To reduce the cost and achieve high diffraction efficiency, a modified moir@ technique for fabricating a large- aperture multi-level Fresnel membrane optic by a novel design of alignment marks is proposed. The modifie...To reduce the cost and achieve high diffraction efficiency, a modified moir@ technique for fabricating a large- aperture multi-level Fresnel membrane optic by a novel design of alignment marks is proposed. The modified moire fringes vary more sensitively with the actual misalignment. Hence, the alignment accuracy is significantly improved. Using the proposed method, a 20 μm thick, four-level Fresnel diffractive polyimide membrane optic with a 200 mm diameter is made, which exhibits over 62% diffraction efficiency into the ＋1 order, and an efficiency root mean square of 0.051.展开更多
The numerical moire method with sensitivity as high as 0.03 nm has been presented. A quantitative displacement and strain analysis program has been proposed by using this method. It is applied to an edge dislocation a...The numerical moire method with sensitivity as high as 0.03 nm has been presented. A quantitative displacement and strain analysis program has been proposed by using this method. It is applied to an edge dislocation and a stacking fault in aluminum. The measured strain of edge dislocation is compared with theoretical prediction given by Peierls-Nabarro dislocation model. The displacement of stacking fault is also obtained.展开更多
A simple and cost-effective multi-wavelength fiber ring laser based on a chirped Moire fiber grating （CMFG） and a semiconductor optical amplifier （SOA） is proposed. Stable triple-wavelength lasing oscillations at ...A simple and cost-effective multi-wavelength fiber ring laser based on a chirped Moire fiber grating （CMFG） and a semiconductor optical amplifier （SOA） is proposed. Stable triple-wavelength lasing oscillations at room temperature are experimentally demonstrated. The measured optical signal-to-noise ratio （SNR） reaches the highest value of 50 dB and the power fluctuation of each wavelength is less than 0.2 dB within a 1-h period. To serve as a wavelength selective element, the CMFG possesses excellent comb-like filtering characteristics including stable wavelength interval and ultra-narrow passband, and its fabrication method is easy and flexible. The lasing oscillation shows a narrower bandwidth than SOA-based multi-wavelength fiber lasers utilizing some other kinds of wavelength selective components. Methods to optimize the laser performance are also discussed.展开更多
Many studies show that semi-arid rangelands throughout the world have been rapidly converted from a grassland state to a bush encroachment state during the past 50 years. Bush encroachment includes the spread of local...Many studies show that semi-arid rangelands throughout the world have been rapidly converted from a grassland state to a bush encroachment state during the past 50 years. Bush encroachment includes the spread of local woody species and/or incursion of woody species introduced from other ecosystems into semi-arid savannas and grassland ecosystems. Rangeland degradation due to bush encroachment causes several challenges, affecting the production of livestock and pastoral people livelihoods in most parts of Africa. Scientists have long been attempting to develop schematic and mathematical theories to explain the observed phenomenon of bush encroachment, and several theories were proposed and developed. The well-regarded theories include： （1） Waiter＇s two-layer model, （2） Moir＇s one-layer model, （3） state-and-transition theory, （4） equilibrium theory, （5） disequilibrium theory, and （6） non-equilibrium theory. Within those theories, the most frequently-indicated driving factors that explain bush encroachment include over-grazing, availability of soil nutrient and moisture, elevated CO2 levels, frequency and intensity of fire, spread of seeds of woody species by livestock and wild animals. It should be stressed that couplings and interactions among diverse driving factors are more often at work in determining the condition of bush encroachment. To summarize, the effort in managing semi-arid ecosystems needs critical knowledge to understand the cause-effect relationships of underlying factors through integrated approach. Therefore, future research on encroachment of woody plants should be multi-discipline oriented and multi-partnership involved.展开更多
The recent discovery of an unconventional insulating phase and an adjacent superconducting (SC) phase in the twisted bilayer graphene (TBG)[1,2] has triggered great excitement (3–11)The two layers of graphene are rot...The recent discovery of an unconventional insulating phase and an adjacent superconducting (SC) phase in the twisted bilayer graphene (TBG)[1,2] has triggered great excitement (3–11)The two layers of graphene are rotated relatively by an angle θ.展开更多
