High-quality factor hybrid metal-dielectric plasma resonance optical filter

Abstract

The invention discloses a high quality factor hybrid metal-dielectric plasma resonance optical filter. The device comprises a substrate layer, and 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, and 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 that of the substrate material; the metal grating layer is of a grating structure arranged periodically and comprises three metal strips in a device period, the width of the middle metal strip is larger than the widths of the two metal strips beside the middle metal strip, and the radiation loss of a coupling mode is adjusted by adjusting the width of the middle metal strip, so that the quality factor of the filter is adjusted. The invention has the advantages of simple structure, high quality factor, compatibility with the traditional integrated circuit preparation process and the like, and has wide application in the fields of optical communication, hyperspectral and multispectral imaging, sensing and the like.

Description

Technical Field

[0001] The invention belongs to the field of nano surface optics, and in particular relates to a high quality factor hybrid metal-dielectric plasma resonance optical filter. Background Art

[0002] Narrowband optical filters are widely used in optical communications, spectral imaging, and sensing. In the past, thin film optical filters based on Fabry-Perot resonance have been studied. Thin film optical filters are based on the principle of thin film interference and are composed of multiple layers of dielectric films or metal films. When the incident light is incident on the device, due to the interaction of reflection, refraction, etc. at the interface of the medium or metal film, multiple interferences and reflections will be formed, and a resonance mode will be formed in the direction of light propagation, so that light of a specific wavelength is selectively reflected or transmitted, achieving a filtering effect. Thin film optical filters have many layers, and in the long-wave band, cracks will occur in the thin film due to the different expansion coefficients of each layer of thin film material. And due to the large number of thin film layers, the preparation process of thin film optical filters is very complicated.

[0003] Optical filters based on nano-surface periodic structures include all-dielectric guided mode resonant optical filters, plasma resonant optical filters, and hybrid metal-dielectric plasma resonant optical filters. Nano-surface periodic structure optical filters use periodic surface structures to provide additional momentum compensation for light in free space, thereby coupling light in free space into the filter structure and exciting a transversely propagating guided mode mode. Due to the coupling effect between the incident light and the guided mode mode, light of a specific wavelength can be transmitted or reflected. Since the dielectric material absorbs little light wave energy, the transmission efficiency of the all-dielectric guided mode resonant optical filter is close to 100%, and the spectral line width can also reach the picometer level. However, since the thickness of the surface structure of the all-dielectric guided mode resonant optical filter is thick enough to provide the corresponding phase compensation, this poses a great challenge to nano-surface processing. Plasmon resonant optical filters use periodic structures on metal surfaces to couple light in free space. The thickness of metal is generally less than 100 nanometers, and the processing technology is simple. However, due to the absorption of light wave energy by metal materials, the spectral line width of the device increases. The quality factor is the ratio of the operating wavelength to the spectral linewidth. From a physical point of view, the quality factor represents the energy stored in the near field of the device. Therefore, the plasma resonance optical filter faces the problems of large light energy loss, wide spectral linewidth, and small quality factor. In order to solve this problem, the hybrid metal-dielectric plasma resonance optical filter was studied. It uses the coupling between the guided mode and the plasma mode to reduce the energy loss of the plasma resonance optical filter. The current quality factor of the hybrid plasma resonance optical filter is up to 500, which is still a technical problem that needs to be solved urgently. Summary of the invention

[0004] In view of the problems existing in the prior art, the purpose of the present invention is to provide a hybrid metal-dielectric plasma resonance optical filter with a simple structure, compatible with the existing integrated circuit manufacturing process and high quality factor.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A high-quality factor hybrid metal-dielectric plasma resonance optical filter comprises a substrate layer, and a high-refractive-index dielectric layer, a low-refractive-index dielectric layer and a metal grating layer sequentially stacked 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 the refractive index of the material of the low refractive index dielectric layer and the refractive index of the material of the substrate layer;

[0009] The metal grating layer is a periodically arranged grating structure, which includes three metal strips in one device period, wherein the width of the middle metal strip is greater than the widths of the two adjacent metal strips, which is equivalent to adding a structural perturbation to the device structure; by adjusting the width of the middle metal strip, the radiation loss of the coupling mode is adjusted, thereby adjusting the quality factor of the filter.

