A high quality factor hybrid metal-dielectric plasmonic resonant optical filter

Abstract

The application discloses a high-quality-factor hybrid metal-dielectric plasmonic resonant optical filter; the filter comprises a substrate layer, a high-refractive-index dielectric layer, a low-refractive-index dielectric layer and a metal grating layer which are sequentially stacked upwards; the substrate layer is made of a low-refractive-index transparent dielectric material; the material of the high-refractive-index dielectric layer has a refractive index greater than the refractive index of the material of the low-refractive-index dielectric layer and the substrate material; the metal grating layer is provided with a grating structure with periodic arrangement and comprises three metal strips in one device period; the width of the middle metal strip is greater than the width of the two metal strips on the sides; the radiation loss of the coupling mode is adjusted by adjusting the width of the middle metal strip, and then the quality factor of the filter is adjusted. The application has the advantages of simple structure, high quality factor, compatibility with traditional integrated circuit preparation processes and the like, and has wide application in the fields of optical communication, hyperspectral and multispectral imaging and sensing and the like.

Description

Technical Field

[0001] This invention belongs to the field of nanosurface optics, specifically relating to a high-quality factor hybrid metal-dielectric plasma resonance optical filter. Background Technology

[0002] Narrowband optical filters have wide applications in optical communication, spectral imaging, and sensing. Previously, thin-film optical filters based on Fabry-Perot resonances have been studied. Thin-film optical filters are based on the principle of thin-film interference and consist of multiple layers of dielectric or metal films. When incident light enters the device, multiple interferences and reflections occur due to the interactions of reflection and refraction at the interfaces of the dielectric or metal films, forming a resonant mode in the direction of light propagation. This allows light of specific wavelengths to be selectively reflected or transmitted, achieving a filtering effect. Thin-film optical filters have many layers, and in the long-wavelength band, the different coefficients of thermal expansion of each layer can cause cracks in the film. Furthermore, due to the large number of layers, the fabrication process of thin-film optical filters is extremely complex.

[0003] Optical filters based on nanoscale surface periodic structures include all-dielectric guided-mode resonant optical filters, plasmon resonance optical filters, and hybrid metal-dielectric plasmon resonance optical filters. Nanoscale surface periodic structure optical filters utilize periodic surface structures to provide additional momentum compensation for light in free space, thereby coupling the light into the filter structure and exciting laterally propagating guided modes. Due to the coupling effect between the incident light and the guided mode, light of a specific wavelength can be transmitted or reflected. All-dielectric guided-mode resonant optical filters achieve near-100% transmission efficiency and picometer-level spectral linewidths due to the low absorption of light energy by the dielectric material. However, the sufficiently thick surface structure required for phase compensation presents a significant challenge to nanoscale surface fabrication. Plasmon resonance optical filters utilize periodic structures on metal surfaces to couple light into free space. The thickness of the metal is typically less than 100 nanometers, making fabrication simple. However, the absorption of light energy by the metal material increases the spectral linewidth of the device. The quality factor, the ratio of operating wavelength to spectral linewidth, essentially represents the energy stored in the near field of a device. Therefore, plasmon resonance optical filters face the problems of high optical energy loss, wide spectral linewidth, and low quality factor. To address this issue, hybrid metal-dielectric plasmon resonance optical filters have been investigated. These filters utilize the coupling between guided mode and plasmon mode to reduce energy loss. Currently, the highest quality factor achieved by hybrid plasmon resonance optical filters is 500, which remains a significant technical challenge. Summary of the Invention

[0004] To address the problems existing in the prior art, the purpose of this invention is to provide a hybrid metal-dielectric plasmon resonance optical filter with a simple structure, compatibility with existing integrated circuit fabrication processes, and high quality factor.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A high-quality factor hybrid metal-dielectric plasmon resonance optical filter includes a substrate layer, and a high-refractive-index dielectric layer, a low-refractive-index dielectric layer, and a metal grating layer stacked sequentially on the substrate layer; wherein:

[0007] The substrate layer is a low-refractive-index transparent dielectric material used to support the main structure of the optical filter;

[0008] The refractive index of the material of the high refractive index dielectric layer is greater than that of the material of the low refractive index dielectric layer and the substrate layer.

[0009] The metal grating layer is a grating structure with a periodic arrangement, including three metal strips in one device cycle. The width of the middle metal strip is greater than the width of the two adjacent metal strips, which is equivalent to adding structural perturbation to the device structure. The radiation loss of the coupling mode is adjusted by adjusting the width of the middle metal strip, thereby adjusting the quality factor of the filter.

