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Most reported metasurfaces based on pancharatnam berry (PB) phase principle mainly focus on cross-polarization wave regulation, but little discussion on co-polarization wave regulation. In this letter, we propose a hollow cross-shaped metal disc metasurface to achieve the modulation of cross-polarization and co-polarization terahertz waves. Under right circularly polarized wave incidence at frequency of 1.4THz, this structure achieves cross-polarization and co-polarization vortex beams with different topological charges, cross-polarization and co-polarization wave deflection beams with different deflection directions, and cross-polarization and co-polarization offset focusing beam. Moreover, the simulation results are consistent with the theoretical calculation predictions. The results indicate that the metasurface can decouple incident circularly polarized waves, regulate cross-polarization and co-polarization reflected waves, and provide a new freedom degree for the control of circularly polarized waves.

Airy beams in microwave and optical region have gained wide attention of researchers due to their remarkable non-diffractive, self-bending and self-healing properties. However, the analysis of Airy beams in terahertz wave band is less. In this letter, we proposed an Airy beam multifunctional terahertz metasurface which can achieve multiple functions under different polarized waves. Under the linearly polarized waves incidence, the designed metasurface generate Airy beams and autofocusing Airy beam. Meanwhile, the same metasurface can also realize diversified functions including focusing, multi-vortex beam, and focusing vortex beam under circularly polarized wave incidence. Our presented structure has potential applications in important scenarios such as biomedicine, imaging, holography, and terahertz multifunctional devices.

Most reported terahertz devices are relatively simple to control by means of external applied electricity, light, heat, magnetism, etc. In the practical application of a terahertz system, the devices with a variety of adjustable methods are promising. In this paper, we design a rectangular-ambulatory-plane metal groove structure on apolymer polydimethylsiloxane (PDMS) substrate with graphene embedded in the metal grooves. The resonance performance of the proposed structure can be controlled by changing the external force and graphene chemical potential. Three electromagnetic-induced reflection peaks of the structure are achieved at 0.54, 0.60, and 0.63 THz with reflectivity of 89.6%, 77.1%, and 75.7%, respectively. Under the change of the applied bias voltage and external force, the reflectivity of the three reflection peaks of the resonator can be regulated freely. In addition, the proposed structure is polarization-insensitive and maintains stable terahertz reflection peaks with incident angles up to 25°.

A switchable metasurface with dual functions of polarization conversion and filtering is proposed in this paper. The designed structure is composed of a medium-metal-medium-metal-medium structure, and VO_2 is embedded in the metal metasurface. When VO_2 is an insulated state, this structure can perform linear polarization conversion under terahertz wave normal incidence, and it has good asymmetric transmission capabilities. At the range from 2.01 to 2.86 THz, the polarization conversion rate is higher than 93%. When VO_2 is in the metallic state, the structure becomes a broadband band-stop filter with a 3 dB bandwidth of 1.73 THz. The polarization conversion and filtering functions of the proposed structure can be converted to each other by changing the external temperature. This switchable multifunction structure provides a new way for the design of terahertz devices.

At present, most of the gradient metasurfaces used to construct surface plasmon polaritons (SPPs)/spoof SPPs (SSPs) couplers are usually compact metal antennas working under reflection and transmission. In reflection mode, meta-couplers link propagating waves and surface waves (SWs), and SWs will undergo significant scattering before coupling to an Eigen SPP in the target system. In transmission mode, metal meta-couplers will encounter complex multilayer designing at the microwave/terahertz region and metal absorption loss at optical frequencies. In this Letter, to the best of our knowledge, a novel design using dielectric gradient metasurfaces instead of metal metasurface couplers is proposed to excite broadband SSPs on the metal groove array. We demonstrate that the well-designed phase dielectric gradient metasurface converts the normal incident terahertz wave to the predetermined angle in the dielectric substrate and then excites the broadband SSPs with the transmission coupling between the dielectric meta-coupler and SSPs surface. This research may open up new avenues in simple and broadband plane dielectric meta-couplers for SSPs in ultra-thin and compact functional devices for versatile applications.

