Analysis of third and one-third harmonic generation in lossy waveguides

A comprehensive study on the requirements for the highly efficient third harmonic generation(THG) and its inverse process, one-third harmonic generation(OTHG), in lossy waveguides is proposed. The field intensity restrictions for both THG and OTHG caused by loss are demonstrated. The effective relative phase ranges, supporting the positive growth of signal fields of THG and OTHG are shrunken by the loss. Furthermore, it turns out that the effective relative phase ranges depend on the intensities of the interacting fields. At last, a modified definition of coherent length in loss situation, which evaluates the phase matching degree more precisely, is proposed by incorporating the shrunken relative phase range and the nonlinear phase mismatch. These theoretical analysis are valuable for guiding the experimental designs for highly efficient THG and OTHG.

Intraspecific trait variation of woody species reduced in a savanna community,southwest China

Plants deploy various ecological strategies in response to environmental heterogeneity.In many forest ecosystems,plants have been reported to have notable inter-and intra-specific trait variation,as well as clear phylogenetic signals,indicating that these species possess a degree of phenotypic plasticity to cope with habitat variation in the community.Savanna communities,however,grow in an open canopy structure and exhibit little species diversification,likely as a result of strong environmental stress.In this study,we hypothesized that the phylogenetic signals of savanna species would be weak,the intraspecific trait variation(ITV)would be low,and the contribution of intraspecific variation to total trait variance would be reduced,owing to low species richness,multiple stresses and relatively homogenous community structure.To test these hypotheses,we sampled dominant woody species in a dry-hot savanna in southwestern China,focusing on leaf traits related to adaptability of plants to harsh conditions(yearround intense radiation,low soil fertility and seasonal droughts).We found weak phylogenetic signals in leaf traits and low ITV(at both individual and canopy-layer levels).Intraspecific variation(including leaf-,layer-and individual-scales)contributed little to the total trait variance,whereas interspecific variation and variation in leaf phenology explained substantial variance.Our study suggests that intraspecific trait variation is reduced in savanna community.Furthermore,our findings indicate that classifying species by leaf phenology may help better understand how species coexist under similar habitats with strong stresses.

High-responsivity on-chip waveguide coupled germanium photodetector for 2 μm waveband

Recently,the emerging 2μm waveband has gained increasing interest due to its great potential for a wide scope of applications.Compared with the existing optical communication windows at shorter wavelengths,it also offers distinct advantages of lower nonlinear absorption,better fabrication tolerance,and larger free carrier plasma effects for silicon photonics,which has been a proven device technology.While much progress has been witnessed for silicon photonics at the 2μm waveband,the primary challenge still exists for on-chip detectors.Despite the maturity and compatibility of the waveguide coupled photodetectors made of germanium,the 2μm regime is far beyond its cutoff wavelength.In this work,we demonstrate an efficient and high-speed on-chip waveguidecoupled germanium photodetector operating at the 2μm waveband.The weak sub-bandgap absorption of epitaxial germanium is greatly enhanced by a lateral separation absorption charge multiplication structure.The detector is fabricated by the standard process offered by a commercial foundry.The device has a benchmark performance with responsivity of 1.05 A/W and 3 dB bandwidth of 7.12 GHz,which is able to receive high-speed signals with up to 20 Gbit/s data rate.The availability of such an efficient and fast on-chip detector circumvents the barriers between silicon photonic integrated circuits and the potential applications at the 2μm waveband.

A platform for integrated spectrometers based on solution-processable semiconductors

Acquiring real-time spectral information in point-of-care diagnosis,internet-of-thing,and other lab-on-chip applications require spectrometers with hetero-integration capability and miniaturized feature.Compared to conventional semiconductors integrated by heteroepitaxy,solution-processable semiconductors provide a much-flexible integration platform due to their solution-processability,and,therefore,more suitable for the multi-material integrated system.However,solution-processable semiconductors are usually incompatible with the micro-fabrication processes.This work proposes a facile and universal platform to fabricate integrated spectrometers with semiconductor substitutability by unprecedently involving the conjugated mode of the bound states in the continuum(conjugated-BIC)photonics.Specifically,exploiting the conjugated-BIC photonics,which remains unexplored in conventional lasing studies,renders the broadband photodiodes with ultra-narrowband detection ability,detection wavelength tunability,and on-chip integration ability while ensuring the device performance.Spectrometers based on these ultra-narrowband photodiode arrays exhibit high spectral resolution and wide/tunable spectral bandwidth.The fabrication processes are compatible with solution-processable semiconductors photodiodes like perovskites and quantum dots,which can be potentially extended to conventional semiconductors.Signals from the spectrometers directly constitute the incident spectra without being computation-intensive,latency-sensitive,and error-intolerant.As an example,the integrated spectrometers based on perovskite photodiodes are capable of realizing narrowband/broadband light reconstruction and in-situ hyperspectral imaging.

