Resonantly enhanced second-and third-harmonic generation in dielectric nonlinear metasurfaces

Nonlinear dielectric metasurfaces provide a promising approach to control and manipulate frequency conversion optical processes at the nanoscale,thus facilitating both advances in fundamental research and the development of new practical applications in photonics,lasing,and sensing.Here,we employ symmetry-broken metasurfaces made of centrosymmetric amorphous silicon for resonantly enhanced second-and third-order nonlinear optical response.Exploiting the rich physics of optical quasi-bound states in the continuum and guided mode resonances,we comprehensively study through rigorous numerical calculations the relative contribution of surface and bulk effects to second-harmonic generation(SHG)and the bulk contribution to third-harmonic generation(THG) from the meta-atoms.Next,we experimentally achieve optical resonances with high quality factors,which greatly boosts light-matter interaction,resulting in about 550 times SHG enhancement and nearly 5000-fold increase of THG.A good agreement between theoretical predictions and experimental measurements is observed.To gain deeper insights into the physics of the investigated nonlinear optical processes,we further numerically study the relation between nonlinear emission and the structural asymmetry of the metasurface and reveal that the generated harmonic signals arising from linear sharp resonances are highly dependent on the asymmetry of the meta-atoms.Our work suggests a fruitful strategy to enhance the harmonic generation and effectively control different orders of harmonics in all-dielectric metasurfaces,enabling the development of efficient active photonic nanodevices.

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.

Recent progress on structural coloration

Structural coloration generates colors by the interaction between incident light and micro-or nanoscale structures.It has received tremendous interest for decades,due to advantages including robustness against bleaching and environmentally friendly properties(compared with conventional pigments and dyes).As a versatile coloration strategy,the tuning of structural colors based on micro-and nanoscale photonic structures has been extensively explored and can enable a broad range of applications including displays,anti-counterfeiting,and coating.However,scholarly research on structural colors has had limited impact on commercial products because of their disadvantages in cost,scalability,and fabrication.In this review,we analyze the key challenges and opportunities in the development of structural colors.We first summarize the fundamental mechanisms and design strategies for structural colors while reviewing the recent progress in realizing dynamic structural coloration.The promising potential applications including optical information processing and displays are also discussed while elucidating the most prominent challenges that prevent them from translating into technologies on the market.Finally,we address the new opportunities that are underexplored by the structural coloration community but can be achieved through multidisciplinary research within the emerging research areas.

All-optical nanoscale thermometry with silicon carbide color centers

All-optical thermometry plays a crucial role in precision temperature measurement across diverse fields.Quantum defects in solids are one of the most promising sensors due to their excellent sensitivity,stability,and biocompatibility.Yet,it faces limitations,such as the microwave heating effect and the complexity of spectral analysis.Addressing these challenges,we introduce a novel approach to nanoscale optical thermometry using quantum defects in silicon carbide(SiC),a material compatible with complementary metal-oxide-semiconductor(CMOS)processes.This method leverages the intensity ratio between anti-Stokes and Stokes emissions from SiC color centers,overcoming the drawbacks of traditional techniques such as optically detected magnetic resonance(ODMR)and zero-phonon line(ZPL)analysis.Our technique provides a real-time,highly sensitive(1.06%K^(-1)),and diffraction-limited temperature sensing protocol,which potentially helps enhance thermal management in the future miniaturization of electronic components.

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.

Improving growth and yield of rice through the use of agricultural Jiaosu in different combinations

Agricultural Jiaosu is rich in nutrients and plant-beneficial microbial communities,and thus has great potential in reducing fertilizer dosage and improving fertilizer use efficiency.This pot experiment was conducted to investigate the effects of agricultural Jiaosu on the growth and yield of rice.The experiment employed three agricultural Jiaosu treatments(i.e.,soil solid agricultural Jiaosu,soil liquid agricultural Jiaosu,leaf liquid agricultural Jiaosu)in various combinations,along with one control treatment(chemical fertilizer).Results revealed that plant height of rive agricultural Jiaosu treatments were higher than that of the control treatment,with the ranks as follows:JT(soil solid agricultural Jiaosu+soil liquid agricultural Jiaosu),JY(soil solid agricultural Jiaosu+leaf liquid agricultural Jiaosu),JA(soil solid agricultural Jiaosu+soil liquid agricultural Jiaosu+leaf liquid agricultural Jiaosu)and F(conventional fertilizer)during harvest.The ratio of panicle biomass to total biomass and straw of three types of Jiaosu treatments was significantly higher than that in the chemical fertilizer treatment,indicating that agricultural Jiaosu treatment altered the dry matter distribution characteristics of rice.The yield per plant of JA treatment(49.35±2.43 g)was 8.51%higher than chemical fertilizer treatment.Additionally,the correlation between soil nutrients and plant growth and yield analysis indicated that agricultural Jiaosu has higher nutrient use efficiency with lower input of N,P,and K.The study highlights the potential function of agricultural Jiaosu in promoting crop growth and yields while reducing the need for fertilization.

High spatial resolution collinear chiral sum-frequency generation microscopy

Chiral sum-frequency generation(SFG)has proven to be a versatile spectroscopic and imaging tool for probing chirality.However,due to polarization restriction,the conventional chiral SFG microscopes have mostly adopted noncollinear beam configurations,which only partially cover the aperture of microscope and strongly spoil the spatial resolution.In this study,we report the first experimental demonstration of collinear chiral SFG microscopy,which fundamentally supports diffraction-limited resolution.This advancement is attributed to the collinear focus of a radially polarized vectorial beam and a linearly polarized(LP)beam.The tightly focused vectorial beam has a very strong longitudinal component,which interacts with the LP beam and produces the chiral SFG.The collinear configuration can utilize the full aperture and thus push the spatial resolution close to the diffraction limit.This technique can potentially boost the understanding of chiral systems.

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.

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.

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.

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.

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.

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.

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.

Warming-driven migration of core microbiota indicates soil property changes at continental scale

Terrestrial species are predicted to migrate northward under global warming conditions,yet little is known about the direction and magnitude of change in microbial distribution patterns.In this continental-scale study with more than 1600 forest soil samples,we verify the existence of core microbiota and lump them into a manageable number of eco-clusters based on microbial habitat preferences.By projecting the abundance differences of eco-clusters between future and current climatic conditions,we observed the potential warming-driven migration of the core microbiota under warming,partially verified by a field warming experiment at Southwest China.Specifically,the species that favor low p H are potentially expanding and moving northward to medium-latitudes(25°–45°N),potentially implying that warm temperate forest would be under threat of soil acidification with warming.The eco-cluster of high-p H with high-annual mean temperature(AMT)experienced significant abundance increases at middle-(35°–45°N)to high-latitudes(>45°N),especially under Representative Concentration Pathway(RCP)8.5,likely resulting in northward expansion.Furthermore,the eco-cluster that favors low-soil organic carbon(SOC)was projected to increase under warming scenarios at low-latitudes(<25°N),potentially an indicator of SOC storage accumulation in warmer areas.Meanwhile,at high-latitudes(>45°N)the changes in relative abundance of this eco-cluster is inversely related with the temperature variation trends,suggesting microbes-mediated soil organic carbon changes are more responsive to temperature variation in colder areas.These results have vital implications for the migration direction of microbial communities and its potential ecological consequences in future warming scenarios.