Dielectric metasurface for realizing stable high-Q-value BIC through eigenfield disturbance

Abstract

The invention belongs to the field of continuum bound states and metasurfaces in nanometer photonics, and discloses a dielectric metasurface for realizing stable high-Q-value BIC through eigenfield disturbance. The dielectric metasurface comprises a dielectric substrate, a plurality of dielectric resonance units and a dielectric wrapping layer, the plurality of dielectric resonance units are periodically arranged on the dielectric substrate, and all the dielectric resonance units are surrounded by the dielectric wrapping layer; the structures of the dielectric resonance units are the same, the whole dielectric resonance units are in a regular quadrangular prism shape, and a vertical round hole is formed in the middle of each dielectric resonance unit. The corresponding positions of the two sides of each dielectric resonance unit in the y-axis direction are each provided with a columnar notch, one notch is a semicircular notch, and the other notch is a semi-elliptical notch. The dielectric metasurface can realize continuum bound state BIC of a high-quality Q factor; the morphology changes on the two sides of the notch are equal, so that the overall structure has a constant effective refractive index, and resonance with stable wavelength is generated; and a way is laid for the application of a low-threshold laser, an ultra-sensitive optical sensor and nonlinear optics.

Description

Technical Field

[0001] The present invention belongs to the fields of bound states in the continuum and metasurfaces in nanophotonics, and particularly relates to a dielectric metasurface for achieving a stable high-Q BIC through intrinsic field perturbation. Background Art

[0002] In recent years, bound states in the continuum (BICs) have been widely studied due to their high Q factors. Different from the traditional concept of bound states located below the light line, BICs are special states with a specific combination of momentum and frequency, where the coupling to the radiation channels is completely cut off, confining photons in a micron- or nanoscale region above the light line in the radiation continuum. They exhibit infinite quality factors, zero linewidths, and low losses, thus attracting great attention, especially in the field of all-dielectric metasurfaces, where using such an ideal platform provides a direct method for achieving ultra-high Q factors. This is of great significance for nanophotonic applications, opening up many promising avenues for enhancing the ability of light-matter interaction, including low-threshold lasers, nonlinear optical effects, solar cells, optical modulators, and highly sensitive sensors.

[0003] A metasurface refers to an artificial layered material with a thickness less than the wavelength, which can flexibly and effectively control characteristics such as the polarization, amplitude, phase, polarization mode, and propagation mode of electromagnetic waves. A metasurface can be regarded as the two-dimensional counterpart of metamaterials. All-dielectric metasurfaces have received extensive attention due to their unique optical properties. They have rich modes, low losses, and are compatible with CMOS processes. These unique properties make them an ideal platform for enhancing light-matter interaction and developing high-performance photonic devices.

[0004] All-dielectric metasurfaces have also been proven to have infinite Q factors in optical BICs, which are non-radiative eigenstates that are completely decoupled from the external environment although they are located in the continuous domain. Usually, they must first be converted into finite but still high-quality quasi-BICs (QBICs) so that they can be excited by external sources. Previous QBICs were mostly induced by introducing in-plane symmetry breaking, also known as symmetry-protected BIC (SP-BIC), such as breaking the structural symmetry, introducing position perturbations, using interference effects, and using oblique incidence. Although these methods have great advantages, the excitation of SP-BIC still encounters some problems: for example, the Q factor decreases with the increase of the asymmetry parameter α, and the change of α will lead to the change of the effective refractive index, making it difficult to obtain the desired stable and high-quality quasi-BIC; during the excitation process, SP-BIC may undergo asymmetric coupling with other radiation modes, resulting in energy leakage; strong field localization may trigger nonlinear effects (such as Kerr effect, multi-photon absorption), changing the system resonance conditions and destroying the stability of BIC; the excitation energy may be dispersed into multiple modes, reducing the local field enhancement effect of the target mode. Summary of the Invention

[0005] Aiming at the problem that the Q factor of SP-BIC cannot meet the ideal requirements in the above, the present invention proposes a dielectric metasurface that realizes a stable high-Q value BIC through intrinsic field perturbation. The designed three-notch nanocube metasurface can effectively perturb the intrinsic field by introducing a central notch, enhance the electric quadrupole resonance, thereby increasing the Q value of multimode interference, and the slight perturbation of the intrinsic field with a constant effective refractive index contributes to robust resonance.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A dielectric metasurface that realizes a stable high-Q value BIC through intrinsic field perturbation, the dielectric metasurface includes a dielectric substrate, dielectric resonant units, and a dielectric cladding layer. A plurality of the dielectric resonant units are periodically arranged on the dielectric substrate, and all the dielectric resonant units are surrounded by the dielectric cladding layer.