[0010] In the present invention, the substrate layer material is 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 the present invention, the metal grating layer material is selected from any one of gold, silver or aluminum. Aluminum is selected for the visible light band, and gold or silver is selected for the near infrared and longer bands.

[0012] In the present invention, in the working band of the device, the high refractive index dielectric layer material has no absorption loss; 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 the present invention, increasing the thickness of the low-refractive-index dielectric layer can reduce the coupling strength of 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 will also decrease significantly. Therefore, the thickness of the low-refractive-index dielectric layer only varies between 10-250nm. This range should be adjusted accordingly according to the different operating wavelengths of the device; in a specific embodiment, when the operating band of the device is in the near-infrared band, when the thickness of the low-refractive-index dielectric layer varies between 30-200nm, as the thickness increases, the quality factor increases.

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

[0015] In the present 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 the present invention, when the device operating wavelength is in the near-infrared band, within one device cycle, the width of the two adjacent metal strips is 600nm, the width of the middle metal grating is between 800-1100nm; and the thickness of the low refractive layer is between 160-235nm.

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

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

[0019] In the optical filter of the present invention, when the 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, and excites the local plasma resonance mode on the metal grating surface. At the same time, since the refractive index of the substrate layer and the low refractive index layer is lower than that of the high refractive index layer, the guided mode mode will be excited in the high refractive index layer. The transmittance peak originates from the coupling between the plasma resonance mode and the guided mode mode. Increasing the thickness of the low refractive index layer reduces the coupling strength between the local plasma resonance mode and the guided mode mode, thereby reducing the spectral line width. From the perspective of the 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 and distributes more in the dielectric layer, thereby reducing the absorption loss of the metal material to the electromagnetic field energy, and then reducing the spectral line width of the device. This is the reason why the absorption loss of the device is reduced and the quality factor is increased. When the width of the grating metal strip is adjusted, that is, within a device cycle, the width of the middle grating strip is increased (adding a perturbation), and a new resonance mode is excited. This mode also originates from the weak coupling between the guided mode mode and the plasma 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 the structural perturbation appears and gradually increases, the dark mode is converted to a bright mode and can leak energy to the far field and thus be detected. The smaller the structural perturbation, that is, the less the width of the middle grating strip increases, the smaller the coupling strength between the incident light and the bright mode, and more energy is stored in the near field and not radiated. Therefore, the smaller the radiation loss of the bright mode, the greater the quality factor of the device. The technical solution proposed in the present invention to increase 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, infrared and other bands.

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

[0021] In the device structure of the present invention, when no structural perturbation is added, the coupling mode of the traditional guided mode mode and the plasma mode is excited, and its quality factor is about 500. When the structural perturbation is added, that is, when the width of the intermediate grating is increased within one device cycle, a new radiation mode is excited. The quality factor of the device can be significantly increased under the adjustment of the structural perturbation and the thickness of the low refractive index layer.

[0022] The resonant optical filter of the present invention has a simple structure and a high quality factor, which can reach a maximum of 4.2×10 3 , which is compatible with existing integrated circuit processing technology, has a wide range of applications in multispectral and hyperspectral imaging, optical communications, and sensing. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0024] Among them, a is a cross-sectional view of a high-quality factor hybrid metal-dielectric plasma 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 using the present invention.

[0026] Figure 3 It is the transmittance spectrum of the hybrid metal-dielectric plasma resonance optical filter when the thickness of the low refractive index dielectric layer (t2) is different.

[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 plasma resonance optical filters under different structural perturbations.

[0029] Figure 6 This is a graph showing how the quality factor changes with the size of the structural perturbation.

[0030] In the figure, the numbers 1 are substrate layer, 2 are high refractive index thin film layer, 3 are low refractive index thin film layer, and 4 are metal grating layer. DETAILED DESCRIPTION

[0031] In order to better understand the present invention, the present invention is further described below in conjunction with embodiments and drawings. The following embodiments are merely for illustration of the present invention rather than for limitation.

