A quasi-bic optical filter and a chiral control method thereof

Abstract

This invention discloses a quasi-BIC optical filter and its chiral modulation method, relating to the field of optical metasurface technology. The method includes: an optical filter based on a metasurface structure, the metasurface comprising several square lattices, each square lattice having an interrupted annular groove etched on it; disrupting the in-plane inversion structural symmetry of the annular grooves in the square lattices of the metasurface to achieve a metasurface-supported quasi-BIC mode, specifically by extending one end of one of the interrupted annular grooves by a certain distance while simultaneously shortening the adjacent ends of adjacent grooves by the same distance, obtaining the transmittance at the quasi-BIC frequency point when switching the transverse magnetic wave in the x-direction of the metasurface to RCP and LCP, and chiral modulation of the circular dichroism of the metasurface based on the transmittance of RCP and LCP.

Description

Technical Field

[0001] This invention relates to the field of optical metasurface technology, specifically to a quasi-BIC optical filter and its chiral control method. Background Technology

[0002] Metasurfaces, chiral metamaterials, and Bound States in the Continuum (BICs) are research areas that have attracted much attention in photonics in recent years. These artificial surfaces or materials, constructed using micro- and nanostructures, have shown significant potential and broad application prospects in optical modulation, polarization control, and optical signal processing. Metasurfaces, through precisely designed micro- and nanostructures, can achieve a high degree of control over parameters such as the phase, amplitude, and polarization of light waves, exhibiting strong flexibility and tunability, and are expected to be applied in imaging, communication, and sensing. Chiral metamaterials, utilizing their unique chiral structure, exhibit unique advantages in optical rotation, polarization conversion, and metamaterial topological states, providing new possibilities for the design and functional expansion of optical filters. Meanwhile, BICs, as a special state, can achieve high quality factor and low energy loss in optical filters, providing an important theoretical foundation and technical support for achieving high-efficiency, low-loss optical modulation and transmission. How to combine metasurfaces, chiral metamaterials, and BIC theory to realize high-efficiency, low-loss optical filters still requires further theoretical exploration and experimental verification.

[0003] Currently, optical filters are constructed using the metasurface structure mentioned in the reference Kim KH, Kim J R. High-Q Chiroptical Resonances by Quasi-Bound States in the Continuumin Dielectric Metasurfaces with Simultaneously Broken In-Plane Inversion and Mirror Symmetries[J]. Advanced Optical Materials, 2021, 9(22): 2101-162. This metasurface is composed of cross-shaped silicon atoms in a unit cell and supported by a silicon dioxide substrate. However, chiral selective optical filters made of a single material often only work for a single wavelength.

[0004] In summary, the performance of optical filters designed using current metasurface structures decreases as the asymmetry of the metasurface structure and its intrinsic modes increases. Summary of the Invention

[0005] To address the shortcomings of existing technologies where the performance of circular dichroism (CD) decreases with increasing asymmetry of the metasurface structure and its intrinsic modes, this invention proposes a quasi-BIC optical filter and its modulation method. By disrupting the in-plane inversion structural symmetry of the annular groove in the metasurface square lattice to a metasurface-supported quasi-BIC mode, a quasi-BIC-supported optical filter is obtained, thus solving the problems existing in the prior art.

[0006] A quasi-BIC optical filter, including C 4v Metasurface structures;

[0007] The C 4v The metasurface structure comprises several square lattices arranged in a planar array. Each square lattice has a hollow annular groove etched on it, and the annular groove is uniformly divided into four equal parts. The structure is achieved by disrupting the C-axis of the annular groove in each square lattice. 2V The structural symmetry is transformed into a metasurface-supported quasi-BIC mode to obtain an optical filter that supports the quasi-BIC. The destruction process specifically involves extending one end of a quarter-annular groove by a certain distance while shortening the end of the adjacent groove by the same distance.

[0008] Furthermore, the metasurface structure exhibits fourfold rotational symmetry.

[0009] Furthermore, the metasurface structure is made of silicon.

[0010] Furthermore, the square lattice has a period of 550 nm and a height of 500 nm.

[0011] Furthermore, the inner diameter of the annular groove is 170 nm and the outer diameter is 200 nm.

[0012] This invention also includes a chiral modulation method for a quasi-BIC optical filter, comprising the following steps:

[0013] Numerical simulations were performed on the band structure of the metasurface constituting the optical filter to calculate the band structure and quality factor distribution for the corresponding frequency bands. The optical filter is constructed by breaking the four-fold rotational symmetry C0. 4v C of metasurface structure 2V Symmetry is obtained;

[0014] Analysis of the distribution diagrams of band structure and quality factor revealed the existence of BIC in the near-infrared band of the metasurface structure.

