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Optical and Photonic Materials|111 Article(s)
Tuning exciton dynamics by the dielectric confinement effect in quasi-two-dimensional perovskites
Minghuan Cui, Chaochao Qin, Yuanzhi Jiang, Shichen Zhang, Changjiu Sun, Mingjian Yuan, Yonggang Yang, and Yufang Liu
The dielectric confinement effect plays an essential role in optoelectronic devices. Existing studies on the relationship between the dielectric confinement and the photoelectric properties are inadequate. Herein, three organic spacers with different dielectric constants are employed to tune the exciton dynamics of quasi-two-dimensional (quasi-2D) Ruddlesden–Popper perovskite films. Femtosecond transient absorption spectroscopy reveals that the small dielectric constant ligand enables a weak dynamic disorder and a large modulation depth of the coherent phonons, resulting in a more complete energy transfer and the inhibition of a trap-mediated nonradiative recombination. Additionally, the increase in the bulk-ligand dielectric constant reduces the corresponding exciton binding energy and then suppresses the Auger recombination, which is beneficial for high-luminance light-emitting diodes. This work emphasizes the importance of dielectric confinement for regulating the exciton dynamics of layered perovskites. The dielectric confinement effect plays an essential role in optoelectronic devices. Existing studies on the relationship between the dielectric confinement and the photoelectric properties are inadequate. Herein, three organic spacers with different dielectric constants are employed to tune the exciton dynamics of quasi-two-dimensional (quasi-2D) Ruddlesden–Popper perovskite films. Femtosecond transient absorption spectroscopy reveals that the small dielectric constant ligand enables a weak dynamic disorder and a large modulation depth of the coherent phonons, resulting in a more complete energy transfer and the inhibition of a trap-mediated nonradiative recombination. Additionally, the increase in the bulk-ligand dielectric constant reduces the corresponding exciton binding energy and then suppresses the Auger recombination, which is beneficial for high-luminance light-emitting diodes. This work emphasizes the importance of dielectric confinement for regulating the exciton dynamics of layered perovskites.
Photonics Research
- Publication Date: Mar. 01, 2024
- Vol. 12, Issue 3, 563 (2024)
Wafer-level substrate-free YIG single crystal film for a broadband tunable terahertz isolator|Editors' Pick
Xilai Zhang, Dan Zhao, Ding Zhang, Qiang Xue, Fei Fan, Yulong Liao, Qinghui Yang, and Qiye Wen
Yttrium iron garnet (YIG) is a promising material for various terahertz applications due to its special optical properties. At present, a high-quality YIG wafer is the desire of terahertz communities and it is still challenging to prepare substrate-free YIG single crystal films. In this work, we prepared wafer-level substrate-free La:YIG single crystal films, for the first time, to our knowledge. Terahertz optical and magneto-optical properties of La:YIG films were characterized by terahertz time domain spectroscopy (THz-TDS). Results show that the as-prepared La:YIG film has an insertion loss of less than 3 dB and a low absorption coefficient of less than 10 cm-1 below 1.6 THz. Benefitting from the thickness of the substrate-free YIG films and low insertion loss, their terahertz properties could be further manipulated by simply using a wafer-stacking technique. When four La:YIG films were stacked, there was an insertion loss of less than 10 dB in the range of 0.1-1.2 THz. The Faraday rotation angle of the four-layer-stacked La:YIG films reached 19°, and the isolation could reach 17 dB. By further increasing the stacking number to eight pieces, a remarkable Faraday rotation angle of 45° was achieved with an isolation of 23 dB, which is important for practical application in the THz band. This material may provide a milestone opportunity to make various non-reciprocal devices, such as isolators and phase shifters. Yttrium iron garnet (YIG) is a promising material for various terahertz applications due to its special optical properties. At present, a high-quality YIG wafer is the desire of terahertz communities and it is still challenging to prepare substrate-free YIG single crystal films. In this work, we prepared wafer-level substrate-free La:YIG single crystal films, for the first time, to our knowledge. Terahertz optical and magneto-optical properties of La:YIG films were characterized by terahertz time domain spectroscopy (THz-TDS). Results show that the as-prepared La:YIG film has an insertion loss of less than 3 dB and a low absorption coefficient of less than 10 cm-1 below 1.6 THz. Benefitting from the thickness of the substrate-free YIG films and low insertion loss, their terahertz properties could be further manipulated by simply using a wafer-stacking technique. When four La:YIG films were stacked, there was an insertion loss of less than 10 dB in the range of 0.1-1.2 THz. The Faraday rotation angle of the four-layer-stacked La:YIG films reached 19°, and the isolation could reach 17 dB. By further increasing the stacking number to eight pieces, a remarkable Faraday rotation angle of 45° was achieved with an isolation of 23 dB, which is important for practical application in the THz band. This material may provide a milestone opportunity to make various non-reciprocal devices, such as isolators and phase shifters.