We propose a tunable and broadband terahertz (THz)-wave absorber based on the graphene metasurface, which consists of a layer of graphene-hollow-petal structure array and a bottom copper film separated by a 20-μm-thick lossless thermo plastic olefin polymer of amorphous structure layer. The mechanism of such a THz-wave absorber is numerically investigated and theoretically analyzed with the aid of a modified Fabry–Perot resonant model and finite element method. A large absorption efficiency of more than 90% in a frequency range of 2.66 THz ∼3.46  THz was obtained up to a THz-wave incident angle as large as 50°. The absorption bandwidth and absorptivity can be tuned by changing the bias voltage of the graphene metasurface Fermi level. This work indicates that our device has potential applications with respect to tunable sensors and smart absorbers.

Drip-irrigation is increasingly applied in maize (Zea maysL.) production in sub-humid region. It is critical to quantify irrigation requirements during different growth stages under diverse climatic conditions. In this study, the Hybrid-Maize model was calibrated and applied in a sub-humid Heilongjiang Province in Northeast China to estimate irrigation requirements for drip-irrigated maize during different crop physiological development stages and under diverse agro-climatic conditions. Using dimensionless scales, the whole growing season of maize was divided into diverse development stages from planting to maturity. Drip-irrigation dates and irrigation amounts in each irrigation event were simulated and summarized in 30-year simulation from 1981 to 2010. The maize harvest area of Heilongjiang Province was divided into 10 agro-climatic zones based on growing degree days, arid index, and temperature seasonality. The simulated results indicated that seasonal irrigation requirements and water stress during different growth stages were highly related to initial soil water content and distribution of seasonal precipitation. In the experimental site, the average irrigation amounts and times ranged from 48 to 150 mm with initial soil water content decreasing from 100 to 20% of the maximum soil available water. Additionally, the earliest drip-irrigation event might occur during 3- to 8-leaf stage. The water stress could occur at any growth stages of maize, even in wet years with abundant total seasonal rainfall but poor distribution. And over 50% of grain yield loss could be caused by extended water stress during the kernel setting window and grain filling period. It is estimated that more than 94% of the maize harvested area in Heilongjiang Province needs to be irrigated although the yield increase varied (0 to 109%) in diverse agro-climatic zones. Consequently, at least 14% of more maize production could be achieved through drip-irrigation systems in Heilongjiang Province compared to rainfed conditions.

Electromagnetic polarization conveys valuable information for signal processing. Manipulation of a terahertz wave polarization state exhibits tremendous potential in developing applications of terahertz science and technology. We propose an approach to efficiently split transverse-electric and transverse-magnetic polarized terahertz waves into different propagation directions over the frequency range from 0.9998 to 1.0007 THz. Both the plane wave expansion method and the finite-difference time-domain method are used to calculate and analyze the transmission characteristics of the proposed device. The present device is very compact and the total size is 1.02  mm×0.99  mm. This polarization beam splitter performance indicates that the structure has a potential application for forthcoming terahertz-wave integrated circuit fields.

Understanding bacterial transportation in unsaturated soil is helpful for reducing and avoiding pathogenic contamination that may be induced by irrigation with reclaimed waste water and for developing better irrigation management practices. Experiments were conducted to study the transport of a typical bacterium, Escherichia coli (E. coli), in a sandy and a sandy loam soil under different application rates and input concentrations. A 30° wedge-shaped plexiglass container was used to represent one twelfth of the complete cylinder in the experiments. The apparent cylindrical application rate varied from 1.05 to 5.76 L h−1and the input concentration of E. colifrom magnitude of 102to 107colony-forming unit (CFU) mL−1. For a given volume of water applied, an increase in application rate resulted in an increase in the wetted radius and a decrease in the wetted depth. In the sandy loam soil, the water spread out in a circular-arc shaped saturated zone on the surface, and the ultimate saturated entry radius increased with the application rate. An increasing application rate of water suspended bacteria allowed a more rapid transport of bacteria, thus accelerating E. colitransport rate and resulting in a larger distributed volume of E. colifor both soil types. For the sandy soil, more than 70% of the E. colithat was detected within the entire wetted volume concentrated in the range of 10 cm from the point source, and the concentration of E. colidecreased greatly as the distance from the point source increased. More than 98% of the E. coliwas detected in a range of 5 cm around the saturated wetted zone for the sandy loam soil. For both soil types tested, an extremely high concentration of E. coliwas observed in the proximity of the point source, and the peak value increased with an increased input concentration. In principle, using an emitter with relative lower application rate would be effective to restrict E. colitransport. To reduce bacterial concentration in the sewage effluent during wastewater treatment is important to decrease the risk of soil contamination caused by irrigation with sewage effluent.

