A BICs metasurface structure sensor based on all-dielectric materials

Abstract

The present invention belongs to the technical field of optical biosensors, and specifically relates to a BICs metasurface structure sensor based on all-dielectric materials. The BICs metasurface structure sensor of the present invention is an array composed of four cubic columns of different sizes arranged in a two-by-two pattern as unit cells and extended in a two-dimensional plane; the heights of the four cubic columns are the same, and the bottom surfaces are squares; the center distance between adjacent cubic columns is P; the period of the unit cell array is 2P; the widths of the four cubic columns are denoted as w1, w2, w3, and w4 in sequence, satisfying: w1 = w3, w2 = w4, and w1 ≠ w2 to achieve the excitation of the quasi-BIC mode; denote Δw = w1 - w2; below the array structure is a waveguide layer made of the same material as the unit cell; below the waveguide layer is a substrate, serving as the support layer of the overall structure. In the present invention, the unit cell structure has a larger surface area compared to other configurations, increasing the depositable area of the biosensor, thereby achieving an increase in surface sensitivity.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical biosensors, and in particular relates to a BICs metasurface structure sensor based on all-dielectric materials. Background Art

[0002] Optical biosensors can detect biomolecules in a label-free, real-time, and highly sensitive manner. Among the many optical biosensors, nano-optical resonators (such as guided mode resonance, surface plasmon resonance, photonic crystal microcavity, continuous spectrum bound state, etc.) have been widely studied due to their extremely strong light-matter interaction characteristics, which makes them very sensitive to changes in the refractive index of the surface of the structure. Recently, with the advancement and development of micro-nano processing technology, high-performance nano-optical resonant devices have been widely used in the field of biosensing.

[0003] Among them, optical resonators based on bound states in the continuum (BICs) stand out from many nano-optical resonator devices because of their infinite Q value in theory. BICs are a non-radiative electromagnetic eigenstate in the radiation region. Ideal BICs have dark field properties and cannot be directly observed by experimental means. In practice, due to processing errors and scattering losses, ideal BICs will become quasi BICs, and this quasi-BICs still has a very high Q value.

[0004] Recently, researchers have discovered BICs modes in structures such as photonic crystals, metasurfaces, and resonant gratings, and have applied them to the field of sensing. The detection of BICs modes in photonic crystals and resonant gratings is generally done by oblique incidence. Although this method can achieve extremely high Q values ​​(greater than 10 5 ), but this oblique incidence detection is more complicated and requires many high-precision instruments, which is not conducive to the miniaturization and integration of the detection system. In the field of metasurfaces, researchers have found that the use of asymmetric configurations can effectively excite quasi-BICs modes with high Q characteristics under normal incidence. This makes it possible to miniaturize and integrate BICs devices.

[0005] For sensors, in addition to high Q value characteristics, the device also needs to have high sensitivity. For biosensors, special attention should be paid to its surface sensitivity. At present, common BICs structure devices are mainly prepared based on lossless or low-loss high-refractive index dielectric materials. The advantage of dielectric materials is that it is easy to modulate the BICs mode and easy to achieve high-Q value resonance in experiments, but the sensitivity of the BICs structure based on all-dielectric materials is limited. Summary of the invention

[0006] The object of the present invention is to provide a BICs metasurface structure sensor based on all-dielectric materials with high surface sensitivity.

[0007] The BICs metasurface structure sensor based on all-dielectric materials provided by the present invention is an array composed of four cubic columns of different sizes arranged in twos as a unit cell, which is extended in a two-dimensional plane. The unit cell is also called a "tetramer". The height of the four cubic columns is the same, denoted as H, and the bottom surface is a square; the distance between the centers of adjacent cubic columns is P; the period of the unit cell array is 2P; the width of the first cubic column is w 1 , the width of the second cubic column is w 2 , the width of the third cubic column is w 3 , the width of the fourth cubic column is w 4 , satisfying: w 1 = w 3 , w 2 =w 4 , and w 1 ≠ w 2 , to achieve the excitation of the quasi-BIC mode; let Δw = w 1 -w 2 ;

[0008] Below the array structure is a waveguide layer made of the same material as the unit cell; below the waveguide layer is a substrate serving as a support layer for the overall structure.

[0009] In the present invention, the material of the unit cell is silicon nitride or titanium dioxide medium material; the refractive index is 1.8-2.5.

[0010] In the present invention, the height H of the cubic column in the unit cell is 50-500 nm, the bottom surface is a square, and the width is 100-350 nm.

[0011] In the present invention, the center spacing P between adjacent cubic columns is 400 nm; and the period 2P of the unit cell is 800 nm.

