Tang Guo-Ying, Sun Zhu-Ling, Jiang Ru-Bin, Li Feng-Quan, Liu Ming-Yuan, Liu Kun, and Qie Xiu-Shu
Characteristics of a triggered bipolar lightning flash obtained in the Shandong triggering lightning experiment (SHATLE) are analyzed based on simultaneous observation results of lightning very high frequency (VHF) interferometer map, channel-base current, fast electric field change and high-speed optical images. The flash lasts about 315 ms with only the initial continuous current (ICC). As the polarity of charges transferred to the ground varies from negative to positive and then to negative, the ICC can be mainly divided into three stages including the first negative ICC stage, positive ICC stage, and second negative ICC stage, respectively, for 152.755 ms, 87.225 ms and 75.02 ms. Charges transferred to the ground during the three stages are about –40.0 C, +13.3 C and –1.0 C, respectively, with the peak current of about –3.8 kA, 1.6 kA and –2.25 kA. According to the VHF interferometer map, during the first negative ICC stage, the upward positive leader (UPL) initiated from the top of the wire as negative charges starts to be transferred to the ground. The UPL develops at a two-dimensional (2D) speed of 3.7 × 104 m/s into the cloud and multiple positive branches develop in the form of small-scale and dense recoil leaders, keeping the increase of negative charge transferred to the ground. Then a negative leader (NL) initiates on a previously ionized positive branch channel and develops into the virgin air horizontally as a floating channel at a 2D propagation speed of 1.59 × 105 m/s. About 28.816 ms later after the NL develops, a negative pulse is detected in the fast electric field change, caused by a negative polarity breakdown discharge from the grounding trunk channel to the floating channel which is observed for the first time. Then about 39 μs later, the first polarity of the channel-base current changes from negative to positive, and rapidly reaches a positive peak in 1.75 ms. Subsequently, with the positive ICC decreasing, the negative leader gradually terminates about 65.85 ms after the first polarity reversal, and then 21.38 ms later the channel-base current slowly changes to the recognizable negative polarity as the second polarity reversal. A recoil leader generating in a previously ionized positive branch channel connects to the trunk channel, resulting in the initial continuous current pulse (ICCP) during the second negative ICC stage. Then several negative recoil leaders occur, tracing back to the previous positive channels without obvious current changes until the flash ends. Based on the analysis, the positive branch channel persistently transfers negative charge to the ground in the whole discharge process by the positive breakdown on the tip or the negative recoil leaders retrograding along the previous positive channels. The trunk channel is connected to the floating channel through negative breakdown discharges, linking at the positive charge accumulation area at the tail end of the negative leader or the positive polar end of a bidirectional leader towards the trunk channel, if the negative leader develops as a bidirectional leader later. Then, the net charge transferred to the ground is dominantly positive and the reversal of first channel current polarity occurs. With the negative leader disappearing, the supply of positive charge ceases, so the current polarity reverses again (the second reversal). Connection of the negative leader to the grounding trunk channel and continuous development of the positive leader are inferred to play an important role in reversing the two current polarities. In this case, the negative leader developing in virgin air might be initiated transversely in an ionized positive channel or from the end of a decayed positive leader branch of the ionized positive channel which is small and undistinguishable from the VHF interferometer map.Characteristics of a triggered bipolar lightning flash obtained in the Shandong triggering lightning experiment (SHATLE) are analyzed based on simultaneous observation results of lightning very high frequency (VHF) interferometer map, channel-base current, fast electric field change and high-speed optical images. The flash lasts about 315 ms with only the initial continuous current (ICC). As the polarity of charges transferred to the ground varies from negative to positive and then to negative, the ICC can be mainly divided into three stages including the first negative ICC stage, positive ICC stage, and second negative ICC stage, respectively, for 152.755 ms, 87.225 ms and 75.02 ms. Charges transferred to the ground during the three stages are about –40.0 C, +13.3 C and –1.0 C, respectively, with the peak current of about –3.8 kA, 1.6 kA and –2.25 kA. According to the VHF interferometer map, during the first negative ICC stage, the upward positive leader (UPL) initiated from the top of the wire as negative charges starts to be transferred to the ground. The UPL develops at a two-dimensional (2D) speed of 3.7 × 104 m/s into the cloud and multiple positive branches develop in the form of small-scale and dense recoil leaders, keeping the increase of negative charge transferred to the ground. Then a negative leader (NL) initiates on a previously ionized positive branch channel and develops into the virgin air horizontally as a floating channel at a 2D propagation speed of 1.59 × 105 m/s. About 28.816 ms later after the NL develops, a negative pulse is detected in the fast electric field change, caused by a negative polarity breakdown discharge from the grounding trunk channel to the floating channel which is observed for the first time. Then about 39 μs later, the first polarity of the channel-base current changes from negative to positive, and rapidly reaches a positive peak in 1.75 ms. Subsequently, with the positive ICC decreasing, the negative leader gradually terminates about 65.85 ms after the first polarity reversal, and then 21.38 ms later the channel-base current slowly changes to the recognizable negative polarity as the second polarity reversal. A recoil leader generating in a previously ionized positive branch channel connects to the trunk channel, resulting in the initial continuous current pulse (ICCP) during the second negative ICC stage. Then several negative recoil leaders occur, tracing back to the previous positive channels without obvious current changes until the flash ends. Based on the analysis, the positive branch channel persistently transfers negative charge to the ground in the whole discharge process by the positive breakdown on the tip or the negative recoil leaders retrograding along the previous positive channels. The trunk channel is connected to the floating channel through negative breakdown discharges, linking at the positive charge accumulation area at the tail end of the negative leader or the positive polar end of a bidirectional leader towards the trunk channel, if the negative leader develops as a bidirectional leader later. Then, the net charge transferred to the ground is dominantly positive and the reversal of first channel current polarity occurs. With the negative leader disappearing, the supply of positive charge ceases, so the current polarity reverses again (the second reversal). Connection of the negative leader to the grounding trunk channel and continuous development of the positive leader are inferred to play an important role in reversing the two current polarities. In this case, the negative leader developing in virgin air might be initiated transversely in an ionized positive channel or from the end of a decayed positive leader branch of the ionized positive channel which is small and undistinguishable from the VHF interferometer map.
