By analyzing monitoring data of atmospheric pollutions in Meiyu season measured by AML-2 mobile lidar, we study the temporal and spatial distribution characteristics of aerosol and ozone in troposphere over Hefei in Meiyu season, and analyze the effect of precipitation on pollution reduction. The results show that the aerosol extinction coefficient is small in Meiyu season and decreases with the increase of altitude on the whole. At 0.5 km, the extinction coefficient is from 0.1 km(-1) to 0.18 km(-1) in multiple days in Meiyu season. Continuous precipitation plays a significant role in reducing aerosol concentration, the mean aerosol extinction coefficient before Meiyu season and in Meiyu season is 0.37 km(-1) and 0.14 km(-1), respectively. The temporal and spatial variation characteristics of ozone are obvious in Meiyu season. The ozone concentration decreases with the increase of altitude and shows large daily variation. In June 20 and 24, 2008, the difference of mass concentration of ozone at 0.4 km is about 59.5 mu g/m(3). Compared with concentration of ozone before Meiyu season, the concentration in Meiyu season is greatly reduced with the maximum difference up to 41.8 mu g/m(3) at the same height.

It is well known that the polarization of a linearly polarized (LP) light would rotate after passing through a single layer graphene under the bias of a perpendicular magnetostatic field. Here we show that a corresponding phase shift could be expected for circularly polarized (CP) light, which can be engineered to design the circular polarization sensitive devices. We theoretically validate that an ultrathin graphene-based absorber with the thickness about λ/76 can be obtained, which shows efficient absorption >90% within incident angles of ±80°. The angle-independent phase shift produced by the graphene is responsible for the nearly omnidirectional absorber. Furthermore, a broadband absorber in frequencies ranging from 2.343 to 5.885 THz with absorption over 90% is designed by engineering the dispersion of graphene.

Terahertz (THz) absorber with dynamically tunable bandwidth possesses huge application value in the field of switches, sensors and THz detection. However, the perfect absorbers based on traditional metamaterials are not intelligent enough to capture the electromagnetic wave in a tunable way. We utilized monolayer graphene and cross-shaped metallic sub-wavelength structure to design broadband absorber with tunable absorption frequency in terahertz regime. The absorption frequency can be turned by changing the chemical potential of graphene which can be easily controlled by the bias voltage supplied to graphene. Simulation results show that 1.3 THz average bandwidth was achieved. And the almost perfect absorption shifted from 2.04 THz~3.53 THz to 3.15 THz~4.24 THz continuously.

Observations of monthly and seasonal nightly water vapor variations over Hefei utilizing L625 lidar water vapor data observed from 2000 to 2008 is the focus of this study. The experimental setup and main parameters of the L625 lidar for water vapor measurement are first presented, then the measurement principle of water vapor and data processing methods are introduced. The water vapor measurement precision of the lidar system was analyzed by comparison with radiosonde. Monthly and seasonal water vapor profiles were built by analyzing 2000–2008 lidar data. In the vertical direction, results show that water vapor content decreases gradually with height. The more the water vapor content in the low atmosphere, the faster the decay rate with altitude. As far as monthly variation, the water vapor content first increases and then decreases with month. The maximum content of water vapor appears in July, at mixing ratio of 15.6g/kg at 1km. The seasonal variability of water vapor content is rather obvious. In summer the water vapor mixing ratio reaches up to 15.0g/kg at 1km, and in winter it is only 3.9g/kg at the same altitude. Interannual variation of water vapor content differs between seasons (as revealed in the standard deviation of data) where summer is least stable and autumn is the most stable. Precipitable water vapor is calculated from water vapor mean profiles at 1–4km and the relationship between precipitable water vapor and precipitation is also investigated. A clear positive correlation is found with Pearson correlation coefficients (R) 0.933 between monthly precipitation and mean precipitable water vapor, as well a clear positive correlation between seasonal precipitation and seasonal mean precipitable water vapor (R=0.988). Precipitation conversion efficiency (PCE) is calculated from precipitation and precipitable water vapor. The monthly PCE reaches its maximum in October at 25.8%, and drops to its minimum in January at 11.5%. Seasonal PCE's minimum is 15.2% in autumn and 23.7% in winter, at maximum.