[0012] In the present invention, the substrate material is silicon, quartz or glass.

[0013] The BICs metasurface structure sensor provided by the present invention, in which the tetramer structure of the unit cell can excite TE (1,1) and TM (1,1) modes with high signal-to-noise ratio, while the BIC mode excited by the general asymmetric metasurface is TE (1,0) or TM (1,0) mode. Compared with the (1,0) order mode, this (1,1) order mode can obtain a higher size / resonance wavelength ratio (i.e., the smallest preparation size in the structure divided by the resonance wavelength). This feature is helpful for the processing and preparation of actual samples. In addition, since the tetramer structure has a larger surface area than other configurations, the surface area on which biomolecular proteins can be deposited is greatly increased, thereby improving the surface sensitivity in the same wavelength band. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of the structure of the tetrameric BICs metasurface unit (unit cell) of the present invention.

[0015] Figure 2 It is a top view of the cubic column structure of the tetrameric BICs metasurface unit (unit cell) of the present invention.

[0016] Figure 3 Reflection spectra of the tetramer metasurface at different Δw.

[0017] Figure 4 Top view of the surface structure of the monomer BICs metastructure.

[0018] Figure 5 Top view of the surface structure of the dimeric BICs metastructure.

[0019] Figure 6 Comparison of TE (1,0) mode spectra under different configurations.

[0020] Figure 7 Comparison of TM (1,1) mode spectra under different configurations.

[0021] Figure 8 Comparison of TE (1,1) mode spectra under different configurations.

[0022] Fig. 9 A top view of protein molecules attached to the surface of the tetrameric superstructure.

[0023] Fig.10 Side view of the protein molecule attached to the surface of the tetrameric superstructure.

[0024] Fig.11 Reflection spectra of the tetramer metasurface at different protein molecule thicknesses.

[0025] Fig.12 Comparison of surface sensitivity of different configurations.

[0026] The numbers in the figure are: 1 is the substrate, 2 is the waveguide layer, 3 is the dielectric square column (or cylinder), 4 is the external environment, and 5 is the protein molecule. DETAILED DESCRIPTION

[0027] The present invention is further described below by way of embodiments in conjunction with the accompanying drawings.

[0028] Embodiment 1:

[0029] There is a waveguide layer with a thickness of 140nm on the support material. On this waveguide layer is a designed tetramer array structure with a tetramer height H of 60nm, an array period of 800nm, and the widths of the first and third cubic columns are equal w 1 =w 3 , the widths of cubic columns two and four are equal w 2 =w 4 ,See Figure 1 , 2. By adjusting the width of adjacent cubic columns and making w 1 ≠w 2 (i.e. Δw≠0) to achieve symmetry breaking within the structural plane, thus generating quasi-BICs mode. Figure 3 It can be seen that when Δw=0nm, there is no resonant mode in the wavelength range of 820-885nm, and when Δw≠0nm, a mode with a very narrow linewidth appears near 825nm and 875nm, respectively, and the linewidths of these two modes (respectively denoted as "quasi-BICs 1" and "quasi-BICs 2") become wider with the increase of Δw, showing that it is a mode with quasi-BICs properties.

[0030] The modes excited by the metasurface in different wavelength ranges are caused by the different diffraction orders generated after the light passes through the diffraction unit. The properties of the mode can be judged by the Rayleigh cutoff wavelength formula, which is: λ = n eff ·Λ / ( i 2 + j 2 ) 1 / 2 , (where Λ is the array period, i and j are the diffraction orders in the x and y directions, n eff is the effective refractive index of the external environment). In this tetramer structure, Λ=2P=800nm, n eff≈1.5. It can be seen that the modes excited near 825nm and 875nm are of the (1,1) order, and generally speaking, the wavelength of the TM mode is smaller than that of the TE mode. Therefore, the above-mentioned "quasi-BICs 1" corresponds to the TM (1,1) mode, and the "quasi-BICs 2" corresponds to the TE (1,1) mode. The BIC mode excited by a general asymmetric metasurface is the TE (1,0) or TM (1,0) mode.

[0031] Embodiment 2:

[0032] In order to further illustrate the advantages and characteristics of the tetramer metasurface, this structure is compared with other metasurface configurations.

[0033] (1) Surface structural parameters of tetramer metastructures

[0034] There is a waveguide layer with a thickness of 140nm on the support material. On this waveguide layer is a designed tetramer array structure with a tetramer height H of 60nm, an array period of 800nm, and a cubic column width w 1 , w 3 =280nm, the width of the second and fourth cubic columns w 2 and w 4 = 200nm, the center distance between adjacent cubic columns is P, see Figure 1 , Figure 2 . Due to w 1 ≠w 2 (i.e., Δw≠0), the in-plane symmetry of the structure is broken, and thus a quasi-BICs mode can be generated.