Photonics Research
- Publication Date: Mar. 01, 2024
- Vol. 12, Issue 3, 505 (2024)
Indium-doped perovskite-related cesium copper halide scintillator films for high-performance X-ray imaging
Rui Liu, Zhiyong Liu, Chengxu Lin, Guangda Niu, Xuning Zhang, Bo Sun, Tielin Shi, and Guanglan Liao
Scintillators are widely utilized in high-energy radiation detection in view of their high light yield and short fluorescence decay time. However, constrained by their current shortcomings, such as complex fabrication procedures, high temperature, and difficulty in the large scale, it is difficult to meet the increasing demand for cost-effective, flexible, and environment-friendly X-ray detection using traditional scintillators. Perovskite-related cesium copper halide scintillators have recently received multitudinous research due to their tunable emission wavelength, high photoluminescence quantum yield (PLQY), and excellent optical properties. Herein, we demonstrated a facile solution-synthesis route for indium-doped all-inorganic cesium copper iodide (Cs3Cu2I5) powders and a high scintillation yield flexible film utilizing indium-doped Cs3Cu2I5 powders. The large area flexible films achieved a PLQY as high as 90.2% by appropriately adjusting the indium doping concentration, much higher than the undoped one (73.9%). Moreover, benefiting from low self-absorption and high PLQY, the Cs3Cu2I5:In films exhibited ultralow detection limit of 56.2 nGy/s, high spatial resolution up to 11.3 lp/mm, and marvelous relative light output with strong stability, facilitating that Cs3Cu2I5:In films are excellent candidates for X-ray medical radiography. Our work provides an effective strategy for developing environment-friendly, low-cost, and efficient scintillator films, showing great potential in the application of high-performance X-ray imaging. Scintillators are widely utilized in high-energy radiation detection in view of their high light yield and short fluorescence decay time. However, constrained by their current shortcomings, such as complex fabrication procedures, high temperature, and difficulty in the large scale, it is difficult to meet the increasing demand for cost-effective, flexible, and environment-friendly X-ray detection using traditional scintillators. Perovskite-related cesium copper halide scintillators have recently received multitudinous research due to their tunable emission wavelength, high photoluminescence quantum yield (PLQY), and excellent optical properties. Herein, we demonstrated a facile solution-synthesis route for indium-doped all-inorganic cesium copper iodide (Cs3Cu2I5) powders and a high scintillation yield flexible film utilizing indium-doped Cs3Cu2I5 powders. The large area flexible films achieved a PLQY as high as 90.2% by appropriately adjusting the indium doping concentration, much higher than the undoped one (73.9%). Moreover, benefiting from low self-absorption and high PLQY, the Cs3Cu2I5:In films exhibited ultralow detection limit of 56.2 nGy/s, high spatial resolution up to 11.3 lp/mm, and marvelous relative light output with strong stability, facilitating that Cs3Cu2I5:In films are excellent candidates for X-ray medical radiography. Our work provides an effective strategy for developing environment-friendly, low-cost, and efficient scintillator films, showing great potential in the application of high-performance X-ray imaging.