In this paper, we demonstrate the design of a low-scattering metamaterial shell with strong backward scattering reduction and a wide bandwidth at microwave frequencies. Low echo is achieved through cylindrical wave expanding theory, and such shell only contains one metamaterial layer with simultaneous low permittivity and permeability. Cut-wire structure is selected to realize the low electromagnetic (EM) parameters and low loss on the resonance brim region. The full-model simulations show good agreement with theoretical calculations, and illustrate that near -20dB reduction is achieved and the -10 dB bandwidth can reach up to 0.6 GHz. Compared with the cloak based on transformation electromagnetics, the design possesses advantage of simpler requirement of EM parameters and is much easier to be implemented when only backward scattering field is cared.

In this paper, the limitation of using (jω)ν as the ideal frequency response of fractional order digital differentiators for non-bandlimited signals is discussed. High frequency error enhanced by (jω)ν, along with the cause of time domain response degradation, is presented. Windows are proposed, which can help to improve the time domain response of fractional order digital differentiator and greatly reduce the filter order if the differentiator is approximated by finite impulse response (FIR) filter. Simulation results show that windowing the output of fractional order digital differentiator in frequency domain is effective in improving the time domain response of signals.

ABSTRACT A method of utilizing frequency selective surface (FSS) to reduce side lobe level (SLL) is proposed. The FSS composed of annular slot arrays loaded with resistors is divided into several regions along the electric field direction. Each region has difference transmission amplitude (TA) but the same transmission phase (TP). Through the appropriate distribution design for each region, the output wave with tapered TA and uniform TP is obtained when the FSS is illuminated by a plane wave. Thus the FSS could be utilized to reduce the antenna SLL. An H‐plane sectorial horn array is adopted as an exciting source to validate its SLL reduction performance. Simulation results show that the proposed FSS superstrate makes the SLL of the horn array in the E‐plane desirably suppressed from −12.4 to −25.9 dB at 10.3GHz with the gain reduced from 18.3 to 16.7 dBi. The measured results agree well with the simulated ones. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1971–1975, 2015

The influence of Al3+ and P5+ ion contents on the redox states of ytterbium ion and the redox mechanisms were investigated, and the dispersion effect of Al3+ and P5+ ions on Yb3+ ions was also discussed.

A novel facet coating technology is presented by studying catastrophic optical mirror damage mechanism of semiconductor laser. In this technology, semiconductor laser are cleaved in the air, and the surface oxide layer is removed with a low energy ion source, immediately flowed by coating the facet with 20nm of thin ZnSe passivation layer. The function of the passivation layer is to protect semiconductor laser facet, and prevent impurity particles diffusing to the facet. Finally the facet is coated with oxidative optical film. The test results of semiconductor laser output power show that output power with the coated ZnSe passivation layer method is 12% higher than coated Si passivation layer, and 36% higher than that coated oxidative optical film. The device coated oxidative optical film is failed when current is 4.1A, and the device coated with Si passivation layer is failed when current is 4.8A, the final failed of the device is coated ZnSe passivation layer. In conclusion, the method of coated ZnSe passivation layer on the semiconductor laser facet can effectively prevent the catastrophic optical mirror damage, and increase the output power of semiconductor lasers.

A method of using frequency selective surface (FSS) to dynamically control beamwidth of antenna is proposed. The unit cell is composed of a two-layer periodic H-shape structure loaded with varactors. By tuning the bias voltage applied to the varactors, the unit cell has two different states that are transparent and absorptive to the incident wave at a certain frequency, respectively. We can control the transparent region of the whole FSS through reasonably setting the capacitance of the varactor loaded on each unit cell, and thus the beamwidth of the transmitted wave could be tuned. The above property has been validated by putting the FSS above the horn antenna array as superstrate. Simulation results show that E-plane beamwidth of the antenna can be enlarged from 13.2 degrees to 31.1 degrees at 5.5 GHz, and experimental results agree well with the simulated ones. In addition, it is found that the operation frequency for beamwidth control can be tuned as well.

Recently, the graphene-based absorber captures much attention due to its bright potential. In this paper, a modeling study of graphene-based absorber is made for variable magnetostatic bias, i.e., the magnetic circular dichroism (MCD). A strong MCD signal about ΔA = 0.94 is obtained in 7 T for a uniform graphene-based absorber. By periodically patterning subwavelength holes on the graphene, the structured absorber exhibits two obvious peaks induced by cavity resonance and localized resonance of meta-surface in 0 T, respectively. As the magnetic field increases, the absorption peaks show quite different electromagnetic response for the two kinds of circularly polarized lights. Two distinct MCDs signals of 0.711 at 2.02 THz and 0.91 at 2.49 THz in 7 T are observed. The results are well explained by a modified equivalent circuit model.