- Jan. 05, 2021
- Acta Physica Sinica
- Vol.69 Issue, 18 189201-1 (2020)
- DOI：10.7498/aps.69.20200374
Liu Zhi-Wei, Zhang Bin, and Chen Yu
To achieve simultaneous protection against both pulsed and continuous wave (CW) or quasi-CW lasers, significant research effort has been devoted to the state-of-the-art optical limiting (OL) materials and processes in an attempt to achieve some measures of protection against such laser beams in the past decades. Two-dimensional (2D) nanomaterials with a lot of unique properties, including graphene, transition metal dichalcogenides, black phosphorus and others, have aroused the extensive research interest of many researchers. In this review paper, we describe systematically the OL mechanisms and the recent achievements in the 2D nanomaterials and their organic/polymeric derivatives for laser protection. In an effort to sustain the advantage of 2D nanomaterials, one can not only introduce the functional molecules or polymers to blend with them to form a complex multi-phase material system, but also embed the soluble 2D nanosheets covalently functionalized with organic/polymeric materials in a polymer host to form host-guest composite materials that are expected to improve the OL performance of the whole system. All in all, an optimized complex multi-component nanomaterial system enormously enhances the performance and applicability of OL devices. In addition, the fundamental studies of the photophysical and photonic properties of 2D nanomaterials and their derivatives in various solid hosts are of significance for modifying the nanomaterials at a molecular level.To achieve simultaneous protection against both pulsed and continuous wave (CW) or quasi-CW lasers, significant research effort has been devoted to the state-of-the-art optical limiting (OL) materials and processes in an attempt to achieve some measures of protection against such laser beams in the past decades. Two-dimensional (2D) nanomaterials with a lot of unique properties, including graphene, transition metal dichalcogenides, black phosphorus and others, have aroused the extensive research interest of many researchers. In this review paper, we describe systematically the OL mechanisms and the recent achievements in the 2D nanomaterials and their organic/polymeric derivatives for laser protection. In an effort to sustain the advantage of 2D nanomaterials, one can not only introduce the functional molecules or polymers to blend with them to form a complex multi-phase material system, but also embed the soluble 2D nanosheets covalently functionalized with organic/polymeric materials in a polymer host to form host-guest composite materials that are expected to improve the OL performance of the whole system. All in all, an optimized complex multi-component nanomaterial system enormously enhances the performance and applicability of OL devices. In addition, the fundamental studies of the photophysical and photonic properties of 2D nanomaterials and their derivatives in various solid hosts are of significance for modifying the nanomaterials at a molecular level.
