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Abstract
This work develops a method to complete the in-flight cross calibration and verification between a radiometer and an imager hosted on aircraft. The in-flight cross calibration is data transmission through time matching, space matching, and spectral matching of two polarization instruments on the same platform, and this method can not only complete the data transfer without considering the surface type to reduce the calibration cycle but also can obtain huge and rich calibration data. The radiometer is the particulate observing scanning polarimeter (POSP), which takes multi-angle, photo-polarimetric measurements in several spectral channels. The POSP measurements in the bands of 670nm and 865nm used in this work are simultaneously measured by the simultaneous imaging polarization camera (SIPC), which is on the same aircraft. The POSP is designed to provide high precision measurements of the atmospheric or earth surface radiation polarization with a substantial along-track spatial coverage, while the SIPC can provide large spatial coverage and high-resolution measurements. Through radiometer-to-imager in-flight cross calibration, the high-precision calibration coefficient of the POSP is transmitted to the SIPC, which can effectively improve the measurement accuracy of the SIPC, and realizes the remote sensing monitoring of atmospheric fine particles with large spatial coverage and high detection precision. First, we deduce the polarization models of the POSP and the SIPC, respectively, and express them in the form of Mueller matrixes, which describe the transformation from incoming polarized radiation to measured signals. Then, we deduce the in-flight cross calibration model of the POSP and the SIPC. Finally, the in-flight experiments have been carried out to validate the radiometer-to-imager in-flight cross calibration model. The results have shown the possibility to minimize the SIPC polarization degree errors with a roughly 0.01 bias relative to POSP on the land.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
By absorbing and reflecting solar radiation, the impact of aerosols on global climate and environment is attracting more and more attention [1–3]. However, in the Earth atmosphere, the distribution and microphysical properties of aerosols are still little known. Therefore, the development of high-precision aerosol space exploration missions is extremely urgent. The NASA’s Aerosol-Cloud-Ecosystem (ACE) mission [4], which is comprised of passive and active sensors (radar and lidar), is nominally planned for a 2021 launch. The Ukrainian space mission Aerosol-UA (UA means Ukraine) [5–8], which is comprised of the multispectral Scanning Polarimeter (ScanPol) [9] and the Multi-Spectral Imaging Polarimeter (MSIP) [10], aims to monitor the distribution of microphysics and atmospheric aerosols in space and plans to launch in 2022. In China, the Polarization Cross-Fire Suite (PCF) [7,11] consists of the Directional Polarization Camera (DPC) and the Particulate Observing Scanning Polarimeter (POSP) that are under preparation for launch in 2020. It's worth mentioning that the China Polarization Cross-Fire Suite (PCF) [7,11] is similar to those of the Aerosol-UA mission [5–8] instruments. For the Aerosol-UA mission instruments, the concept of the MSIP and ScanPol polarimeters cross-calibration is discussed, the results show that through cross-calibrating the MSIP using ScanPol, the polarization errors of the MSIP can be within 1% [8]. Unfortunately, they only carry out numerical verification experiment and don't conduct the in-flight experiment verification.
The DPC [12,13], which is the type of polarization and directionality of the Earth’s reflectance instrument (POLDER) polarimeter, is a multispectral wide-angle imaging polarimeter, while the POSP [14] is the type of aerosol polarimetry sensor (APS) polarimeter, which is important for the DPC data corrections. The PCF instruments, combined together, can realize remote sensing monitoring of atmospheric fine particles with large spatial coverage and high detection precision. Since the PCF instruments are hosted on the same satellite platform, the fields of view of the PCF instruments are partly overlapped during atmospheric detection. Therefore, cross-calibration of the PCF instruments can be used for transmit the high-precision calibration coefficient of the POSP to the DPC for the overlapping part, effectively improving the measurement accuracy of the DPC. In order to verify the feasibility of the PCF instruments cross-calibration method, we have carried out the in-flight experiments. However, the DPC uses optical wedge motion compensation to achieve pixel registration, it is designed for specific orbital heights and requires a stable satellite platform. In addition, the high specific velocity of the aircraft is different from that of the satellite platform, and the method of compensation pixel registration for the optical wedge motion of DPC is difficult to realize, so the DPC is not suitable for the in-flight experiment. Therefore, during the in-flight experiments, we replace the DPC with a SIPC, which is a push broom imager with an about 650m×850 m ground sample distance, and formed the PCF-type instruments with the POSP, which is scanning in the along-track direction with approximately 52 m instantaneous field of view for the nadir measurements used in this work. Through the cooperative observation of the POSP and the SIPC, the high precision polarization data of POSP can be transmitted to SIPC by the in-flight cross calibration method of the same platform polarization instruments, which can effectively improve the data accuracy of SIPC.
