rough sidewalls in the high-confinement Si3N4 waveguide. By incorporating modified Euler bends, and a weakly tapered gap directional coupler, adiabatic transmission with simultaneous ultra-low loss and compact size is achieved even in 7-µm wide waveguide. Results show that the intrinsic quality factor Qi of MRR is (6.8 ± 0.4) × 106 at the wavelength of 1550 nm, which is approximately four times higher than the previously reported by the same fabrication process. An ultra-low loss of 0.051 ± 0.003 dB/cm is achieved based on the standard LIGENTEC-AN800 technology. This accomplishment addresses a critical challenge in high-confinement waveguides. Our work provides new insights into the low propagation loss in Si3N4 waveguides and provides a broader prospect for integrated photonics in the ultra-high-Q regime.

40 Gb/s multimode all-optical regenerator based on the low-loss silicon-based nanowaveguide.

With the increasing demand for communication capacity, all-optical regeneration of multimode signals is a helpful technology of network nodes and optical signal processors. However, the difficulty of regenerating signal in higher-order modes hinders the practical application of multimode all-optical regenerators. In this study, we experimentally demonstrate the 40 Gb/s all-optical regeneration of NRZ-OOK signal in TE0 and TE1 modes via four-wave mixing (FWM) in the low-loss silicon-based nanowaveguide. By optimizing the parameters of waveguide section to enhance FWM conversion efficiency of two modes, and introducing Euler bending to reduce crosstalk between modes, the transmission loss of the silicon waveguide is 0.3 dB/cm, and the FWM conversion efficiency of the multimode regenerator is as high as -9.6 dB (TE0) and -13.0 dB (TE1). Both modes achieve extinction ratio enhancement of about 6 dB after regeneration. This silicon-based all-optical regenerator has great application potential in all-optical signal processing systems.

Prevalence of occupational hypersensitivity pneumonitis: a systematic review and meta-analysis.

In this meta-analysis, we aimed to evaluate the prevalence of occupational hypersensitivity pneumonitis (OHP) among different occupations globally. Our search was conducted on MEDLINE via PubMed, Scopus, Web of Science, and Cochrane CENTRAL from inception to September 2023. Eligible studies were observational in nature and focused on several specific occupations. A total of 46 articles were included (n = 2,826,420 participants). The overall prevalence of OHP was found to be 4.2% (95% CI: 2.1% to 8.0%), but this varied significantly based on occupation and geographic location. Printers had the highest OHP prevalence at 57.14%, followed by tobacco workers (26.32%), and water-related workers (24.10%). South America showed the highest prevalence of 16.71%, compared to Asia (15.19%), and North America (8.52%). Significant variations in OHP prevalence by occupation and region were found, with the highest rates in printers and tobacco workers. Age and smoking were identified as contributing factors to the prevalence variability.

Developing an early warning system for detecting sepsis in patients with trauma.

The purpose of this study was to analyse the risk factors for sepsis in patients with trauma and develop a new scoring system for predicting sepsis in patients with trauma based on these risk factors. This will provide a simple and effective early warning method for the rapid and accurate detection and evaluation of the probability of sepsis in patients with trauma to assist in planning timely clinical interventions. We undertook a retrospective analysis of the clinical data of 216 patients with trauma who were admitted to the emergency intensive care unit of the emergency medicine department of the Hebei Medical University Third Hospital, China, between November 2017 and October 2022. We conducted a preliminary screening of the relevant factors using univariate logistic regression analysis and included those factors with a p value of <0.075 in the multivariate logistic regression analysis, from which the risk factors were screened and assigned, and obtained a total score, which was the sepsis early warning score. The incidence of sepsis in patients in the intensive care unit with trauma was 36.9%, and the mortality rate due to sepsis was 19.4%. We found statistically significant differences in several factors for patients with sepsis. The risk factors for sepsis in patients with trauma were the activated partial thromboplastin time, the New Injury Severity Score, growth differentiation factor-15 levels, shock, mechanical ventilation and the Acute Physiology and Chronic Health Evaluation II score. The area under the receiver operating characteristic curve of the sepsis early warning score for predicting sepsis in patients with trauma was 0.725. When the cutoff value of the early warning score was set at 5.0 points, the sensitivity was 69.9% and the specificity was 60.3%. The incidence of sepsis in patients with trauma can be reduced by closely monitoring patients' hemodynamics, implementing adequate fluid resuscitation promptly and by early removal of the catheter to minimize the duration of unnecessary invasive mechanical ventilation. In this study, we found that the use of the sepsis early warning score helped in a more accurate and effective evaluation of the prognosis of patients with trauma.

