Asymmetry spin-polarization Fermi arc and topological chiral beam splitter with the triply degenerate point in metamaterial.

Dirac points (DPs) and Weyl points (WPs) have received much attention in photonic crystals (PhCs) and three-dimensional (3D) metamaterials research due to the robust surface states and Fermi arcs. In this work, two pairs of triply degenerate points (TDPs) have been proposed in a 3D metamaterial by breaking the time reversal symmetry (T) with an external magnetic field. Based on these TDPs, two pairs of asymmetric surface states with spin-polarization are revealed, and a topological chiral beam splitter is demonstrated showing the different propagating directions of the right-handed polarization (RCP) and left-handed polarization (LCP) lights. Remarkably, we can achieve unidirectional propagation with RCP or LCP even excited by a linear source owing to the asymmetry surface state. Our work provides a new, to the best of our knowledge, platform to study spin-polarization surface states and the enhanced spin photonic Hall effect in the metamaterials.

396 MHz CPA femtosecond laser system based on a single crystal fiber rod-type amplifier.

The paper presents the results of multiplying the repetition frequency of the seed source to 396 MHz using an extra-cavity fiber multiplication method. The single crystal fiber rod-type amplifier was able to boost the average output power to approximately 15.9 W. Using a transmission gratings-based pulse compressor, an average output power of 9.56 W was achieved, corresponding to a burst energy of up to 382 µJ and a compression efficiency of 81.7%. The shortest pulse duration was optimized to be 594 fs; however, an obvious pedestal was also observed. This system reduces the impact of nonlinear effects on the device by increasing the repetition frequency, thereby increasing the injection energy of a single process in industrial processing.

Local OAM manipulation of a terahertz wave from the air filament by chirping the few-cycle vortex pump laser.

We propose a scheme to manipulate the local orbital angular momentum (OAM) of the ultra-broadband (0.1-30 THz) terahertz (THz) waves from the laser-induced short air filament via chirping the few-cycle vortex laser pump. The simulation results show that either the THz vortex pulses with linear azimuth-dependent phases or the THz angular accelerating vortex beams (AAVBs) with nonlinear azimuth-dependent phases can be produced by tuning the chirp parameter of the pump. Thus, the dominant physical mechanism for THz generation can be determined. The THz temporal and transverse spatial distributions can be also controlled by the chirp parameter. Furthermore, their local OAM density distributions present very complex structures because most of the modulated azimuthal intensity and the corresponding local angular helicity distributions are not able to cancel out completely. Via analyzing the simulated THz results at the different pump intensities, we classify the initial pump intensity into three cases. For the low intensity case, the Kerr effect comes into prominence, so the generated THz radiation shall be vortex pulses. While for the high intensity case, the leading plasma effect dominates. In contrast, when the pump intensity is at the medium level, the Kerr nonlinearity and the plasma effect may be comparable and competitive. Basically, THz AAVBs are generated for both high and medium intensity cases. Our study will provide the possibility for studying the optically induced rotation technology more intuitively from the perspective of angular momentum transfer.

Orthogonality of diffractive deep neural network.

Some rules of the diffractive deep neural network (D2NN) are discovered. They reveal that the inner product of any two optical fields in D2NN is invariant and the D2NN acts as a unitary transformation for optical fields. If the output intensities of the two inputs are separated spatially, the input fields must be orthogonal. These rules imply that the D2NN is not only suitable for the classification of general objects but also more suitable for applications aimed at optical orthogonal modes. Our simulation shows the D2NN performs well in applications like mode conversion, mode multiplexing/demultiplexing, and optical mode recognition.

Ultra-Small 2D PbS Nanoplatelets: Liquid-Phase Exfoliation and Emerging Applications for Photo-Electrochemical Photodetectors.

2D PbS nanoplatelets (NPLs) form an emerging class of photoactive materials and have been proposed as robust materials for high-performance optoelectronic devices. However, the main drawback of PbS NPLs is the large lateral size, which inhibits their further investigations and practical applications. In this work, ultra-small 2D PbS NPLs with uniform lateral size (11.2 ± 1.7 nm) and thickness (3.7 ± 0.9 nm, ≈6 layers) have been successfully fabricated by a facile liquid-phase exfoliation approach. Their transient optical response and photo-response behavior are evaluated by femtosecond-resolved transient absorption and photo-electrochemical (PEC) measurements. It is shown that the NPLs-based photodetectors (PDs) exhibit excellent photo-response performance from UV to the visible range, showing extremely high photo-responsivity (27.81 mA W-1 ) and remarkable detectivity (3.96 × 1010 Jones), which are figures of merit outperforming currently reported PEC-type PDs. The outstanding properties are further analyzed based on the results of first-principle calculations, including electronic band structure and free energies for the oxygen evolution reaction process. This work highlights promising applications of ultra-small 2D PbS NPLs with the potential for breakthrough developments also in other fields of optoelectronic devices.