In this paper, the polarization models of the POSP and the SIPC have been deduced, respectively. And then, we deduce the in-flight cross calibration model of the PCF instruments, which is comprised of the POSP and the SIPC. Finally, the in-flight experiments have been carried out to validate the radiometer-to-imager in-flight cross calibration model.
2. Cross calibration method
In this paper, the PCF-type instruments consist of the POSP and the SIPC, which are shown in Fig. 1, where POSP and SIPC are mounted on the same installation substrate, and the second and sixth parts in Fig. 1 are the field of view of POSP and SIPC, respectively. In addition, the scan mirrors of POSP are used to obtain data, and the filter wheel is in front of the lens and serve as filter for the optical system of SIPC to separate the light of different bands. The PCF-type instruments coordinated observation through time matching, space matching, and spectral matching, which are the basis for the radiometer-to-imager in-flight cross calibration, to obtain the high precision detection of comprehensive parameters of aerosol. Through radiometer-to-imager in-flight cross calibration, the high-precision calibration coefficients of the POSP are transmitted to the SIPC, which can effectively improve the measurement accuracy of the SIPC. Next, we will introduce the in-flight cross calibration method of the POSP and the SIPC.
2.1 Measurement model of the POSP
The optical measurement principle of the POSP is shown in Fig. 2, which is a multispectral scanning polarimeter with nine spectral channels in the near ultraviolet (NUV), visible (VIS) and shortwave infrared (SWIR) spectral channels. These spectral channels are centered at the wavelengths 380, 410, 443, 490, 670,865, 1380, 1610 and 2250 nm. Its optical system consists of orthogonal reflector groups, front-view telephoto lens, Wollaston prisms, color separation films, detector assemblies, etc. The target signal is introduced by the rotation of the orthogonal reflector groups to realize multi-angle observation. The Wollaston prisms are located behind the front-view telephoto lens and serve as polarization analyzers for the optical system to subdivide information from each band into four polarizations (0°, 45°, 90°, and 135°). The color separation films are used to separate the light of different bands, which are received by different detector assemblies.
Polarization measurement principle of the POSP is based on Pickering method [15] to measure stokes vector. The stokes vector is used to describe any state of polarized light, where the first parameter represents the total intensity of the light field and the remaining three parameters describe the polarization state. The Wollaston prism acts as a polarization analyzer for the optical system. These target light beams are analyzed by the pair of the Wollaston prisms, whose relative polarization azimuthal angles are at 45 °, and one Wollaston prism is used for analyze the 0° and 90° polarization components, while the other Wollaston prism is used for analyze the 45° and 135° polarization components. In this way, the pair of the Wollaston prisms divide an initial light beam spatially into two pairs of orthogonally polarized intensity signals ${S_{0^\circ }}$ and ${S_{90^\circ }}$, ${S_{45^\circ }}$ and ${S_{135^\circ }}$, which are received by the two double pixel detector assemblies. The Stokes parameters ($I,Q,U$) can be obtained by the calibration model of the POSP. The POSP measurement model [14] can be described by Eq. (1):
In Eq. (1) the subscripts 0 °, 90 °, 45 ° and 135 ° correspond to four light beams registered by the two double pixel detector assemblies, ${S_{0^\circ }}$ and ${S_{90^\circ }}$, ${S_{45^\circ }}$ and ${S_{135^\circ }}$ are the response Digital Number(DN) values of the four polarization directions of the POSP in the same band after corrected by the dark reference value. For the POSP, the values ${T_1},{T_2}$ are intra-telescope calibration factors that equalize non-polarized input responses:
$\xi (p )$ can be iterated until it converges to an acceptable level for edit distance on real sequence (EDR) retrieval:
2.2 Measurement model of the SIPC
The optical measurement principle of the SIPC is shown in Fig. 3, which is a push broom imager with the field of view of 10°×7.6°. The SIPC has six spectral channels, which centered at the wavelengths 565,670,763,865,910nm. Its optical system consists of filter wheel, lens, dispersion prism, three linear polarizers and three CCD detectors. The filter wheel is in front of the lens and serve as filter for the optical system to separate the light of different bands. The dispersion prism is located behind the lens and serve as the beam splitter for the optical system to divide the target signal into three beams with the same amplitude. These target light beams are analyzed by three linear polarizers, and producing three polarized intensity signals ${R_{0^\circ }}$, ${R_{45^\circ }}$ and ${R_{90^\circ }}$, which are received by the three CCD detectors.