Ultra-efficient second harmonic generation via mode phase matching in integrated lithium niobate racetrack resonators.

High-efficiency second harmonic generation (SHG) relying solely on intermodal dispersion engineering remains a challenge. Here, we realize highly efficient SHG using a double-waveguide coupled racetrack microring resonator on X-cut lithium niobate on insulator (LNOI), where both pump and second harmonic (SH) approach critical coupling. Through precise temperature tuning, simultaneous pump and SH resonance is attained in the resonator, dramatically enhancing SHG efficiency. With low pump power, a normalized conversion efficiency of 9972%/W is achieved. Moreover, the resonator provides a 25.73 dB enhancement in SHG efficiency compared to a 4 mm straight waveguide with identical phase matching in our experiment. This work enables efficient wavelength conversion and quantum state generation on integrated X-cut LNOI platforms.

Large group delay and low loss optical delay line based on chirped waveguide Bragg gratings.

On-chip optical delay lines (ODLs) based on chirped waveguide Bragg gratings (CWBG) have attracted much attention in recent years. Although CWBGs are well developed, the CWBG which have large group delay (GD), large delay-bandwidth product and low loss while is circulator-free have little been investigated so far. In this work, we propose and experimentally demonstrate such a CWBG-based ODL. This device is fabricated on a low-loss 800-nm-height silicon nitride platform, combining 20.11-cm long index-chirped multi-mode spiral waveguide antisymmetric Bragg gratings with a directional coupler. The bandwidth of this circulator-free ODL is 23 nm. The total GD is 2864 ps and the delay-bandwidth product is 65.87 ns·nm, which both are the largest values achieved by on-chip CWBG reported to our knowledge. Its loss is 1.57 dB/ns and the total insertion loss of the device is 6 dB at the central wavelength near 1550 nm. This integrated CWBG can be explored in practical applications including microwave photonics, temporal optics, and optical communication.

Dual-band achromatic metalens-assisted grating couplers for wavelength demultiplexing.

The design of grating couplers (GCs) that can (de)multiplex and couple arbitrarily defined spatial light into photonic devices is crucial for miniaturized integrated chips. However, traditional GCs have a limited optical bandwidth due to their wavelength's dependency on the coupling angle. In this paper, we propose a device that addresses this limitation by combining a dual-broadband achromatic metalens (ML) with two focusing GCs. By controlling the frequency dispersion, the waveguide-mode-based ML achieves excellent dual-broadband achromatic convergence and separates broadband spatial light into opposing directions at normal incidence. The focused and separated light field matches the grating diffractive mode field and is then coupled into two waveguides by the GCs. This ML-assisted GCs device exhibits a good broadband property with -3 dB bandwidths of 80 nm at 1.31 µm (CE ∼ -6 dB) and 85 nm at 1.51 µm (CE ∼ -5 dB), which almost covers the entire designed working bands, representing an improvement over traditional spatial light-GC coupling. This device can be integrated into optical transceivers and dual-band photodetectors to enhance the bandwidth of wavelength (de)multiplexing.

Compact and fast-response optical switch based on complex refractive index engineering.

The optical switch is a crucial device in integrated photonic circuits. Among the various types of optical switches available, the on-off Mach-Zehnder interferometer is one of the most widely used structures. However, compared with other structures, such as a microring, the large footprint of a Mach-Zehnder interferometer significantly restricts the integration density. In this paper, we propose a compact Mach-Zehnder interferometer based on complex refractive index engineering. By manipulating the complex index of the material in the structure, the lateral size of the device can be compressed down to only 3.25 µm. Moreover, the reducing of the space between heaters and waveguides leads to a fast response of only 1.9 µs. Our work offers a new, to the best of our knowledge, approach of a compact integrated optical switch, and opens a new avenue for application of absorbing materials.

Complex-valued matrix-vector multiplication system for a large-scale optical FFT.

