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Nanophotonics and Photonic Crystals|68 Article(s)
Strong light–matter interactions based on excitons and the abnormal all-dielectric anapole mode with both large field enhancement and low loss
Yan-Hui Deng, Yu-Wei Lu, Hou-Jiao Zhang, Zhong-Hong Shi, Zhang-Kai Zhou, and Xue-Hua Wang
The room temperature strong coupling between the photonic modes of micro/nanocavities and quantum emitters (QEs) can bring about promising advantages for fundamental and applied physics. Improving the electric fields (EFs) by using plasmonic modes and reducing their losses by applying dielectric nanocavities are widely employed approaches to achieve room temperature strong coupling. However, ideal photonic modes with both large EFs and low loss have been lacking. Herein, we propose the abnormal anapole mode (AAM), showing both a strong EF enhancement of ∼70-fold (comparable to plasmonic modes) and a low loss of 34 meV, which is much smaller than previous records of isolated all-dielectric nanocavities. Besides realizing strong coupling, we further show that by replacing the normal anapole mode with the AAM, the lasing threshold of the AAM-coupled QEs can be reduced by one order of magnitude, implying a vital step toward on-chip integration of nanophotonic devices. The room temperature strong coupling between the photonic modes of micro/nanocavities and quantum emitters (QEs) can bring about promising advantages for fundamental and applied physics. Improving the electric fields (EFs) by using plasmonic modes and reducing their losses by applying dielectric nanocavities are widely employed approaches to achieve room temperature strong coupling. However, ideal photonic modes with both large EFs and low loss have been lacking. Herein, we propose the abnormal anapole mode (AAM), showing both a strong EF enhancement of ∼70-fold (comparable to plasmonic modes) and a low loss of 34 meV, which is much smaller than previous records of isolated all-dielectric nanocavities. Besides realizing strong coupling, we further show that by replacing the normal anapole mode with the AAM, the lasing threshold of the AAM-coupled QEs can be reduced by one order of magnitude, implying a vital step toward on-chip integration of nanophotonic devices.
Photonics Research
- Publication Date: Apr. 01, 2024
- Vol. 12, Issue 4, 854 (2024)
Dielectric metasurface evolution from bulk to monolayer by strong coupling of quasi-BICs for second harmonic boosting
Yinong Xie, Qianting Chen, Jin Yao, Xueying Liu, Zhaogang Dong, and Jinfeng Zhu
2D materials are promising candidates as nonlinear optical components for on-chip devices due to their ultrathin structure. In general, their nonlinear optical responses are inherently weak due to the short interaction thickness with light. Recently, there has been great interest in using quasi-bound states in the continuum (q-BICs) of dielectric metasurfaces, which are able to achieve remarkable optical near-field enhancement for elevating the second harmonic generation (SHG) emission from 2D materials. However, most studies focus on the design of combining bulk dielectric metasurfaces with unpatterned 2D materials, which suffer considerable radiation loss and limit near-field enhancement by high-quality q-BIC resonances. Here, we investigate the dielectric metasurface evolution from bulk silicon to monolayer molybdenum disulfide (MoS2), and discover the critical role of meta-atom thickness design on enhancing near-field effects of two q-BIC modes. We further introduce the strong-coupling of the two q-BIC modes by oblique incidence manipulation, and enhance the localized optical field on monolayer MoS2 dramatically. In the ultraviolet and visible regions, the MoS2 SHG enhancement factor of our design is 105 times higher than that of conventional bulk metasurfaces, leading to an extremely high nonlinear conversion efficiency of 5.8%. Our research will provide an important theoretical guide for the design of high-performance nonlinear devices based on 2D materials. 2D materials are promising candidates as nonlinear optical components for on-chip devices due to their ultrathin structure. In general, their nonlinear optical responses are inherently weak due to the short interaction thickness with light. Recently, there has been great interest in using quasi-bound states in the continuum (q-BICs) of dielectric metasurfaces, which are able to achieve remarkable optical near-field enhancement for elevating the second harmonic generation (SHG) emission from 2D materials. However, most studies focus on the design of combining bulk dielectric metasurfaces with unpatterned 2D materials, which suffer considerable radiation loss and limit near-field enhancement by high-quality q-BIC resonances. Here, we investigate the dielectric metasurface evolution from bulk silicon to monolayer molybdenum disulfide (MoS2), and discover the critical role of meta-atom thickness design on enhancing near-field effects of two q-BIC modes. We further introduce the strong-coupling of the two q-BIC modes by oblique incidence manipulation, and enhance the localized optical field on monolayer MoS2 dramatically. In the ultraviolet and visible regions, the MoS2 SHG enhancement factor of our design is 105 times higher than that of conventional bulk metasurfaces, leading to an extremely high nonlinear conversion efficiency of 5.8%. Our research will provide an important theoretical guide for the design of high-performance nonlinear devices based on 2D materials.