We propose an efficient way to realize Fano-type resonances at optical frequencies based on a low-loss dielectric nanorod array. An ultrahigh Q factor (larger than 10000) is numerically demonstrated, which is attributed to the mode interference between the broadband Fabry–Perot (FP) resonance and the narrowband guided mode stemming from coupled quadrupoles. A wide gap region for field enhancement is formed between adjacent rods, making such a structure an ideal platform for related applications such as biological sensing and nonlinear devices.

The feasibility of 360 ∘ scanning Luneburg lens based on electric-controlled metamaterial is analyzed. When the line source is fixed in the center of the lens, the direction of radiated beam can be tuned by adjusting the bias voltages distribution applied to the varactors in each unit cell. By introducing an artificial reflective plane, the gain of the antenna can be further enhanced. The minimal step of scanning angle obtained in simulation is only 7 ∘ , which can be further decreased by increasing the dimension and the number of unit cell used in Luneburg lens.

In this paper, a novel design of metamaterial is reported for realizing low sidelobe level (SLL) in the antenna array. This metamaterial is divided into some rows which are composed of periodic annular slots with different transmission controlled by its dimension. The transmission distribution of all the rows can be designed as Taylor amplitude distribution (TAD), by properly adjusting the dimension of annular slots. To prove the validity of this metamaterial, it is located above the traditional linear array with uniform amplitude and phase as surperstrate. The simulation result shows that the employment of metamaterial can effectively reduce the SLL of the antenna array from -13.4dB to -23.6dB at 10.284GHz, which is very close to the designed SLL of -26dB. This approach has some potential application in the antenna arrays, avoiding complicated design of unequal feeding network.

As highlighted by recent articles [Phys. Rev. Lett. 105, 053901 (2010) and Science 331, 889-892 (2011)], the coherent control of narrowband perfect absorption in intrinsic silicon slab has attracted much attention. In this paper, we demonstrate that broadband coherent perfect absorber (CPA) can be achieved by heavily doping an ultrathin silicon film. Two distinct perfect absorption regimes are derived with extremely broad and moderately narrow bandwidth under symmetrical coherent illumination. The large enhancement of bandwidth may open up new avenues for broadband applications. Subsequently, interferometric method is used to control the absorption coherently with extremely large contrast between the maximum and minimum absorptance. Compared with the results in literatures, the thin film CPAs proposed here show much more flexibility in both operation frequency and bandwidth.

Highly efficient absorber is of particular importance in terahertz regime as naturally occurring materials with frequency-selective absorption in this frequency band is difficult to find. Here we present the design and characterization of a broadband terahertz absorber based on heavily Boron-doped silicon (0.7676 Ω cm) grating. It is numerically demonstrated by utilizing both the zero- and first order diffraction in the doped silicon wafer, relative absorption bandwidth larger than 100% can be achieved. Furthermore, the design can be easily extended to higher frequencies as the optical property of doped silicon is tunable through changing the doping concentration.

The location, structure, working principle and the assembling method of a reflex light-tube in a photoel'ectric pointing system are introduced, meanwhile the effects of installation error and assembling error of the reflex light-tube between two plane mirrors on the accuracy of azimuth transfer are analyzed. Matlab soft- ware is used to compute and draw the statistic histograms of the azimuth transfer error and the elevation error, and the results provide a guidance for the assembling reflex fight-tube. According to the results from calcula- tion and analysis, the main factor affecting azimuth transfer error of reflex light-tube is the assembling error be- tween the two plane mirrors in the reflex light-tube. As long as the assemble error between the two plane mir- rors satisfies the technical requirements of azimuth error ≤4" and elevation error ≤ 0. 5', the azimuth transfer error of the reflex light-tube can be ≤ 10" even the installation error extrema in three directions of the reflex light-tube reach 15°

We demonstrated numerically a multilayered broadband nearly perfect absorber in terahertz frequencies. The structure is composed of multilayered doped silicon film. Transfer matrix method and genetic algorithm are utilized to simulate and optimize the absorption property. The 20 dB attenuation bandwidth of this structure is larger than 94%, while 40 dB attenuation bandwidth is also larger than 50%. Compared with previous multilayered absorbers based on resistive metal, the structure proposed here shows better performance and is easier to fabricate.

An electrical tunable L-band absorbing material for two polarisations is presented. The proposed absorber consists of a metal resonator and four surrounded metal lines which are connected by pin diodes, and exhibits the tunable range of reflectivity reaching to -40 dB for both polarisations. In particular, for TM polarisation, the working frequency is also tunable from 1.9 to 2.1 GHz due to the conversion between two distinct resonance modes supported by the resonator and surrounding lines, respectively.