[0015] C that disrupts the square lattice of metasurfaces 2VThe structure is symmetric to support the metasurface in a quasi-BIC mode. A three-axis coordinate system is established, with the square lattice positioned on the xoy plane. The origin of the coordinate system coincides with the center of the annular groove. A transverse magnetic wave is added in the x-direction, perpendicular to the xoy plane and pointing upwards. Periodic boundary conditions are added to the xoy plane, and a perfectly matched layer is constructed along the z-axis. One end of one of the interrupted annular grooves is extended by a certain distance, while the adjacent end of the next groove is simultaneously shortened by the same distance. The extension distance is defined as δ. Excitation is performed using a transverse magnetic wave in the x-direction, and the transmittance at the quasi-BIC frequency point is obtained when the transverse magnetic wave in the x-direction of the metasurface is switched between RCP and LCP.

[0016] Chiral modulation of the circular dichroism of metasurfaces was performed based on the transmittance of RCP and LCP.

[0017] Furthermore, the BIC present in the near-infrared band of the metasurface structure exists at the center of the first Brillouin zone, and its quality factor tends to infinity.

[0018] Furthermore, the chiral modulation of the circular dichroism of the optical filter based on the transmittance of RCP and LCP is described, whereby the circular dichroism CD is represented as follows:

[0019] CD T =(T LCP -T RCP ) / (T LCP +T RCP )

[0020] Where T represents transmittance, T RCP and T LCP These represent the right-handed and left-handed circular polarization of the incident light, respectively.

[0021] This invention provides a quasi-BIC optical filter and its chiral modulation method, which has the following beneficial effects:

[0022] This invention breaks the four-fold rotational symmetry C 4v C of metasurface structure 2V Symmetry is used to obtain an optical filter that supports a quasi-BIC; simultaneously, by adjusting the C that constitutes the optical filter... 4v Numerical simulations of the band structure of the metasurface were performed to analyze the variation characteristics of the quasi-BIC transmission peak, thereby obtaining a high Q-factor quasi-BIC and high-performance circular dichroism. This invention improves the sensitivity of optical filters by applying the metasurface structure to optical filtering, and it also has the potential to be applied to high-spectral-resolution chiral biosensors, low-threshold lasers, and related security applications. Attached Figure Description

[0023] Figure 1 C is an embodiment of the present invention.4v Schematic diagram of metasurface structure;

[0024] Figure 2 C is an embodiment of the present invention. 4v Model distribution of band structure and Q factor of relevant intrinsic modes near the BIC frequency of metasurface;

[0025] Figure 3 This is a schematic diagram illustrating the variation trend of Q-factor as a function of δ and the variation trend of the wavelength of the intrinsic mode with the asymmetric parameter in the embodiments of the present invention.

[0026] Figure 4 This is a distribution diagram of the Q factor of the metasurface under different losses in the fully symmetrical case of the present invention.

[0027] Figure 5 This is a schematic diagram of the transmission spectra of the metasurface under different degrees of symmetry disruption in an embodiment of the present invention;

[0028] Figure 6 This is a schematic diagram of the transmission spectra of RCP and LCP used on the C4v metasurface with symmetry broken at δ = 20 nm, and the circular dichroism values ​​near the quasi-BIC in an embodiment of the present invention.

[0029] Figure 7 This is a schematic diagram of the transmission spectrum when the asymmetric perturbation factor δ ranges from 10 nm to 60 nm in an embodiment of the present invention;

[0030] Figure 8 This is a schematic diagram illustrating the metasurface chirality effect considering losses in an embodiment of the present invention. Detailed Implementation

[0031] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0032] This invention proposes a quasi-BIC optical filter, based on a silicon material with interrupted circular slots and C... 4v Metasurface structures such as Figure 1 As shown, the optical filter is created by breaking a fourfold rotational symmetry (C0). 4v C of metasurface structures 2V Obtained through symmetry; Figure 1 (a) C is composed of annular grooves with interruptions. 4v Metasurfaces are made of silicon. Figure 1 (b) is a schematic diagram of a single cell of the metasurface, with the structural parameter being periodic P. x =P y=550nm, height h =500nm, and the radii of the circles forming the groove are a1 =170nm and a2 =200nm respectively. Here, the invention defines the perturbation method. Figure 1 (c) shows that as the air slot on the left extends to the right, the slot on the right shortens by the same distance. To achieve in-plane and out-of-plane symmetry, the substrate is assumed to be air in the simulation calculations, i.e., substrate medium n=1. The refractive index of the metasurface is taken from Palik E D. Handbook of Optical Constants of Solids[M]. Academic Press, New York, 1985: xvii-xviii. This invention uses transverse magnetic waves in the x-direction to excite the BIC mode. Simultaneously, periodic boundary conditions are added in the horizontal xoy plane, and a perfectly matched layer (PML) is constructed in the vertical z-direction.

