Angle-insensitive strong-dispersion dielectric grating guided-mode resonance optical filter

Abstract

The invention discloses a strong dispersion dielectric grating guided mode resonance optical filter insensitive to angles. The filter is based on a strong-dispersion sub-wavelength dielectric grating structure, and the resonant wavelength shows the characteristic of being insensitive to the change of an incident angle. By reducing the thickness of the grating layer and the grating line width, the angle sensitivity of the device can be further reduced. The insensitivity of the wavelength of the device to the angle mainly depends on the intrinsic dispersion characteristic of an excited optical mode in the device, that is, the frequency of the optical mode with strong dispersion changes slowly along with in-plane wave vectors, so that the change of the far-field frequency response along with the angle is obviously slowed down. Based on the principle, an angle-insensitive metasurface device can be designed by using a strong dispersion optical mode. The design method provided by the invention is not only suitable for an optical filter, but also can be popularized to other metasurface devices with strict requirements on angle sensitivity, and has wide application prospects in the fields of optical communication, hyperspectral and multispectral imaging, sensing and the like.

Description

Technical Field

[0001] This invention belongs to the field of nano-surface optical technology, specifically, it relates to a strong dispersive dielectric grating guided mode resonant optical filter that is insensitive to angle. Background Technology

[0002] Narrowband optical filters have wide applications in optical communication, spectral imaging, and sensing. Optical filters based on periodic nanostructures mainly include all-dielectric guided-mode resonant optical filters, plasmon resonance optical filters, and hybrid metal-dielectric plasmon resonance optical filters. These filters operate by utilizing the periodic surface structure to provide additional momentum compensation for light in free space, thereby coupling the light into the filter structure and exciting transversely propagating guided modes. Through the coupling between the incident light and the guided mode, light of specific wavelengths can be selectively transmitted or reflected. Specifically, the operating wavelength of a nanostructured periodic optical filter corresponds to the wavelength at which the sum of the transverse component of the incident light wave vector and the effective wave vector provided by the periodic surface structure matches the wave vector of the guided mode. Therefore, the operating wavelength of the filter is closely related to the tilt angle of the incident light.

[0003] Studies have shown that the center wavelength of all-dielectric guided-mode resonant narrowband filters and hybrid metal-dielectric guided-mode resonant filters is extremely sensitive to changes in the incident light tilt angle, and this change is almost linear. However, this angle sensitivity limits the filter's performance in some applications. For example, in the display field, wavelength variations in narrowband filters can cause screen color distortion at different viewing angles, affecting display quality. In the imaging field, the filter's transmission efficiency decreases with increasing incident light tilt angle, leading to reduced light intensity received by the detector and consequently affecting image quality. Furthermore, when performing spectral measurements on angle-sensitive filters, precise alignment of the optical system is required; otherwise, changes in the resonant wavelength can lead to inaccurate wavelength measurements, reducing measurement accuracy.

[0004] To address the angle sensitivity of filter wavelengths, researchers have proposed several solutions, such as mode interferometry (MIM) and local plasmon resonance (LPR). However, MIM devices are typically complex, while LPR is only applicable to metallic devices and not to all-dielectric materials. Therefore, designing a simple, angle-insensitive narrowband optical filter has become a pressing technical challenge. Solving this problem will broaden the applications of narrowband optical filters in displays, imaging, and spectral measurements. Summary of the Invention

[0005] To address the problems of existing technologies, the purpose of this invention is to provide a strongly dispersive dielectric grating guided-mode resonant optical filter with a simple structure, compatible with existing integrated circuit fabrication processes, and insensitive to incident angle. This filter is based on a strongly dispersive subwavelength dielectric grating structure, and its resonant wavelength exhibits insensitivity to changes in the incident angle. By reducing the thickness of the grating layer and the grating linewidth, the angle sensitivity of the device can be further reduced. The insensitivity of the device's wavelength to angle primarily depends on the intrinsic dispersion characteristics of the excited optical modes in the device; that is, the frequency of strongly dispersive optical modes changes relatively slowly with the in-plane wave vector, resulting in a significant slowdown in the far-field frequency response with angle variation. Based on this principle, angle-insensitive metasurface devices can be designed using strongly dispersive optical modes. The design method proposed in this invention is not only applicable to optical filters but can also be extended to other metasurface devices with strict requirements for angle sensitivity, showing broad application prospects in optical communication, hyperspectral and multispectral imaging, and sensing.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows.

[0007] This invention discloses an angle-insensitive strongly dispersive dielectric grating guided mode resonant optical filter, comprising, from bottom to top, a substrate layer and a subwavelength dielectric grating layer; the substrate layer is made of a transparent dielectric material, and the subwavelength dielectric grating layer is a periodically arranged grating structure, wherein the refractive index of the subwavelength dielectric grating layer material is higher than that of the substrate layer material; during operation, a strongly dispersive optical mode is excited from the subwavelength dielectric grating layer, and the filter operates in the strongly dispersive spectral frequency region, that is, the frequency region near the bandgap edge (photonic crystal band edge) of the resonant frequency-propagation constant curve. At this time, the response wavelength of the filter changes little with the incident angle, that is, the filter is insensitive to angle.

[0008] In this invention, the angle tolerance of the filter is increased by reducing the thickness of the subwavelength dielectric grating layer.

[0009] In this invention, the angle tolerance of the filter is increased by reducing the width of the subwavelength dielectric grating layer.

[0010] In this invention, the angle tolerance of the filter is increased by using a higher refractive index material in the subwavelength dielectric grating layer.