Photonics Research
- Publication Date: Apr. 01, 2024
- Vol. 12, Issue 4, 784 (2024)
Observation of accurately designed bound states in the continuum in momentum space
Jiaju Wu, Jingguang Chen, Xin Qi, Zhiwei Guo, Jiajun Wang, Feng Wu, Yong Sun, Yunhui Li, Haitao Jiang, Lei Shi, Jian Zi, and Hong Chen
Bound states in the continuum (BICs) in artificial photonic structures have received considerable attention since they offer unique methods for the extreme field localization and enhancement of light-matter interactions. Usually, the symmetry-protected BICs are located at high symmetric points, while the positions of accidental BICs achieved by tuning the parameters will appear at some points in momentum space. Up to now, to accurately design the position of the accidental BIC in momentum space is still a challenge. Here, we theoretically and experimentally demonstrate an accurately designed accidental BIC in a two-coupled-oscillator system consisting of bilayer gratings, where the optical response of each grating can be described by a single resonator model. By changing the interlayer distance between the gratings to tune the propagation phase shift related to wave vectors, the position of the accidental BIC can be arbitrarily controlled in momentum space. Moreover, we present a general method and rigorous numerical analyses for extracting the polarization vector fields to observe the topological properties of BICs from the polarization-resolved transmission spectra. Finally, an application of the highly efficient second harmonic generation assisted by quasi-BIC is demonstrated. Our work provides a straightforward strategy for manipulating BICs and studying their topological properties in momentum space. Bound states in the continuum (BICs) in artificial photonic structures have received considerable attention since they offer unique methods for the extreme field localization and enhancement of light-matter interactions. Usually, the symmetry-protected BICs are located at high symmetric points, while the positions of accidental BICs achieved by tuning the parameters will appear at some points in momentum space. Up to now, to accurately design the position of the accidental BIC in momentum space is still a challenge. Here, we theoretically and experimentally demonstrate an accurately designed accidental BIC in a two-coupled-oscillator system consisting of bilayer gratings, where the optical response of each grating can be described by a single resonator model. By changing the interlayer distance between the gratings to tune the propagation phase shift related to wave vectors, the position of the accidental BIC can be arbitrarily controlled in momentum space. Moreover, we present a general method and rigorous numerical analyses for extracting the polarization vector fields to observe the topological properties of BICs from the polarization-resolved transmission spectra. Finally, an application of the highly efficient second harmonic generation assisted by quasi-BIC is demonstrated. Our work provides a straightforward strategy for manipulating BICs and studying their topological properties in momentum space.