Photonics Research
- Publication Date: Feb. 01, 2024
- Vol. 12, Issue 2, 369 (2024)
Visible-frequency nonvolatile reconfigurable Janus metasurfaces for dual-wavelength-switched and spin-asymmetric holograms|Editors' Pick
Huan Yuan, Zheqiang Zhong, and Bin Zhang
Janus metasurface holography with asymmetric transmission characteristics provides new degrees of freedom for multiplexing technologies. However, earlier metasurfaces with asymmetrical transmission faced limitations in terms of tunability and multifunctionality. In this study, we propose a metasurface color holographic encryption scheme with dynamic switching and asymmetric transmission at visible frequencies using a low-loss nonvolatile optical phase-change material, Sb2S3. Using a modified holographic optimization strategy, we achieved high-fidelity asymmetric holographic imaging of a nanostructured metasurface. By controlling the incident direction and wavelength of visible light, as well as the level of crystallization of Sb2S3, this reconfigurable metasurface enables the precise manipulation of tunable color holographic image displays. In particular, in the semi-crystalline state of Sb2S3, the encoded information can be securely encrypted using a two-channel color-holographic image, whereas only a preset camouflaged image is displayed in the crystalline or amorphous state of Sb2S3. The proposed multiencrypted Janus metasurface provides a potential approach for dynamic holographic displays with ultrahigh capacity, holographic encryption, and information storage. Janus metasurface holography with asymmetric transmission characteristics provides new degrees of freedom for multiplexing technologies. However, earlier metasurfaces with asymmetrical transmission faced limitations in terms of tunability and multifunctionality. In this study, we propose a metasurface color holographic encryption scheme with dynamic switching and asymmetric transmission at visible frequencies using a low-loss nonvolatile optical phase-change material, Sb2S3. Using a modified holographic optimization strategy, we achieved high-fidelity asymmetric holographic imaging of a nanostructured metasurface. By controlling the incident direction and wavelength of visible light, as well as the level of crystallization of Sb2S3, this reconfigurable metasurface enables the precise manipulation of tunable color holographic image displays. In particular, in the semi-crystalline state of Sb2S3, the encoded information can be securely encrypted using a two-channel color-holographic image, whereas only a preset camouflaged image is displayed in the crystalline or amorphous state of Sb2S3. The proposed multiencrypted Janus metasurface provides a potential approach for dynamic holographic displays with ultrahigh capacity, holographic encryption, and information storage.
Photonics Research
- Publication Date: Feb. 01, 2024
- Vol. 12, Issue 2, 356 (2024)
Circular polarization-selective optical, photothermal, and optofluidic effects in chiral metasurfaces
Cuiping Ma, Peng Yu, Zhimin Jing, Yisong Zhu, Peihang Li, Wenhao Wang, Hongxing Xu, Yanning Zhang, Liang Pan, Tae-Youl Choi, Arup Neogi, Alexander O. Govorov, and Zhiming Wang
Circular dichroism (CD) is extensively used in various material systems for applications including biological detection, enantioselective catalysis, and chiral separation. This paper introduces a chiral absorptive metasurface that exhibits a circular polarization-selective effect in dual bands—positive and negative CD peaks at short wavelengths and long wavelengths, respectively. Significantly, we uncover that this phenomenon extends beyond the far-field optical response, as it is also observed in the photothermal effect and the dynamics of thermally induced fluid motion. By carefully engineering the metasurface design, we achieve two distinct CD signals with high g factors (∼1) at the wavelengths of 877 nm and 1045 nm, respectively. The findings presented in this study advance our comprehension of CD and offer promising prospects for enhancing chiral light–matter interactions in the domains of nanophotonics and optofluidics. Circular dichroism (CD) is extensively used in various material systems for applications including biological detection, enantioselective catalysis, and chiral separation. This paper introduces a chiral absorptive metasurface that exhibits a circular polarization-selective effect in dual bands—positive and negative CD peaks at short wavelengths and long wavelengths, respectively. Significantly, we uncover that this phenomenon extends beyond the far-field optical response, as it is also observed in the photothermal effect and the dynamics of thermally induced fluid motion. By carefully engineering the metasurface design, we achieve two distinct CD signals with high g factors (∼1) at the wavelengths of 877 nm and 1045 nm, respectively. The findings presented in this study advance our comprehension of CD and offer promising prospects for enhancing chiral light–matter interactions in the domains of nanophotonics and optofluidics.