[0008] The structure of each dielectric resonant unit is the same, and the overall shape is a regular quadrangular prism with a vertical circular hole in the middle. There is a columnar notch at the corresponding positions on both sides of each dielectric resonant unit in the y-axis direction, one of which is a semi-circular notch and the other is a semi-elliptical notch.

[0009] In the above technical solution, the excitation method of the intrinsic field perturbation is: by changing the geometric dimensions of the upper and lower semi-circular notches and the radius of the central perforation of the dielectric resonant unit, the structural symmetry is broken and the intrinsic field is perturbed, enhancing the destructive interference between the electric dipole (ED) and the electric quadrupole (EQ), effectively suppressing the radiation loss, and resulting in a high-Q resonance of SP-BIC.

[0010] In a further technical solution, the dielectric resonant units are periodically arranged in the x-direction and y-direction on the dielectric substrate, and the period is P.

[0011] In a further technical solution, the radius of the semi-circular notch of the dielectric resonant unit is r1, and the angle between the radius and the chord (i.e., the chord between the midpoint of the semi-circular arc and one of its endpoints) is θ1, which is used to describe the side shape of the semi-circular notch; the distance between the midpoint of the semi-elliptical notch of the dielectric resonant unit and the midpoint of the semi-elliptical arc is the semi-major axis or semi-minor axis r2, and the angle between it and the chord (i.e., the chord between the midpoint of the semi-elliptical arc and one of its endpoints) is θ2, which is used to describe the side shape of the semi-elliptical notch. The asymmetry parameter of the semi-circular notch and the semi-elliptical notch is defined as α = |θ2 - θ1| / θ1.

[0012] By tuning the radius of the semi-circular notch in the y direction and adjusting one of the semi-circular notches to a semi-elliptical notch, the infinite Q-factor and invisible BIC are transformed into a finite Q-factor and visible quasi-BIC. Since the morphological changes on both sides of the semi-elliptical notch are equal to those of the semi-circular notch, the effective refractive index of the structure remains constant, making the resonant wavelength position of the quasi-BIC very stable. After testing, when α = 0.11, the Q-factor can reach 3×10 4 , exceeding most of the literature reports in the visible spectrum.

[0013] In addition, the introduction of the central perforation is the origin of the Q-factor enhancement. It causes a large perturbation of the eigenfield, thereby increasing EQ, enhancing the destructive interference between ED and EQ, effectively suppressing the radiation loss, and resulting in a high-Q resonance of SP-BIC.

[0014] At the same time, by changing the radius r0 of the central circular hole, the transmission wavelength of the quasi-BIC can be tuned. As the radius of the circular hole increases, the improvement of the Q-factor becomes more obvious, and the resonance position gradually undergoes a blue shift.

[0015] To reveal the potential physical origin of the resonance, the scattering powers of different multipoles such as the electric dipole (ED), magnetic dipole (MD), toroidal dipole (TD), electric quadrupole (EQ), and magnetic quadrupole (MQ) in the Cartesian coordinate system are analyzed and demonstrated. It is found that the contribution of ED always dominates. Due to the asymmetric distribution of the notches on both sides disturbing the eigenfield of the structure, it can be inferred from the electric field distributions of ED and EQ that SP-BIC mainly generates peaks through the destructive interference between ED and EQ.

[0016] Based on the dielectric metasurface described in the present invention, under the vertical excitation of y-polarized light, when there is no perturbation, i.e., θ1 = θ2 = 45°, the transmission spectrum of the metasurface is flat in the wavelength range of 650 nm - 750 nm. When θ2 increases from 45° to 67.8°, a sharp peak appears at a wavelength of 708 nm and gradually intensifies, and its resonance position is stable with the change of θ2.

[0017] In a further technical solution, the dielectric substrate is made of one of quartz wafers, magnesium fluoride, K9 glass, and BF33 glass with a refractive index of 1.4 - 1.5. Since the refractive indices of these materials are extremely close, they have selection universality; the dielectric resonant unit is selected from one of silicon, silicon nitride, silicon carbide, and silicon dioxide with a refractive index of 2 - 4; the dielectric cladding layer is selected from one of air, organic substances, and optical liquids.