[0033] This invention performs numerical simulations of the band structure of the metasurface constituting the optical filter, and obtains the following results: Figure 2 The pattern shown conforms to BIC characteristics in the near-infrared band. For a completely symmetric supercell at this point, due to C... 4v Due to the unique nature of the structure, and the absence of inherent losses, the symmetrically protected band structure (BIC) generated by the structure cannot couple with the external environment. This is caused by a mismatch with the external environment. To prove the existence of the BIC mode in the structure, this invention calculates the band structure and quality factor distribution for the corresponding frequency band. Figure 2 The band structure and Q factor of the relevant intrinsic modes near the BIC frequency of a planar silicon metasurface were simulated. Figure 2 (a) is the band structure of the mode existing in the first Brillouin zone, while Figure 2 (b) shows the quality factor distribution of the mode in momentum space. The quality factor tends to infinity at k=0. The inset shows the electric field distribution at the BIC frequency point of the structure. At the center of the first Brillouin zone (gamma point), the quality factor tends to infinity. As the distance from the gamma point increases, the quality factor gradually decreases in a quadratic exponential manner. Based on the band structure and quality factor distribution diagrams, this invention confirms the existence of BIC in the near-infrared band of this structure.

[0034] Due to the structural symmetry of the metasurface, the structure possesses a BIC mode, which is a dark mode with an infinite quality factor. This BIC mode mismatches with external modes, preventing coupling with the outside world. Therefore, this invention disrupts the structural symmetry, causing the BIC mode to evolve into a quasi-BIC mode, thus enabling radiation channels. Here, this invention aims to break the in-plane C-axis of the structure. 2vSymmetry is defined in a way that breaks symmetry: the annular groove on the left side of the cell extends to the right, while the annular groove on the right side shortens by the same distance, and the distance between the two annular grooves remains unchanged. This invention defines the displacement as δ. This invention calculates the changes in quality factor and eigenmode for different values ​​of δ. Figure 3 (a) shows the trend of Q-factor as a function of δ. For a perfectly symmetric metasurface, i.e., δ = 0 nm, the quality factor reaches 10⁹. This is due to the symmetry-protected BIC resulting from the structural symmetry, which possesses an infinite quality factor. Once the symmetry is broken, the BIC transforms into a quasi-BIC, and the quality factor gradually decreases with increasing degree of symmetry breaking. This means that the amount of eigenmodes coupled into the metasurface structure from the external far-field also gradually increases. The inset shows the defined mode of symmetry breaking. Figure 3 (b) shows the trend of the wavelength of the intrinsic mode as asymmetric parameter changes. For δ values ​​of 0-60 nm, the intrinsic mode appears in the range of 1550 nm to 1580 nm. As δ increases, the wavelength of the intrinsic mode increases, which means that a redshift occurs.

[0035] Optical loss in materials is unavoidable due to defects and surface roughness caused by process and manufacturing precision errors in actual processing. Therefore, this invention simulates the quality factor distribution of the intrinsic mode under actual loss conditions. This invention performs simulation calculations by increasing the extinction coefficient k of the material, which includes both absorption and scattering. Figure 4 The distribution of the calculated Q-factor is given for different extinction coefficients (the first Brillouin zone of the lattice is plotted in the upper right corner of each figure). When the value of k becomes 10... -9 10 -7 and 10 -5 When the corresponding maximum value of the Q factor exceeds 10, 9 10 7 and 10 5 This proves the actual existence of quasi-BIC in practice. It is worth noting that when the extinction coefficient k is set to 0, the quasi-BIC becomes a BIC, and at the frequency of the BIC, due to the absence of loss, the radiation Q factor diverges to infinity. Figure 4 The distribution of the Q-factor of a metasurface under different losses in the case of complete symmetry, where the imaginary part k of the refractive index is reduced to... Figure 4 (a)10 -9 (b)10 -7 (c)10 -5 .

[0036] This invention involves varying degrees of symmetry disruption of metasurface cells and calculates their transmission spectra, such as... Figure 5As shown. In the case of complete symmetry, the structure contains a bipolar interphase (BIC), so the intrinsic modes cannot couple with the outside, and therefore no transmission peak appears. Furthermore, in the wavelength range of 1548-1560 nm, the metasurface structure exhibits a near-1 high transmittance for vertically incident linearly polarized light.