It is assumed that the maximum transmittance of polarizer is ${t_x}$ and the minimum transmittance is ${t_y}$. In this case, the Mueller matrix ${M_P}(\theta )$ for a polarizer in an arbitrary orientation $\theta $ can be described by Eq. (7):
The measurement equation of the SIPC for the input light polarization ${S_{scene}}$ can be expressed as:
The measurement matrix of the SIPC can be obtained as follows:
2.3 In-flight cross calibration model
The POSP is equipped with in-flight calibration units that can maintain the polarization measurement accuracy while in the flight [16]. These in-flight calibration units can guarantee the long-term in-flight polarimetric measurement accuracy and measurement reliability of the POSP. However, the in-flight SIPC calibration is not provided because of the technical difficulties in the calibration units design. In addition, the long-term in-flight polarimetric measurement accuracy and measurement reliability of the SIPC will be reduced due to the lens contamination and polarizer degradation. Therefore, it is necessary to transmit the high-precision calibration coefficient of the POSP to the SIPC to effectively improve the measurement accuracy and measurement reliability of the SIPC.
The POSP and the SIPC are hosted on the same aircraft with the same centered wavelengths at 670 and 865 nm, which allows the radiance of scene simultaneously detected by the both radiometer and imager to be compared at the same viewing and solar angles, and in the nearly identical spectral bands. At the same time, the fields of view of the POSP and the SIPC are partially overlapped, which make the radiometer-to-imager in-flight cross calibration possible using the POSP data. The radiometer-to-imager in-flight cross calibration model is considered below.
From Eq. (12), we can see that Eq. (12) can be re-written as:
3. Polarimetric accuracy validation
The polarimetric accuracy validation of the POSP and the SPIC consist of two approaches, one is the laboratory polarimetric accuracy validation, and the other is the on-ground polarimetric accuracy validation. From the above analysis, we can see that the POSP and the SIPC share two channels with polarization sensitivity, roughly centered at 670 and 865nm, which can be used as the basis for the polarimetric accuracy validation. However, the extinction ratio of the APIR29G020 polarizer used in the three channels of the SIPC decreasing obviously in the spectral band of 865 nm, as shown in Fig. 4, which causes the SIPC to be unable to accurately analyze the polarization information in this spectral band, so that the experimental data are analyzed and compared only in the spectral band of 670nm in the later data processing.
3.1 Laboratory polarimetric accuracy validation
Before conducting the laboratory polarimetric accuracy validation, the laboratory calibration procedure of SIPC is briefly introduced and the calibration procedure of POSP can refer to the literature [14], which will not be introduced here. The goal of the laboratory calibration of SIPC is to determine the response matrix that links any combination of measurements of the uncalibrated normalized Stokes parameters to their true values. Through Section 2.2, we can get the measurement model of SIPC, which is shown in Eq. (15). Therefore, the calibration parameters for SIPC mainly include the depolarization factors $\gamma {\; }and{\; }\tau $ of polarizers, the orientation error $\varepsilon $ of the linear polarizer, the gain coefficients ${K_1}{\; }and{\; }{K_2}$ in the corresponded beam paths and the absolute radiometric calibration coefficient ${A_r}$.