[0018] The present invention also discloses a preparation method of the dielectric metasurface that realizes stable high-Q BIC through eigenfield perturbation, and the specific preparation method is as follows:

[0019] Step 1: Substrate cleaning, cleaning and processing the surface of the quartz wafer.

[0020] Step 2: Thin film deposition, depositing a dielectric thin film on the substrate to form an optical element of the metasurface.

[0021] Step 3: Hard mask preparation, transferring the photoresist image to the hard mask, and then etching and transferring the final pattern to the substrate through the hard mask.

[0022] Step 4: Etching, etching the photoresist and the hard mask into the required structure.

[0023] In a further technical solution, the preparation method further includes the following steps:

[0024] Step 5: Detection, the prepared metasurface needs to undergo strict quality inspection and evaluation, including optical performance testing, mechanical performance testing, and environmental adaptability testing.

[0025] In a further technical solution, the thin film deposition method used in the second step is one of chemical vapor deposition CVD, plasma-enhanced chemical vapor deposition PECVD, and magnetron sputtering.

[0026] In a further technical solution, the etching method used in the fourth step is one of inductively coupled plasma etching ICP, reactive ion etching technology IRE, and dry etching.

[0027] The beneficial effects of the present invention are as follows. In previous studies, the resonance wavelength position of BICs would change with the change of the asymmetry parameter, resulting in the excitation of BICs deviating from the Γ point, and the excitation intensity of the resonance peak weakening and the Q factor decreasing. In the present invention, since the morphological changes on both sides of the notch are equal, the effective refractive index of the structure also remains constant, making the quasi-BIC resonance wavelength position very stable; the introduction of the central perforation in the structure can effectively perturb the eigenfield, enhance the resonance of the electric quadrupole, and enhance the multimode interference, thereby increasing the Q value. In addition, by tuning the radius of the middle circular hole r0, the tuning of the quasi-BIC resonance wavelength and the Q value can be achieved. In addition, the structure of the invention is easier to prepare, and the material is not easily oxidized and corroded. It opens up many promising ways for realizing the ability to enhance the light-matter interaction at the nanoscale in indirect electrically pumped lasers, nonlinear optical effects, solar cells, optical modulators, and highly sensitive sensors and for technological progress. Description of the Drawings

[0028] Figure 1 It is a three-dimensional structure schematic diagram and a unit cell schematic diagram of a dielectric metasurface for realizing a stable high-Q value BIC through eigenfield perturbation according to the present invention.

[0029] Figure 2Transmission spectrum variation with θ of a dielectric metasurface for realizing a stable high-Q BIC through intrinsic field perturbation in the present invention.

[0030] Figure 3 Power dissipation of different multipoles of a dielectric metasurface for realizing a stable high-Q BIC through intrinsic field perturbation in the present invention. (Wherein, Figure 3 (a) is the power dissipation of different multipoles, Figure 3 (b) is the simulated electric field distribution of ED in the x-y plane, Figure 3 (c) is the simulated electric field distribution of EQ in the x-y plane).

[0031] Figure 4 Transmission spectra of a dielectric metasurface for realizing a stable high-Q BIC through intrinsic field perturbation in the present invention at different central circular hole radii r0. (Wherein, Figure 4 (a) is a schematic diagram of the metasurface with different central circular hole radii r0, Figure 4 (b) is the variation of the transmission spectra of the metasurface calculated at central circular hole radii r0 = 0, 10, 20, 25, and 30 nm with r0).

[0032] Figure 5 Q-factor variation curves of BIC at different r0 of a dielectric metasurface for realizing a stable high-Q BIC through intrinsic field perturbation in the present invention. Detailed implementation manners

[0033] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation manners.

[0034] Example 1:

[0035] As Figure 1 shown, this example relates to a design structure of a dielectric metasurface for realizing a stable high-Q BIC through intrinsic field perturbation, including a dielectric substrate, a plurality of dielectric resonant units arranged on the dielectric substrate, the plurality of dielectric resonant units being arranged periodically, and the overall structure being surrounded by a dielectric cladding layer.

[0036] In this example, the material selected for the dielectric substrate is SiO2 with a refractive index of 1.45, the material selected for the dielectric resonant units is Si with a refractive index of 3.2, and the material selected for the dielectric cladding layer is air with a refractive index of 1.