Recent advancements in optical convolutional neural networks (CNNs) and radar signal processing systems have brought an increasing need for the adoption of optical fast Fourier transform (OFFT). Presently, the fast Fourier transform (FFT) is executed using electronic means within prevailing architectures. However, this electronic approach faces limitations in terms of both speed and power consumption. Concurrently, existing OFFT systems struggle to balance the demands of large-scale processing and high precision simultaneously. In response, we introduce a novel, to the best of our knowledge, solution: a complex-valued matrix-vector system harnessed through wavelength selective switches (WSSs) for the realization of a 24-input optical FFT, achieving a high-accuracy level of 5.4 bits. This study capitalizes on the abundant wavelength resources available to present a feasible solution for an optical FFT system with a large N.

Pushing photon-pair generation rate in microresonators by Q factor manipulation.

Photon pairs generated by employing spontaneous nonlinear effects in microresonators are critically essential for integrated optical quantum information technologies, such as quantum computation and quantum cryptography. Microresonators featuring high-quality (Q) factors can offer simple yet power-efficient means to generate photon pairs, thanks to the intracavity field enhancement. In microresonators, it is known that the photon-pair generation rate (PGR) is roughly proportional to the cubic power of the Q factor. However, the upper limit on PGR is also set by the Q factor: a higher Q factor brings a longer photon lifetime, which in turn leads to a lower repetition rate allowing for photon flow emitted from the microresonator, constrained by the Fourier-transform limit. Exceeding this limit will result in the overlap of photon wave packets in the time domain, thus degrading the quantum character of single-photon light beams. To push the limit of PGR in a single resonator, we propose a method by harnessing the resonance linewidth-manipulated microresonators to improve the maximum achievable photon repetition rate while keeping the power efficiency. The maximum achievable PGR and power efficiency are thus balanced by leveraging the combination of low and high-Q resonances.

Impact of Hypoxia-Hyperoxia Exposures on Cardiometabolic Risk Factors and TMAO Levels in Patients with Metabolic Syndrome.

Along with the known risk factors of cardiovascular diseases (CVDs) constituting metabolic syndrome (MS), the gut microbiome and some of its metabolites, in particular trimethylamine-N-oxide (TMAO), are actively discussed. A prolonged stay under natural hypoxic conditions significantly and multi-directionally changes the ratio of gut microbiome strains and their metabolites in feces and blood, which is the basis for using hypoxia preconditioning for targeted effects on potential risk factors of CVD. A prospective randomized study included 65 patients (32 females) with MS and optimal medical therapy. Thirty-three patients underwent a course of 15 intermittent hypoxic-hyperoxic exposures (IHHE group). The other 32 patients underwent sham procedures (placebo group). Before and after the IHHE course, patients underwent liver elastometry, biochemical blood tests, and blood and fecal sampling for TMAO analysis (tandem mass spectrometry). No significant dynamics of TMAO were detected in both the IHHE and sham groups. In the subgroup of IHHE patients with baseline TMAO values above the reference (TMAO ≥ 5 µmol/l), there was a significant reduction in TMAO plasma levels. But the degree of reduction in total cholesterol (TCh), low-density lipoprotein (LDL), and regression of liver steatosis index was more pronounced in patients with initially normal TMAO values. Despite significant interindividual variations, in the subgroup of IHHE patients with MS and high baseline TMAO values, there were more significant reductions in cardiometabolic and hepatic indicators of MS than in controls. More research is needed to objectify the prognostic role of TMAO and the possibilities of its correction using hypoxia adaptation techniques.

Parity-time symmetry in monolithically integrated graphene-assisted microresonators.

Recently, optical systems with parity-time (PT) symmetry have attracted considerable attention due to its remarkable properties and promising applications. However, these systems usually require separate photonic devices or active semiconductor materials. Here, we investigate PT symmetry and exceptional points (EPs) in monolithically integrated graphene-assisted coupled microresonators. Raman effect and graphene cladding are utilized to introduce the balanced gain and loss. We show that PT-symmetry breaking and EPs can be achieved by changing the pump power and the chemical potential. In addition, the intracavity field intensities experience suppression and revival as the graphene-induced loss increases. Due to the unique distribution of optical field, tunable nonreciprocal light transmission is theoretically demonstrated when introducing the gain saturation nonlinearity. The maximum isolation ratio can reach 26 dB through optimizing the relevant parameters. Our proposed scheme is monolithically integrated, CMOS compatible, and exhibits remarkable properties for microscale light field manipulation. These superior features make our scheme has promising applications in optical communication, computing and sensing.