The aim of drip fertigation is synchronising the application of water and nutrients with crop requirements, and maintaining the proper concentration and distribution of nutrient and water in the soil. The wetting patterns and nutrient distributions under drip fertigation have been proved to be closely related to the fertigation strategies. In order to find out the critical factors that affect the nutrient distribution under different drip fertigaiton strategies, a computer simulation model HYDRUS2D/3D was used to simulate the water and nitrate distribution for various fertigation strategies from a surface point source. Simulation results were compared with the observed ones from our previous studies. A 15° wedge-shaped plexiglass container was used in our experiment to represent one-twenty-fourth of the complete cylinder. The height of container is 40 cm, and the radius is 41 cm. The ammonium nitrate solution was added through a no. 7 needle connected to a Mariotte tube with a flexible hose. The soil water content, nitrate and ammonium concentrations were measured. The comparison of simulated and observed data demonstrated that the model performed reliably. The numerical analysis for various fertigation strategies from a surface point source showed that: (1) The total amount of irrigation water, the concentration of the fertilizer solution and the amount of pure water used to flush the pipeline after fertilizer solution application are the three critical factors influencing the distribution of water and fertilizer nitrogen in the soil. (2) The fresh water irrigation duration prior to fertigation has no obvious effect on nitrate distribution. The longer flushing time period after fertigation resulted in nitrate accumulation closer to the wetting front. From the point of avoiding the possibility of nitrate loss from the root zone, we recommended that the flushing time period should be as shorter as possible. (3) For a given amount of fertilizer, higher concentration of the fertilizer applied solution reduces the potential of nitrate leaching in drip irrigation system. While, lower concentration of the fertilizer solution resulted in an uniform distribution of nitrate band closer to the wetted front.

A series of novel malonate derivatives-calix[4]arene conjugates were synthesized through Knoevenagel condensation reaction, and the structures of these functionalized calix[4]arenes have been determined.

A compact terahertz tunable multichannel filter based on two cascaded photonic crystal waveguide directional couplers, formed by two closely spaced line defect waveguides in a two-dimensional photonic crystal of silicon rods in the air, is numerically studied using plane wave expansion and finite difference time domain methods. The filtering properties of the present terahertz wave filter, including channel number and frequency, can be tuned by adjusting excitation of external applied magnetic field. The dynamical shift of channels with external applied magnetic field intensity is studied in detail. The transmittances of the present filter are higher than 95% for each terahertz wave frequency with a compact size of 3.8 mm $\times$ 10 mm. The designed device can possess both multichannel and tuning features, which could be of technical use in the terahertz wave communications system applications.

Chlorination is usually an economical method for treating clogging in drip emitters during sewage application. Appropriate assessment of the responses of soil and crop is essential for determining an optimal chlorination scheme. During 2008 to 2009, field experiments were conducted in a solar-heated greenhouse for tomato drip irrigated with secondary sewage effluent, to investigate the influences of chlorine injection intervals and levels on soil chemical properties and nitrogen uptake. Injection intervals ranging from two to eight weeks and injection concentrations ranging from 2 to 50 mg L−1were used. A salinity factor and a nutrient factor were extracted from the pool of the nine soil chemical constituents using factor analysis method. The results demonstrated that chlorination practices increased the residual Cl in the soil, resulting in an increased salinity factor, especially for the frequent chlorination at a high injection concentration. Chlorination weakened the accumulation of nutrients factor in the upper soil layer. Nitrogen uptake of the tomato plants also was inhibited by the increased salinity in the upper soil layer caused by high chlorination levels. In order to reduce the unfavorable effect on soil chemical properties and nitrogen uptake, chlorination scheme with concentrations of lower than 20 mg L−1was recommended.