[0037] Wherein, Figure 1The structural parameters of the dielectric resonator unit are provided, which are uniformly arranged in a square grid with a period P = 400 nm. The height H of the dielectric resonator unit is 320 nm, and each side length is equal, L = 250 nm. The radius of the central circular hole is r0 = 30 nm, and the semi - circular radii are r1 = r2 = 53 nm. The angle θ between the radius and the chord is used to describe the side shape of the notch, where θ1 = θ2 = 45°, and the asymmetry parameter is defined as α = |θ2 - θ1| / θ1.

[0038] Example 2:

[0039] This example relates to the preparation process flow of the above - mentioned stable high - Q BIC dielectric metasurface, which is as follows:

[0040] 1) Substrate cleaning: Clean and process the surface of the quartz wafer.

[0041] 2) Thin - film deposition: Deposit a layer of dielectric film on the substrate to form the optical element of the metasurface.

[0042] 3) Hard - mask preparation: Transfer the photoresist image to the hard mask, and then transfer the final pattern to the substrate through the hard mask.

[0043] 4) Etching: Etch the photoresist and the hard mask into the required structure.

[0044] 5) Detection: The prepared metasurface needs to undergo strict quality inspection and evaluation, including optical performance testing, mechanical performance testing, and environmental adaptability testing.

[0045] In step 2) of this example, the coating method used is plasma - enhanced chemical vapor deposition (PECVD); in step 4), the etching method used is inductively coupled plasma etching (ICP).

[0046] In addition, the above - mentioned preparation method may involve multiple methods. In Example 2, only one of the methods is adopted. When products are prepared using other preparation processes, their implementation effects are very close to those of Example 2.

[0047] Example 3:

[0048] As Figure 2 shown, on the basis of Example 1, this example introduces symmetry breaking to convert the non - radiative BIC into a quasi - BIC.

[0049] The symmetry - breaking position changes the magnitude of θ1 of one of the semi - circular notches. Figure 2The transmission spectrum of the metasurface under y-polarized light illumination is shown. In the absence of perturbation, i.e., when θ1 = 45°, in the range of 650 nm to 750 nm, the transmission spectrum is almost flat. At this time, the BIC exhibits an optical resonance with an infinitely narrow linewidth (infinite Q factor), making it invisible in the transmission spectrum. As θ1 increases, a sharp peak appears at 708 nm and gradually intensifies, and its resonance position remains stable as θ1 changes.

[0050] Figure 3 (a) Describes the power dissipation of different multipoles, indicating that the contribution of the electric dipole ED accounts for the largest proportion, while the contributions of other dipole moments are almost negligible. Figure 3 (b) and (c) are the simulated electric field distributions of ED and EQ in the x - y plane, from which it can be inferred that the SP - BIC mainly generates sharp peaks through destructive interference between ED and EQ.

[0051] Example 4:

[0052] To reveal that the high - Q resonance of the SP - BIC is caused by multimode interference due to the introduction of the central circular hole defect, in this example, based on the above Example 3, the transmission spectra of the metasurfaces with different central circular hole radii r0 at α = 0.45 are calculated, as shown in Figure 4 (a) and 4(b). Figure 4 (a) Describes the schematic diagrams of metasurfaces with different central circular hole radii r0. Figure 4 (b) Calculates the variation of the transmission spectra of the metasurfaces at the central circular hole radii r0 = 0, 10, 20, 25, and 30 nm respectively with the change of r0 (the arrow points to the change of r0 from 0 to 30 nm). For the case of r0 = 0, there is no circular hole at the center of the nanocube, and the excited resonance exhibits a wide linewidth and weak intensity. However, as r0 increases, both the resonance intensity and linewidth in the transmission spectrum are significantly improved, and the peak position undergoes a blue shift, indicating that the perturbation of the central circular hole to the eigenfield can lead to a significant modulation of the Q value.

[0053] Subsequently, for different central circular hole radii, the scattering powers and contribution ratios of the ED, MD, TD, EQ, and MQ components in the nanocube are calculated. The contribution of the EQ response is significantly enhanced by introducing the central circular hole, which points to the origin of the Q - factor enhancement. Since no defect is introduced at the center of the nanocube, the relatively weak destructive interference between ED and EQ is difficult to suppress the radiation loss. However, the introduction of the central circular hole causes a large perturbation to the eigenfield, thereby increasing EQ, enhancing the destructive interference between ED and EQ, effectively suppressing the radiation loss, and resulting in the high - Q resonance of the SP - BIC.

