Preface to the special topic: Nonlinear optics and devices of low-dimensional materials
  • Sep. 20, 2020
  • Acta Physica Sinica
  • Vol. 69, Issue 18, 180101-1 (2020)
  • DOI:10.7498/aps.69.180101
Data traffic capability of double-layer network based on coupling strength
Jin-Long Ma, Chang-Feng Du, Wei Sui, and Xiang-Yang Xu
The two-layer network model offers us a new viewpoint to observe the traffic dynamics of multilayer network systems. An efficient coupling mechanism is of great importance for alleviating the traffic congestion on two-layer networks. In order to reduce the network congestion and improve network transmission performance, the coupling mechanism between two layers of network and three coupling methods, which are random coupling, disassortative coupling and assortative coupling, are studied based on degree correlation. The packet transmission process is analyzed with both the shortest path routing strategy and degree-based weight routing strategy. The influences of the coupling mode and its corresponding routing strategy on the traffic capacity of the two-layer network are studied. In this paper, two scale-free networks are used to construct the two-layer network for simulation experiments. The network scale is in a range from 200 to 2400 with the value of average degree being 8. We focus on the traffic dynamics of two-layer network, and analyze the relationship between the traffic capacity and the three coupling modes, which are random coupling, disassortative coupling and assortative coupling, under the constraints of the shortest path routing strategy and the weight-based routing strategy. According to the characteristics of the coupling connection between the two layers of network, the best coupling method which is suitable for a certain routing strategy should be investigated. The suitable coupling connection between the two layers can effectively increase the traffic capacity. Both numerical result and analytical result show that the packet generation rate, average transmission time, and average throughput can be obviously improved under the shortest path routing strategy with the disassortative coupling method. When the degree-based static weight routing strategy is used, the traffic performance parameters such as packet generation rate, average transmission time, and average throughput can reach the optimal values with the assortative coupling method. It makes the traffic flow uniform that the routing strategy is chosen with the most suitable coupling method on the two-layer network, and the network traffic capacity may be effectively enhanced. More generally, the results indicate that the coupling modes can give rise to traffic behavior that relies subtly on the routing strategy on the two-layer network. Our work may shed some light on the design and optimization of some real traffic or communication networks.
  • Sep. 20, 2020
  • Acta Physica Sinica
  • Vol. 69, Issue 18, 188901-1 (2020)
  • DOI:10.7498/aps.69.20200181
Research progress of low-dimensional semiconductor materials in field of nonlinear optics
Rui-Xue Bai, Jue-Han Yang, Da-Hai Wei, and Zhong-Ming Wei
Since the first ruby laser was invented, researchers have focused their attention on how to achieve a strong laser light source, which cannot be produced by the ordinary light sources. Since then, the rich and colorful characteristics of nonlinear optical materials have been discovered, such as the saturation absorption, reverse saturation absorption and nonlinear refraction. They are applied to optoelectronic devices, optical switching devices and optical communication. At the same time, with the increase of the requirements for device integration performance in industrial production, ordinary three-dimensional devices are difficult to meet the production requirements, and the advent of low-dimensional semiconductor devices effectively solves this problem. Therefore, the combination of nonlinear optics and low-dimensional semiconductor materials is a general trend. The emergence of quantum dots, quantum wire lasers, and amplifiers confirms this. In this paper, we summarize the frontier work on nonlinear optics by selecting several special low-dimensional structures and several materials, providing some references for future research. However, due to the fact that the instability and low filling ratio of low-dimensional materials remain to be improved, further relevant research is still required.
  • Sep. 20, 2020
  • Acta Physica Sinica
  • Vol. 69, Issue 18, 184211-1 (2020)
  • DOI:10.7498/aps.69.20200206
Tuning optical force of dielectric/metal core-shell placed above Au film
Jia-Chen Zhang, Wei-Xing Yu, Fa-Jun Xiao, and Jian-Lin Zhao
Manipulating the core-shell structure with the optical force has been extensively studied, giving birth to applications such as particle sorting, biomarkers and drug delivery. Tailoring the optical force exerted on the core-shell above the metallic film remains unexplored, despite the obvious benefits for both fundamental research and applications including strong coupling, surface enhanced spectroscopy, nanolaser, and nanoscale sensing. In this work, we systematically investigate the optical force exerted on a dielectric/metal core-shell above a gold film by utilizing the Maxwell stress tensor formalism. It is found that at the present gold substrate, the optical force on the core-shell can be one order of magnitude larger than that on the individual core-shell due to the strong coupling between the core-shell and the gold film. Interestingly, the direction of the optical force can be reversed from positive to negative by distributing the local field from the upside of core-shell to the structure gap through changing the excitation wavelength. Furthermore, we demonstrate that the magnitude and peak wavelength of the optical force can be well controlled by altering the structure gap, the size and refractive index of the core. More specifically, it is found that the coupling strength between the core-shell and the gold film decreases with the gap size increasing. As a result, we observe the blue shift of bonding mode and the decrease of local field in the gap, which leads the force peak wavelength to be blue-shifted and the force peak magnitude to decrease, respectively. Also, by increasing the radius and refractive index of the core, a red shift of force peak is accompanied with the red shift of the bonding mode. In addition, the force peak magnitude follows the same trend as the total local field enhancement factor when the radius and refractive index of the core change. We hope that our results open the way to control the cavity size of particle on film structure, which would be beneficial for tailoring the light matter interaction even down to single molecular level and promises to have the applications in novel functional photonic devices.