Generation and evolution of different terahertz singular beams from long gas-plasma filaments.

We theoretically and numerically investigate the generation and evolution of different pulsed terahertz (THz) singular beams with an ultrabroad bandwidth (0.1-40 THz) in long gas-plasma filaments induced by a shaped two-color laser field, i.e., a vortex fundamental pulse (ω0) and a Gaussian second harmonic pulse (2ω0). Based on the unidirectional propagation model under group-velocity moving reference frame, the simulating results demonstrate that three different THz singular beams, including the THz necklace beams with a π-stepwise phase profile, the THz angular accelerating vortex beams (AAVBs) with nonlinear phase profile, and the THz vortex beams with linear phase profile, are generated. The THz necklace beams are generated first at millimeter-scale length. Then, with the increase of the filament length, THz AAVBs and THz vortex beams appear in turn almost periodically. Our calculations confirm that all these different THz singular beams result from the coherent superposition of the two collinear THz vortex beams with variable relative amplitudes and conjugated topological charges (TCs), i.e., +2 and -2. These two THz vortex beams could come from the two four-wave mixing (FWM) processes, respectively, i.e., ω0+ω0-2ω0→ωTHz and -(ω0+ω0) + 2ω0→ωTHz. The evolution of the different THz singular beams depends on the combined effect of the pump ω0-2ω0 time delay and the separate, periodical, and helical plasma channels. And the TC sign of the generated THz singular beams can be easily controlled by changing the sign of the ω0-2ω0 time delay. We believe that these results will deepen the understanding of the THz singular beam generation mechanism and orbital angular momentum (OAM) conversion in laser induced gas-filamentation.

Enhance terahertz radiation and its polarization- control with two paralleled filaments pumped by two-color femtosecond laser fields.

We present experimentally an obvious enhancement of the terahertz (THz) radiation with two paralleled filaments pumped by two-color laser fields for a full use of a high laser power, compared with single filament. By mapping the 3-dimensional electric trajectories of generated THz fields with a (111) ZnTe crystal, we observe that the total THz polarization from two filaments can be manipulated by varying the time delay between the two orthogonally polarized pumps, which agrees well with the simulations under the photocurrent model. Notably, the power and spectrum of the THz field almost keep unchanged while manipulating the ellipticity of the THz polarization, which is important for a polarization-controllable THz source.

All-optical high spatial-temporal resolution photography with raster principle at 2 trillion frames per second.

A novel single-shot ultrafast all-optical photography with raster principle (OPR) that can capture real-time imaging of ultrafast phenomena is proposed and demonstrated. It consists of a sequentially timed module (STM), spectral-shaping module (SSM), and raster framing camera (RFC). STM and SSM are used for linearly encoding frequency-time mapping and system calibration, respectively. The function of the RFC is sampling the target by microlens arrays and framing on the basis of frequency-time-spatial positions conversion. We demonstrated the recording of transient scenes with the spatial resolution of ∼90lp/mm, the frame number of 12 and the frame rate of 2 trillion frames per second (Tfps) in single-shot. Thanks to its high spatial-temporal resolution, high frame rate (maximum up to 10 Tfps or more) and sufficient frame number, our OPR can observe the dynamic processes with complex spatial structure at the atomic time scale (10 fs∼1ps), which is promising for application in plasma physics, shock waves in laser-induced damage, and dynamics of condensed matter materials.

Liquid crystal-based order electrically controlled q-plate system.

This paper proposes a liquid crystal-based order adjustable q-plate system. The system, which is solid-state and electrically controlled without any mechanical components, consists of several bit cells and one symbol cell. The bit cells can be electrically selected whether to modulate the beam. The magnitude of the order of the q-plate system can be controlled by activating specific bit cells. And the sign of the order can be changed by controlling the voltages in the symbol cell. The whole system can realize the function of the order adjustable q-plate with the order ranging from -2n + 1 to 2n-1 with n bit cells. In our experiment, the system with 4 bits is verified. Based on the q-plate system, the vector beams and optical vortexes with the orders ranging from -15 to 15 can be generated.

Deep learning based atmospheric turbulence compensation for orbital angular momentum beam distortion and communication.