[0010] In this invention, the substrate material is either a quartz glass substrate or a magnesium fluoride substrate; the high refractive index layer material is selected from any one of titanium dioxide, silicon nitride, or silicon; and the low refractive index layer material is selected from any one of magnesium fluoride or silicon dioxide.

[0011] In this invention, the metal grating layer material is selected from gold, silver, or aluminum. Aluminum is selected for the visible light band, while gold or silver is selected for the near-infrared and longer bands.

[0012] In this invention, the high-refractive-index dielectric layer material has no absorption loss in the operating wavelength range of the device; the light absorption rates of the substrate layer, the high-refractive-index dielectric layer, and the low-refractive-index dielectric layer are close to zero.

[0013] In this invention, increasing the thickness of the low-refractive-index dielectric layer can reduce the coupling strength between the plasma mode and the guided mode, thereby reducing the absorption loss of the device. However, as the coupling between the plasma mode and the guided mode decreases, the transmittance of the device also decreases significantly. Therefore, the thickness of the low-refractive-index dielectric layer only varies between 10-250 nm. This range should be adjusted accordingly depending on the operating wavelength of the device; in a specific embodiment, when the device operates in the near-infrared band, when the thickness of the low-refractive-index dielectric layer varies between 30-200 nm, the quality factor increases with increasing thickness.

[0014] In this invention, when the device operates in the near-infrared band, the perturbation in the structure changes, that is, within one device cycle, the width of the two adjacent metal strips is 600nm, and the width of the middle metal grating varies between 600-1100nm; in a specific embodiment, when the width of the middle metal grating varies between 800nm-1100nm, the quality factor increases as the perturbation decreases.

[0015] In this invention, the thickness of the metal grating is in the range of 40-60 nm; a thickness greater than 60 nm will increase the sideband of the filter.

[0016] In this invention, when the device operates in the near-infrared band, within one device cycle, the width of the two adjacent metal strips is 600 nm, the width of the middle metal grating is between 800-1100 nm, and the thickness of the low-refractive layer is between 160-235 nm.

[0017] In this invention, the device is polarization dependent, with the electric field direction of the incident light along the direction of the periodic arrangement of the grating, and the magnetic field direction along the direction of the grating strips.

[0018] In this invention, the resonant wavelength of the optical filter is linearly adjusted by changing the period of the metal grating.

[0019] In the optical filter of this invention, when incident light is incident on the filter structure, the periodic metal grating structure provides additional momentum compensation for the incident light in the lateral direction, exciting local plasmon resonance modes on the surface of the metal grating. Simultaneously, since the refractive indices of the substrate layer and the low-refractive-index layer are relatively lower than those of the high-refractive-index layer, guided-mode modes are excited in the high-refractive-index layer. The transmittance peak originates from the coupling between the plasmon resonance mode and the guided-mode mode. Increasing the thickness of the low-refractive-index layer reduces the coupling strength between the local plasmon resonance mode and the guided-mode mode, thereby reducing the spectral linewidth. From the perspective of local electromagnetic field distribution, increasing the thickness of the low-refractive-index layer pulls the energy of the local electromagnetic field away from the metal surface, distributing it more in the dielectric layer, thereby reducing the absorption loss of electromagnetic field energy by the metal material, and thus reducing the spectral linewidth of the device. This is the reason for the reduced absorption loss and increased quality factor of the device. Furthermore, when the width of the grating metal strip is adjusted, i.e., increasing the width of the middle grating strip within one device cycle (adding a perturbation), a new resonance mode is excited. This mode also originates from the weak coupling between the guided-mode mode and the plasmon mode. When the width of the middle grating strip is equal to the width of the other two grating strips, the newly excited mode is in a dark mode state and cannot be detected by the far-field detector. When structural perturbations appear and gradually increase, the dark mode converts into a bright mode, which can leak energy to the far field and thus be detected. The smaller the structural perturbation, i.e., the less the width of the middle grating strip increases, the weaker the coupling strength between the incident light and the bright mode. More energy is stored in the near field and not radiated out. Therefore, the radiation loss of the bright mode is smaller, and the quality factor of the device is larger. The technical solution proposed in this invention, which increases the quality factor of the device by increasing the thickness of the low refractive index layer and the width of the middle strip, is universal and applicable to visible light, near-infrared, and infrared bands.