Photonics Research
- Publication Date: Mar. 13, 2024
- Vol. 12, Issue 4, 638 (2024)
Observation of gapless corner modes of photonic crystal slabs in synthetic translation dimensions
Wen-Jin Zhang, Hao-Chang Mo, Wen-Jie Chen, Xiao-Dong Chen, and Jian-Wen Dong
Second-order topological photonic crystals support localized corner modes that deviate from the conventional bulk-edge correspondence. However, the frequency shift of corner modes spanning the photonic band gap has not been experimentally reported. Here, we observe the gapless corner modes of photonic crystal slabs within a parameter space by considering translation as an additional synthetic dimension. These corner modes, protected by topological pumping in synthetic translation dimensions, are found to exist independently of the specific corner configuration. The gapless corner modes are experimentally imaged via the near-field scanning measurement and validated numerically by full-wave simulations. We propose a topological rainbow with gradient translation, demonstrating the ability to extract and separate specific frequency components of light into different spatial locations. Our work contributes to the advancement of topological photonics and provides valuable insights into the exploration of gapless corner modes in synthetic dimensions. Second-order topological photonic crystals support localized corner modes that deviate from the conventional bulk-edge correspondence. However, the frequency shift of corner modes spanning the photonic band gap has not been experimentally reported. Here, we observe the gapless corner modes of photonic crystal slabs within a parameter space by considering translation as an additional synthetic dimension. These corner modes, protected by topological pumping in synthetic translation dimensions, are found to exist independently of the specific corner configuration. The gapless corner modes are experimentally imaged via the near-field scanning measurement and validated numerically by full-wave simulations. We propose a topological rainbow with gradient translation, demonstrating the ability to extract and separate specific frequency components of light into different spatial locations. Our work contributes to the advancement of topological photonics and provides valuable insights into the exploration of gapless corner modes in synthetic dimensions.
Photonics Research
- Publication Date: Feb. 26, 2024
- Vol. 12, Issue 3, 444 (2024)
Controllable shaping of high-index dielectric nanoparticles by exploiting the giant optical force of femtosecond laser pulses
Yuheng Mao, Shuwen Bai, Mingcheng Panmai, Lidan Zhou, Shimei Liu, Shulei Li, Haiying Liu, Haihua Fan, Jun Dai, and Sheng Lan
Nanoparticles made of different materials usually support optical resonances in the visible to near infrared spectral range, such as the localized surface plasmons observed in metallic nanoparticles and the Mie resonances observed in dielectric ones. Such optical resonances, which are important for practical applications, depend strongly on the morphologies of nanoparticles. Laser irradiation is a simple but effective way to modify such optical resonances through the change in the morphology of a nanoparticle. Although laser-induced shaping of metallic nanoparticles has been successfully demonstrated, it remains a big challenge for dielectric nanoparticles due to their larger Young’s modulus and smaller thermal conductivities. Here, we proposed and demonstrated a strategy for realizing controllable shaping of high-index dielectric nanoparticles by exploiting the giant optical force induced by femtosecond laser pulses. It was found that both Si and Ge nanoparticles can be lit up by resonantly exciting the optical resonances with femtosecond laser pulses, leading to the luminescence burst when the laser power exceeds a threshold. In addition, the morphologies of Si and Ge nanoparticles can be modified by utilizing the giant absorption force exerted on them and the reduced Young’s modulus at high temperatures. The shape transformation from sphere to ellipsoid can be realized by laser irradiation, leading to the blueshifts of the optical resonances. It was found that Si and Ge nanoparticles were generally elongated along the direction parallel to the polarization of the laser light. Controllable shaping of Si and Ge can be achieved by deliberately adjusting the excitation wavelength and the laser power. Our findings are helpful for understanding the giant absorption force of femtosecond laser light and are useful for designing nanoscale photonic devices based on shaped high-index nanoparticles. Nanoparticles made of different materials usually support optical resonances in the visible to near infrared spectral range, such as the localized surface plasmons observed in metallic nanoparticles and the Mie resonances observed in dielectric ones. Such optical resonances, which are important for practical applications, depend strongly on the morphologies of nanoparticles. Laser irradiation is a simple but effective way to modify such optical resonances through the change in the morphology of a nanoparticle. Although laser-induced shaping of metallic nanoparticles has been successfully demonstrated, it remains a big challenge for dielectric nanoparticles due to their larger Young’s modulus and smaller thermal conductivities. Here, we proposed and demonstrated a strategy for realizing controllable shaping of high-index dielectric nanoparticles by exploiting the giant optical force induced by femtosecond laser pulses. It was found that both Si and Ge nanoparticles can be lit up by resonantly exciting the optical resonances with femtosecond laser pulses, leading to the luminescence burst when the laser power exceeds a threshold. In addition, the morphologies of Si and Ge nanoparticles can be modified by utilizing the giant absorption force exerted on them and the reduced Young’s modulus at high temperatures. The shape transformation from sphere to ellipsoid can be realized by laser irradiation, leading to the blueshifts of the optical resonances. It was found that Si and Ge nanoparticles were generally elongated along the direction parallel to the polarization of the laser light. Controllable shaping of Si and Ge can be achieved by deliberately adjusting the excitation wavelength and the laser power. Our findings are helpful for understanding the giant absorption force of femtosecond laser light and are useful for designing nanoscale photonic devices based on shaped high-index nanoparticles.