For the calibration of the depolarization factors $\gamma {\; }and{\; }\tau $ of polarizers, it can be seen from Eq. (8) that the depolarization factors $\gamma {\; }and{\; }\tau $ of polarizers are mainly related to extinction ratio e of polarizer, and the extinction ratio e can reach 103 in the spectral band of 670 nm. Therefore, it can be calculated that the depolarization factors $\gamma {\; }and{\; }\tau $ of polarizers are roughly 0.998 and 0.998, respectively. For the calibration of the orientation error $\varepsilon $ of the linear polarizer, we can perform a rotating extinction experiment on the reference polarizer and the polarizer integrated in the SIPC to obtain the corresponding angular deviation. First, the reference polarizer is mounted on the precision turntable as a reference. Then, one of the directions is used as the direction of 0°detection, and the precision turntable is rotated continuously. When the direction of the transmission axis of the reference polarizer is parallel to that of the polarizer integrated in the SIPC, the light intensity of the detector is the largest, and the angle difference between the angle and the reference coordinate system is recorded. Finally, the true transmission axis directions of other directions are obtained so as to complete the calibration of the orientation error $\varepsilon $ of the linear polarizer. For the calibration of the gain coefficients ${K_1}{\; }and{\; }{K_2}$ in the corresponded beam paths and the absolute radiometric calibration coefficient ${A_r}$, this can be calculated using Eq. (17). The calibration results of the laboratory are shown in Table 1.
Table 1. Results of the laboratory calibration factors.
The VPOLS-II, a type of high-precision reference source with variable linear polarization degree at large dynamic range, aims to generate a specific degree of polarization of the light source, which is used for the laboratory polarimetric accuracy validation of the POSP and the SIPC. The VPOLS-II produces linear polarization rays by four parallel glass plates. First, the radiation source passes through the integrating sphere and a beam expander collimation system to form incident light with higher uniformity and parallelism, and then passes through the polarization state regulator to generate different linearly polarized light with a DoLP of 0,0.05,0.10,0.15,0.20,0.25, or 0.30, which is obtained by the POSP and the SIPC.
Results of the laboratory polarimetric accuracy validation of the POSP and the SIPC using the VPOLS-II are shown in Fig. 5 along with an independent, unpolarized (DoLP < 0.0005) validation measurement. For the laboratory polarimetric accuracy validation, the DoLP error in Fig. 5 is defined as the root mean square (RMS) of the measurements obtained by the POSP and the SIPC, and the error bars depict their extreme values. The DoLP errors, showing no dependence on the DoLP values itself, are on average (0.9 ± 0.2,0.6 ± 0.2) ×10−3 at 670 and 865nm obtained by POSP and on average (3 ± 0.3) ×10−3 at 670 nm obtained by SIPC, respectively.
3.2 On-ground polarimetric accuracy validation
Before conducting the on-ground polarimetric accuracy validation, the on-ground calibration procedure of SIPC is briefly introduced. For the SIPC, the calibration parameters of the depolarization factors $\gamma {\; }and{\; }\tau $ of polarizers and the orientation error $\varepsilon $ of the linear polarizer are mainly related to device processing and assembly, which is basically relatively stable. Therefore, there is no need to recalibrate these parameters during the on-ground experiment. Here, we only need to recalibrate the gain coefficients ${K_1}{\; }and{\; }{K_2}$ in the corresponded beam paths and the absolute radiometric calibration coefficient ${A_r}$, which may change because of changes in the environment and other factors. For the calibration of the gain coefficients ${K_1}{\; }and{\; }{K_2}$ in the corresponded beam paths and the absolute radiometric calibration coefficient ${A_r}$, this can be calculated using Eq. (17). The calibration results of the on-ground are shown in Table 2.
Table 2. Results of the on-ground calibration factors.
The on-ground polarimetric accuracy validation is designed to obtain the radiance and polarization of the sky by the POSP and the SIPC through scanning the sky at the same time and compare the data for verification [17]. The basic experimental conditions are as follows:
- (a) Test time: November 28, 2018, 14: 00-16: 00.
- (b) Location: Science Island, Hefei, Anhui Province, China.
- (c) Weather conditions: clear and cloudless.
- (d) Environmental conditions: temperature 20 ° C, humidity 42%.
The POSP and the SIPC are mounted on the same substrate, as shown in Fig. 6, and scan the main plane of the sun continuously to obtain the radiance and polarization of the sky. The filter wheel is used to switch the spectral band of the SIPC.
After the on-ground experimental, the data of the radiance and polarization of the sky obtained by the POSP and the SIPC at the same time are selected for processing. Through the measurement models of the POSP and the SIPC introduced above (see Section 2.1 and Section 2.2), the sky radiation luminance and degree of polarization obtained by the POSP and the SIPC can be calculated and the compare verification results are shown in Fig. 7.