[0032] The high quality factor hybrid metal-dielectric plasma resonance optical filter proposed in the present invention has a structural schematic diagram as shown in FIG. Figure 1 As shown in (a), it 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 are arranged on the substrate layer. The thickness of the high refractive index layer is t3, the thickness of the low refractive index layer is t2, and the thickness of the metal grating layer is t1. Figure 1As shown in (b), there are three metal grating strips in one device period, the device period is Λ, and the width of the grating strip is l. The structural perturbation of the device described in the present invention is to add an additional width Δl to the width l of the middle metal grating strip. In the present invention, the two-dimensional time-domain finite difference method (FDTD) is used to design the geometric structure of the hybrid metal-dielectric plasma resonance optical filter, and the transmittance spectrum of the device is calculated. In the calculation process, magnesium fluoride is used as the material of the substrate and the low refractive index layer, and the refractive index is set to 1.38. Silicon is used as the material of the high refractive index layer, and the refractive index is set to 3.48. Gold is used as the material of the metal grating layer. The reason why silicon is selected as the material of the high refractive index layer and magnesium fluoride is selected as the material of the low refractive index layer is that the difference in the refractive index of the two materials is large, which can make the guided mode more localized, or the energy of the electromagnetic field can be pulled away from the surface of the metal and distributed more in the dielectric layer, which is beneficial to increase the quality factor of the device. In the simulation setting, the simulation area uses periodic boundary conditions at the two edges along the x direction and absorptive boundary conditions at the two edges along the z direction. A plane wave light source is used as the light source, and the electric field direction of the plane wave is along the x direction.

[0033] Example 1

[0034] The reflectivity spectrum of the hybrid metal-dielectric plasma resonance optical filter is calculated using the finite-difference time-domain method, such as 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 the high / low refractive index dielectric layers. In this embodiment, the device operating wavelength is 1362nm, the thickness t3 of the high refractive index layer is set to 215nm, the thickness t2 of the low refractive index layer is set to 235nm, and the thickness t1 of the gold grating layer is set to 40nm. The period Λ of the device is 1740nm, the width of the middle grating strip is 900nm, and the width l of the grating strips on both sides is 600nm, that is, the structural perturbation Δl is 300nm. It can be seen that two transmittance peaks appear in the reflectivity spectrum. The transmittance peak at the resonant wavelength of 1560nm corresponds to the coupling mode of the traditional plasma and guided mode. The transmission peak at the resonant wavelength of 1364nm corresponds to the new coupling mode excited by the structural perturbation. The spectral line width of the transmittance peak of the traditional coupling mode is greater than the spectral line width 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 unchanged, the structural perturbation Δl is 300nm, and the thickness t2 of the low refractive index layer is changed to 30nm, 60nm, 90nm, 120nm, 150nm and 180nm respectively. The transmittance spectrum of the optical filter under different thicknesses of the low refractive index layer is calculated using the time-domain finite difference method, as shown in FIG. Figure 3 As shown. It can be seen that with the increase in the thickness of the low refractive index layer, the central wavelength of the transmittance spectrum is gradually blue-shifted, and the line width of the transmittance spectrum is significantly reduced. Since the coupling mode based on structural perturbation belongs to the Fano resonance, the Fano line shape is used to fit the transmittance spectrum under various structural parameters, and then the quality factor of the filter is obtained. The expression of the Fano line shape is shown in formula (1),

[0037]

[0038] Where a, b and c are constants. ω is the frequency, 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 using the relationship between the quality factor of the mode and the damping coefficient, that is:

[0039]

[0040] Figure 4 The graph of the quality factor of the filter as a function of thickness when the thickness t2 of the low refractive index layer is changed is shown. It can be seen that as the thickness of the low refractive index layer increases, the quality factor gradually increases. This is because changing the thickness of the low refractive index layer can regulate the distribution of the local electromagnetic field. When 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 is more distributed in the dielectric material. This reduces the absorption loss of the electromagnetic field of the device, corresponding to the decrease in the damping coefficient in formula (2). Therefore, the quality factor of the device increases. However, it is worth noting that as the thickness of the low refractive index layer increases, the transmittance of the filter will gradually decrease. This is because the increase in the thickness of the low refractive index layer causes the plasma mode and the guided mode to gradually enter the weak coupling mechanism. The field distribution profiles of the two modes overlap less and less in space, and the transmittance of the device will gradually decrease.