- Jan. 05, 2021
- Acta Physica Sinica
- Vol.69 Issue, 18 184201-1 (2020)
- DOI：10.7498/aps.69.20200313
Wang Ju-Shan, Ma Jin-Peng, Zhao Xiang-Yong, Chen Ming-Zhu, Wang Fei-Fei, Wang Tao, Tang Yan-Xue, Cheng Wei, Lin Di, and Luo Hao-Su
Relaxor ferroelectric single crystal piezoelectric materials have become the core components of new piezoelectric devices such as ultrasonic transducers used in high-end medical ultrasound diagnostic and therapeutic equipment. High-element density array technology and micro-electro-mechanical systems have developed rapidly. For the new generation of 20–80 MHz medical high-frequency ultrasound transducers, the thickness of high-frequency piezoelectric composite material is usually 20–60 μm, and the width of each piezoelectric column is about 5–15 μm. However, the kerf of traditional cutting-and-filling method is too wide, and it is difficult to reduce the size of the array element, which is not conducive to the density of the array element and the demand for higher frequency applications with higher resolution. In this work, a micromechanical fabrication method based on deep reactive ion etching is used to reduce the slit width and increase the array density. We study the fabrication technology of novel and high-performance relaxor ferroelectric single crystal Mn doped Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (Mn-PIMNT) micro scale piezoelectric array. The influence of the parameters of lithography and deep reactive ion etching on the morphology of piezoelectric array are studied. We obtain the formation mechanisms of different kerfs, different shapes of piezoelectric array element and the relationship among etching rate of Mn-PIMNT single crystal with antenna power, bias power and etching gas ratio. Finally, the size of piezoelectric array element is less than 10 μm, the etching depth is more than 20 μm, the kerf width is less than 5 μm, the angle is controllable, and the maximum is more than 87°. The ferroelectric domain structure and the regulation of electric field effect of micro scale piezoelectric elements are studied by means of piezoelectric force microscope. The variation rules of piezoelectric properties and micro scale are obtained. This method can effectively bypass the shortcomings of the wide kerf and the destruction of the crystal orientation by the traditional cutting-and-filling method. It provides a new preparation technology for the development of high-frequency piezoelectric composites, high-density ultrasonic transducer arrays and new piezoelectric micro mechanical systems. This project presents the guidance and reference for the new micromachining technology of ferroelectric materials, and also lays the foundation for the high-frequency piezoelectric composite and high-frequency ultrasonic transducer.Relaxor ferroelectric single crystal piezoelectric materials have become the core components of new piezoelectric devices such as ultrasonic transducers used in high-end medical ultrasound diagnostic and therapeutic equipment. High-element density array technology and micro-electro-mechanical systems have developed rapidly. For the new generation of 20–80 MHz medical high-frequency ultrasound transducers, the thickness of high-frequency piezoelectric composite material is usually 20–60 μm, and the width of each piezoelectric column is about 5–15 μm. However, the kerf of traditional cutting-and-filling method is too wide, and it is difficult to reduce the size of the array element, which is not conducive to the density of the array element and the demand for higher frequency applications with higher resolution. In this work, a micromechanical fabrication method based on deep reactive ion etching is used to reduce the slit width and increase the array density. We study the fabrication technology of novel and high-performance relaxor ferroelectric single crystal Mn doped Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (Mn-PIMNT) micro scale piezoelectric array. The influence of the parameters of lithography and deep reactive ion etching on the morphology of piezoelectric array are studied. We obtain the formation mechanisms of different kerfs, different shapes of piezoelectric array element and the relationship among etching rate of Mn-PIMNT single crystal with antenna power, bias power and etching gas ratio. Finally, the size of piezoelectric array element is less than 10 μm, the etching depth is more than 20 μm, the kerf width is less than 5 μm, the angle is controllable, and the maximum is more than 87°. The ferroelectric domain structure and the regulation of electric field effect of micro scale piezoelectric elements are studied by means of piezoelectric force microscope. The variation rules of piezoelectric properties and micro scale are obtained. This method can effectively bypass the shortcomings of the wide kerf and the destruction of the crystal orientation by the traditional cutting-and-filling method. It provides a new preparation technology for the development of high-frequency piezoelectric composites, high-density ultrasonic transducer arrays and new piezoelectric micro mechanical systems. This project presents the guidance and reference for the new micromachining technology of ferroelectric materials, and also lays the foundation for the high-frequency piezoelectric composite and high-frequency ultrasonic transducer.
- Jan. 05, 2021
- Acta Physica Sinica
- Vol.69 Issue, 18 187701-1 (2020)
- DOI：10.7498/aps.69.20200544
Fang Yun-Tuan, Wang Zhang-Xin, Fan Er-Pan, Li Xiao-Xue, and Wang Hong-Jin
Two kinds of two-dimensional photonic crystal with hexagonal honeycomb lattices are constructed in which the scatterer and the matrix materials are reversed. Due to the symmetry of special point group, the lattices have p and d orbitals in the center of Brillouin region, which are similar to those in the electronic system. With the structure reversal, the p and d orbitals are also directly inverted. Quantitative analysis shows that the orbital inversion is due to the inversion of air band and medium band because of the local resonance effect in the low frequency bands. Based on the parity properties of p and d orbitals, the pseudo spin states are constructed by analogy to the quantum spin Hall effect in electronic systems. The analysis of the effective Hamiltonian at Γ point shows that the topological phase transition caused by orbital inversion is revealed. The pseudo spin edge states construct an optimal structure. The electromagnetic wave simulations and energy flow vector analysis show that the structure edge takes on the properties of quantum spin Hall effect, namely, the propagation direction is locked by the spin direction and the propagation is topologically protected. The results also show that the quantum spin Hall effect can be realized without undergoing the closing of gap. The comparison among similar researches indicates that the realization of the pseudo spin states does not need the deformation of lattice, and the structure proposed in this work possesses the characteristics of simple design, wide band gap and strong edge localization.Two kinds of two-dimensional photonic crystal with hexagonal honeycomb lattices are constructed in which the scatterer and the matrix materials are reversed. Due to the symmetry of special point group, the lattices have p and d orbitals in the center of Brillouin region, which are similar to those in the electronic system. With the structure reversal, the p and d orbitals are also directly inverted. Quantitative analysis shows that the orbital inversion is due to the inversion of air band and medium band because of the local resonance effect in the low frequency bands. Based on the parity properties of p and d orbitals, the pseudo spin states are constructed by analogy to the quantum spin Hall effect in electronic systems. The analysis of the effective Hamiltonian at Γ point shows that the topological phase transition caused by orbital inversion is revealed. The pseudo spin edge states construct an optimal structure. The electromagnetic wave simulations and energy flow vector analysis show that the structure edge takes on the properties of quantum spin Hall effect, namely, the propagation direction is locked by the spin direction and the propagation is topologically protected. The results also show that the quantum spin Hall effect can be realized without undergoing the closing of gap. The comparison among similar researches indicates that the realization of the pseudo spin states does not need the deformation of lattice, and the structure proposed in this work possesses the characteristics of simple design, wide band gap and strong edge localization.