We design and demonstrate an all-optical temporal differentiator based on a simple Moire fiber grating operated in reflection. The simulation results prove that a single Moire fiber grating with only one π-phase shif...We design and demonstrate an all-optical temporal differentiator based on a simple Moire fiber grating operated in reflection. The simulation results prove that a single Moire fiber grating with only one π-phase shifted point can act as the first-order temporal differentiator and that a Moire fiber grating incorporating two symmetrical π-phase shifted points can act as the second-order temporal differentiator. A practical Moire fiber grating is fabricated, thereby proving that such a grating can act as the first-order temporal differentiator. Our results verify the feasibility, flexibility, and accuracy of the proposed method.展开更多
A geometrical moir method for displaying directly isochromatics and isoclinics of a diametrically pressed circular disk is presented. It is demonstrated that by using two identical or different Wulff nets (or grids) n...A geometrical moir method for displaying directly isochromatics and isoclinics of a diametrically pressed circular disk is presented. It is demonstrated that by using two identical or different Wulff nets (or grids) not only the stress fields of a circular disk can be displayed, but the u -and v -displacement fields can also be produced.展开更多
A polarization modulator based on splitting with a Savart plate and rotation of an analyzer for a moiré system with grating imaging is presented, and its modulation principle is analyzed. The polarization modulat...A polarization modulator based on splitting with a Savart plate and rotation of an analyzer for a moiré system with grating imaging is presented, and its modulation principle is analyzed. The polarization modulator is simple and achromatic. It is composed of a polarizer, a Savart plate, and an analyzer. The polarizer and the Savart plate are placed in front of the index grating to split the image of the scale grating in the moiré system. The analyzer is placed behind the grating and rotated to realize the modulation of the moiré signal. The analyzer can be rotated either continually with high speed or step by step with low speed to form different modulation modes. The polarization modulator makes the moiré system insensitive to the change of initial intensity. In experiments, we verified the usefulness of the polarization modulator.展开更多
We report on mid-infrared superabsorbers based on quasi-periodic moiré metasurfaces in metal-insulator-metal form. By varying the spacer thickness, moiré rotation angle, and filling factor of the superabsorb...We report on mid-infrared superabsorbers based on quasi-periodic moiré metasurfaces in metal-insulator-metal form. By varying the spacer thickness, moiré rotation angle, and filling factor of the superabsorbers, we can tune narrowband or broadband absorption in a systematic way. With their high tunability of near-unity absorption and simple fabrication, in combination with decoupled mode theory for an efficient design, moiré superabsorbers are well-suited for a wide range of applications in sensing, imaging, and communication.展开更多
基金supported by the National Natural Science Foundation of China under Grant No.11375175
文摘To reduce the cost and achieve high diffraction efficiency, a modified moir@ technique for fabricating a large- aperture multi-level Fresnel membrane optic by a novel design of alignment marks is proposed. The modified moire fringes vary more sensitively with the actual misalignment. Hence, the alignment accuracy is significantly improved. Using the proposed method, a 20 μm thick, four-level Fresnel diffractive polyimide membrane optic with a 200 mm diameter is made, which exhibits over 62% diffraction efficiency into the ＋1 order, and an efficiency root mean square of 0.051.
基金the National Natural Science Foundation of China under Grants No.10562003 and 10272054
文摘The numerical moire method with sensitivity as high as 0.03 nm has been presented. A quantitative displacement and strain analysis program has been proposed by using this method. It is applied to an edge dislocation and a stacking fault in aluminum. The measured strain of edge dislocation is compared with theoretical prediction given by Peierls-Nabarro dislocation model. The displacement of stacking fault is also obtained.
基金supported by the National Natural Science Foundation of China (No.60771008)the National "863" Program of China (No.2007AA01Z258)+1 种基金the Support Plan for New Century Excellent Talents (No.NCET-05-0091)the Science Fund of Beijing Jiaotong University (No.2007XM003)
文摘A simple and cost-effective multi-wavelength fiber ring laser based on a chirped Moire fiber grating （CMFG） and a semiconductor optical amplifier （SOA） is proposed. Stable triple-wavelength lasing oscillations at room temperature are experimentally demonstrated. The measured optical signal-to-noise ratio （SNR） reaches the highest value of 50 dB and the power fluctuation of each wavelength is less than 0.2 dB within a 1-h period. To serve as a wavelength selective element, the CMFG possesses excellent comb-like filtering characteristics including stable wavelength interval and ultra-narrow passband, and its fabrication method is easy and flexible. The lasing oscillation shows a narrower bandwidth than SOA-based multi-wavelength fiber lasers utilizing some other kinds of wavelength selective components. Methods to optimize the laser performance are also discussed.