It can be seen from Fig. 7 that the trends of the sky radiation luminance and degree of polarization obtained by the POSP and the SIPC in the spectral band of 670 nm have good consistency, and the sky radiation luminance and degree of polarization change slowly, indicating that the atmospheric state is relatively uniform and stable. By comparing the trends of the sky radiation luminance and degree of polarization of the POSP and the SIPC, we can see the correctness of measurement model of the POSP and the SIPC under natural target.
4. Cross calibration verification in the in-flight experiment
The POSP and the SIPC are deployed on the same aircraft in 2019 to participate the in-flight experiment, together with positioning POS system and other auxiliary equipment. Among them, the POSP and the SIPC work together to obtain surface polarized radiation data at the same time. The positioning POS system is used to obtain the useful information of the POSP and the SIPC, such as precision attitude angle, positioning coordinates and altitude, which are used for subsequent auxiliary data processing. The other auxiliary equipment is mainly used to acquire and store experiment data. The main objective of the in-flight experiment is to validate the radiometer-to-imager in-flight cross calibration model. The in-flight experiment is performed during March 2019, and includes a total of five flights, targeting predominantly northeast Hebei Province, which is shown in Fig. 8. Flights are performed on clear sky and cloudy days, over ocean, coastal, flat terrain, towns, mountains, etc.
The POSP, being an along-track scanner, and the SIPC, being a push broom imager, differ in spatial, angular, and resolution, and these differences must be accounted for in a point-to-point cross calibration of the data. For the radiometer-to-imager in-flight cross calibration, the spatial response matching of the POSP and the SIPC is a core problem. Fortunately, there is sufficient overlap for a point-to-point cross calibration between the POSP and the SIPC. In order to identify gridded SIPC data within the larger POSP footprint, and to process those data for cross calibration, we use the following steps.
- 1. Resolution matching. The spatial resolution of a SIPC image at nadir is 0.6×0.6m, whereas the instantaneous POSP footprint at nadir is 52 m in diameter, which is smeared by an additional 52 m in the flight direction due to rotation of the scan mirror during the exposure, and creating an elliptical resolution element with a non-uniform response. Thus, the coefficients of a non-uniform response should be created and applied to merge the SIPC pixel so as to match the POSP resolution.
- 2. Pixel alignment. Systematic errors in geolocation are corrected by optimizing the alignment of surface features in the matched pixels. Through pixel alignment, the POSP pixel and the SIPC pixel corresponding to the same point on the surface are associated, and correcting the positional relationship between the nadir of the POSP pixel and the image pixel of the SIPC.
After the resolution matching and pixel alignment of the POSP and the SIPC pixel, the radiometer-to-imager in-flight cross calibration factors can be calculated through the in-flight cross calibration model. To ensure the accuracy of the in-flight cross calibration factors, only uniform nadir data of the POSP and the SIPC are selected for calculation in this paper. The in-flight cross calibration factors are shown in Fig. 9, where Fig. 9(a) is the in-flight cross calibration factors over land, and Fig. 9(b) is the in-flight cross calibration factors over ocean.
The RMS of the above results can be obtained as shown in the Table 3.
Table 3. Results of the in-flight cross calibration factors.
Based on the cross calibration factors calculated above, two typical surfaces, such as land and ocean, are selected to verify the in-flight cross calibration model, and the results are shown in Fig. 10, where Fig. 10(a) is the calculation result of land radiance of the POSP and the SIPC, Fig. 10(b) is the calculation result of land polarization degree of the POSP and the SIPC, Fig. 10(c) is the calculation result of ocean radiance of the POSP and the SIPC, Fig. 10(d) is the calculation result of ocean polarization degree of the POSP and the SIPC.
It can be seen from the Fig. 10 that the land has higher reflectivity and lower polarization, while the ocean has lower reflectivity and higher polarization. The blue line represents the deviation of radiance or polarization degree from SIPC relative to POSP, and ΔL and ΔP are the RMS of these deviation values, respectively. The RMS of the above results can be obtained as shown in the Table 4.
Table 4. Verification results of the radiometer-to-imager in-flight cross calibration.