[0037] Once the symmetry is broken, the BIC (Brillouin Interval) evolves into a quasi-BIC, and transmission peaks appear in the transmission spectrum. Furthermore, with increasing degree of symmetry disruption, the transmission peaks gradually redshift, and the peak width also increases. This is due to increased leakage caused by moving away from the first Brillouin zone. Here, the position of the transmission peak represents the magnitude of the real part of the refractive index, while the peak width represents the imaginary part of the dielectric constant. It is noteworthy that the amplitude of all transmission peaks in the figure is around 0.5. This is because linearly polarized light can be decomposed into a superposition of left-handed and right-handed polarized light, and the quasi-BIC induced by the asymmetric perturbation factor possesses chiral filtering capabilities, meaning that at the quasi-BIC frequency, only right-handed circularly polarized light passes through, while left-handed light undergoes total internal reflection.

[0038] This invention extends the discussion of the quasi-BIC mechanism to strongly chiral conversions with high Q-factors. For the case of δ = 20 nm, the transverse magnetic (TM) wave in the x-direction is switched to LCP and RCP, and the transmission spectrum is then calculated, yielding the following results: Figure 6 As shown, Figure 6 (a) is C that violates the symmetry at δ = 20 nm. 4v Transmission spectra of RCP and LCP are used for metasurfaces. (b) Circular dichroism values ​​near the quasi-BIC, with a minimum value of -0.93.

[0039] Here, the present invention is based on the definition of circular dichroism (CD):

[0040] CD T =(T LCP -T RCP ) / (T LCP +T RCP )

[0041] Where T represents transmittance, T RCP and T LCP These represent the right-handed and left-handed circular polarization of the incident light, respectively. Figure 6 (a) shows the transmission spectra of the metasurface after symmetry destruction to perpendicularly incident RCP and LCP. At the quasi-BIC frequency point, it can be seen that the metasurface has a high transmittance for RCP light and a low transmittance for LCP light, with a minimum value of 0.03. Figure 6 (b) shows the CD value derived from the calculated LCP and RCP transmission. It can be seen that the chiral effect is more pronounced in the quasi-BIC band, with a minimum CD value reaching -0.93. This demonstrates the control of this C value by BIC. 4vMetasurfaces exhibit excellent chiral filtering capabilities.

[0042] like Figure 7 The transmission spectra are shown for asymmetric perturbation factors δ from 10 nm to 60 nm. Figure 7 As shown in (a) and (b), under all symmetry disruption conditions, the metasurface exhibits low transmittance for perpendicularly incident LCP light in the quasi-BIC frequency band, with the lowest transmittance (0.05) at δ = 20 nm. For perpendicularly incident RCP light, the metasurface demonstrates high transmittance in the quasi-BIC frequency band, with the most significant transmittance at δ = 20 nm, reaching a maximum of 0.95. Furthermore, as the degree of symmetry disruption exceeds 20 nm, the transmittance of LCP light near the quasi-BIC frequency gradually increases with increasing δ, while the transmittance of RCP light gradually decreases. This indicates that the CD value also decreases, and the chiral filtering ability of the metasurface gradually diminishes. This is because increasing the degree of disruption leads to a decrease in the quality factor, thereby reducing circular dichroism. Figure 7 (c) shows the CD value calculated based on the obtained transmission curves of the metasurface for LCP and RCP. It can be clearly seen that CD is minimum at δ = 20 nm, with a minimum value of -0.93. This indicates that the metasurface has the best chiral control effect for light under this level of degradation. This is because, under this condition, the ellipticity of the topological charge and the quality factor of the structure are exactly the best for the chiral control of the metasurface.

[0043] In actual manufacturing, due to limitations in process precision and purification purity, metasurfaces experience certain losses during processing. To investigate losses caused by defects or surface roughness during manufacturing, the imaginary part of silicon's refractive index is set to 10. -5 To approximate the actual situation, and to calculate the chiral effect of the metasurface under this condition, the results are as follows: Figure 8 As shown. Figure 8 (a) Considering losses, the absorption, reflection and transmission of linear light by the metasurface when its cellular symmetry is broken by 20 nm can be clearly seen: in the quasi-BIC band, the silicon metasurface has a certain absorption of light, which is caused by losses, so the transmission amplitude at this frequency will be reduced. Figure 8 (b) shows the calculated electric field diagram of the cell considering losses; the red arrows indicate the magnetic field vectors. The electric field strength is stronger compared to the lossless version, which is due to material absorption. Finally, we used LCP and RCP light to illuminate the lossy silicon metasurface, respectively, and obtained the transmission spectra as follows: Figure 8 (c) At the quasi-BIC frequency, the material still exhibits high transmittance for RCP light, but low transmittance for LCP light. Their CD values ​​are then calculated as follows: Figure 8(d) As expected, the CD value of the lossy metasurface decreased to about -0.92. This value is still considerable, indicating that the silicon metasurface is very feasible in experiments and manufacturing.