A terahertz wave polarization beam splitter, based on two cascaded multimode interference structures with different widths, is designed and numerically demonstrated. The numerical calculation results show that the designed polarization beam splitter can split transverse-electric (TE) and transverse-magnetic (TM) -polarized terahertz waves into different propagation directions with high efficiency over a frequency range from 6.40 to 6.50 THz. This polarization beam splitter shows more than a 22.06 dB extinction ratio for TE-polarization and a 31.65 dB extinction ratio for TM-polarization. Using such a polarization beam splitter, the whole length of the polarization beam splitter is reduced to about 1/12 that of a conventional design. This enables the polarization beam splitter to be used in terahertz wave integrated circuit fields.

Over the past ten years, electromagnetic terahertz (THz) frequencies region from 100 GHz to 10 THz (or wavelengths of 30m-3mm) have received extensive attention and investigation. Terahertz wave detection enables direct calculations of both the imaginary and the real parts of the refractive index without using the Kramers-Kronig relations. There are many potential applications such as radio astronomy, atmospheric studies, remote sensing, and plasma diagnostics. Photonic crystal (PC) is a low-loss periodic dielectric medium. With special design and construct the PCs can control the propagation of THz wave in certain directions with specified frequencies. In this letter, we present the numerical design and analysis of three kinds of compact terahertz wave beam splitter based on photonic crystals structure. The novel terahertz wave polarizing beam splitter has been designed and calculated through finite element method. The simulation results show that the proposed polarizing beam splitter has high efficiency and a high extinction ratio. We confirm theoretically that the photonic crystal structures can be used for separating TE and TM-polarized modes of the electromagnetic waves in the terahertz range.

Terahertz radiations, which refer to the frequencies from 100GHz to 10THz, lie in the frequency gap between the infrared and microwave, have received considerable attention during the past decades. Due to their special prosperties, THz radiations have been applied in many fields such as gases, semiconductors, explosives materials, and environment pollutants. The technique is based on recording the time dependence of the electric field of a short electromagnetic pulse transmitted through a sample. The ratio of the Fourier transforms of the data recorded with and without the sample yields the complex transmission coefficient of the sample in the frequency domain. The absorption coefficient and the refractive index of the material studied are directly related to the amplitude and phase respectively of the transmitted field. Terahertz wave filter, a frequency-selective surface structure, has been characterized by terahertz time-domain spectroscopy in the region from 0.1 to 3THz. We have compared THz-TDS measurement and calculation results of the mode-matching theory of the terahertz wave filter, and find that the two data sets agree very closely. The peak of the transmittance of about 90.5%occurs at 0.45THz for the first case and the peak of the transmittance of about 89.4% occurs at 0.79THz for the second case.

Terahertz wave is a kind of electromagnetic wave ranging from 0.1~10THz, between microwave and infrared, which occupies a special place in the electromagnetic spectrum. Terahertz radiation has a strong penetration for many media materials and nonpolar substance, for example, dielectric material, plastic, paper carton and cloth. In recent years, researchers around the world have paid great attention on terahertz technology, such as safety inspection, chemical biology, medical diagnosis and terahertz wave imaging, etc. Transmission properties of two-dimensional metal microstrip structures in the terahertz regime are presented and tested. Resonant terahertz transmission was demonstrated in four different arrays of subwavelength microstrip structure patterned on semiconductor. The effects of microstrip microstrip structure shape were investigated by using terahertz time-domain spectroscopy system. The resonant terahertz transmission has center frequency of 2.05 THz, transmission of 70%.