As can be seen from the Table 4, on the land, the radiance have a roughly 2.5% bias of SIPC relative to POSP and the polarization degree have a roughly 0.01 bias of SIPC relative to POSP, while on the ocean, the radiance have a roughly 9.3% bias of SIPC relative to POSP and the polarization degree have a roughly 0.04 bias of SIPC relative to POSP. We also find that observations of ocean scenes, where the deviation of polarization degree and radiance is larger than that of land, but the range of deviation is acceptable, which can illustrate the correctness of the radiometer-to-imager in-flight cross calibration model.
5. Discussion
The China Polarization Cross-Fire Suite (PCF) [7,11] concept consists of the directional polarization camera (DPC) [12,13] and the particulate observing scanning polarimeter (POSP) [14], which is similar to those of the Aerosol-UA mission [5–8] instruments concept composed of the multispectral imaging polarimeter (MSIP) [10] and the multispectral Scanning Polarimeter (ScanPol) [9]. However, the ScanPol polarimeter allows for multiangle scanning along the track, similar to the APS / Glory concept, while the POSP polarimeter provides cross-track scanning. Obviously, the data obtained by these two instruments through different scanning methods have their own advantages. For example, using along-track scanning can obtain multi-angle data, while using cross-track scanning can obtain larger width data. For the PCF system composed of POSP and DPC, because of the large field of view of DPC, the multi-angle data can be obtained. Therefore, in this case, the POSP can be set to cross-track scanning mode, so that the larger width multi-angle data can be obtained. However, the method of compensation pixel registration for the optical wedge motion of DPC is difficult to realize during the in-flight experiment, so the DPC is not suitable for the in-flight experiment. Therefore, during the in-flight experiments, we replace the DPC with a SIPC, which is a simultaneous imaging polarization camera (SIPC), and there is no problem of optical wedge motion compensation registration. But the field of view of SIPC is small, in order to get more multi-angle data, it is reasonable to set the POSP to the along-track scanning mode during the in-flight experiment. In addition, it is a pity that the POSP and the SIPC have only one spectral band of 670 nm to be used for cross calibration.
In the future, the high-precision spatial detection of atmospheric aerosols with polarization remote sensors on the same platform are the direction of future development. It is meaningful to obtain rich polarization remote sensing data by rationally using two or more polarization remote sensors or polarization detection methods to work together, and through Multi-sensors data fusion to acquire polarization remote sensing data incorporating multi-angle, multi-spectral, high-precision and high spatial resolution, which has more advantages than the polarization remote sensing data obtained with a single polarization remote sensor.
6. Summary
In this paper, the radiometer-to-imager in-flight cross calibration methods are discussed and validated through the in-flight experiment. First, the polarization models of the POSP and the SIPC, as well as the radiometer-to-imager in-flight cross calibration model are deduced. Then, the polarimetric accuracy validation experiment, including the laboratory polarimetric accuracy validation and the on-ground polarimetric accuracy validation, are carried out to validate the correctness of the polarization models of the POSP and the SIPC. The laboratory polarimetric accuracy validation of the POSP and the SIPC are performed using the VPOLS-II, demonstrating excellent polarimetric performance: the POSP DoLP errors recorded at two wavelengths (670 and 865 nm) are on average (0.9 ± 0.2,0.6 ± 0.2) ×10−3 and the SIPC DoLP error recorded at one wavelength (670 nm) are on average (3 ± 0.3)×10−3, respectively, while the results of the on-ground polarimetric accuracy validation show that the trends of the sky radiation luminance and degree of polarization obtained by the POSP and the SIPC in the spectral band of 670 nm have good consistency. Finally, the in-flight experiments have been carried out to validate the radiometer-to-imager in-flight cross calibration model. The results have shown that the radiance have a roughly 2.5% bias of SIPC relative to POSP and the polarization degree have a roughly 0.01 bias of SIPC relative to POSP on the land, while on the ocean, the radiance have a roughly 9.3% bias of SIPC relative to POSP and the polarization degree have a roughly 0.04 bias of SIPC relative to POSP, which can illustrate the correctness of the radiometer-to-imager in-flight cross calibration model.
Funding
International Team of Advanced Polarization Remote Sensing Technology and Application (GJTD-2018-15).
Acknowledgments
We thank the POSP team for their contributions to the project and the manuscript. We also thank the large team involved in the successful conduct of the in-flight experiment. Part of this research is performed at Anhui Institute of Optics and Fine Mechanics, CAS, Hefei, China.
Disclosures
The authors declare no conflicts of interest.
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