[0020] Compared with the prior art, the advantages of the present invention are:

[0021] In the device structure of this invention, without structural perturbation, a coupled mode of conventional guided mode and plasmonic mode is excited, with a quality factor of approximately 500. When structural perturbation is added, i.e., increasing the width of the intermediate grating within one device cycle, a new radiation mode is excited. The quality factor of the device can be significantly increased by adjusting the structural perturbation and the thickness of the low-refractive-index layer.

[0022] The resonant optical filter of this invention has a simple structure and a high quality factor, which can reach up to 4.2 × 10⁻⁶. 3 Its compatibility with existing integrated circuit manufacturing processes allows it to be widely used in multispectral and hyperspectral imaging, optical communication, and sensing. Attached Figure Description

[0023] Figure 1This is a schematic diagram of the structure of the high-quality factor hybrid metal-dielectric plasma resonant optical filter of the present invention.

[0024] In this diagram, a is a cross-sectional view of the high-quality factor hybrid metal-dielectric plasmon resonance optical filter, and b is a top view of the device.

[0025] Figure 2 The transmittance spectrum of the high-quality factor hybrid metal-dielectric plasma resonance optical filter of the present invention.

[0026] Figure 3 Transmittance spectra of hybrid metal-dielectric plasmon resonance optical filters with varying low-refractive-index dielectric layer thicknesses (t2).

[0027] Figure 4 This is a graph showing the relationship between the quality factor and the thickness of the low-refractive-index layer.

[0028] Figure 5 Transmittance spectra of hybrid metal-dielectric plasmon resonance optical filters under different structural perturbations.

[0029] Figure 6 This is a graph showing the relationship between the quality factor and the size of the structural perturbation.

[0030] The labels in the diagram are: 1 for the substrate layer, 2 for the high refractive index thin film layer, 3 for the low refractive index thin film layer, and 4 for the metal grating layer. Detailed Implementation

[0031] To better understand the present invention, the present invention will be further described below with reference to the embodiments and accompanying drawings. The following embodiments are only for illustration and not for limiting the present invention.

[0032] The high-quality factor hybrid metal-dielectric plasmon resonance optical filter proposed in this invention has the following structural schematic diagram: Figure 1 As shown in (a), the structure includes a substrate layer 1, a high-refractive-index dielectric layer 2, a low-refractive-index dielectric layer 3, and a metal grating layer 4 disposed on the substrate layer. The thickness of the high-refractive-index layer is denoted as t3, the thickness of the low-refractive-index layer as t2, and the thickness of the metal grating layer as t1. Figure 1As shown in (b), there are three metal grating strips within one device period, denoted as Λ, and the width of the grating strips is l. The structural perturbation of the device described in this invention involves adding an additional width Δl to the width l of the middle metal grating strip. In this invention, the geometric structure of the hybrid metal-dielectric plasmon resonance optical filter is designed using the two-dimensional finite-difference time-domain (FDTD) method, and the transmittance spectrum of the device is calculated. During the calculation, magnesium fluoride is used for both the substrate and the low-refractive-index layer, with a refractive index of 1.38. Silicon is used for the high-refractive-index layer, with a refractive index of 3.48. Gold is used for the metal grating layer. The reason for choosing silicon for the high-refractive-index layer and magnesium fluoride for the low-refractive-index layer is that the large difference in refractive index between the two materials can make the guided mode more localized, or can pull the energy of the electromagnetic field away from the metal surface and distribute it more in the dielectric layer, which is beneficial for increasing the device's quality factor. In the simulation settings, periodic boundary conditions are used at the two edges along the x-direction of the simulation region, and absorbing boundary conditions are used at the two edges along the z-direction. The light source is a plane wave source, and the electric field of the plane wave is along the x-direction.

[0033] Example 1

[0034] The reflectance spectrum of a hybrid metal-dielectric plasmon resonance optical filter was calculated using the finite-difference time-domain method, as follows: Figure 2 As shown. The operating wavelength of the device determines the optimal thickness of the low-refractive-index dielectric layer and the high-refractive-index dielectric layer. The longer the operating wavelength, the thicker the optimal thickness of both the high- and low-refractive-index dielectric layers. In this embodiment, the operating wavelength of the device is 1362 nm, the thickness t3 of the high-refractive-index layer is set to 215 nm, the thickness t2 of the low-refractive-index layer is set to 235 nm, and the thickness t1 of the gold grating layer is set to 40 nm. The device period Λ is 1740 nm, the width of the middle grating strip is 900 nm, and the width l of the two outer grating strips is 600 nm, i.e., the structural perturbation Δl is 300 nm. It can be seen that two transmittance peaks appear in the reflectance spectrum. The transmittance peak at the resonance wavelength of 1560 nm corresponds to the traditional plasma-guided mode coupling mode. The transmittance peak at the resonance wavelength of 1364 nm corresponds to the new coupling mode excited by the structural perturbation. The spectral linewidth of the transmittance peak of the traditional coupling mode is greater than that of the coupling mode excited by the structural perturbation.