With the realization of terahertz wave generator and detector, electromagnetic terahertz wave wavelength range (30m~3000m) attracted significant attention and has been extensively investigated. Many potential applications of terahertz waves have been dramatically explored including medical diagnosis, security screening, military detection, radio astronomy, atmospheric studies, and communication. In this paper, we proposed a kind of photonic crystal fiber with Panda eyes structure. Using finite element analysis method, the transmission properties of Panda eyes photonic crystal fiber in terahertz wave regime was analyzed. The results show that terahertz photonic crystal waveguide with a low transmission loss can be designed if there are suitable parameters. When the transmission frequency f=0.5THz and d/=0.8, the minimum transmission loss is less than 0.5dB/m.

The terahertz (THz) band, which refers to the spectral region between 0.1 and 10THz, covers the fingerprints of many chemical and biological materials. Within the past few years, there are increasing demands for experiments in terahertz frequencies, in different areas such as biotechnology, nanotechnology, space science, security, chemical and biological sensing, terahertz wave communications, and medical diagnostics. For potential applications, the functional devices, such as beam polarizers, switchs and filters, are crucial components for a terahertz system. Terahertz wave filter based on two kinds of microstrip resonant structures, has been characterized by terahertz time-domain spectroscopy in the region from 0.1 to 3THz. The experimental results for the frequency dependence of the transmittance of the terahertz wave filter show that the terahertz wave transmittance peak is of 79.5% at 0.5THz and 82.5% at 0.81THz.

There are increasing demands for experiments in terahertz frequencies, in different areas such as biotechnology, nanotechnology, space science, security, terahertz wave communications, and plasma diagnostics. For potential applications, the functional devices, such as beam polarizers, modulators and filters, are crucuial components for a terahertz system. As a dispensable device for ultrafast information processing and interconnection of terahertz wave communication, terahertz wave filter has attracted considerable attention. In this paper, we design the terahertz wave filter structure based on three kinds of photonic crystals structures. The finite-difference time-domain (FDTD) has been employed to analysis the performances of these terahertz wave filters. The simulation results show that these designed filters exhibit excellent transmission performance such as high transmission at the central frequency, adjustable bandpass, and good rejection of the sideband frequencies. Three kinds of compact and integrated terahertz wave filters are obtained.

Pulsed THz time-domain spectroscopy is a coherent technique, in which both the amplitude and the phase of a THz pulse are measured. Recently, material characterization using THz spectroscopy has been applied to biochemicals, pharmaceuticals, polymers and semiconductors and has given us important information. Moreover, THz imaging has progressed and is expected to be applicable for the identification of narcotics and explosives. The most important and characteristic point of THz spectroscopy is said to be its ability to observe intermolecular vibrations in contrast to infrared spectroscopy (IR), which observes intramolecular vibrations. Coherent detection enables direct calculations of both the imaginary and the real parts of the refractive index without using the Kramers-Kronig relations. Terahertz wave spectroscopy has been used to study the properties and absorption spectra characteristic of materials. In this paper, the spectral characteristics of cow skin, pig skin sheep skin, horse skin and deer skin have been measured with terahertz time-domain spectroscopy in the range of 0.1~2.0THz. The results show that THz-TDS technology provides an important tool for quality analysis and detection of leathers.

To find a kind nanoparticles can be used as additive in lubricants, oil-soluble calcium borate nanoparticles were prepared in the present of Na2B4O7.10H2O, CaCl2, and oleic acid as modifier by in-situ method, and the effects of different reaction conditions on the coil-solubility of the nanoparticles were investigated. The results indicate that the surface of calcium borate was altered from hydrophilicity to hydrophobicity after the modification with oleic acid, and so the nanoparticles possess high solubility in mineral base oils. Moreover, TT-IR confirms that the bond of oleic acid onto the surface of calcium borate nanoparticles exists.

To develop a potential substitute for conventional lubricant additive, pure calcium borate nanoparticles (PCBN) and calcium borate nanoparticles modified with oleic acid (MCBN) were prepared. The microstructures of the as-obtained samples were characterized using X-ray power diffraction (XRD), transmission electron microscope (TEM) and infrared spectra (IR), and the dispersing stability of oil containing nanoparticles was also evaluated. Tribological properties of MCBN used as lubricating additive were evaluated using four-ball tribometer and compared with that of PCBN. The results indicate that MCBN show better antiwear and friction-reducing property than PCBN. The worn surface of the steel ball lubricated with MCBN was investigated by Polarized microscope (PM) and X-ray photoelectron spectroscopy (XPS), and the action mechanism of MCBN during friction process was preliminarily discussed.