- Jan. 05, 2021
- Acta Physica Sinica
- Vol.69 Issue, 18 184101-1 (2020)
- DOI：10.7498/aps.69.20200415
Zeng Zhou-Xiao-Song, Wang Xiao, and Pan An-Lian
Two-dimensionl (2D) layered transition metal dichalcogenides (TMDCs) have received great attention in integrated on-chip photonic and photoelectric applications due to their unique physical properties including indirect-to-direct optical bandgap transition, broad bandgap from visible band to near-infrared band, as well as their excellent optoelectric properties derived from the 2D confinement. Recently, with the in-depth study of their fundament nonlinear optical properties, these 2D layered TMDCs have displayed significant potential applications in nonlinear optical devices. In this review, we focus on recent research progress of second harmonic generation (SHG) studies of TMDCs. Firstly, we briefly introduce the basic theory of nonlinear optics (mainly about SHG). Secondly, the several intrinsic SHG relative properties in TMDCs including layer dependence, polarization dependence, exciton resonance effect, valley selection rule are discussed. Thirdly, the latest SHG modulation and enhancement studies are presented, where the electric field, strain, plasmonic structure and micro-cavity enhancement are covered. Finally, we will summarize and give a perspective of possible research direction in the future. We believe that a more in-depth understanding of the SHG process in 2D layered TMDCs as well as the material structure and modulation effects paves the way for further developing the ultra-thin, multifunctional 2D nonlinear optical devices.Two-dimensionl (2D) layered transition metal dichalcogenides (TMDCs) have received great attention in integrated on-chip photonic and photoelectric applications due to their unique physical properties including indirect-to-direct optical bandgap transition, broad bandgap from visible band to near-infrared band, as well as their excellent optoelectric properties derived from the 2D confinement. Recently, with the in-depth study of their fundament nonlinear optical properties, these 2D layered TMDCs have displayed significant potential applications in nonlinear optical devices. In this review, we focus on recent research progress of second harmonic generation (SHG) studies of TMDCs. Firstly, we briefly introduce the basic theory of nonlinear optics (mainly about SHG). Secondly, the several intrinsic SHG relative properties in TMDCs including layer dependence, polarization dependence, exciton resonance effect, valley selection rule are discussed. Thirdly, the latest SHG modulation and enhancement studies are presented, where the electric field, strain, plasmonic structure and micro-cavity enhancement are covered. Finally, we will summarize and give a perspective of possible research direction in the future. We believe that a more in-depth understanding of the SHG process in 2D layered TMDCs as well as the material structure and modulation effects paves the way for further developing the ultra-thin, multifunctional 2D nonlinear optical devices.