[0035] Example 2

[0036] Based on the structural parameters of Example 1, while keeping other parameters constant, the structural perturbation Δl is 300 nm, and the thickness t2 of the low-refractive-index layer is varied to 30 nm, 60 nm, 90 nm, 120 nm, 150 nm, and 180 nm. The transmittance spectra of the optical filter with different low-refractive-index layer thicknesses are calculated using the finite-difference time-domain method, as shown below. Figure 3 As shown, it can be seen that increasing the thickness of the low refractive index layer gradually causes the center wavelength of the transmittance spectrum to blue shift, and the spectral linewidth of the transmittance spectrum to decrease significantly. Since the coupling mode based on structural perturbation belongs to Fano resonance, the transmittance spectrum under each structural parameter is fitted using the Fano line shape, and then the quality factor of the filter is obtained. As shown in Equation (1), the expression of the Fano line shape is given.

[0037]

[0038] Where a, b, and c are constants. ω is the frequency, and ω0 is the center frequency of the resonance. γ is the damping coefficient of the resonance mode. The damping coefficient of the resonance mode can be obtained by fitting the transmittance spectrum using formula (1). The quality factor of the mode can be obtained by using the relationship between the quality factor of the mode and the damping coefficient, that is:

[0039]

[0040] Figure 4 The graph shows the variation of the filter's quality factor with thickness t2 when the thickness of the low-refractive-index layer is changed. It can be seen that the quality factor gradually increases with increasing low-refractive-index layer thickness. This is because changing the thickness of the low-refractive-index layer can modulate the distribution of the local electromagnetic field. As the thickness of the low-refractive-index layer gradually increases, the local electromagnetic field is pulled away from the surface of the gold grating and distributed more within the dielectric material. This reduces the absorption loss of the device's electromagnetic field, corresponding to a decrease in the damping coefficient in formula (2), thus increasing the device's quality factor. However, it is worth noting that the filter's transmittance gradually decreases with increasing low-refractive-index layer thickness. This is because increasing the thickness of the low-refractive-index layer causes the plasma mode and guided mode to gradually enter a weak coupling mechanism, resulting in a smaller spatial overlap between the field distribution profiles of the two modes, leading to a gradual decrease in the device's transmittance.

[0041] Example 3

[0042] Based on the structural parameters of Example 1, with other parameters remaining constant, the thickness of the low-refractive-index layer was 235 nm. The magnitude Δl of the structural perturbation was varied to 0 nm, 100 nm, 200 nm, 300 nm, 400 nm, and 500 nm. The transmittance spectra of the device under different structural perturbation conditions were calculated using the finite-difference time-domain method, as shown below. Figure 5As shown. It can be seen that when the structural perturbation is zero, that is, when the width of each of the three metal gratings is 600 nm, there is only one peak in the transmittance spectrum, which corresponds to the traditional coupled-mode. However, when the width of the middle metal grating increases and structural perturbation is introduced, a new coupled-mode appears in the transmittance spectrum. As the structural perturbation increases, the spectral linewidth of the newly excited coupled-mode gradually increases. Using the Fano formula (1) and the quality factor expression (2) in Example 2, the quality factor of the device under different structural perturbation conditions is calculated, such as Figure 6 As shown in the diagram, the rhombus-shaped dots represent the relationship between the quality factor of the new coupled-mode modes and the structural perturbation, while the circular dots represent the relationship between the quality factor of the traditional coupled-mode modes and the structural perturbation. It can be seen that as the structural perturbation increases, the quality factor of the newly emerging coupled-mode modes gradually increases, reaching a maximum of 4.2 × 10⁻⁶. 3 The quality factor of traditional coupled-mode devices remains almost unchanged under different structural perturbations, approximately 417. This demonstrates that adding structural perturbations, while keeping other structural parameters constant, can increase the device's quality factor by an order of magnitude. This is because introducing structural perturbations into the device structure excites new coupled-modes. Without perturbations, these modes are in a dark mode state, unable to radiate energy into the far field and thus undetectable. With the introduction of structural perturbations, the dark mode transforms into a bright mode, radiating energy into the far field. Furthermore, as the structural perturbation increases, the mode radiates more energy into the far field, resulting in greater radiation loss. Conversely, as the structural perturbation decreases, the mode radiates less energy into the far field, storing more energy in the near field, leading to lower radiation loss and a higher quality factor.