Photonics Research
- Publication Date: Feb. 01, 2024
- Vol. 12, Issue 2, 331 (2024)
Electrically tunable phase-change metasurface for dynamic infrared thermal camouflage
Yufeng Xiong, Yunzheng Wang, Chao Feng, Yaolan Tian, Liang Gao, Jun-Lei Wang, Zhuang Zhuo, and Xian Zhao
Dynamic infrared thermal camouflage technology has attracted extensive attention due to its ability to thermally conceal targets in various environmental backgrounds by tuning thermal emission. The use of phase change materials (PCMs) offers numerous advantages, including zero static power, rapid modulation rate, and large emissivity tuning range. However, existing PCM solutions still encounter several practical application challenges, such as temperature uniformity, amorphization achievement, and adaptability to different environments. In this paper, we present the design of an electrically controlled metal-insulator-metal thermal emitter based on a PCM metasurface, and numerically investigate its emissivity tunability, physical mechanisms, heat conduction, and thermal camouflage performance across different backgrounds. Furthermore, the influence of the quench rate on amorphization was studied to provide a guidance for evaluating and optimizing device structures. Simulation results reveal that the thermal emitter exhibits a wide spectral emissivity tuning range between 8 and 14 μm, considerable quench rates for achieving amorphization, and the ability to provide thermal camouflage across a wide background temperature range. Therefore, it is anticipated that this contribution will promote the development of PCM-based thermal emitters for practical dynamic infrared thermal camouflage technology with broad applications in both civilian and military domains. Dynamic infrared thermal camouflage technology has attracted extensive attention due to its ability to thermally conceal targets in various environmental backgrounds by tuning thermal emission. The use of phase change materials (PCMs) offers numerous advantages, including zero static power, rapid modulation rate, and large emissivity tuning range. However, existing PCM solutions still encounter several practical application challenges, such as temperature uniformity, amorphization achievement, and adaptability to different environments. In this paper, we present the design of an electrically controlled metal-insulator-metal thermal emitter based on a PCM metasurface, and numerically investigate its emissivity tunability, physical mechanisms, heat conduction, and thermal camouflage performance across different backgrounds. Furthermore, the influence of the quench rate on amorphization was studied to provide a guidance for evaluating and optimizing device structures. Simulation results reveal that the thermal emitter exhibits a wide spectral emissivity tuning range between 8 and 14 μm, considerable quench rates for achieving amorphization, and the ability to provide thermal camouflage across a wide background temperature range. Therefore, it is anticipated that this contribution will promote the development of PCM-based thermal emitters for practical dynamic infrared thermal camouflage technology with broad applications in both civilian and military domains.
Photonics Research
- Publication Date: Feb. 01, 2024
- Vol. 12, Issue 2, 292 (2024)
Giant two-photon absorption of anatase TiO2 in Au/TiO2 core-shell nanoparticles
Lijie Wang, Tsz Him Chow, Malte Oppermann, Jianfang Wang, and Majed Chergui
We report on deep-to-near-UV transient absorption spectra of core-shell Au/SiO2 and Au/TiO2 nanoparticles (NPs) excited at the surface plasmon resonance of the Au core, and of UV-excited bare anatase TiO2 NPs. The bleaching of the first excitonic transition of anatase TiO2 at ∼3.8 eV is a signature of the presence of electrons/holes in the conduction band (CB)/valence band (VB) of the material. We find that while in bare anatase TiO2 NPs, two-photon excitation does not occur up to the highest used fluences (1.34 mJ/cm2), it takes place in the TiO2 shell at moderate fluences (0.18 mJ/cm2) in Au/TiO2 core-shell NPs, as a result of an enhancement due to the plasmon resonance. We estimate the enhancement factor to be of the order of ∼108–109. Remarkably, we observe that the bleach of the 3.8 eV band of TiO2 lives significantly longer than in bare TiO2, suggesting that the excess electrons/holes in the conduction/valence band are stored longer in this material. We report on deep-to-near-UV transient absorption spectra of core-shell Au/SiO2 and Au/TiO2 nanoparticles (NPs) excited at the surface plasmon resonance of the Au core, and of UV-excited bare anatase TiO2 NPs. The bleaching of the first excitonic transition of anatase TiO2 at ∼3.8 eV is a signature of the presence of electrons/holes in the conduction band (CB)/valence band (VB) of the material. We find that while in bare anatase TiO2 NPs, two-photon excitation does not occur up to the highest used fluences (1.34 mJ/cm2), it takes place in the TiO2 shell at moderate fluences (0.18 mJ/cm2) in Au/TiO2 core-shell NPs, as a result of an enhancement due to the plasmon resonance. We estimate the enhancement factor to be of the order of ∼108–109. Remarkably, we observe that the bleach of the 3.8 eV band of TiO2 lives significantly longer than in bare TiO2, suggesting that the excess electrons/holes in the conduction/valence band are stored longer in this material.