- Jan. 05, 2021
- Acta Physica Sinica
- Vol.69 Issue, 18 184210-1 (2020)
- DOI：10.7498/aps.69.20200452
Fei Hong-Ming, Yan Shuai, Xu Yu-Cheng, Lin Han, Wu Min, Yang Yi-Biao, Chen Zhi-Hui, Tian Yuan, and Zhang Ya-Min
Recently, quantum computing and information processing based on photons has become one research frontier, attracting significant attentions. The optical asymmetric transmission devices (OATD), having similar function to the diode in electric circuitry, will find important applications. In particular, the OATDs based on nanophotonic structures are preferred due to their potential applications in the on-chip integration with other photonic devices. Therefore, there have been numerous applications of OATDs based on different nanostructures, including composite grating structures, metasurfaces, surface plasmon polaritons, metamaterials, photonic crystals (PhCs). However, in general, those designs show relatively low forward transmittance ( 0.5 for both TE and TM polarized light. Meanwhile, the backward transmittance can be effectively cut off by the unique dispersion properties of the PhC heterostructures. In this way, the heterostructure is able to achieve polarization independent asymmetric transmission of light waves in a broad wavelength range. To visualize the light propagation in the PhC heterostructure, we use the finite-difference-time-domain method to calculate the electric intensity distributions of the forward and backward propagation light of both TE and TM polarization at a wavelength of 1550 nm. Strong self-collimation effect of forward propagation light and the nearly complete blockage of backward propagation light can be identified unambiguously in the intensity plots, confirming the theoretical analysis. The calculation of transmittance and contrast ratio spectra show that the asymmetric transmission wavelength bandwidth can reach 532 nm with the forward transmittance and contrast ratio being 0.693 and 0.946 at an optical communication wavelength of 1550 nm for TE polarized light. On the other hand, for the TM polarized light, the asymmetric transmission wavelength bandwidth is 128 nm, the forward transmittance and contrast ratio are 0.513 and 0.972, respectively, at 1550 nm wavelength. Thus, it is confirmed that the PhC heterostructure achieves highly efficient, broadband and polarization independent asymmetric transmission. Finally, to further improve the forward transmittance of the TE polarized light, we modulate the radius of the front row of photonic lattice of PhC 1 at the interface. It shows that the forward transmittance can be further improved to a record high value of 0.832 with a bandwidth of 562 nm for TE polarized light. Our design opens up new possibilities for designing OATDs based on PhCs, and will find broad applications, for the design can be realized by current nanofabrication techniques.Recently, quantum computing and information processing based on photons has become one research frontier, attracting significant attentions. The optical asymmetric transmission devices (OATD), having similar function to the diode in electric circuitry, will find important applications. In particular, the OATDs based on nanophotonic structures are preferred due to their potential applications in the on-chip integration with other photonic devices. Therefore, there have been numerous applications of OATDs based on different nanostructures, including composite grating structures, metasurfaces, surface plasmon polaritons, metamaterials, photonic crystals (PhCs). However, in general, those designs show relatively low forward transmittance (< 0.5) and narrow working bandwidth (< 100 nm), and they are able to work with only one polarization state. This makes the current OATDs unsuitable for many applications. To solve this challenge, here we design a two-dimensional (2D) PhC heterostructure based on the self-collimating effect and bandgap properties. The PhC heterostructure is composed of two square lattice 2D PhCs (PhC 1 and PhC 2) on a silicon substrate with different lattice shapes and lattice constants. The PhC 1 is composed of periodically arranged silicon cylinders in air. Meanwhile, the PhC 2 is an square air hole array embedding in silicon. The two PhCs are integrated with an inclined interface with an angle of 45° with respect to the direction of incident light. The plane wave expansion method is used to calculate the band diagrams and equal frequency contours (EFCs) of the two PhCs. As the propagation directions of light waves in PhCs are determined by the gradient direction of the EFCs, we are able to control the light propagation by controlling the EFCs of PhCs. By engineering the EFCs, the PhC 2 shows strong self-collimation effect in a broad wavelength range with a central wavelength of 1550 nm for both TE and TM polarization. By self-collimating the forward incident light from different incident angles to couple to the output waveguide, we are able to significantly increase the forward transmittance to > 0.5 for both TE and TM polarized light. Meanwhile, the backward transmittance can be effectively cut off by the unique dispersion properties of the PhC heterostructures. In this way, the heterostructure is able to achieve polarization independent asymmetric transmission of light waves in a broad wavelength range. To visualize the light propagation in the PhC heterostructure, we use the finite-difference-time-domain method to calculate the electric intensity distributions of the forward and backward propagation light of both TE and TM polarization at a wavelength of 1550 nm. Strong self-collimation effect of forward propagation light and the nearly complete blockage of backward propagation light can be identified unambiguously in the intensity plots, confirming the theoretical analysis. The calculation of transmittance and contrast ratio spectra show that the asymmetric transmission wavelength bandwidth can reach 532 nm with the forward transmittance and contrast ratio being 0.693 and 0.946 at an optical communication wavelength of 1550 nm for TE polarized light. On the other hand, for the TM polarized light, the asymmetric transmission wavelength bandwidth is 128 nm, the forward transmittance and contrast ratio are 0.513 and 0.972, respectively, at 1550 nm wavelength. Thus, it is confirmed that the PhC heterostructure achieves highly efficient, broadband and polarization independent asymmetric transmission. Finally, to further improve the forward transmittance of the TE polarized light, we modulate the radius of the front row of photonic lattice of PhC 1 at the interface. It shows that the forward transmittance can be further improved to a record high value of 0.832 with a bandwidth of 562 nm for TE polarized light. Our design opens up new possibilities for designing OATDs based on PhCs, and will find broad applications, for the design can be realized by current nanofabrication techniques.
