Hao Ouyang, Si-Yang Hu, Man-Ling Shen, Chen-Xi Zhang, Xiang-Ai Cheng, and Tian Jiang
Germanium diselenide (GeSe2), a layered IV-VI semiconductor, has an in-plane anisotropic structure and a wide band gap, exhibiting unique optical, electrical, and thermal properties. In this paper, polarization axis Raman spectrum and linear absorption spectrum are used to characterize the crystal axis orientation and energy band characteristics of GeSe2 flake, respectively. Based on the results, a micro-domain I scan system is used to study the optical nonlinear absorption mechanism of GeSe2 near the resonance band. The results show that the nonlinear absorption mechanism in GeSe2 is a superposition of saturation absorption and excited state absorption, and is strongly dependent on the polarization and wavelength of incident light. Under near-resonance excitation (450 nm), the excited state absorption is more greatly dependent on polarization. With different polarizations of incident light, the modulation depth can be changed from 4.6% to 9.9%; for non-resonant excitation (400 nm), the modulation depth only changes from 7.0% to 9.7%. At the same time, compared with saturation absorption, the polarization-dependent excited state absorption is greatly affected by the distance away from the resonance excitation wavelength.Germanium diselenide (GeSe2), a layered IV-VI semiconductor, has an in-plane anisotropic structure and a wide band gap, exhibiting unique optical, electrical, and thermal properties. In this paper, polarization axis Raman spectrum and linear absorption spectrum are used to characterize the crystal axis orientation and energy band characteristics of GeSe2 flake, respectively. Based on the results, a micro-domain I scan system is used to study the optical nonlinear absorption mechanism of GeSe2 near the resonance band. The results show that the nonlinear absorption mechanism in GeSe2 is a superposition of saturation absorption and excited state absorption, and is strongly dependent on the polarization and wavelength of incident light. Under near-resonance excitation (450 nm), the excited state absorption is more greatly dependent on polarization. With different polarizations of incident light, the modulation depth can be changed from 4.6% to 9.9%; for non-resonant excitation (400 nm), the modulation depth only changes from 7.0% to 9.7%. At the same time, compared with saturation absorption, the polarization-dependent excited state absorption is greatly affected by the distance away from the resonance excitation wavelength.
- Sep. 20, 2020
- Acta Physica Sinica
- Vol. 69, Issue 18, 184212-1 (2020)
- DOI：10.7498/aps.69.20200443
Xiang Li, Yong Chen, Hao Feng, and Lei Qi
Acoustically-excited bubble dynamics is the foundation of pipeline bubble detection based on acoustic technology. Due to the existence of multiple bubbles in pipeline flow, the Bjerknes forces among arbitrary bubbles under acoustic excitation may enforce bubble-bubble interaction and then change the features of bubble dynamics. Based on traditional free bubble’s Rayleigh-Plesset (R-P) model, this paper tries to establish bubble-bubble interaction model in consideration of the second Bjerknes force and bubble distribution in the pipeline axial direction. Meanwhile, the influence of finite wave speed in compressible fluid is considered. The proposed model is numerically calculated by the fourth-order Runge-Kutta method. Firstly, the differences in bubble feature between the free bubble’s R-P model and bubble-bubble interaction model are compared under excitation with different frequencies and amplitudes. Results show that the differences in bubble dynamics are minor when the bubble’s distance is large enough. When the bubble’s distance is fixed, the differences are significant on condition that the frequency of acoustic excitation is nearly the resonant frequency of bubbles. Secondly, through establishing compressible model and incompressible fluid model, we compare the differences between the two models. Numerical calculations show that the second Bjerknes force under the compressible assumption acts as an external force and forces the bubble to vibrate. On the other hand, the second Bjerknes force under the incompressible assumption changes the dynamics of bubble-bubble interaction as well as the resonant features. Finally, we study the effect of bubble-bubble distance and bubble’s axial position on bubble vibration characteristics. The bubble-bubble distance affects the second Bjerknes force and may lead the bubbles to vibrate nonlinearly. The bubble’s axial position changes the phase of external acoustic force and leads to the difference in initial vibration feature. When this difference is coupled with the second Bjerknes force, the bubble-bubble interaction may be changed even into nonlinear vibration, leading the bubble’s oscillation spectrum to differ from linear vibrations significantly. These results demonstrate that the resonant state of a small bubble may be converted into nonlinear vibration state if the second Bjerknes force is present. On the other hand, the resonant state of a large bubble can keep linear vibration when the second Bjerknes force is not obvious.Acoustically-excited bubble dynamics is the foundation of pipeline bubble detection based on acoustic technology. Due to the existence of multiple bubbles in pipeline flow, the Bjerknes forces among arbitrary bubbles under acoustic excitation may enforce bubble-bubble interaction and then change the features of bubble dynamics. Based on