文摘Many studies show that semi-arid rangelands throughout the world have been rapidly converted from a grassland state to a bush encroachment state during the past 50 years. Bush encroachment includes the spread of local woody species and/or incursion of woody species introduced from other ecosystems into semi-arid savannas and grassland ecosystems. Rangeland degradation due to bush encroachment causes several challenges, affecting the production of livestock and pastoral people livelihoods in most parts of Africa. Scientists have long been attempting to develop schematic and mathematical theories to explain the observed phenomenon of bush encroachment, and several theories were proposed and developed. The well-regarded theories include： （1） Waiter＇s two-layer model, （2） Moir＇s one-layer model, （3） state-and-transition theory, （4） equilibrium theory, （5） disequilibrium theory, and （6） non-equilibrium theory. Within those theories, the most frequently-indicated driving factors that explain bush encroachment include over-grazing, availability of soil nutrient and moisture, elevated CO2 levels, frequency and intensity of fire, spread of seeds of woody species by livestock and wild animals. It should be stressed that couplings and interactions among diverse driving factors are more often at work in determining the condition of bush encroachment. To summarize, the effort in managing semi-arid ecosystems needs critical knowledge to understand the cause-effect relationships of underlying factors through integrated approach. Therefore, future research on encroachment of woody plants should be multi-discipline oriented and multi-partnership involved.
基金supported by the National Key R&D Program of China (2018YFA0305800)National Natural Science Foundation of China (11804337)+1 种基金Strategic Priority Research Program of CAS (XDB28000000)Beijing Municipal Science & Technology Commission (Z181100004218001)
文摘The recent discovery of an unconventional insulating phase and an adjacent superconducting (SC) phase in the twisted bilayer graphene (TBG)[1,2] has triggered great excitement (3–11)The two layers of graphene are rotated relatively by an angle θ.
基金supported by the National Natural Science Foundation of China (No. 60871067)the National Natural Science Association Foundation of China (No. 11076028)the Projects of Science and Technology Commission of Shanghai Municipality (No.08DZ2230400)
文摘We design and demonstrate an all-optical temporal differentiator based on a simple Moire fiber grating operated in reflection. The simulation results prove that a single Moire fiber grating with only one π-phase shifted point can act as the first-order temporal differentiator and that a Moire fiber grating incorporating two symmetrical π-phase shifted points can act as the second-order temporal differentiator. A practical Moire fiber grating is fabricated, thereby proving that such a grating can act as the first-order temporal differentiator. Our results verify the feasibility, flexibility, and accuracy of the proposed method.
文摘A geometrical moir method for displaying directly isochromatics and isoclinics of a diametrically pressed circular disk is presented. It is demonstrated that by using two identical or different Wulff nets (or grids) not only the stress fields of a circular disk can be displayed, but the u -and v -displacement fields can also be produced.
文摘A polarization modulator based on splitting with a Savart plate and rotation of an analyzer for a moiré system with grating imaging is presented, and its modulation principle is analyzed. The polarization modulator is simple and achromatic. It is composed of a polarizer, a Savart plate, and an analyzer. The polarizer and the Savart plate are placed in front of the index grating to split the image of the scale grating in the moiré system. The analyzer is placed behind the grating and rotated to realize the modulation of the moiré signal. The analyzer can be rotated either continually with high speed or step by step with low speed to form different modulation modes. The polarization modulator makes the moiré system insensitive to the change of initial intensity. In experiments, we verified the usefulness of the polarization modulator.
基金financial support of the Office of Naval Research Young Investigator Program (No. N00014-17-1-2424)
文摘We report on mid-infrared superabsorbers based on quasi-periodic moiré metasurfaces in metal-insulator-metal form. By varying the spacer thickness, moiré rotation angle, and filling factor of the superabsorbers, we can tune narrowband or broadband absorption in a systematic way. With their high tunability of near-unity absorption and simple fabrication, in combination with decoupled mode theory for an efficient design, moiré superabsorbers are well-suited for a wide range of applications in sensing, imaging, and communication.