[0035] (2) Surface structural parameters of single-mer metastructures

[0036] There is a waveguide layer with a thickness of 140nm on the support material. On this waveguide layer is a designed monomer array structure with a monomer height H of 60nm and an array period 2P of 800nm. The monomer is composed of a cubic column 1 and a cubic column 2, with widths w and w, respectively. 1 , w 2 =280nm, see Figure 4 . Due to w 1 ≠w 2 (i.e., Δw≠0), the in-plane symmetry of the structure is broken, and thus a quasi-BICs mode can be generated.

[0037] (3) Surface structural parameters of dimer metastructures

[0038] There is a waveguide layer with a thickness of 140nm on the support material. On this waveguide layer is a designed monomer array structure, with a monomer height H of 60nm and an array period 2P of 800nm. The dimer is composed of cubic column 1 and cubic column 2. The center interval between cubic column 1 and cubic column 2 is P, and the widths are w 1 , w 2 =280nm, see Figure 5 . Due to w 1 ≠w 2 (i.e., Δw≠0), the in-plane symmetry of the structure is broken, and thus a quasi-BICs mode can be generated.

[0039] Figure 6 The quasi-BIC modes excited by three configurations near 1180nm are shown. It can be seen that both monomers and dimers can produce obvious BIC modes near this wavelength range, while tetramers have no mode in this band. According to the Rayleigh cutoff wavelength formula, the mode in this wavelength range belongs to the TE (1,0) mode, which is also a common quasi-BIC mode excited by asymmetric metasurfaces, generally called the fundamental mode, and is the easiest to excite.

[0040] Figure 7 , Figure 8 The BIC mode reflection spectra of the three configurations excited near the TM (1,1) mode and the TE (1,1) mode are shown respectively. It can be seen that although both monomers and dimers can produce BIC modes near this wavelength range, their Q values ​​are much lower than those of tetramers. Among them, the Q values ​​of the TM (1,1) mode of the monomer and dimer are 3860 and 3890 respectively, while the Q value of the TM (1,1) mode of the tetramer is 34600, which is one order of magnitude higher. In addition, the Q value of the TE (1,1) mode of the dimer is 1030, while the Q value of the TM (1,1) mode of the tetramer is 7110, which is about 7 times higher. It is worth emphasizing that since the (1,1) order mode belongs to a high-order mode and is generally not easy to excite, the resonance intensity of the quasi-BIC mode of the monomer and dimer here is very low, which is not conducive to practical application.

[0041] The tetramer structure does not have this problem. The reason can be understood as follows: the diagonal cubic columns in the designed tetramer structure have the same size (i.e., the widths of cubic columns one and four are equal; the widths of cubic columns two and three are equal). Therefore, this structure can be further regarded as a "dimer" structure arranged diagonally, and the center spacing between the diagonal cubic columns is P, the period of the structure can be considered to be P. At this time, combined with the Rayleigh cutoff wavelength formula, it can be obtained that the mode with a resonance wavelength of 827nm also corresponds to the (1,0) order, which is also the fundamental mode.

[0042] In summary, the proposed tetramer structure can be viewed as a diagonally arranged "dimer" structure, where the diagonal spacing is larger than that of adjacent structures, making the actual period of the tetramer ( P) is smaller than the monomer and dimer structures (2P). It is worth noting that the minimum preparation size of the three structures is 200nm, but the actual available resonance wavelengths are distributed around 827nm (tetramer) and 1180nm (monomer, dimer). Therefore, the tetramer structure has a larger size-to-wavelength ratio (the size-to-wavelength ratio is defined as the minimum preparation size in the structure divided by the resonance wavelength). The significance of this parameter is that a large processing size will improve the stability of the structure preparation while reducing the preparation cost, but if the large size will cause the resonance wavelength of the structure to be significantly red-shifted (for example, to 1.5μm), and water has a strong absorption effect on light at long wavelengths, which is not conducive to sensing detection. Therefore, it is also very important for the device to excite a smaller resonance wavelength under a large structural size structure. The tetramer structure has just this feature.

[0043] Embodiment 3:

[0044] In order to verify the characteristics of the tetramer metasurface, the surface sensitivity of this structure was compared with that of other metasurface designs.