- Jan. 05, 2021
- Acta Physica Sinica
- Vol.69 Issue, 18 184214-1 (2020)
- DOI：10.7498/aps.69.20200538
Long Hui, Hu Jian-Wei, Wu Fu-Gen, and Dong Hua-Feng
As the substance carrier of nonlinear optical phenomenon, saturable absorber is an essential material for generating the ultrafast pulse laser. The saturable absorbers based on graphene, transition metal sulfides, topological insulators, black phosphorus and other two-dimensional (2D) materials exhibit different optical advantages. However, limitations of those single 2D materials as saturable absorbers exist. The nanomaterial heterojunction structure can combine the advantages of different 2D materials to achieve optical complementarity, and it also provides new ideas for generating the ultrafast laser with ultrashort pulse duration and high peak power. Here in this paper, the preparation methods, band alignment and the electronic transition mechanism of heterojunction saturable absorbers are summarized, and the recent research progress of ultrafast lasers based on 2D nano-heterostructures are also reviewed, including the wavelength, pulse width, repetition frequency and pulse energy. Therefore, 2D nano-heterostructure exhibits great potential applications in future optical modulator and optical switch.As the substance carrier of nonlinear optical phenomenon, saturable absorber is an essential material for generating the ultrafast pulse laser. The saturable absorbers based on graphene, transition metal sulfides, topological insulators, black phosphorus and other two-dimensional (2D) materials exhibit different optical advantages. However, limitations of those single 2D materials as saturable absorbers exist. The nanomaterial heterojunction structure can combine the advantages of different 2D materials to achieve optical complementarity, and it also provides new ideas for generating the ultrafast laser with ultrashort pulse duration and high peak power. Here in this paper, the preparation methods, band alignment and the electronic transition mechanism of heterojunction saturable absorbers are summarized, and the recent research progress of ultrafast lasers based on 2D nano-heterostructures are also reviewed, including the wavelength, pulse width, repetition frequency and pulse energy. Therefore, 2D nano-heterostructure exhibits great potential applications in future optical modulator and optical switch.
- Jan. 05, 2021
- Acta Physica Sinica
- Vol.69 Issue, 18 188102-1 (2020)
- DOI：10.7498/aps.69.20201235
Zhang Yun-Gang, Liu Huang-Tao, Gao Qiang, Zhu Zhi-Feng, Li Bo, and Wang Yong-Da
SF6 is widely used in gas insulated switchgear due to its excellent insulating and arcing performance. SF6 arc plasma has been extensively studied, but time-resolved spectral characteristics of SF6 arc plasma have not been reported. In this paper, the optical filament generated from focused femtosecond laser is used to guide the high-voltage discharge for generating SF6 plasma in SF6 environment. The SF6 plasma spectrum is obtained in a wavelength range of 300–820 nm, and the identification and attribution of the spectral lines are investigated. The S and F lines are mainly in the 300–550 nm band and 600–800 nm band, respectively. The analysis shows that the S and F atoms are mainly directly or indirectly generated by the collision between SF6 and high-energy electrons during the SF6 decomposition caused by discharge. The S ions are generated by the collision of S atoms with high-energy electrons. The time-resolved spectrum of the SF6 plasma superimposed by the continuous spectrum and the line spectrum is given, and its intensity increases and then decreases. The continuous spectrum is mainly generated by the combined effect of bremsstrahlung and recombination radiation. The recombination radiation is mainly generated by the collision of electron with ions and the recombination between molecular and atoms after SF6 decomposition. The fluorescence lifetime of S ion at 409.91 nm is 57 ns, and the fluorescence lifetime of F atom at 685.60 nm is 341 ns. The evolution law of electron temperature and density with time are given. The electron temperature reaches 2047 K in the early stage of plasma formation. After that, the electron temperature quickly falls to about 1600 K within 300 ns due to the rapid expansion of the plasma and the increase in energy loss during electron movement. At the beginning of discharge, a large number of electrons are generated due to the rapid decomposition of SF6, and the electron density is highest ( $ 10.1 \times {10^{17}}\;{\rm{c}}{{\rm{m}}^{ - {\rm{3}}}}$). After that, the electron density drops rapidly within 200 ns because the recombination between electrons and ions decreases with delay time. Finally, it is proved that the SF6 plasma is in local thermal equilibrium based on the Mc Whirter criterion. The results are of great significance for studying the decomposition mechanism of SF6 and the on-line monitoring technique of high-voltage equipment.SF6 is widely used in gas insulated switchgear due to its excellent insulating and arcing performance. SF6 arc plasma has been extensively studied, but time-resolved spectral characteristics of SF6 arc plasma have not been reported. In this paper, the optical filament generated from focused femtosecond laser is used to guide the high-voltage discharge for generating SF6 plasma in SF6 environment. The SF6 plasma spectrum is obtained in a wavelength range of 300–820 nm, and the identification and attribution of the spectral lines are investigated. The S and F lines are mainly in the 300–550 nm band and 600–800 nm band, respectively. The analysis shows that the S and F atoms are mainly directly or indirectly generated by the collision between SF6 and high-energy electrons during the SF6 decomposition caused by discharge. The S ions are generated by the collision of S atoms with high-energy electrons. The time-resolved spectrum of the SF6 plasma superimposed by the continuous spectrum and the line spectrum is given, and its intensity increases and then decreases. The continuous spectrum is mainly generated by the combined effect of bremsstrahlung and recombination radiation. The recombination radiation is mainly generated by the collision of electron with ions and the recombination between molecular and atoms after SF6 decomposition. The fluorescence lifetime of S ion at 409.91 nm is 57 ns, and the fluorescence lifetime of F atom at 685.60 nm is 341 ns. The evolution law of electron temperature and density with time are given. The electron temperature reaches 2047 K in the early stage of plasma formation. After that, the electron temperature quickly falls to about 1600 K within 300 ns due to the rapid expansion of the plasma and the increase in energy loss during electron movement. At the beginning of discharge, a large number of electrons are generated due to the rapid decomposition of SF6, and the electron density is highest ( $ 10.1 \times {10^{17}}\;{\rm{c}}{{\rm{m}}^{ - {\rm{3}}}}$ ![]()