Photonics Research
- Publication Date: Jun. 28, 2023
- Vol. 11, Issue 7, 1303 (2023)
Broadband omnidirectional visible spectral metamaterials
Jing Zhao, Xianfeng Wu, Di Cao, Mingchao Zhou, Zhijie Shen, and Xiaopeng Zhao
Optical metamaterials offer the possibility of controlling the behavior of photons similarly to what has been done about electrons in semiconductors. However, most optical metamaterials are narrowband, and they achieve negative refraction within a small window of incident angles, making them impractical for common visible light systems that operate effectively over a wide range of frequencies and directions. Considerable resistive loss at the resonant frequency of these metamaterials further prevents them from being deployed in the real world. Here, we develop a novel metamaterial randomly assembled by a list of narrowband, omnidirectional, and ultralow-loss meta-cluster systems using a bottom-up approach. Weak interactions among numerous meta-cluster sets greatly broaden the effective bandwidth of the overall structure, exhibiting frequency selectivity and spatial modulation when responding to white-light illumination. We observe negative refraction in the 490–730 nm band, and observe an inverse Doppler effect at green, yellow, and red frequencies, across most of the visible spectrum. Our method allows for low-cost fabrication of sizable broadband omnidirectional three-dimensional metamaterial samples, which opens the door to the rapid development of optical metamaterials, micro–nano assembly and preparation, tunable optical device engineering, etc. Optical metamaterials offer the possibility of controlling the behavior of photons similarly to what has been done about electrons in semiconductors. However, most optical metamaterials are narrowband, and they achieve negative refraction within a small window of incident angles, making them impractical for common visible light systems that operate effectively over a wide range of frequencies and directions. Considerable resistive loss at the resonant frequency of these metamaterials further prevents them from being deployed in the real world. Here, we develop a novel metamaterial randomly assembled by a list of narrowband, omnidirectional, and ultralow-loss meta-cluster systems using a bottom-up approach. Weak interactions among numerous meta-cluster sets greatly broaden the effective bandwidth of the overall structure, exhibiting frequency selectivity and spatial modulation when responding to white-light illumination. We observe negative refraction in the 490–730 nm band, and observe an inverse Doppler effect at green, yellow, and red frequencies, across most of the visible spectrum. Our method allows for low-cost fabrication of sizable broadband omnidirectional three-dimensional metamaterial samples, which opens the door to the rapid development of optical metamaterials, micro–nano assembly and preparation, tunable optical device engineering, etc.
Photonics Research
- Publication Date: Jun. 23, 2023
- Vol. 11, Issue 7, 1293 (2023)
Completely spin-decoupled geometric phase of a metasurface
Xinmin Fu, Jie Yang, Jiafu Wang, Yajuan Han, Chang Ding, Tianshuo Qiu, Bingyue Qu, Lei Li, Yongfeng Li, and Shaobo Qu
Metasurfaces have provided an unprecedented degree of freedom (DOF) in the manipulation of electromagnetic waves. A geometric phase can be readily obtained by rotating the meta-atoms of a metasurface. Nevertheless, such geometric phases are usually spin-coupled, with the same magnitude but opposite signs for left- and right-handed circularly polarized (LCP and RCP) waves. To achieve independent control of LCP and RCP waves, it is crucial to obtain spin-decoupled geometric phases. In this paper, we propose to obtain completely spin-decoupled geometric phases by engineering the surface current paths on meta-atoms. Based on the rotational Doppler effect, the rotation manner is first analyzed, and it is found that the generation of a geometric phase lies in the rotation of the surface current paths on meta-atoms. Since the induced surface current paths under the LCP and RCP waves always start oppositely and are mirror-symmetrical with each other, it is natural that the geometric phases have the same magnitude and opposite signs when the meta-atoms are rotated. To obtain spin-decoupled geometric phases, the induced surface current under one spin should be rotated by one angle while the current under the other spin is rotated by a different angles. In this way, LCP and RCP waves can acquire different geometric phase changes. Proof-of-principle prototypes were designed, fabricated, and measured. Both the simulation and experiment results verify spin-decoupled geometric phases. This work provides a