[0045] (1) Surface structural parameters of tetramer metastructures

[0046] There is a waveguide layer with a thickness of 140nm on the support material. On this waveguide layer is a designed tetramer array structure with a tetramer height H of 60nm, an array period 2P of 800nm, and a cubic column width w 1 , w 3 =280nm, the width of the second and fourth cubic columns w 2 and w 4 = 200nm, the center distance between adjacent cubic columns is P, see Figure 1 , Figure 2 . Due to w 1 ≠w 2 (i.e., Δw≠0), the in-plane symmetry of the structure is broken, and thus a quasi-BICs mode can be generated.

[0047] (2) Surface structural parameters of single-mer metastructures

[0048] There is a waveguide layer with a thickness of 140nm on the support material. On this waveguide layer is a designed monomer array structure with a monomer height H of 60nm and an array period 2P of 800nm. The monomer is composed of a cubic column 1 and a cubic column 2, with widths w and w, respectively. 1 , w 2 =280nm, see Figure 4 . Due to w 1 ≠w 2 (i.e., Δw≠0), the in-plane symmetry of the structure is broken, and thus a quasi-BICs mode can be generated.

[0049] (3) Surface structural parameters of dimer metastructures

[0050] There is a waveguide layer with a thickness of 140nm on the support material. On this waveguide layer is a designed monomer array structure, with a monomer height H of 60nm and an array period 2P of 800nm. The dimer is composed of cubic column 1 and cubic column 2. The center distance between cubic column 1 and cubic column 2 is P, and the width is w 1 , w 2 =280nm, see Figure 5 . Due to w 1 ≠w 2 (i.e., Δw≠0), the in-plane symmetry of the structure is broken, and thus a quasi-BICs mode can be generated.

[0051] This metasurface is placed in a liquid environment to achieve biosensing detection. Here, biosensing can be performed by attaching protein molecules of different thicknesses to the surface of the structure to represent protein molecules with different concentrations. Taking tetramers as an example, the specific attachment method is shown in Figure 6 (Top view) and Figure 7 (Side view). When the refractive index (the refractive index of the protein molecule is 1.5) and thickness of the structure surface change. By simulating different thicknesses (the thickness of the protein molecule is 0nm, 10nm), the peak position of the resonance mode will move, see Figure 8 In the tetramer structure, the sensitivity of the quasi-BIC 1 mode was 0.24nm / nm, and that of the quasi-BIC 2 mode was 0.22nm / nm.

[0052] The same processing method was used to calculate the surface sensitivity of monomers and dimers. The specific comparison results are shown in Fig. 9 . In the comparison of quasi-BIC 1 mode, the surface sensitivity of the monomer is: 0.201nm / nm, and the sensitivity of the dimer is: 0.205nm / nm, both of which are lower than the tetramer structure. In the comparison of quasi-BIC 2 mode, the sensitivity of the dimer is: 0.192nm / nm, which is also lower than the tetramer structure. The surface sensitivity of the quasi-BIC 1 mode and quasi-BIC 2 mode with tetramer structure is increased by 20% and 15% respectively compared with the design of other configurations. This is all due to the fact that the tetramer structure has a larger surface area than other configurations, so theoretically the area where the protein can be deposited is increased, thereby achieving an increase in surface sensitivity.

Claims

1. A BICs metasurface structure sensor based on all-dielectric materials, characterized in that: It is an array composed of four cubic columns arranged in twos as a unit cell, which is extended in a two-dimensional plane. This unit cell is also called a "tetramer"; The four cubic columns have the same height, denoted as H, and the bottom surface is a square; the center spacing of adjacent cubic columns is P; the period of the unit cell array is 2P; the width of the first cubic column is w1, the width of the second cubic column is w2, the width of the third cubic column is w3, and the width of the fourth cubic column is w4, satisfying: w1= w3, w2=w4, and w1≠ w2, so as to realize the excitation of the quasi-BIC mode; Let Δw = w1- w2; The diagonally oriented cubic pillars in the tetrameric structure have the same size; Below the array structure is a waveguide layer made of the same material as the unit cell; below the waveguide layer is a substrate serving as a support layer for the overall structure.

2. The BICs metasurface structure sensor based on all-dielectric materials according to claim 1 is characterized in that: The material of the unit cell is silicon nitride or titanium dioxide medium material; the refractive index is 1.8-2.

5.

3. The BICs metasurface structure sensor based on all-dielectric materials according to claim 2 is characterized in that: The height H of the cubic columns in the unit cell is 50-500 nm, and the width is 100-350 nm.

4. The BICs metasurface structure sensor based on all-dielectric materials according to claim 1, characterized in that: The center spacing P of adjacent cubic columns is 400nm; the period 2P of the unit cell is 800nm.

5. The BICs metasurface structure sensor based on all-dielectric materials according to claim 1, characterized in that: The substrate material is silicon, quartz or glass.

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