  • Sep. 20, 2020
  • Acta Physica Sinica
  • Vol. 69, Issue 18, 184206-1 (2020)
  • DOI:10.7498/aps.69.20200214
Two-dimensional nanomaterials and their derivatives for laser protection
Zhi-Wei Liu, Bin Zhang, and Yu Chen
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.
  • Sep. 20, 2020
  • Acta Physica Sinica
  • Vol. 69, Issue 18, 184201-1 (2020)
  • DOI:10.7498/aps.69.20200313
Ag@SiO2 coupled structure’s design and regulation and control of response to thin film solar cells
Bu-Qing Gong, Xiao-Yu Chen, Wei-Peng Wang, Zhi-Ye Wang, Hua Zhou, and Xiang-Qian Shen
The coupled nano-structure Ag@SiO2 has both plasmon excitation like metallic nanoparticles and diffraction scattering like a dielectric nanosphere, which effectively controls the propagation path and the energy distribution of incident light and shows great potential applications in light trapping for thin film solar cells. In this work, we construct a three-dimensional electromagnetic model based on the finite-difference time-domain (FDTD) and rigorous coupled-wave analysis (RCWA) method to investigate the regulation mechanism of Ag@SiO2 coupling structure to the spectral response of amorphous silicon cells. By being optimally designed, a high-efficiency cell device is achieved. The results show that the transmitted light into the active layer reaches a maximum value when Ag and SiO2 have their feature sizes of 18 and 150 nm, respectively. The absorption spectrum of the corresponding cell device also arrives at its maximum value. The photoelectric conversion efficiency is enhanced from 7.19% to 7.80%, with an increment of 8.48% compared with the flat solar cell with an equivalent thickness of absorbing layer.
  • Sep. 20, 2020
  • Acta Physica Sinica
  • Vol. 69, Issue 18, 188801-1 (2020)
  • DOI:10.7498/aps.69.20200334
Bismuth nanosheets based saturable-absorption passively Q-switching mid-infrared single-crystal fiber laser
Qian-Qian Hao, Meng-Yu Zong, Zhen Zhang, Hao Huang, Feng Zhang, Jie Liu, Dan-Hua Liu, Liang-Bi Su, and Han Zhang
As a new two-dimensional material, bismuth nanosheet is an effective modulator for realizing a mid-infrared pulsed laser, which benefits from its suitable band gap, higher carrier mobility and better room temperature stability, as well as its excellent electrical and optical properties. The mid-infrared single-crystal fiber is a preferable gain medium for high-power laser because of its advantages of both crystal and fiber. In this paper, a bismuth nanosheet saturable absorber is successfully prepared by the ultrasonic method and used for the first time in a diode-pumped Er:CaF2 single-crystal fiber mid-infrared passively Q-switching pulsed laser. A compact concave planar linear resonator is designed to study the Q-switching Er:CaF2 single-crystal fiber laser with bismuth nanosheets serving as saturable absorbers. The pump source is a fiber-coupled semiconductor laser with a core diameter of 105 μm, a numerical aperture of 0.22, and a central emission wavelength of 976 nm. The pump light is focused onto the front end of the gain medium through a coupled collimating system with a coupling ratio of 1∶2. The gain medium is a 4 at.% Er3+:CaF2 single-crystal fiber grown by the temperature gradient method, and this fiber has two polished but not coated ends, a diameter of 1.9 mm, and a length of 10 mm. To reduce the thermal effect, the single-crystal fiber is tightly wrapped with indium foil and mounted on a copper block with a constant temperature of 12 ℃. The input mirror has a high reflection coating at 2.7–2.95 μm and an antireflection coating at 974 nm, with a curvature radius of 100 mm. A group of partially transmitting plane mirrors are used as output couplers, respectively, with transmittances of 1%, 3%, and 5% at 2.7–2.95 μm. The total length of the resonant cavity is 26 mm. By inserting the bismuth nanosheet into the resonator and carefully adjusting its position and angle, a stable mid-infrared Q-switching laser is obtained. At the absorbed pump power of 1.52 W, a pulsed laser with an average output power of 190 mW is obtained for an output mirror with a transmittance of 3%. The shortest pulse width is 607 ns, the repetition frequency is 58.51 kHz, and the corresponding single pulse energy and peak power are 3.25 μJ and 5.35 W, respectively.