[0044] This invention proposes a quasi-BIC optical filter, based on a silicon material with interrupted circular slots and C... 4v Metasurfaces, by breaking the C of the metasurface cell 2v By restoring symmetry, a quasi-BIC with a high Q-factor and a circular dichroism as high as -0.93 was obtained. The existence of BIC was confirmed through band structure and quality factor calculations, and the influence of specific losses on the quality factor of the silicon metasurface was analyzed. Secondly, by disrupting the symmetry of the structure to different degrees, the variation characteristics of the quasi-BIC transmission peak were obtained. Based on this, the transmission spectra after structural symmetry disruption were calculated under LCP and RCP incident conditions. It was found that the chiral effect of the optical filter was most pronounced when the disruption reached 20 nm, with a CD of -0.93, and the chiral effect decreased with further increases in disruption degree. Then, the structure was parametrically analyzed to obtain the influence of loss and groove width on circular dichroism. Finally, the chiral structure was applied to chiral sensing, and the calculated sensitivity of the optical filter reached 0.0027, demonstrating good performance of the chiral filter. The results of this work have the potential for high-spectral-resolution chiral biosensing, low-threshold lasers, and related security applications.

[0045] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A quasi-BIC optical filter, characterized in that, include: C 4v Metasurface structures; The C 4v The metasurface structure comprises several square lattices arranged in a planar array, each square lattice having a hollow annular groove etched on it, and the annular groove being uniformly divided into four equal parts; the C 4v The metasurface structure exhibits fourfold rotational symmetry. The square lattice has a period of 550 nm and a height of 500 nm. The annular grooves have an inner diameter of 170 nm and an outer diameter of 200 nm. This is achieved by disrupting the C-axis of the annular grooves in each square lattice. 2V The structural symmetry is transformed into a metasurface-supported quasi-BIC mode to obtain an optical filter that supports the quasi-BIC. The destruction process specifically involves extending one end of a quarter-annular groove by a certain distance while shortening the end of the adjacent groove by the same distance.

2. The quasi-BIC optical filter according to claim 1, characterized in that, The C 4v The metasurface structure is made of silicon.

3. A chiral modulation method for a quasi-BIC optical filter as described in claim 1, characterized in that, Includes the following steps: For C that constitutes an optical filter 4v Numerical simulations were performed on the band structure of the metasurface structure to obtain the band structure and quality factor distribution maps for the corresponding frequency bands; the optical filter is obtained by breaking the four-fold rotational symmetry C 4v C of metasurface structure 2V Symmetry is obtained; By analyzing the distribution diagrams of band structure and quality factor, we obtained the following results in C 4v BIC exists in the near-infrared band of metasurface structures; Destroy C 4v C of metasurface structure 2V The structure is symmetric to support the quasi-BIC mode of the metasurface. A three-axis coordinate system is established, with the square lattice positioned on the xoy plane. The origin of the coordinate system coincides with the center of the annular groove. A transverse magnetic wave is added in the x-direction, perpendicular to the xoy plane and pointing upwards. Periodic boundary conditions are added to the xoy plane, and a perfectly matched layer is constructed in the z-axis direction. One end of one of the interrupted annular grooves is extended by a certain distance, while the adjacent end of the adjacent groove is simultaneously shortened by the same distance. The extension distance is defined as δ. Excitation is performed using a transverse magnetic wave in the x-direction, and the transmittance at the quasi-BIC frequency point is obtained when the transverse magnetic wave in the x-direction of the metasurface is switched between RCP and LCP. Chiral modulation of the circular dichroism of metasurfaces was performed based on the transmittance of RCP and LCP.

4. The chiral modulation method for a quasi-BIC optical filter according to claim 3, characterized in that, The C 4v The BIC in the near-infrared band of metasurface structures exists at the center of the first Brillouin zone, and its quality factor tends to infinity.

5. The chiral modulation method for a quasi-BIC optical filter according to claim 3, characterized in that, The circular dichroism (CD) of the optical filter is chirally modulated based on the transmittance of RCP and LCP, and the circular dichroism CD is represented as follows: ； Where T represents transmittance. and These represent the right-handed and left-handed circular polarization of the incident light, respectively.

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