Hundredfold increase of stimulated Brillouin-scattering bandwidth in whispering-gallery mode resonators

Backward stimulated Brillouin scattering(SBS)is widely exploited for various applications in optics and optoelectronics.It typically features a narrow gain bandwidth of a few tens of megahertz in fluoride crystals.Here we report a hundredfold increase of SBS bandwidth in whispering-gallery mode resonators.The crystalline orientation results in a large variation of the acoustic phase velocity upon propagation along the periphery,from which a broad Brillouin gain is formed.Over 2.5 GHz wide Brillouin gain profile is theoretically found and experimentally validated.SBS phenomena with Brillouin shift frequencies ranging from 11.73 to 14.47 GHz in ultrahigh Q Z-cut magnesium fluoride cavities pumped at the telecommunication wavelength are demonstrated.Furthermore,the Brillouin-Kerr comb in this device is demonstrated.Over 400 comb lines spanning across a spectral window of 120 nm are observed.Our finding paves a new way for tailoring and harnessing the Brillouin gain in crystals.

Emerging opportunities for ultra-high Q whispering gallery mode microcavities

Whispering gallery modes (WGMs) were first discovered for sound waves in the whispering gallery of St Paul’s Cathedral and explained by Rayleigh [1] in 1878. In 1961, Garrett et al.[2] applied the concept of WGMs to optical systems and realized stimulated emissions in Sm2+-doped CaF2 spheres.Since then, WGMs have been widely and intensively studied in a range of micro-sized systems, including microdroplets,microspheres, microtoroids, microdisks, and microtubes.

High-speed silicon photonic Mach–Zehnder modulator at 2 μm

Recently,2-μm wave band has gained increasing interest due to its potential application for next-generation optical communication.But the development of 2-μm optical communications is substantially hampered by the modulation speed due to the device bandwidth constraints.Thus,a high-speed modulator is highly demanded at 2μm.Motivated by this prospect,we demonstrate a high-speed silicon Mach–Zehnder modulator for a 2-μm wave band.The device is configured as a single-ended push–pull structure with waveguide electrorefraction via the free carrier plasma effect.The modulator was fabricated via a multiproject wafer shuttle run at a commercial silicon photonic foundry.The modulation efficiency of a single arm is measured to be 1.6 V·cm.The high-speed characterization is also performed,and the modulation speed can reach 80 Gbit/s with 4-level pulse amplitude modulation(PAM-4)formats.

Emerging material platforms for integrated microcavity photonics

Many breakthroughs in technologies are closely associated with the deep understanding and development of new material platforms.As the main material used in microelectronics,Si also plays a leading role in the development of integrated photonics.The indirect bandgap,absence ofχ(2)nonlinearity and the parasitic nonlinear absorptions at the telecom band of Si imposed technological bottlenecks for further improving the performances and expanding the functionalities of Si microcavities in which the circulating light intensity is dramatically amplified.The past two decades have witnessed the burgeoning of the novel material platforms that are compatible with the complementary metal-oxide-semiconductor(COMS)process.In particular,the unprecedented optical properties of the emerging materials in the thin film form have resulted in revolutionary progress in microcavity photonics.In this review article,we summarize the recently developed material platforms for integrated photonics with the focus on chip-scale microcavity devices.The material characteristics,fabrication processes and device applications have been thoroughly discussed for the most widely used new material platforms.We also discuss open challenges and opportunities in microcavity photonics,such as heterogeneous integrated devices,and provide an outlook for the future development of integrated microcavities.