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- Jan. 05, 2021
- Acta Physica Sinica
- Vol.69 Issue, 18 185201-1 (2020)
- DOI：10.7498/aps.69.20200636
Fan Zeng-Hua, Rong Wei-Bin, Liu Zi-Xiao, Gao Jun, and Tian Ye-Bing
Liquid droplet is a prerequisite for micro-robot based on liquid medium. The investigation of the migration characteristics of condensed droplets on the end surface of a single-finger microgripper is of significance for obtaining stable droplets. The principle of flexible operation for micro-components using droplet condensation is analyzed first. The liquid bridge force acting on a microsphere is derived. A growth model of condensed droplet on the tip of a single-finger microgripper is established, including single-droplet growth, droplet coalesce, droplet movement, and pining effect. Condensation process on the tip of single-finger microgripper with a diameter of 130－400 μm is observed experimentally. Small droplets are formed by directly growing with a big growth rate in the initial stage, then the droplet growth is determined by droplet coalesce. The experimental results show that a single droplet is formed on the end surface after direct growth and droplets coalesce. The maximum droplet volume of 5.5 nL appears on the tip of a single-finger actuator with a diameter of 400 μm under the conditions of surface temperature of –5 °C, room temperature of 24 °C and humidity of 37%. The stability of the formed droplets is dominated by temperature gradients and edge effects during growth process. The distribution of condensed droplets is asymmetric while the microgripper is placed on a cooling surface with temperature gradient. A big growth rate is shown in a low temperature range. A single asymmetric droplet with an offset of 13 μm with respect to the axis of the actuator is formed, which is caused by the temperature gradient. A stable contact angle of 112° is obtained on the end surface of a single-finger microgripper with a diameter of 137 μm because of edge effect using the ambient temperature of 24 °C and humidity of 42%. Condensed droplets located on the end surface of hydrophobic microgripper are more stable than the untreated microgripper. Compared with the droplet formation (0.3 nL) on an untreated microgripper with a diameter of 150 μm, a lager stable droplet of 0.4 nL is obtained on the end face of a small microgripper with a diameter of 130 μm because of the hydrophobic action. The validity of theoretical analysis is verified by experimental results. The experimental investigation of the migration characteristics of condensed droplets on the end surface of a single-finger microgripper shows that the droplet shape can be changed by adjusting the temperature gradient and hydrophilic/hydrophobic performance, which plays an important role in achieving a stable droplet on the end surface.Liquid droplet is a prerequisite for micro-robot based on liquid medium. The investigation of the migration characteristics of condensed droplets on the end surface of a single-finger microgripper is of significance for obtaining stable droplets. The principle of flexible operation for micro-components using droplet condensation is analyzed first. The liquid bridge force acting on a microsphere is derived. A growth model of condensed droplet on the tip of a single-finger microgripper is established, including single-droplet growth, droplet coalesce, droplet movement, and pining effect. Condensation process on the tip of single-finger microgripper with a diameter of 130－400 μm is observed experimentally. Small droplets are formed by directly growing with a big growth rate in the initial stage, then the droplet growth is determined by droplet coalesce. The experimental results show that a single droplet is formed on the end surface after direct growth and droplets coalesce. The maximum droplet volume of 5.5 nL appears on the tip of a single-finger actuator with a diameter of 400 μm under the conditions of surface temperature of –5 °C, room temperature of 24 °C and humidity of 37%. The stability of the formed droplets is dominated by temperature gradients and edge effects during growth process. The distribution of condensed droplets is asymmetric while the microgripper is placed on a cooling surface with temperature gradient. A big growth rate is shown in a low temperature range. A single asymmetric droplet with an offset of 13 μm with respect to the axis of the actuator is formed, which is caused by the temperature gradient. A stable contact angle of 112° is obtained on the end surface of a single-finger microgripper with a diameter of 137 μm because of edge effect using the ambient temperature of 24 °C and humidity of 42%. Condensed droplets located on the end surface of hydrophobic microgripper are more stable than the untreated microgripper. Compared with the droplet formation (0.3 nL) on an untreated microgripper with a diameter of 150 μm, a lager stable droplet of 0.4 nL is obtained on the end face of a small microgripper with a diameter of 130 μm because of the hydrophobic action. The validity of theoretical analysis is verified by experimental results. The experimental investigation of the migration characteristics of condensed droplets on the end surface of a single-finger microgripper shows that the droplet shape can be changed by adjusting the temperature gradient and hydrophilic/hydrophobic performance, which plays an important role in achieving a stable droplet on the end surface.