Integrated photonic devices enabled by silicon traveling wave-like Fabry-Perot resonators.

Integrated photonic devices play a key role in modern optical communications, of which optical resonators are important fundamental structures. This work proposes and experimentally demonstrates compact integrated photonic devices based on a traveling wave-like Fabry-Perot (TW-like FP) resonator(s) coupled with waveguides. Add-drop filters based on a single TW-like FP resonator have been realized with a high drop efficiency and the same output direction for the through and drop ports. Particularly, their transmission response can be either symmetric Lorentzian or asymmetric Fano line shape, through adjusting the shift between the two bus waveguides and the waveguide widths. Fano resonance has been demonstrated in a TW-like FP resonator with a very high extinction ratio and large slope rate. The second-order optical filter exhibits low-loss flat-top passbands with small ripples. Owing to the compact size, easy scalability, and large flexibility, TW-like FP cavity-based devices using Fano and Lorentzian resonances will provide new potential applications in integrated photonics.

Coexistence of circular dichroism and asymmetric transmission in Babinet-complementary metamaterials.

Chiral metamaterials with circular dichroism (CD) or asymmetric transmission (AT) draw enormous attention for their attractive applications in polarization transformers, circular polarizers, and biosensing. In this study, a feasible trilayer chiral metamaterials (TCM) is designed and investigated in theory and simulation. The proposed TCM is composed of a nanoslit layer and a Babinet-complementary nanorod layer separated by a nanoslit spacer. Owing to symmetry breaking by the tilted nanoslit in metal film, the TCM shows simultaneous CD and AT effects in the near-infrared region. The simulated electric charge distributions prove that the chirality arises from the excitation of asymmetric electric dipole resonant modes due to the coupling of adjacent unit cells. Moreover, CD and AT can be tuned by the tilted angle of the nanoslit and the thickness of the spacer, the fitting functions of which are consistent with the theoretical formulas based on transmittance matrix analysis. The proposed nanostructure offers a potential strategy for manipulating metamaterials with simultaneous CD and AT effects, allowing a multitude of exciting applications such as ultra-sensitive polarization transformer and biosensor.

Dual-comb based time-stretch optical coherence tomography for large and segmental imaging depth.

Optical coherence tomography based on time-stretch enables high frame rate and high-resolution imaging for the inertia-free wavelength-swept mechanism. The fundamental obstacle is still the acquisition bandwidth's restriction on imaging depth. By introducing dual-comb with slightly different repetition rates, the induced Vernier effect is found to be capable of relieving the problem. In our work, a dual-comb based time-stretch optical coherence tomography is proposed and experimentally demonstrated, achieving a 1.5-m imaging depth and 200-kHz A-scan rate. Moreover, about a 33.4-µm resolution and 25-µm accuracy are achieved. In addition, by adjusting the frequency detuning of the dual-comb, the A-scan rate can be further boosted to video-rate imaging. With enlarged imaging depth, this scheme is promising for a wide range of applications, including light detection and ranging.

Fast and high-resolution spectroscopy based on asynchronous optical sampling.

Dispersive time stretch has made many ultrafast applications possible owing to its high frame rate, as compared to conventional spectroscopies. By further introducing a converging time lens, this spectroscopy can resolve arbitrary emission spectra within the aperture. However, a spectral resolution of tens of picometers hinders its high-precision application. There are two limitations: the temporal aperture of the acquired signal and the actual acquisition bandwidth. To overcome these restrictions, two approaches were developed. First, a large-aperture time lens, with higher-order dispersion compensation, is used to overcome the fundamental limit of the time-bandwidth product. Second, asynchronous optical sampling, based on two frequency combs, overcomes the technical limit of the acquisition bandwidth. As a result, in this study, time-stretch spectroscopy achieved a 1-pm spectral resolution, 24-nm observation bandwidth, and 1-kHz frame rate. Moreover, it was used to observe some spectral dynamics of the random lasing process and devices with narrow spectral widths. This scheme provides essential improvement for time-stretch spectroscopy to achieve high precision.