Direct observation of chaotic resonances in optical microcavities

Optical microcavities play a significant role in the study of classical and quantum chaos.To date,most experimental explorations of their internal wave dynamics have focused on the properties of their inputs and outputs,without directly interrogating the dynamics and the associated mode patterns inside.As a result,this key information is rarely retrieved with certainty,which significantly restricts the verification and understanding of the actual chaotic motion.Here we demonstrate a simple and robust approach to directly and rapidly map the internal mode patterns in chaotic microcavities.By introducing a local index perturbation through a pump laser,we report a spectral response of optical microcavities that is proportional to the internal field distribution.With this technique,chaotic modes with staggered mode spacings can be distinguished.Consequently,a complete chaos assisted tunneling(CAT)and its time-reversed process are experimentally verified in the optical domain with unprecedented certainty.

Dispersion engineering and measurement in crystalline microresonators using a fiber ring etalon

Dispersion engineering and measurement are significant for nonlinear photonic applications using whispering gallery mode microresonators.Specifically,the Kerr microresonator frequency comb as an important example has attracted a great amount of interest in research fields due to the potential capability of full integration on a chip.A simple and cost-efficient way for dispersion measurements is thereby in high demand for designing such a microcomb device.Here,we report a dispersion measurement approach using a fiber ring etalon reference.The free spectral range of the etalon is first measured through sideband modulation,and the dispersion of the etalon is inferred by binary function fitting during the dispersion measurement.This method is demonstrated on two MgF_(2) disk resonators.Experimental results show good agreement with numerical simulations using the finite element method.Dispersion engineering on such resonators is also numerically investigated.

Design of a barcode-like waveguide nanostructure for efficient chip-fiber coupling

A barcode-like waveguide nanostructure with discretized multilevel pixel lines is designed and optimized by a nonlinear search algorithm. We obtain the design of a one-dimensional multilevel nanostructure with-1.04 d B efficiency for surface normal coupling to a standard single-mode fiber. Another design is achieved from the automatic optimization process, which enables polarization-independent coupling to a single-mode fiber. The optimum coupling efficiency is simulated to be-2.83 dB for TE and-3.49 for TM polarization centered near the 1550 nm wavelength. Polarization-dependent loss of less than 1 dB over 45.3 nm is achieved.

Observational study of the physical and chemical characteristics of the winter radiation fog in the tropical rainforest in Xishuangbanna, China

We conducted a three-month field experiment focusing on the physical and chemical characteristics of fog in a tropical rainforest in Xishuangbanna,Southwest China,in the winter of 2019.In general,the fog would form at midnight and persist because of the increased long-wave radiative cooling combined with the high relative humidity,gentle breeze,and a relatively low aerosol number concentration in the forest;the fog would dissipate before noon due to the increasing turbulence near the surface.This diurnal cycle is typical for radiation fog.The microphysical fog properties included a relatively low number concentration of the fog droplet,large droplet size,high liquid water content,narrow droplet number-size distribution,and high supersaturation.The chemical properties showed that the fog water was slightly alkaline with low electrical conductivity,whereas the highest proportions of anions and cations therein were Cl^(−)and Ca^(2+),respectively;the chemical components were enriched in small fog droplets.In addition,we indirectly calculated the fog supersaturation according to theκ-Köhler theory.We found that condensation broadens the droplet number-size distribution at relatively low supersaturation,which is positively correlated with the fog-droplet number concentration and negatively correlated with the droplet mean-volume diameter;this affects the key microphysical processes of fog.

Lanthanide-doped nanocrystals in high-Q microtoroids for stable on-chip white-light lasers

The plentiful energy states of lanthanide(Ln^(3+))-doped nanomaterials make them very promising for on-chip integrated white-light lasers.Despite the rapid progresses,the Ln^(3+)-based white upconversion emissions are strongly restricted by their low upconversion quantum efficiency and the color stability.Herein,we combine the CaF_(2):Yb_(35)Tm_(1.5)Er_(0.5)nanocrystals and the high-Q microtoroids,and experimentally demonstrate the chip-integrated stable white-light laser.By optimizing the sizes,density,and distributions of Ln^(3+)-doped nanocrystals,the Q factors of Ln^(3+)-doped microtoroids are maintained as high as 5×10^(5).The strong light matter interaction in high-Q microtoroids greatly enhances the upconversion emission and dramatically reduces the laser thresholds at 652 nm,545 nm,and 475 nm to similarly low values(1.89-2.10 m J cm^(-2)).Consequently,robust white-light microlaser has been experimentally achieved from a single microtoroid.This research has paved a solid step toward the chip-scale integrated broadband microlasers.