  • Sep. 20, 2020
  • Acta Physica Sinica
  • Vol. 69, Issue 18, 184205-1 (2020)
  • DOI:10.7498/aps.69.20200337
Dual-wavelength self-starting mode-locking Er-doped fiber laser with MnPS3 saturable absorber
Qiang Yu, Kun Guo, Jie Chen, Tao Wang, Jin Wang, Xin-Yao Shi, Jian Wu, Kai Zhang, and Pu Zhou
As a member of the metal phosphorus trichalcogenide family, MPS3 is widely used in nonlinear optics and devices, which can be regarded as a significant benefit for the excellent photonic and optoelectronic properties. In this work, the MnPS3 nanosheet is prepared by the chemical vapor transport method and the MnPS3 saturable absorber is demonstrated by modifying mechanical exfoliation. To the best of our knowledge, the dual-wavelength self-starting mode-locking erbium-doped fiber laser with MnPS3 saturable absorber is demonstrated for the first time. The dual wavelength mode-locked laser with a pulse repetition rate of 5.102 MHz at 1565.19 nm and 1565.63 nm is proposed. Its maximum output power at the dual-wavelength is 27.2 MW. The mode-locked laser can self-start and stably run for more than 280 h.
  • Sep. 20, 2020
  • Acta Physica Sinica
  • Vol. 69, Issue 18, 184208-1 (2020)
  • DOI:10.7498/aps.69.20200342
Mechanism of longitudinal magnetic field suppressed Richtmyer-Meshkov instability
Sha Sha, Huan-Hao Zhang, Zhi-Hua Chen, Chun Zheng, Wei-Tao Wu, and Qi-Chen Shi
Based on the ideal magneto-hydro-dynamic equations (MHD) and adopting the CTU (Corner Transport Upwind) + CT (Constrained Transport) algorithm, the Richtmyer-Meshkov (RM) instability of the Gaussian distribution R22 gas column generated by the interaction of a planar shock wave with the gas column in the presence of magnetic field is investigated numerically. The numerical results show that the evolution of shock wave structure and interface instability during the interaction between shock wave and gas column are consistent with the experimental results of Haas and Sturtevant (Hass J F, Sturtevant B 1987 J. Fluid Mech. 181 41) in the absence of a magnetic field. In the presence of a longitudinal magnetic field, the evolution of the shock structures is almost unaffected, while the density interface appears very smooth and interfacial vortex sequences vanish, which is a clear indication of the RMI suppresion. Moreover, as the shock wave impinges on the interface, the magnetic field lines refract on the interface, resulting in the generation of magnetic field gradient in the flow field, which is not consistent with the scenario in the above case. As a result, a stronger magnetic tension is produced on the inner and outer vorticity layer. Simultaneously, the magnetic tension produces a moment on the interface fluid, which is opposite to the rolling direction of the velocity shear. Therefore, the interface instability and the main vortex rolling-up are effectively suppressed. In addition, the non-uniform distribution of magnetic tensions along the interface changes the aggregation degree of the magnetic lines, amplifying the magnetic strength, and finally enhancing the suppression effect of the magnetic field on the interface instability.
  • Sep. 20, 2020
  • Acta Physica Sinica
  • Vol. 69, Issue 18, 184701-1 (2020)
  • DOI:10.7498/aps.69.20200363
Characteristics and mechanism of a triggered lightning with two polarity reversals of charges transferred to ground
Guo-Ying Tang, Zhu-Ling Sun, Ru-Bin Jiang, Feng-Quan Li, Ming-Yuan Liu, Kun Liu, and Xiu-Shu Qie
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.
  • Sep. 20, 2020
  • Acta Physica Sinica
  • Vol. 69, Issue 18, 189201-1 (2020)
  • DOI:10.7498/aps.69.20200374