Photonics Research
- Publication Date: Feb. 01, 2024
- Vol. 12, Issue 2, 282 (2024)
Refractive index sensing based on a twisted nano-kirigami metasurface
Shuqi Qiao, Xiaochen Zhang, Qinghua Liang, Yang Wang, Chang-Yin Ji, Xiaowei Li, Lan Jiang, Shuai Feng, Honglian Guo, and Jiafang Li
Plasmonic sensing technology has attracted considerable attention for high sensitivity due to the ability to effectively localize and manipulate light. In this study, we demonstrate a refractive index (RI) sensing scheme based on open-loop twisted meta-molecule arrays using the versatile nano-kirigami principle. RI sensing has the features of a small footprint, flexible control, and simple preparation. By engineering the morphology of meta-molecules or the RI of the ambient medium, the chiral surface lattice resonances can be significantly enhanced, and the wavelength, intensity, and sign of circular dichroism (CD) can be flexibly tailored. Utilizing the relation between the wavelength of the CD peak and the RI of the superstrate, the RI sensor achieves a sensitivity of 1133 nm/RIU. Additionally, we analyze these chiroptical responses by performing electromagnetic multipolar decomposition and electric field distributions. Our study may serve as an ideal platform for applications of RI measurement and provide new insights into the manipulation of chiral light–matter interactions. Plasmonic sensing technology has attracted considerable attention for high sensitivity due to the ability to effectively localize and manipulate light. In this study, we demonstrate a refractive index (RI) sensing scheme based on open-loop twisted meta-molecule arrays using the versatile nano-kirigami principle. RI sensing has the features of a small footprint, flexible control, and simple preparation. By engineering the morphology of meta-molecules or the RI of the ambient medium, the chiral surface lattice resonances can be significantly enhanced, and the wavelength, intensity, and sign of circular dichroism (CD) can be flexibly tailored. Utilizing the relation between the wavelength of the CD peak and the RI of the superstrate, the RI sensor achieves a sensitivity of 1133 nm/RIU. Additionally, we analyze these chiroptical responses by performing electromagnetic multipolar decomposition and electric field distributions. Our study may serve as an ideal platform for applications of RI measurement and provide new insights into the manipulation of chiral light–matter interactions.
Photonics Research
- Publication Date: Jan. 29, 2024
- Vol. 12, Issue 2, 218 (2024)
Three-dimensional nanoscale vortex line visualization and chiral nanostructure fabrication of tightly focused multi-vortex beams via direct laser writing|On the Cover
Mengdi Luo, Jisen Wen, Pengcheng Ma, Qiuyuan Sun, Xianmeng Xia, Gangyao Zhan, Zhenyao Yang, Liang Xu, Dazhao Zhu, Cuifang Kuang, and Xu Liu
Optical singularity is pivotal in nature and has attracted wide interest from many disciplines nowadays, including optical communication, quantum optics, and biomedical imaging. Visualizing vortex lines formed by phase singularities and fabricating chiral nanostructures using the evolution of vortex lines are of great significance. In this paper, we introduce a promising method based on two-photon polymerization direct laser writing (2PP-DLW) to record the morphology of vortex lines generated by tightly focused multi-vortex beams (MVBs) at the nanoscale. Due to Gouy phase, the singularities of the MVBs rotate around the optical axis and move towards each other when approaching the focal plane. The propagation dynamics of vortex lines are recorded by 2PP-DLW, which explicitly exhibits the evolution of the phase singularities. Additionally, the MVBs are employed to fabricate stable three-dimensional chiral nanostructures due to the spiral-forward property of the vortex line. Because of the obvious chiral features of the manufactured nanostructures, a strong vortical dichroism is observed when excited by the light carrying orbital angular momentum. A number of applications can be envisioned with these chiral nanostructures, such as optical sensing, chiral separation, and information storage. Optical singularity is pivotal in nature and has attracted wide interest from many disciplines nowadays, including optical communication, quantum optics, and biomedical imaging. Visualizing vortex lines formed by phase singularities and fabricating chiral nanostructures using the evolution of vortex lines are of great significance. In this paper, we introduce a promising method based on two-photon polymerization direct laser writing (2PP-DLW) to record the morphology of vortex lines generated by tightly focused multi-vortex beams (MVBs) at the nanoscale. Due to Gouy phase, the singularities of the MVBs rotate around the optical axis and move towards each other when approaching the focal plane. The propagation dynamics of vortex lines are recorded by 2PP-DLW, which explicitly exhibits the evolution of the phase singularities. Additionally, the MVBs are employed to fabricate stable three-dimensional chiral nanostructures due to the spiral-forward property of the vortex line. Because of the obvious chiral features of the manufactured nanostructures, a strong vortical dichroism is observed when excited by the light carrying orbital angular momentum. A number of applications can be envisioned with these chiral nanostructures, such as optical sensing, chiral separation, and information storage.