traditional free bubble’s Rayleigh-Plesset (R-P) model, this paper tries to establish bubble-bubble interaction model in consideration of the second Bjerknes force and bubble distribution in the pipeline axial direction. Meanwhile, the influence of finite wave speed in compressible fluid is considered. The proposed model is numerically calculated by the fourth-order Runge-Kutta method. Firstly, the differences in bubble feature between the free bubble’s R-P model and bubble-bubble interaction model are compared under excitation with different frequencies and amplitudes. Results show that the differences in bubble dynamics are minor when the bubble’s distance is large enough. When the bubble’s distance is fixed, the differences are significant on condition that the frequency of acoustic excitation is nearly the resonant frequency of bubbles. Secondly, through establishing compressible model and incompressible fluid model, we compare the differences between the two models. Numerical calculations show that the second Bjerknes force under the compressible assumption acts as an external force and forces the bubble to vibrate. On the other hand, the second Bjerknes force under the incompressible assumption changes the dynamics of bubble-bubble interaction as well as the resonant features. Finally, we study the effect of bubble-bubble distance and bubble’s axial position on bubble vibration characteristics. The bubble-bubble distance affects the second Bjerknes force and may lead the bubbles to vibrate nonlinearly. The bubble’s axial position changes the phase of external acoustic force and leads to the difference in initial vibration feature. When this difference is coupled with the second Bjerknes force, the bubble-bubble interaction may be changed even into nonlinear vibration, leading the bubble’s oscillation spectrum to differ from linear vibrations significantly. These results demonstrate that the resonant state of a small bubble may be converted into nonlinear vibration state if the second Bjerknes force is present. On the other hand, the resonant state of a large bubble can keep linear vibration when the second Bjerknes force is not obvious.
- Sep. 20, 2020
- Acta Physica Sinica
- Vol. 69, Issue 18, 184703-1 (2020)
- DOI：10.7498/aps.69.20200546
Bin Liu, Peng-Xiang Zhao, Xia Zhao, Yue Luo, and Li-Chao Zhang
Underwater optical imaging is the key technology to explore the underwater mystery. However, due to the absorption and backscattering effects of the media in the underwater environment, the image acquired by the detector will be severely degraded. In order to obtain the effective underwater scene information, it is necessary to restore the acquired underwater image. The restoration technology based on differential polarization is one of the main methods of restoring the underwater images, which can suppress the background scattered light by the common-mode suppression between orthogonal polarization graphs, thus realizing the restoration of underwater image. However, the relevant research shows that the restoration effect of this method is general for the underwater non-uniform light field. The main reason is that the estimation errors of polarization degree and background scattering intensity under the condition of the non-uniform underwater light field are large. Out of the above problem, in this paper we present the multiple aperture underwater imaging technology of fused polarization information. The method uses the camera array to realize the large virtual aperture imaging system, thus obtaining the wide-angle light field information, and then to fuse the depth information of the scene to realize the accurate estimation of background scattering light intensity and polarization degree under the condition of underwater non-uniform light field. The estimated parameter value can better reflect the global characteristics of the scene. Through the imaging experiments on the targets with different polarization degrees in the turbid underwater environment, comparing with the current advanced restoration algorithm, the results show that the proposed method can effectively solve the problems of background scattering and polarization degree significant estimation error caused by non-uniform underwater light field, and obtain high-quality restoration results. Through the contrast imaging experiment of the target in the underwater environment with different turbidity concentrations, the results show that with the increase of turbidity concentration of the water, the image recovery effect of the method in this paper is gradually weakened. However, it still has a good restoration effect at a large concentration. At the same time, imaging experiments are conducted on targets in underwater environments with different sediment concentrations. The results show that the method proposed in this paper can also obtain a better restoration image in the turbid water environment containing sediment.Underwater optical imaging is the key technology to explore the underwater mystery. However, due to the absorption and backscattering effects of the media in the underwater environment, the image acquired by the detector will be severely degraded. In order to obtain the effective underwater scene information, it is necessary to restore the acquired underwater image. The restoration technology based on differential polarization is one of the main methods of restoring the underwater images, which can suppress the background scattered light by the common-mode suppression between orthogonal