Observation of a manifold in the chaotic phase space of an asymmetric optical microcavity

Chaotic dynamics in optical microcavities, governed dominantly by manifolds, is of great importance for both fundamental studies and photonic applications. Here, we report the experimental observation of a stable manifold characterized by energy and momentum evolution in the nearly chaotic phase space of an asymmetric optical microcavity. By controlling the radius of a fiber coupler and the coupling azimuth of the cavity, corresponding to the momentum and position of the input light, the injected light can in principle excite the system from a desired position in phase space. It is found that once the input light approaches the stable manifold, the angular momentum of the light experiences a rapid increase, and the energy is confined in the cavity for a long time.Consequently, the distribution of the stable manifold is visualized by the output power and the coupling depth to high-Q modes extracted from the transmission spectra, which is consistent with theoretical predictions by the ray model. This work opens a new path to understand the chaotic dynamics and reconstruct the complex structure in phase space, providing a new paradigm of manipulating photons in wave chaos.

Enhancing magnetic dipole emission with magnetic metamaterials

Magnetic dipole（MD） transitions are important for a range of technologies from quantum light sources and displays to lasers and bio-probes. However, the typical MD transitions are much weaker than their electric counterparts and are usually neglected in practical applications. Herein, we experimentally demonstrate that the MD transitions can be significantly enhanced by the well-developed magnetic metamaterials in the visible optical range. The magnetic metamaterials consist of silver nanostrips and a thick silver film, which are separated with an Eu3＋：polymethyl methacrylate（PMMA） film. By controlling the thickness of the Eu3＋：PMMA film, the magnetic resonance has been tuned to match the emission wavelength of MDs. Consequently,the intensity of MD emission has been significantly increased by around 30 times at the magnetic resonance wavelength, whereas the intensity of electric dipole emission is well-preserved. The corresponding numerical calculations reveal that the enhancement is directly generated by the magnetic resonance, which strongly increases the magnetic local density of states around the MD emitter and can efficiently radiate the MD emission into the far field. This is the first demonstration, to the best of our knowledge, that MD transitions can be improved by an additional degree of magnetic freedom, and we believe this research shall pave a new route towards bright magnetic emitters and their potential applications.

增温驱动的核心微生物的迁移可指示土壤属性的改变

尽管微生物-气候的相互作用已得到越来越多的研究者和决策者的认可,但微生物的高多样性和对气候环境变化多变量的响应导致预测微生物在未来气候背景下的分布格局非常困难.本研究依托于中国土壤微生物组计划,基于采集自中国东部森林的1600多个样品的16S r RNA基因测序数据,首先证实了微生物群落组成和多样性的纬度分布规律且温度对微生物群落组成有显著的直接作用.其次,利用核心微生物代替整体群落来进行多样性的缩减,并将这些核心微生物根据其对环境的偏好性划分为不同的生态集群,这些生态集群在空间上的热点区域,即高丰度区域相互不重叠.此外,通过Cubist模型预测未来不同气候变化情景下(RCP2.6和RCP8.5)各生态集群的丰度变化并将其投影到中国森林生态系统分布区域,通过与现在的分布格局做对比得到增温驱动的生态集群空间分布格局的变化.这些变化一方面可以指示集群内微生物对未来气候变化的适应性,另一方面考虑到每一类生态集群所代表的环境偏好性,这些变化也可进一步用来指示未来气候变化背景下土壤属性的变化.

Guiding flow of light with supersymmetry

The continuous supersymmetry transformation is applied to the silicon waveguides,and the guidance and conversion of any mode in a wide spectral range are successfully realized in experiments.This proves its great potential in optical spatial mode modulation and space division multiplexing in optical communication.

Suppressing meta-holographic artifacts by laser coherence tuning

A metasurface hologram combines fine spatial resolution and large viewing angles with a planar form factor and compact size.However,it suffers coherent artifacts originating from electromagnetic cross-talk between closely packed meta-atoms and fabrication defects of nanoscale features.Here,we introduce an efficient method to suppress all artifacts by fine-tuning the spatial coherence of illumination.Our method is implemented with a degenerate cavity laser,which allows a precise and continuous tuning of the spatial coherence over a wide range,with little variation in the emission spectrum and total power.We find the optimal degree of spatial coherence to suppress the coherent artifacts of a meta-hologram while maintaining the image sharpness.This work paves the way to compact and dynamical holographic displays free of coherent defects.