Photonics Research
- Publication Date: Dec. 21, 2023
- Vol. 12, Issue 1, 70 (2024)
Generating a nanoscale blade-like optical field in a coupled nanofiber pair
Yuxin Yang, Jiaxin Gao, Hao Wu, Zhanke Zhou, Liu Yang, Xin Guo, Pan Wang, and Limin Tong
An optical field with sub-nm confinement is essential for exploring atomic- or molecular-level light-matter interaction. While such fields demonstrated so far have typically point-like cross-sections, an optical field having a higher-dimensional cross-section may offer higher flexibility and/or efficiency in applications. Here, we propose generating a nanoscale blade-like optical field in a coupled nanofiber pair (CNP) with a 1-nm-width central slit. Based on a strong mode coupling-enabled slit waveguide mode, a sub-nm-thickness blade-like optical field can be generated with a cross-section down to ∼0.28 nm×38 nm at 1550 nm wavelength (i.e., a thickness of ∼λ0/5000) and a peak-to-background intensity ratio (PBR) higher than 20 dB. The slit waveguide mode of the CNP can be launched from one of the two nanofibers that are connected to a standard optical fiber via an adiabatical fiber taper, in which a fundamental waveguide mode of the fiber can be converted into a high-purity slit mode with high efficiency (>98%) within a CNP length of less than 10 μm at 1550 nm wavelength. The wavelength-dependent behaviors and group velocity dispersion in mode converting processes are also investigated, showing that such a CNP-based design is also suitable for broadband and ultrafast pulsed operation. Our results may open up new opportunities for studying light-matter interaction down to the sub-nm scale, as well as for exploring ultra-high-resolution optical technology ranging from super-resolution nanoscopy to chemical bond manipulation. An optical field with sub-nm confinement is essential for exploring atomic- or molecular-level light-matter interaction. While such fields demonstrated so far have typically point-like cross-sections, an optical field having a higher-dimensional cross-section may offer higher flexibility and/or efficiency in applications. Here, we propose generating a nanoscale blade-like optical field in a coupled nanofiber pair (CNP) with a 1-nm-width central slit. Based on a strong mode coupling-enabled slit waveguide mode, a sub-nm-thickness blade-like optical field can be generated with a cross-section down to ∼0.28 nm×38 nm at 1550 nm wavelength (i.e., a thickness of ∼λ0/5000) and a peak-to-background intensity ratio (PBR) higher than 20 dB. The slit waveguide mode of the CNP can be launched from one of the two nanofibers that are connected to a standard optical fiber via an adiabatical fiber taper, in which a fundamental waveguide mode of the fiber can be converted into a high-purity slit mode with high efficiency (>98%) within a CNP length of less than 10 μm at 1550 nm wavelength. The wavelength-dependent behaviors and group velocity dispersion in mode converting processes are also investigated, showing that such a CNP-based design is also suitable for broadband and ultrafast pulsed operation. Our results may open up new opportunities for studying light-matter interaction down to the sub-nm scale, as well as for exploring ultra-high-resolution optical technology ranging from super-resolution nanoscopy to chemical bond manipulation.