polarization graphs, thus realizing the restoration of underwater image. However, the relevant research shows that the restoration effect of this method is general for the underwater non-uniform light field. The main reason is that the estimation errors of polarization degree and background scattering intensity under the condition of the non-uniform underwater light field are large. Out of the above problem, in this paper we present the multiple aperture underwater imaging technology of fused polarization information. The method uses the camera array to realize the large virtual aperture imaging system, thus obtaining the wide-angle light field information, and then to fuse the depth information of the scene to realize the accurate estimation of background scattering light intensity and polarization degree under the condition of underwater non-uniform light field. The estimated parameter value can better reflect the global characteristics of the scene. Through the imaging experiments on the targets with different polarization degrees in the turbid underwater environment, comparing with the current advanced restoration algorithm, the results show that the proposed method can effectively solve the problems of background scattering and polarization degree significant estimation error caused by non-uniform underwater light field, and obtain high-quality restoration results. Through the contrast imaging experiment of the target in the underwater environment with different turbidity concentrations, the results show that with the increase of turbidity concentration of the water, the image recovery effect of the method in this paper is gradually weakened. However, it still has a good restoration effect at a large concentration. At the same time, imaging experiments are conducted on targets in underwater environments with different sediment concentrations. The results show that the method proposed in this paper can also obtain a better restoration image in the turbid water environment containing sediment.
- Sep. 20, 2020
- Acta Physica Sinica
- Vol. 69, Issue 18, 184202-1 (2020)
- DOI：10.7498/aps.69.20200471
Yi-Quan Xu, and Cong Wang
The leap in communication technology in recent years has brought new challenges to the compactness, modulation speed, working bandwidth and control efficiency of modulation equipment. The discovery of graphene has led the two-dimensional materials to develop rapidly, and a series of new materials have continuously emerged, such as MXene, black phosphorus, transition metal sulfides, etc. These new two-dimensional materials have excellent nonlinear optical effects, strong light-matter interaction, and ultra-wide working bandwidth. Using their thermo-optic effect, nonlinear effect and the combination with optical structure, the needs of ultra-fast modulation in optical communication can be met. Compact, ultra-fast, and ultra-wide will become the tags for all-optical modulation of two-dimensional materials in the future. This article focuses on all-optical devices based on thermo-optical effects and non-linear effects of two-dimensional materials, and introduces fiber-type Mach-Zehnder interferometer structures, Michelson interferometer structures, polarization interferometer structures, and micro-ring structures. In this paper, the development status of all-optical devices is discussed from the perspectives of response time, loss, driving energy, extinction ratio, and modulation depth. Finally, we review the latest developments, analyze the challenges and opportunities faced by all-optical devices, and propose that all-optical devices should be developed in the direction of ring resonators and finding better new two-dimensional materials. We believe that all-optical devices will maintain high-speed development, acting as a cornerstone to promote the progress of all-optical systems.The leap in communication technology in recent years has brought new challenges to the compactness, modulation speed, working bandwidth and control efficiency of modulation equipment. The discovery of graphene has led the two-dimensional materials to develop rapidly, and a series of new materials have continuously emerged, such as MXene, black phosphorus, transition metal sulfides, etc. These new two-dimensional materials have excellent nonlinear optical effects, strong light-matter interaction, and ultra-wide working bandwidth. Using their thermo-optic effect, nonlinear effect and the combination with optical structure, the needs of ultra-fast modulation in optical communication can be met. Compact, ultra-fast, and ultra-wide will become the tags for all-optical modulation of two-dimensional materials in the future. This article focuses on all-optical devices based on thermo-optical effects and non-linear effects of two-dimensional materials, and introduces fiber-type Mach-Zehnder interferometer structures, Michelson interferometer structures, polarization interferometer structures, and micro-ring structures. In this paper, the development status of all-optical devices is discussed from the perspectives of response time, loss, driving energy, extinction ratio, and modulation depth. Finally, we review the latest developments, analyze the challenges and opportunities faced by all-optical devices, and propose that all-optical devices should be developed in the direction of ring resonators and finding better new two-dimensional materials. We believe that all-optical devices will maintain high-speed development, acting as a cornerstone to promote the progress of all-optical systems.