Atmospheric transmission distortion is one of the main challenges hampering the practical application of a vortex beam (VB) which carries orbital angular momentum (OAM). In this work, we propose and investigate a deep learning based atmospheric turbulence compensation method for correcting the distorted VB and improving the performance of OAM multiplexing communication. A deep convolutional neural network (CNN) model, which can automatically learn the mapping relationship of the intensity distributions of input and the turbulent phase, is well designed. After trained with loads of studying samples, the CNN model possesses a good generalization ability in quickly and accurately predicting equivalent turbulent phase screen, including the untrained turbulent phase screens. The results show that through correction, the mode purity of the distorted VB improves from 39.52% to 98.34% under the turbulence intensity of Cn2 = 1 × 10-13. Constructing an OAM multiplexing communication link, the bit-error-rate (BER) of the transmitted signals in each OAM channel is reduced by almost two orders of magnitude under moderate-strong turbulence, and the demodulated constellation diagram also converges well after compensated by the CNN model.

Improve polarization topological order sorting with the diffractive splitting method.

Vector beams with different polarization topological orders (PTOs) are the eigenmodes of traditional optical fibers and are orthogonal to each other, so the PTO multiplexing channel is a promising candidate for the oncoming generation of optical communication. Here we demonstrate experimentally a PTO sorting system with high separation resolution based on the diffractive splitting (DS) method. Our experiments show that our design with the DS method helps to enhance the separation resolution to 77.5% from 58%, compared to a design without the application of a DS method. Theoretically, to increase the copy number can promote the separation resolution further. This Letter provides a high-resolution way to decode information from PTO division multiplexing.

Generation of terahertz vortex pulses without any need of manipulation in the terahertz region.

Converting a Gaussian mode to a vortex beam is much more inconvenient in the terahertz (THz) region than in the near-infrared (NIR) region due to underdevelopment of THz components and strong THz diffraction. This Letter reports the direct generation of THz vortex pulses by optical difference-frequency between two NIR chirped pulses with different topological charges (TCs). By designing a passive and transmissive device for a collinear NIR pulse pair with conjugated TCs, we have experimentally obtained stable THz vortex pulses with a TC value of 2 or -2. The process needs no THz components and so is flexible to be realized and has promising applications in the THz field.

Common-path spectral interferometry for single-shot terahertz electro-optics detection: publisher's note.

This publisher's note corrects an error in one of the authors' names and an error in one of the grant numbers in Opt. Lett.42, 4263 (2017).

Electro-optical coupling of a circular Airy beam in a uniaxial crystal.

This paper investigates theoretically and numerically on the electro-optical coupling (EOC) for a circular Airy beam (CAB) propagating along the optical axis of a uniaxial crystal after deducing the wave coupling equations of EOC. For a circularly polarized incident CAB, EOC can be used to generate vortex beam by coupling the incident left-handed component into the right-handed vortex component with a vortex topological charge of 2. What's more, EOC plays important role in enhancing or suppressing the abrupt autofocusing, the most important property of CABs, for both left-handed and right-handed components. Near the focal plane, EOC can result in electrically controllable optical "needle" and "cage", which shall be interesting in micromanipulation. In addition, EOC can influence or even forbid the exchange between spin angular momentum (SAM) and orbit angular momentum (OAM). For a linearly polarized incident CAB, its two Cartesian field components of the beam cannot only couple their powers to each other, but also lead to the changes of the intensity pattern and polarization distributions. The polarization state becomes spatially inhomogeneous, and possesses vortex phase with a topological charge of 2 during propagation. EOC presents a new way to control an Airy beam fast and efficiently.

Common-path spectral interferometry for single-shot terahertz electro-optics detection.

We propose a common-path spectral interferometer for single-shot terahertz (THz) electro-optics (EO) detection, where a probe pulse pair with orthogonal polarizations and a relative time delay are generated by simply using a birefringent plate. One of them, as the object, transmits through a THz EO crystal with THz phase modulation, while the other goes through the crystal without any phase imposed by target the THz field as the reference. The co-axial propagation of the pulse pair can effectively reduce the noises due to mechanical vibrations, air turbulences, and temperature fluctuations in the traditional non-common-path spectral interferometers. Our experiments show that, for a given target THz pulse field, the measured THz signals in a single-shot mode have a signal-to-noise ratio (SNR) of 41.2 with our THz common-path spectral interferometer, but 7.91 with a THz Mach-Zehnder spectral interferometer; thus, our design improves the SNR of the THz signal by about 5.2 times.

Propagation dynamics of a circular Airy beam in a uniaxial crystal.

This paper investigates theoretically and numerically the propagation characteristics of a circular Airy beam (CAB) in a uniaxial crystal in detail. The beam loses its boundary cylindrical symmetry during propagation because of the medium anisotropy, although it propagates along the optical axis. This effect of anisotropy on the propagating beam becomes increasingly evident with the increase of the propagation distance. Another main influential factor of the propagation characteristics is the ratio of the extraordinary refractive index to the ordinary refractive index (ne/no). The more the value deviates from 1, the worse the symmetry of the beam intensity distribution becomes. The polarization becomes notably complicated, but possesses a vortex state with a topological charge of 2 during propagation. The abruptly autofocusing characteristic, the most important property of CABs, also appears when the crystal length is long enough, which is greatly different from that characteristic in isotropic media. This work is helpful for the design of optical devices based on uniaxial crystals for beams with some special wavefronts.