[0043] The present invention discloses a high-quality factor hybrid metal-dielectric plasmon resonance optical filter device, comprising a transparent substrate layer 1, a high-refractive-index dielectric layer 2, a low-refractive-index dielectric layer 3, and a metal grating layer 4. When structural perturbations are introduced into the metal grating, new resonant modes are excited. By adjusting the magnitude of the structural perturbations, the radiation loss of the device is reduced, thereby improving the device's quality factor. Furthermore, the metal grating, acting as a free-space optical coupler, can couple light into the device, exciting plasmon resonance modes on the surface of the metal grating and guided-mode resonance modes in the high-refractive-index dielectric layer. The essence of the new resonance modes is the weak coupling between the guided-mode and the plasmon modes. This weak coupling pulls the energy of the electromagnetic field away from the surface of the metal grating, thereby reducing the absorption loss and improving the device's quality factor. In this invention, the high- and low-refractive-index dielectrics and the metal grating with structural perturbations work synergistically to reduce the absorption and radiation losses of the filter, thus improving the quality factor of the plasmon resonance optical filter. This invention has the advantages of simple structure, high quality factor and compatibility with traditional integrated circuit fabrication processes, and has wide applications in optical communication, hyperspectral and multispectral imaging and sensing.

Claims

1. A high-quality factor hybrid metal-dielectric plasmon resonance optical filter, characterized in that it comprises a substrate layer, and a high-refractive-index dielectric layer, a low-refractive-index dielectric layer, and a metal grating layer sequentially stacked upwards on the substrate layer; wherein: The substrate layer is a low-refractive-index transparent dielectric material used to support the main structure of the optical filter; The refractive index of the high refractive index dielectric layer material is greater than that of the low refractive index dielectric layer material and the substrate layer material. The metal grating layer is a grating structure with a periodic arrangement, including three metal strips in one device cycle. The width of the middle metal strip is greater than the width of the two adjacent metal strips, which is equivalent to adding structural perturbation to the device structure. The radiation loss of the coupling mode is adjusted by adjusting the width of the middle metal strip, thereby adjusting the quality factor of the filter. The device operates in the near-infrared band. Within one device cycle, the width of the two adjacent metal strips is 600 nm, and the width of the central metal grating is between 800 and 1100 nm; the thickness of the low-refractive layer is between 160 and 235 nm.

2. The high-quality factor hybrid metal-dielectric plasmon resonance optical filter according to claim 1, characterized in that, The substrate material is either a quartz glass substrate or a magnesium fluoride substrate; the high refractive index dielectric layer material is selected from any one of titanium dioxide, silicon nitride, or silicon; the low refractive index dielectric layer material is selected from any one of magnesium fluoride or silicon dioxide.

3. The high-quality factor hybrid metal-dielectric plasmon resonance optical filter according to claim 1, characterized in that, The material for the metal grating layer is selected from either gold or silver.

4. The high-quality factor hybrid metal-dielectric plasmon resonance optical filter according to claim 1, characterized in that, In the operating wavelength range of the device, the high refractive index dielectric layer material has no absorption loss, and the light absorption rate of the substrate layer, the high refractive index dielectric layer, and the low refractive index dielectric layer is close to zero.

5. The high-quality factor hybrid metal-dielectric plasmon resonance optical filter according to claim 1, characterized in that, The thickness of the metal grating ranges from 40 to 60 nm.

6. The high-quality factor hybrid metal-dielectric plasmon resonance optical filter according to claim 1, characterized in that, The device is polarization dependent, with the electric field direction of the incident light along the direction of the periodic arrangement of the grating, and the magnetic field direction along the direction of the grating strips; the light source is a plane wave source.

7. The high-quality factor hybrid metal-dielectric plasmon resonance optical filter according to claim 1, characterized in that, The resonant wavelength of the optical filter is linearly adjusted by changing the period of the metal grating.

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