[0054] Figure 5Depicts the variation of the Q factor calculated at different α with r0, demonstrating that the Q factor in this three-notch system can be further improved by reducing α. At α = 0.11, the Q factor can reach 3×10 4 magnitude, exceeding most of the literature reports in the visible spectrum.

[0055] It should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the protection scope of the present invention. Any combination or equivalent transformation made on the basis of the above embodiments belongs to the protection scope of the present invention.

Claims

1. A dielectric metasurface for achieving stable high-Q BIC through intrinsic field perturbation, characterized in that: The dielectric metasurface includes a dielectric substrate, a dielectric resonance unit and a dielectric wrapping layer, and a plurality of the dielectric resonance units are periodically arranged on the dielectric substrate. Periodically arranged dielectric resonance units are provided, and all dielectric resonance units are surrounded by a dielectric wrapping layer; each dielectric resonance unit has the same structure, and is in the shape of a regular quadrangular prism as a whole, with a vertical circular hole in the middle; each dielectric resonance unit has a columnar notch at corresponding positions on both sides of the y-axis direction, one of which is a semicircular notch and the other is a semi-elliptical notch.

2. The dielectric metasurface for achieving stable high-Q BIC through intrinsic field perturbation according to claim 1, characterized in that: The dielectric resonance units are periodically arranged in the x direction and the y direction of the dielectric substrate, and the period is P.

3. The dielectric metasurface for achieving stable high-Q BIC through intrinsic field perturbation according to claim 1, characterized in that: The radius of the semicircular notch of the dielectric resonance unit is r1, and the angle between the radius and the chord - the chord between the midpoint of the semicircular arc and one of its end points - is θ1; the midpoint of the semi-elliptical notch of the dielectric resonance unit and the midpoint of the semi-elliptical arc is the semi-major axis or semi-minor axis r2, and the angle between it and the chord - the chord between the midpoint of the semi-elliptical arc and one of its end points - is θ2. The asymmetry parameters of the semicircular notch and the semi-elliptical notch are defined as α = |θ2-θ1| / θ1.

4. A dielectric metasurface for achieving stable high-Q BIC through intrinsic field perturbation according to claim 1 or 3, characterized in that: The dielectric substrate is made of one of quartz glass, magnesium fluoride, K9 glass and BF33 glass.

5. The dielectric metasurface for achieving stable high-Q BIC through intrinsic field perturbation according to claim 1 or 3, characterized in that: The dielectric resonance unit is made of one of silicon, silicon nitride, silicon carbide and silicon dioxide.

6. The dielectric metasurface for achieving stable high-Q BIC through intrinsic field perturbation according to claim 1, characterized in that: The material used for the medium wrapping layer is one of air, organic matter and optical liquid.

7. A method for preparing a dielectric metasurface for achieving stable high-Q BIC through intrinsic field perturbation as claimed in any one of claims 1 to 6, characterized in that: The preparation method is as follows: 1) Substrate cleaning: clean and treat the surface of the quartz wafer; 2) Thin film deposition: depositing a dielectric thin film on a substrate to form an optical element of the metasurface; 3) Hard mask preparation: transfer the photoresist image to the hard mask, and then transfer the final pattern to the substrate through the hard mask; 4) Etching: Etch the photoresist and hard mask into the desired structure.

8. The method for preparing a dielectric metasurface for achieving stable high-Q BIC through intrinsic field perturbation according to claim 7, characterized in that: The preparation method further comprises the following steps: 5) Testing: The prepared metasurface needs to undergo rigorous quality testing and evaluation, including optical performance testing, mechanical performance testing, and environmental adaptability testing.

9. The method for preparing a dielectric metasurface for achieving stable high-Q BIC through intrinsic field perturbation according to claim 7, characterized in that: The thin film deposition method used in step 2) is one of chemical vapor deposition CVD, plasma enhanced chemical vapor deposition PECVD, and magnetron sputtering.

10. The method for preparing a dielectric metasurface for achieving stable high-Q BIC through intrinsic field perturbation according to claim 7, characterized in that: The etching method used in step 4) is one of inductively coupled plasma etching (ICP), reactive ion etching (IRE) and dry etching.

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