We designed a compact terahertz (THz) wave polarizing beam splitter based on a periodic bilayer structure, which operates over a wide THz wavelength range. Within a short length (about 1?mm), this polarizing beam splitter separates THz wave TE- and TM-polarized modes into orthogonal output waveguides. Results of simulations with the finite-element method show that 99.99% of the TE-polarized THz wave is deflected by the periodic bilayer structure (with 39.9?dB extinction ratio), whereas 99.58% of the TM-polarized THz wave propagates through the structure (with a 23.7?dB extinction ratio). Tolerance analysis reveals a large tolerance to fabrication errors.

A new power divider, composed of a novel composite right/left-handed (CRLH) transmission line (TL) unit, is proposed. The properties of the power divider based on four CRLH TL unit cells are investigated theoretically. By adjusting the parameters of the capacitors and the inductors, the power divider shows perfectly symmetric power division at 5.13 GHz, return loss up to −24 dB, with the transmitted power being close to −3.1 dB. The phenomena are demonstrated by simulation results. Being compact in size and low-cost, the proposed power divider is very suitable for microwave and millimeter wave integrated circuits.

This publisher’s note contains corrections to Opt. Lett.46, 290 (2021)OPLEDP0146-959210.1364/OL.412229.

In the past decade, spectroscopy and imaging in the terahertz region (0.1-10 THz) of the electromagnetic spectrum has been applied in both basic research and potential industrial applications, such as medical diagnosis, security screening, radio astronomy, atmospheric studies, short-range indoor communication, chemical, biological sensing, medical and biological imaging, and detection of explosives. In this paper, we design a narrow bandpass terahertz wave filter using three kinds of two-dimensional photonic crystals. By using finite-difference time-domain (FDTD) method, we examined the transmittance spectra for the proposed terahertz wave filter. The simulated results show that the proposed filter exhibit excellent transmission performance such as high transmission at the central frequency, adjustable bandpass, and good rejection of the sideband frequencies.

The terahertz transmission spectra of three different persistent organic pollutants (POPs) (aldrin, dieldrin, and endrin) are measured by using terahertz time-domain spectroscopy (THz-TDS) in the terahertz region of 0.2-1.8THz. The spectral dependence of the absorption for individual three kinds of POPs is extracted from the transmission data. Computational chemistry using the B3LYP density functional method is used to study structure and internal rotations in POPs, where results strongly suggest that frequencies of the POPs internal rotor correspond to the observed spectra. The absorption coefficient for the soil/aldrin mixture is 19.64cm-1at 1.41THz, 20.91cm-1at 1.57THz, 13.69cm-1at 1.78THz. For soil/dieldrin mixture, the frequency positions of prominent absorption features at 1.29, 1.4 and 1.56THz are observed. The absorption coefficient for the soil/dieldrin mixture is 16.73cm-1 at 1.29THz, 18.85cm-1at 1.4THz, 24.5cm-1at 1.56THz. There exhibits three strong absorption peaks coefficient of the soil/endrin mixture at the frequency of 1.58THz and 1.67THz. The absorption coefficient of the soil/endrin mixture is 6.24cm-1at 1.58THz, 12.58cm-1at 1.67THz. There is reasonably good agreement between theory and experiment. The results show that the THz-TDS can be used to study POPs in soil quality evaluation or safety inspection further.

We propose a novel Z-cut lithium niobate electro-optic modulator based on a Si2N2O substrate. The structure is analyzed using the finite element method. The novel modulator with 3-dB optical bandwidth, half-wave voltage, and characteristic impedance at 1.5-μm wavelength are 120 GHz, 3.5 V and 50.2□, respectively. The simulation results show that the modulator can achieve wide bandwidth, low half-wave voltage, and a good impedance match. The modulator has good potential for high-speed optical transmission systems.