Photonics Research
- Publication Date: Dec. 22, 2023
- Vol. 12, Issue 1, 154 (2024)
Quenching of second-harmonic generation by epsilon-near-zero media|On the Cover
Chenglin Wang, Ran Shi, Lei Gao, Alexander S. Shalin, and Jie Luo
Epsilon-near-zero (ENZ) media were demonstrated to exhibit unprecedented strong nonlinear optical properties including giant second-harmonic generation (SHG) due to their field-enhancement effect. Here, on the contrary, we report the quenching of SHG by the ENZ media. We find that when a tiny nonlinear particle is placed very close to a subwavelength ENZ particle, the SHG from the nonlinear particle can be greatly suppressed. The SHG quenching effect originates from the extraordinary prohibition of electric fields occurring near the ENZ particle due to evanescent scattering waves, which is found to be universal in both isotropic and anisotropic ENZ particles, irrespective of their shapes. Based on this principle, we propose a kind of dynamically controllable optical metasurface exhibiting switchable SHG quenching effect. Our work enriches the understanding of optical nonlinearity with ENZ media and could find applications in optical switches and modulators. Epsilon-near-zero (ENZ) media were demonstrated to exhibit unprecedented strong nonlinear optical properties including giant second-harmonic generation (SHG) due to their field-enhancement effect. Here, on the contrary, we report the quenching of SHG by the ENZ media. We find that when a tiny nonlinear particle is placed very close to a subwavelength ENZ particle, the SHG from the nonlinear particle can be greatly suppressed. The SHG quenching effect originates from the extraordinary prohibition of electric fields occurring near the ENZ particle due to evanescent scattering waves, which is found to be universal in both isotropic and anisotropic ENZ particles, irrespective of their shapes. Based on this principle, we propose a kind of dynamically controllable optical metasurface exhibiting switchable SHG quenching effect. Our work enriches the understanding of optical nonlinearity with ENZ media and could find applications in optical switches and modulators.
Photonics Research
- Publication Date: Aug. 01, 2023
- Vol. 11, Issue 8, 1437 (2023)
Electrical manipulation of lightwaves in the uniaxially strained photonic honeycomb lattices under a pseudomagnetic field
Zhipeng Qi, Hao Sun, Guohua Hu, Chunyu Deng, Wanghua Zhu, Bo Liu, Ying Li, Shaopeng Liu, Xuechao Yu, and Yinping Cui
The realization of pseudomagnetic fields for lightwaves has attained great attention in the field of nanophotonics. Like real magnetic fields, Landau quantization could be induced by pseudomagnetic fields in the strain-engineered graphene. We demonstrated that pseudomagnetic fields can also be introduced to photonic crystals by exerting a linear parabolic deformation onto the honeycomb lattices, giving rise to degenerate energy states and flat plateaus in the photonic band structures. We successfully inspire the photonic snake modes corresponding to the helical state in the synthetic magnetic heterostructure by adopting a microdisk for the unidirectional coupling. By integrating heat electrodes, we can further electrically manipulate the photonic density of states for the uniaxially strained photonic crystal. This offers an unprecedented opportunity to obtain on-chip robust optical transports under the electrical tunable pseudomagnetic fields, opening the possibility to design Si-based functional topological photonic devices. The realization of pseudomagnetic fields for lightwaves has attained great attention in the field of nanophotonics. Like real magnetic fields, Landau quantization could be induced by pseudomagnetic fields in the strain-engineered graphene. We demonstrated that pseudomagnetic fields can also be introduced to photonic crystals by exerting a linear parabolic deformation onto the honeycomb lattices, giving rise to degenerate energy states and flat plateaus in the photonic band structures. We successfully inspire the photonic snake modes corresponding to the helical state in the synthetic magnetic heterostructure by adopting a microdisk for the unidirectional coupling. By integrating heat electrodes, we can further electrically manipulate the photonic density of states for the uniaxially strained photonic crystal. This offers an unprecedented opportunity to obtain on-chip robust optical transports under the electrical tunable pseudomagnetic fields, opening the possibility to design Si-based functional topological photonic devices.
Photonics Research
- Publication Date: Jun. 26, 2023
- Vol. 11, Issue 7, 1294 (2023)
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