- Jan. 05, 2021
- Acta Physica Sinica
- Vol.69 Issue, 18 186801-1 (2020)
- DOI：10.7498/aps.69.20200463
Qinghim , and Naranmandula
In this paper, we observe and record the phenomenon of multi-bubble sonoluminescence in phosphoric acid solution which helium is injected into. It is found that a large number of cavitation bubbles are produced in the solution, which can form bubble groups with different shapes and structures, among them the spherical bubble group is a typical one. By the Rayleigh-Plesset equation of bubble group and any bubble in the group with the combination of van der Waals process equation, we study the variations of the bubble radius, temperature of the bubble, pressure pulse in the groups which are composed, respectively, of the bubbles with the same size and the same gas, the bubbles with the same size and different gases, the bubbles with different sizes and the same gas and the bubbles with different sizes and different gases. The results show that for the bubble group composed of the bubbles with the same size and the same gas and the bubble group composed of the bubbles with the same size and different gases, the gas contained in the bubble has a significant effect on the temperature inside the bubble, but has little effect on the bubble radius and the pressure pulse peak within the cluster. Bubble collapse depth of gas with low molecular weight is deeper than that of gas with high molecular weight, but the radius change of rebound stage is smaller than that of the latter. For the bubble group composed of the bubbles with different sizes and the same gas and the bubble group composed of the bubbles with different sizes and different gases, when the total number of bubbles in a bubble group is constant, for the case where there is only one large bubble in the group, the temperature in the large bubble is the highest, which can be higher than the temperature in a single bubble with the same size and the same gas; with the increase of the number of large bubbles in the group, the temperatures in the large and small bubbles both decrease rapidly: the temperature in the large bubbles approaches to the temperature in the bubbles of the bubble group composed of large bubbles with the same gas and the same size, and the temperature in the small bubble gradually approaches to the temperature in small bubble with the same gas under the radiation of many large bubbles. With the increase of the number of large bubbles, the peak value of the pressure pulse in the bubble group first decreases sharply to the inflection point, and then increases steadily to the peak value of the pressure pulse in the bubble group composed of large bubbles with the same gas and the same size. The proportion of large bubbles number in the bubble group has an important influence on the cavitation characteristics of the bubble group, only when the proportion of large bubbles reaches a certain value, can the bubbles of different sizes in the bubble group collapse at the same time, and this conclusion improve and perfect the previous conclusion. The results of this paper will provide some theoretical guidance and help to further explore the cavitation effect and luminescent mechanism of the bubble group.In this paper, we observe and record the phenomenon of multi-bubble sonoluminescence in phosphoric acid solution which helium is injected into. It is found that a large number of cavitation bubbles are produced in the solution, which can form bubble groups with different shapes and structures, among them the spherical bubble group is a typical one. By the Rayleigh-Plesset equation of bubble group and any bubble in the group with the combination of van der Waals process equation, we study the variations of the bubble radius, temperature of the bubble, pressure pulse in the groups which are composed, respectively, of the bubbles with the same size and the same gas, the bubbles with the same size and different gases, the bubbles with different sizes and the same gas and the bubbles with different sizes and different gases. The results show that for the bubble group composed of the bubbles with the same size and the same gas and the bubble group composed of the bubbles with the same size and different gases, the gas contained in the bubble has a significant effect on the temperature inside the bubble, but has little effect on the bubble radius and the pressure pulse peak within the cluster. Bubble collapse depth of gas with low molecular weight is deeper than that of gas with high molecular weight, but the radius change of rebound stage is smaller than that of the latter. For the bubble group composed of the bubbles with different sizes and the same gas and the bubble group composed of the bubbles with different sizes and different gases, when the total number of bubbles in a bubble group is constant, for the case where there is only one large bubble in the group, the temperature in the large bubble is the highest, which can be higher than the temperature in a single bubble with the same size and the same gas; with the increase of the number of large bubbles in the group, the temperatures in the large and small bubbles both decrease rapidly: the temperature in the large bubbles approaches to the temperature in the bubbles of the bubble group composed of large bubbles with the same gas and the same size, and the temperature in the small bubble gradually approaches to the temperature in small bubble with the same gas under the radiation of many large bubbles. With the increase of the number of large bubbles, the peak value of the pressure pulse in the bubble group first decreases sharply to the inflection point, and then increases steadily to the peak value of the pressure pulse in the bubble group composed of large bubbles with the same gas and the same size. The proportion of large bubbles number in the bubble group has an important influence on the cavitation characteristics of the bubble group, only when the proportion of large bubbles reaches a certain value, can the bubbles of different sizes in the bubble group collapse at the same time, and this conclusion improve and perfect the previous conclusion. The results of this paper will provide some theoretical guidance and help to further explore the cavitation effect and luminescent mechanism of the bubble group.
- Jan. 05, 2021
- Acta Physica Sinica
- Vol.69 Issue, 18 184301-1 (2020)
- DOI：10.7498/aps.69.20200381
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