Photonics Research, Volume. 12, Issue 12, 2972(2024)

Magnetically tunable bound states in the continuum with arbitrary polarization and intrinsic chirality

Qing-An Tu1、†, Hongxin Zhou1、†, Dong Zhao1, Yan Meng2,3、*, Maohua Gong1,4、*, and Zhen Gao1,5、*
Author Affiliations
  • 1State Key Laboratory of Optical Fiber and Cable Manufacturing Technology, Department of Electronic and Electrical Engineering, Guangdong Key Laboratory of Integrated Optoelectronics Intellisense, Southern University of Science and Technology, Shenzhen 518055, China
  • 2School of Electrical Engineering and Intelligentization, Dongguan University of Technology, Dongguan 523808, China
  • 3e-mail: mengyan@dgut.edu.cn
  • 4e-mail: gongmh@sustech.edu.cn
  • 5e-mail: gaoz@sustech.edu.cn
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    Figures & Tables(13)
    MO PhC slab supporting magnetically tunable symmetry-protected BICs at the Γ point. (a) Schematic of a 2D MO PhC slab under external magnetic fields along the z direction. Arbitrary far-field polarization can be achieved by tuning the external magnetic fields. (b) Simulated band structure of the MO PhC slab with δ=0 (without external magnetic fields). The TE-like band of interest is highlighted in red. The inset shows the magnetic field profile of the eigenstate (|Hz|) at the Γ point. (c) The far-field polarization around the BICs evolves from linear (lower panel) to elliptical (middle panel) and finally to circular (upper panel) as the external magnetic field (B) increases. The topological polarization singularity at the Γ point indicates the existence of symmetry-protected BICs. (d) Calculated Q factor near the BICs.
    Stability of the band structures and Q factors under different external magnetic field strengths. (a) Band structures and (b) Q factors of the square MO PhC slab as the external magnetic field δ increases from 0 to 0.3. Inset in (a): zoom-in of the band structures near the Γ point. Inset in (b): magnetic field profile of the eigenstate (|Hz|) at the Γ point. (c), (d) The far-field scattered power is dominated by the electric quadrupole (EQ) moment for δ=0 in (c) and δ=0.3 in (d). (e) In-plane electric field distributions of the MO PhC slab at the Γ point for δ=0 (left panel) and δ=0.3 (right panel). (f) Evolution of each component of the EQ moment with (kx,ky)=(0.03π/a,0) under increasing magnetic field strengths.
    Magnetically tunable BICs with arbitrary polarization in the MO PhC slab. (a)–(d) Evolution of far-field polarization of the MO PhC slab and (e)–(h) their corresponding ellipticity with increasing external magnetic fields δ=0, 0.1, 0.2, and 0.3, respectively. The far-field polarization gradually evolves from linear to circular as δ increases from 0 to 0.3. (i) Far-field polarization of the iso-frequency contour in momentum space [red dashed circles in (a)–(d)] mapped on the Poincaré sphere with δ=0, 0.1, 0.2, and 0.3, respectively. (j) Simulated (red line) and analytical (gray stars) ellipticity of the far-field polarization with different external magnetic field strengths at the gray triangles in (a)–(d) with (kx,ky)=(0.03π/a,0).
    Off-Γ chiral emission of the MO PhC slab over a broad frequency range and multiple incident angles. (a) Far-field polarization around the symmetry-protected BICs at the Γ point with δ=0.3. The red dashed line represents a straight path with fixed ky=0, and kx varying from −0.05π/a to 0.05π/a. (b), (c) Reflection spectra of the MO PhC slab under (b) RCP and (c) LCP incidences along the red dashed line in (a), respectively. (d) The red dashed circle represents the closing path of the iso-frequency contour at f=153 THz in momentum space. (e), (f) Reflection spectra of the MO PhC slab illustrated by (e) RCP and (f) LCP incidences along the red dashed circle in (d), respectively.
    At-Γ intrinsic chiral BICs generated by breaking the C2 and TRS symmetries simultaneously. (a) Schematic of an MO PhC slab under perpendicular external magnetic fields. Inset: unit cell of the MO PhC slab. (b), (c) Far-field polarization of the MO PhC slab with (b) δ=0 and (c) δ=0.24, respectively. When δ=0, a pair of C points (q=−1/2) with opposite chirality is split from the Γ-point BICs by breaking C2 symmetry. When δ=0.24, an intrinsic chiral BIC is generated by moving one C points to the Γ point. (d), (e) Simulated (d) azimuthal angle map and (e) Q factor of the far-field polarization with δ=0.24. The π-phase change around the Γ point and the high-Q factor confirm the existence of the intrinsic chiral BICs at the Γ point. (f), (g) Reflection spectra of the MO PhC slab with δ=0.24 for (f) LCP and (g) RCP incidences, respectively. The vanishing points of the reflection spectra are indicated by the red and blue dashed circles.
    At-Γ intrinsic chiral BICs with a downward external magnetic field. (a) Far-field polarization of the MO PhC slab with δ=0; a pair of C points (q=−1/2) with opposite chirality is split from the Γ-point BICs by breaking C2 symmetry. (b) Far-field polarization for δ=−0.24; an intrinsic chiral BIC is generated by moving one C point to the Γ point. (c), (d) Reflection spectra of the MO PhC slab with δ=−0.24 for (c) RCP and (d) LCP incidences, respectively. The vanishing points of the reflection spectra are indicated by the blue and red dashed circles, respectively.
    Component diagram of the EQ moment along a straight path (ky=0, kx=0−0.05π/a) under an increasing external magnetic field.
    Evolution of far-field polarization and ellipticity under tunable external magnetic fields along the negative z direction.
    Evolution of far-field polarization and ellipticity with an increasing δ along the z direction.
    Magnetically tunable BICs with arbitrary polarization in a hexagonal MO PhC slab.
    Off-Γ chiral emission of the MO PhC slab.
    Complete evolution process of the intrinsic chiral BICs.
    Transmittance and circular dichroism spectra of the square MO PhC slab.
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    Qing-An Tu, Hongxin Zhou, Dong Zhao, Yan Meng, Maohua Gong, Zhen Gao, "Magnetically tunable bound states in the continuum with arbitrary polarization and intrinsic chirality," Photonics Res. 12, 2972 (2024)

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    Paper Information

    Category: Nanophotonics and Photonic Crystals

    Received: Aug. 20, 2024

    Accepted: Oct. 8, 2024

    Published Online: Dec. 2, 2024

    The Author Email: Yan Meng (mengyan@dgut.edu.cn), Maohua Gong (gongmh@sustech.edu.cn), Zhen Gao (gaoz@sustech.edu.cn)

    DOI:10.1364/PRJ.539830

    CSTR:32188.14.PRJ.539830

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