Source regulation symmetry breaks polarized excitons

hyperbolic exciting,

Half broken, half tao.

I don’t know who cut the symmetry,

The source dipole moment resembles scissors.

As one of the ideal carriers for information transmission in the “post-Moore era”, photons are highly respected for their advantages of fast transmission speed, low energy consumption and high capacity. However, photons themselves are not charged, cannot be directly controlled electrically like electrons, and their volatility is limited by the optical diffraction limit. Therefore, how to achieve highly localized and precise manipulation of photons at the nanoscale has become an important and challenging scientific problem. As a “half-light-half-matter” quasiparticle generated by the interaction between light and matter, polarion is expected to be applied to the construction of the next generation of high-efficiency, high-performance, low-energy integrated nanophotonic devices due to its advantages of strong local light field and low propagation loss. In particular, hyperbolic polaritons generated in highly anisotropic media can theoretically support infinitely large wave vector propagation without considering loss, thereby compressing the light field to an infinitesimal scale.

The excitation and propagation of polarizon is closely related to the symmetry of the material. Recent studies have shown that there is a special hyperbolic shear polaritons in low-symmetric monoclinic crystals, and due to the non-diagonalization of the permittivity tensor of monoclinic crystals, its hyperbolic wavefront shows obvious mirror symmetry breaking, which further enhances the directionality of polarion propagation. However, low-symmetry crystals have the problem of large propagation loss, and intrinsic hyperbolic polaritons in traditional high-symmetry crystals always show mirror symmetry, which is not conducive to the efficient directional propagation of polaritons.

In response to the above problems, the team of Zhang Xinliang and Li Peining of Huazhong University of Science and Technology, in cooperation with Professor Dai Zhigao of China University of Geosciences (Wuhan), proposed a new strategy to break the mirror symmetry of hyperbolic polaritons by controlling the polarization characteristics of the excitation source and realize the directional transmission of nanoscale light fields.

By studying the influence mechanism of near-field excitation source on hyperbolic polaritons in the plane, they theoretically and experimentally proved that under the condition of symmetry independent of crystal structure, symmetrically broken hyperbolic polaritons in hyperbolic materials can be directly induced by in-plane dipole moments, so as to realize the advantageous fusion of hyperbolic polaritons in high-symmetrical crystals with low loss and high orientation in low-symmetrical crystals.

The work was published in the new journal eLight of the Excellence Program High Start under the title “Source-configured symmetry-broken hyperbolic polaritons”. The co-first authors of this paper are Hu Caixing, Sun Tian and Zeng Ying, doctoral students of Huazhong University of Science and Technology, and this research was supported by the National Natural Science Foundation of China, the National Key Research and Development Program, the Natural Science Foundation of Hubei Province and the Innovation Fund of Wuhan National Research Center for Optoelectronics.

In this work, as shown in Figure 1a, the authors continue the low-loss, high-symmetry calcite (CaCO3) crystal discovered in the previous stage as an excellent hyperbolic polarion observation platform, and use electron beam lithography to prepare high-quality nano-sized gold disk antennas on their surfaces. As shown in Figures 1a and b, the in-plane electric dipole moment is generated in the disk under the polarization of the metal disk by the plane wave polarized by p, which further excites the hyperbolic polarizon in the calcite crystal. The authors find that by applying far-field plane wave excitation with different linear polarization states, the in-plane polarization direction of the electric dipole moment can be adjusted, which in turn directly affects the symmetry of the near-field intensity distribution of hyperbolic polaritons. The experimental results show that by this regulatory method, the free switching of hyperbolic polarized excitons symmetric/asymmetric can be achieved in highly symmetric crystal calcite (Figures 1c and d). Moreover, compared with the symmetry state, the energy directional transmission efficiency of hyperbolic polarizers in the symmetry-breaking state is significantly improved (Figure 1e). Further research shows that the symmetry-breaking propagation of hyperbolic polaritons can be precisely controlled over a wide frequency range.

Figure 1: Optical dipole antenna excitation and regulation of asymmetric hyperbolic polarizers. (a) Schematic diagram of near-field experiments; (b) Near-field image of the phase distribution of the dipole electric field on the gold disk antenna; Simulation results of hyperbolic polarion (left) and near-field imaging (right) in calcite crystal under different electric dipole moment directions. (e) Asymmetric amplitude comparison plots extracted from dotted positions in simulation and experimental data

To explain this phenomenon, the research team established a theoretical model to describe the relationship between the horizontal electric dipole angular spectral distribution and the hyperbolic isofrequency line while considering the conservation of momentum, as shown in Figure 2. It can be seen that the mirror-symmetry-breaking hyperbolic polarizon (Figure 2c) is essentially caused by the asymmetric source field distribution (Figure 2a) excited by its in-plane electric dipole moment, and does not depend on the intrinsic hyperbolic dispersion properties of the in-plane anisotropic material (Figure 2b). Therefore, the experimental results can be extended to different types of polarized exciton systems (e.g., plasmonized excitons, etc.). The authors also discuss the influence of different types of point sources on the propagation orientation of hyperbolic polarizons.

Figure 2: Interaction between the excitation source and the in-plane anisotropic medium leads to a polarion symmetry break. (a) Calculate the angular spectral distribution of the horizontal electric dipole momentum space (the field source intensity is zero at the yellow dashed line). (b) Hyperbolic isofrequency lines in highly symmetrical crystals. (c) Interaction of the source field with hyperbolic isofrequency lines. When the yellow dashed line coincides with the asymptote of the hyperbolic isofrequency line (i.e., the electric dipole moment is perpendicular to the hyperbolic asymptote), hyperbolic polarion asymmetry is greatest (the red dotted line represents the unexcited mode on the hyperbolic isofrequency).

The field source control method proposed in this study is simple and easy to adjust, which provides a new manipulation method for optical conduction at the nanoscale, which is expected to be applied to new integrated photonics devices such as optical modulation micro-nano waveguides, optical switches, and optical isolators, and promote the development of optical computing, optical sensing and quantum optics. While realizing the miniaturization of optoelectronic devices, it is also easy to combine with other control methods to realize multiphysics control of polaritons. (Source: China Optics WeChat public account)

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