- Sep. 20, 2020
- Acta Physica Sinica
- Vol. 69, Issue 18, 184216-1 (2020)
- DOI：10.7498/aps.69.20200654
Yan-Ju Wei, Jie Zhang, Sheng-Cai Deng, Ya-Jie Zhang, Ya-Jing Yang, Sheng-Hua Liu, and Hao Chen
Atomization of droplets is ubiquitous in many natural and industrial processes, such as falling rain drops, inkjet printing, fuel injection in automotive and gas-turbine engines. Acoustic irradiation provides a very effective method of atomizing fluid. However, the acoustic atomization of acoustically levitated droplet is seldom studied. To assess the possibility of achieving ultrafine atomization, we, in this paper, systematically study the atomization of an acoustically levitated droplet placed in a hot gas of a flame. High speed camera is utilized to investigate the atomization characteristics of various droplets with diameters ranging from 0.5 mm to 3.5 mm.The experimental results show that the sound pressure of the resonance acoustic field has the ability to atomize the droplet when it is suddenly bathed in hot gas. Here the heating acts as a switch to convert the droplet surface from an acoustic isolator to conductor by heating the surface to strong evaporation. The presence of a high concentration of vapor molecules surrounding the droplet caused the acoustic field to change, thus, a much larger pressure gradient is established along the droplet surface, resulting in the atomization of droplet from the equator. Furthermore, Faraday wave stimulation and discretization on the film cause the droplet to further disintegrate when the droplet diameter is large enough. The atomization consists of three different styles, i.e. rim spray (RS), film disintegration (FD) and normal sputtering (NS). When exposed to hot gas, the droplets with equivalent diameter D0 D0 > 3.2 mm undergo further film buckling, forming a closed bubble due to the Helmholtz resonator effect and NS at the bottom. This sound driven atomization of droplets enriches the understanding of fluid mechanism in multi-physical fields, and may provide new ideas for relative application research.
- Sep. 20, 2020
- Acta Physica Sinica
- Vol. 69, Issue 18, 184702-1 (2020)
- DOI：10.7498/aps.69.20200562
Jing Tian, Mei-Jiang Hou, Yang Jiang, Hong-Xu Zhang, Guang-Fu Bai, and Hao Feng
A novel fiber sensor based compound ring laser cavity with linear variation of frequency is proposed and demonstrated experimentally. The compound ring laser cavity is comprised of a ring cavity and a straight cavity. This system can generate the beat frequency spectrum by employing an erbium doped fiber amplifier, a fiber Bragg grating is used as a sensor head and the straight cavity reflector, a π phase shifted fiber Bragg grating serves as a microwave photonic passband filter. The principle of the proposed sensor is theoretically analyzed, showing that as the displacement increases the beat frequency decreases, and there exists a linear relationship between displacement change and beat frequency shift. In experiment, it is shown that the sensor has a high sensitivity of about 86.19 kHz/mm and can achieve a good linear response (R2 = 0.9973), and that the minimum monitored displacement is about 10 μm. The measurement results demonstrate that the sensor is accurate, sensitive, and the proposed sensor system has a compact and simple structure, which makes it convenient for more applications in future.A novel fiber sensor based compound ring laser cavity with linear variation of frequency is proposed and demonstrated experimentally. The compound ring laser cavity is comprised of a ring cavity and a straight cavity. This system can generate the beat frequency spectrum by employing an erbium doped fiber amplifier, a fiber Bragg grating is used as a sensor head and the straight cavity reflector, a π phase shifted fiber Bragg grating serves as a microwave photonic passband filter. The principle of the proposed sensor is theoretically analyzed, showing that as the displacement increases the beat frequency decreases, and there exists a linear relationship between displacement change and beat frequency shift. In experiment, it is shown that the sensor has a high sensitivity of about 86.19 kHz/mm and can achieve a good linear response (R2 = 0.9973), and that the minimum monitored displacement is about 10 μm. The measurement results demonstrate that the sensor is accurate, sensitive, and the proposed sensor system has a compact and simple structure, which makes it convenient for more applications in future.
- Sep. 20, 2020
- Acta Physica Sinica
- Vol. 69, Issue 18, 184217-1 (2020)
- DOI：10.7498/aps.69.20200385
Zeng-Hua Fan, Wei-Bin Rong, Zi-Xiao Liu, Jun Gao, and Ye-Bing Tian
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.
- Sep. 20, 2020
- Acta Physica Sinica
- Vol. 69, Issue 18, 186801-1 (2020)
- DOI：10.7498/aps.69.20200463
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