robust means to obtain a spin-dependent geometric phase and can be readily extended to higher frequency bands such as the terahertz, IR, and optical regimes. Metasurfaces have provided an unprecedented degree of freedom (DOF) in the manipulation of electromagnetic waves. A geometric phase can be readily obtained by rotating the meta-atoms of a metasurface. Nevertheless, such geometric phases are usually spin-coupled, with the same magnitude but opposite signs for left- and right-handed circularly polarized (LCP and RCP) waves. To achieve independent control of LCP and RCP waves, it is crucial to obtain spin-decoupled geometric phases. In this paper, we propose to obtain completely spin-decoupled geometric phases by engineering the surface current paths on meta-atoms. Based on the rotational Doppler effect, the rotation manner is first analyzed, and it is found that the generation of a geometric phase lies in the rotation of the surface current paths on meta-atoms. Since the induced surface current paths under the LCP and RCP waves always start oppositely and are mirror-symmetrical with each other, it is natural that the geometric phases have the same magnitude and opposite signs when the meta-atoms are rotated. To obtain spin-decoupled geometric phases, the induced surface current under one spin should be rotated by one angle while the current under the other spin is rotated by a different angles. In this way, LCP and RCP waves can acquire different geometric phase changes. Proof-of-principle prototypes were designed, fabricated, and measured. Both the simulation and experiment results verify spin-decoupled geometric phases. This work provides a robust means to obtain a spin-dependent geometric phase and can be readily extended to higher frequency bands such as the terahertz, IR, and optical regimes.
Photonics Research
- Publication Date: Jun. 16, 2023
- Vol. 11, Issue 7, 1162 (2023)
Blue perovskite single-mode lasing in a rubidium lead bromide microcubic cavity
Bo Li, Wangqi Mao, Shuang Liang, Yifeng Shi, Hongxing Dong, and Long Zhang
Lead halide perovskite microlasers have shown impressive performance in the green and red wavebands. However, there has been limited progress in achieving blue-emitting perovskite microlasers. Here, blue-emitting perovskite-phase rubidium lead bromide (RbPbBr3) microcubes were successfully prepared by using a one-step chemical vapor deposition process, which can be utilized to construct optically pumped whispering gallery mode microlasers. By regulating the growth temperature, we found that a high-temperature environment can facilitate the formation of the perovskite phase and microcubic morphology of RbPbBr3. Notably, blue single-mode lasing in a RbPbBr3 microcubic cavity with a narrow linewidth of 0.21 nm and a high-quality factor (∼2200) was achieved. The obtained lasing from RbPbBr3 microlasers also exhibited an excellent polarization state factor (∼0.77). By modulating the mixed-monovalent cation composition, the wavelength of the microlaser could be tuned from green (536 nm) to pure blue (468 nm). Additionally, the heat stability of the mix-cation perovskite was better than that of conventional CsPbBr3. The stable and high-performance blue single-mode microlasers may thus facilitate the application of perovskite lasers in blue laser fields. Lead halide perovskite microlasers have shown impressive performance in the green and red wavebands. However, there has been limited progress in achieving blue-emitting perovskite microlasers. Here, blue-emitting perovskite-phase rubidium lead bromide (RbPbBr3) microcubes were successfully prepared by using a one-step chemical vapor deposition process, which can be utilized to construct optically pumped whispering gallery mode microlasers. By regulating the growth temperature, we found that a high-temperature environment can facilitate the formation of the perovskite phase and microcubic morphology of RbPbBr3. Notably, blue single-mode lasing in a RbPbBr3 microcubic cavity with a narrow linewidth of 0.21 nm and a high-quality factor (∼2200) was achieved. The obtained lasing from RbPbBr3 microlasers also exhibited an excellent polarization state factor (∼0.77). By modulating the mixed-monovalent cation composition, the wavelength of the microlaser could be tuned from green (536 nm) to pure blue (468 nm). Additionally, the heat stability of the mix-cation perovskite was better than that of conventional CsPbBr3. The stable and high-performance blue single-mode microlasers may thus facilitate the application of perovskite lasers in blue laser fields.
Photonics Research
- Publication Date: May. 30, 2023
- Vol. 11, Issue 6, 1067 (2023)
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