Biocontrol of post-harvest Alternaria alternata decay of cherry tomatoes with rhamnolipids and possible mechanisms of action.

BACKGROUND: Rhamnolipids were reported to have evident antifungal activity. The efficacy of rhamnolipids against Alternaria alternata and their possible mechanisms involved were investigated. RESULT: The decay incidences of A. alternata of cherry tomatoes (Lycopersicon esculentum) treated by rhamnolipids were significantly reduced. The in vitro assays showed that rhamnolipids inhibited fungal growth on solid medium and prevented spore germination and mycelium growth in liquid medium. In addition, the combination of rhamnolipids and essential oil had a synergistic effect leading to the decrease of fungicidal concentrations of laurel oil. Scanning electron microscopy and transmission electron microscopy observations of the pathogen revealed significant morphological and cell structural alterations in the hyphae. Compared to the control, the content of nucleic acid in supernatant of the suspension of A. alternata increased, while the content of DNA and protein of mycelium decreased, which was in agreement with electrolyte leakage experiments. CONCLUSION: Rhamnolipids could be an alternative to chemicals for controlling post-harvest phytopathogenic fungi on fruits and vegetables.

Modified THz electro-optic sampling for high optical modulation depth, large dynamical range, and low background noises.

We present a modified THz electro-optic sampling method to combine the advantages of its two traditional counterparts at near 0° and 45° optical biases: excellent ability to cancel the background noises, high optical modulation, and large dynamical range. The first advantage results from the method's symmetrical layout to get dynamical noise cancellation. By setting the static birefringent phases of the two balanced beams with a pair of opposite numbers, our setup can record THz waveforms without distortion with its maximal modulation depth, thus optimal signal-to-noise ratio (SNR). The setting also releases the linearity of the measured signal from the static birefringence, thus enlarging greatly the linear dynamical range. For a given THz field, the recorded SNR with our setup, without a lock-in, is more than 10 times higher than that with the "crossed and balanced" design [IEEE Trans. Microwave Theory Tech. 47, 2644 (1999)].

In vitro and in vivo control of Alternaria alternata in cherry tomato by essential oil from Laurus nobilis of Chinese origin.

BACKGROUND: Many essential oils were reported to be used as natural, environmental friendly antimicrobial agents. The antifungal activity in vitro and in vivo of an essential oil extracted from Chinese local Laurus nobilis leaves against Alternaria alternata has been studied. RESULT: The main components of the essential oil were investigated by means of gas chromatography­mass spectrometry and 14 components were identified. The main ones were eugenol, caryophyllene and cinnamaldehyde. The antifungal test showed that at 800 µg mL−1 of L. nobilis oil completely inhibited the growth of A. alternata. In addition, the conidial germination of the pathogen was significantly inhibited at 200 µgmL−1, and the weight of mycelia efficiently decreased at 500 µgmL−1. The in vivo assay indicated that 500 µg mL−1 L. nobilis oil was effective in protecting cherry tomatoes (Lycopersicon esculentum)from infection of A. alternata, with an inhibition ratio of 33.9%. Scanning electron microscopy observations of the pathogen revealed significant morphological alterations in the hyphae. CONCLUSION: This work suggested that L. nobilis oil could be used as a potential fungicide to control the post-harvest disease caused by A. alternata.

Effect of citronella essential oil on the inhibition of postharvest Alternaria alternata in cherry tomato.

BACKGROUND: Essential oils such as citronella oil exhibit antifungal activity and are potential alternative inhibitors to chemical synthetic fungicides for controlling postharvest diseases. In this study the antifungal activity of citronella oil against Alternaria alternata was investigated. RESULTS: In vitro, citronella oil showed strong inhibition activity against A. alternata. The minimum inhibitory concentration in potato dextrose agar and potato dextrose broth medium was determined as 1 and 0.8 µL mL(-1) respectively. In vivo the disease incidence of Lycopersicon esculentum (cherry tomato) treated with citronella oil was significantly (P < 0.05) reduced compared with the control after 5 days of storage at 25 °C and 95% relative humidity. The disease incidence at oil concentrations of 0.2-1.5 µL mL(-1) was 88-48%. The most effective dosage of the oil was 1.5 µL mL(-1), with 52% reduction, and the oil had no negative effect on fruit quality. Scanning electron microscopy observation revealed considerably abnormal mycelial morphology. CONCLUSION: Citronella oil can significantly inhibit A. alternata in vitro and in vivo and has potential as a promising natural product for controlling black rot in cherry tomato.