[0041] Example 3

[0042] Based on the structural parameters of Example 1, the thickness of the low refractive index layer is 235nm while maintaining other parameters unchanged. The size of the structural perturbation Δl is changed to 0nm, 100nm, 200nm, 300nm, 400nm, and 500nm. The transmittance spectra of the device under different structural perturbations are calculated using the finite difference time domain method, as shown in FIG. Figure 5As shown. It can be seen that when the structural perturbation is zero, that is, when the width of the three metal gratings is 600nm, there is only one peak in the transmittance spectrum, which corresponds to the traditional coupled mode. When the width of the middle metal grating increases and the 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 is calculated under different changes in the structural perturbation, as shown in FIG. Figure 6 As shown in the figure, the diamond points represent the relationship between the quality factor of the new coupled mode and the structural perturbation, and the circular points represent the relationship between the quality factor of the traditional coupled mode and the structural perturbation. It can be seen that as the structural perturbation increases, the quality factor of the newly emerged coupled mode gradually increases, and the maximum value can reach 4.2×10 3 . The quality factor of the traditional coupled mode mode remains almost unchanged under different structural perturbations, which is about 417. It can be seen that the addition of structural perturbations can increase the quality factor of the device by an order of magnitude when other structural parameters of the device are the same. This is because the introduction of structural perturbations in the device structure excites new coupled modes. In the absence of perturbations, the mode is in a dark mode state and cannot radiate energy to the far field and be detected. When structural perturbations are introduced, the dark mode is converted into a bright mode and radiates to the far field. As the structural perturbations increase, the more energy the mode radiates to the far field, the greater the radiation loss. Conversely, as the structural perturbations decrease, the less energy the mode radiates to the far field, and more energy is stored in the near field. The smaller the radiation loss, the higher the quality factor.

[0043] As mentioned above, the present invention discloses a high-quality factor hybrid metal-dielectric plasma resonance optical filter device, which includes 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 a structural perturbation is introduced into the metal grating, a new resonance mode is excited, and the radiation loss of the device is reduced by adjusting the size of the structural perturbation, thereby improving the quality factor of the device; in addition, the metal grating, as an optical coupler in free space, can couple light into the device to excite a plasma resonance mode on the surface of the metal grating and excite a guided mode resonance mode in the high-refractive index dielectric layer, and the essence of the new resonance mode is the weak coupling effect between the guided mode mode and the plasma mode. The weak coupling effect between the guided mode mode and the plasma pulls the energy of the electromagnetic field in the device away from the surface of the metal grating, thereby reducing the absorption loss of the device and improving the quality factor of the device; in the present invention, the high-low refractive index dielectric pair and the metal grating with structural perturbation work together to reduce the absorption loss and radiation loss of the filter, thereby improving the quality factor of the plasma resonance optical filter. The present invention has the advantages of simple structure, high quality factor, and compatibility with traditional integrated circuit manufacturing processes, and is widely used in the fields of optical communication, hyperspectral and multispectral imaging, and sensing.

Claims

1. A high quality factor hybrid metal-dielectric plasma 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 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 material of the high refractive index dielectric layer is greater than the refractive index of the material of the low refractive index dielectric layer and the refractive index of the substrate layer; The metal grating layer is a periodically arranged grating structure, which includes three metal strips in one device period, wherein the width of the middle metal strip is greater than the widths of the two adjacent metal strips, which is equivalent to adding a structural perturbation to the device structure; by adjusting the width of the middle metal strip, the radiation loss of the coupling mode is adjusted, thereby adjusting the quality factor of the filter.

2. The high quality factor hybrid metal-dielectric plasma resonance optical filter according to claim 1, characterized in that: The substrate layer material is 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.

3. The high quality factor hybrid metal-dielectric plasma resonance optical filter according to claim 1, characterized in that: The metal grating layer material is selected from any one of gold, silver or aluminum; when the working band of the device is the visible light band, the metal grating layer material is aluminum, and when the working band of the device is the near infrared and longer bands, gold or silver is selected.

4. The high quality factor hybrid metal-dielectric plasma resonance optical filter according to claim 1, characterized in that: In the working band 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 plasma resonance optical filter according to claim 1, characterized in that: The absorption loss of the optical filter is reduced by increasing the thickness of the low refractive index dielectric layer, thereby increasing the quality factor of the filter; the thickness of the low refractive index layer varies between 10-250 nm.

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

7. The high quality factor hybrid metal-dielectric plasma resonance optical filter according to claim 1, characterized in that: The device operates at a near-infrared wavelength. Within one device cycle, the width of the two adjacent metal strips is 600 nm, and the width of the middle metal grating is between 800-1100 nm; the thickness of the low-refractive layer is between 160-235 nm.

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

9. The high quality factor hybrid metal-dielectric plasma 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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