[0011] In this invention, the substrate is a MgF2 substrate; the refractive index of the dielectric grating layer material is greater than 2.8. In specific embodiments, the refractive index of the dielectric grating layer material is between 2.8 and 3.4, the tilt angle of the incident angle is less than 4°, the center wavelength of the filter reflectivity peak remains almost unchanged, and the wavelength of the strong dispersive subwavelength grating filter is not sensitive to the incident angle.

[0012] In this invention, the thickness of the dielectric grating layer is between 420-460 nm, the tilt angle of the incident angle is less than 4°, the center wavelength of the filter reflectivity peak remains almost unchanged, and the wavelength of the strong dispersive subwavelength grating filter is not sensitive to the incident angle.

[0013] In this invention, the width of the grating strip is between 340 and 380 nanometers, the tilt angle of the incident angle is less than 4°, the center wavelength of the filter reflectivity peak remains almost unchanged, and the wavelength of the strong dispersive subwavelength grating filter is not sensitive to the incident angle.

[0014] In this invention, the device is polarization dependent, the electric field direction of the incident light is along the grating strip direction, and the magnetic field direction is along the periodic arrangement direction of the grating; the light source is a plane wave light source.

[0015] In this invention, a dielectric nanofiber grating is used to design a narrow-linewidth optical resonant filter. When a plane wave is incident on the dielectric grating filter structure, multiple orders of diffracted waves are excited within the grating. The diffracted waves travel along the surface... x The propagation constant in the direction can be expressed as: (1) Where Λ is the grating period. m K is the order of the diffracted wave. x This represents the wave vector of the optical mode. It is only when the propagation constant of the diffracted wave is equal to the wave vector K of the waveguide resonance mode that the wave vector is expressed. x Only when (ω) matches can the filter reflect light of a specific frequency. As can be seen from formula (1), changes in the incident angle directly affect the transverse component of the incident light wave vector. k x The change in , in turn, leads to the change in the wave vector K of the optical resonance mode that matches the diffracted wave. x (ω) changes. Whether the frequency of the resonant mode changes with the matched wave vector depends on the dispersion relation of the grating structure, i.e., ω changes with K. x The relationship between (ω) and its variation. Differentiating both sides of equation (1), (2) By organizing, we can obtain: (3) As can be seen from equation (3), when K'(ω) approaches infinity, dω / dθ approaches zero. This indicates that the waveguide mode resonant frequency varies with the incident angle depending on the wave vector K. x The strong dispersion of the optical mode results in a small change in frequency with the incident angle. Therefore, the angular dispersion of the filter based on the coupling of the incident light with the high-dispersion guided mode can be significantly reduced. Specifically, the frequency of the grating structure varies with the wave vector K. xThe change in (ω) determines the angular sensitivity of the filter. If the frequency of the grating structure changes with K... x If the angle (ω) changes slowly, the filter's operating frequency is less sensitive to changes in the incident angle. By carefully designing the grating structure and optimizing its geometric parameters, a flat dispersion band structure can be achieved, thereby effectively controlling the filter's dispersion characteristics. The high refractive index contrast grating structure designed in this invention has a band curve that is nearly flat near the band gap edge, meaning the frequency changes slowly with the optical mode wave vector. The frequency region near the band edge is defined as the high-dispersion spectral region (also known as the strong-dispersion region). Filters operating in the high-dispersion spectral region exhibit angle insensitivity.

[0016] Compared with the prior art, the advantages of the present invention are: The resonant optical filter of this invention has a simple structure, is insensitive to the incident angle, and is compatible with existing integrated circuit processing technology. It has wide applications in multispectral and hyperspectral imaging, optical communication, and sensing. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of a strongly dispersive dielectric grating guided-mode resonant optical filter that is insensitive to angle. Layer 1 is the dielectric grating layer, and layer 2 is the substrate layer.

[0018] Figure 2 This is a diagram of the band structure of a high-refractive-index dielectric grating.

[0019] Figure 3 (a) is the thickness of the dielectric grating layer. h Band structure diagrams of high refractive index gratings under different conditions. Figure 3 (b) represents the thickness of the dielectric grating layer. h The relationship between the center wavelength of a strongly dispersive dielectric grating guided mode resonant optical filter and the incident angle under different conditions. Figure 3 (c) shows the reflectance spectrum of the strongly dispersive dielectric grating guided mode resonant optical filter under different incident angles, where the thickness of the grating layer is 440 nm.

[0020] Figure 4 (a) is the width of the grating strip ( w Band structure diagrams of high refractive index gratings under different conditions. Figure 4 (b) is the width of the grating strip ( w The relationship between the center wavelength of a strongly dispersive dielectric grating guided mode resonant optical filter and the incident angle under different conditions. Figure 4 (c) shows the reflectance spectrum of the strongly dispersive dielectric grating guided mode resonant optical filter under different incident angles, where the width of the grating strip is 380 nm.

[0021] Figure 5 (a) is the band structure diagram of a high refractive index grating with different grating materials (n). Figure 5 (b) shows the relationship between the center wavelength of the strongly dispersive dielectric grating guided mode resonant optical filter and the incident angle when the grating material (n) is different. Figure 5 (c) shows the reflectance spectrum of the strongly dispersive dielectric grating guided mode resonant optical filter under different incident angles, where the refractive index of the grating material is 3.2.

[0022] In the diagram, 1 represents the dielectric grating layer, and 2 represents the substrate layer. Detailed Implementation

[0023] 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.

[0024] The angle-insensitive, strongly dispersive dielectric grating guided-mode resonant optical filter proposed in this invention is shown in the schematic diagram below. Figure 1 As shown, the device includes a substrate layer 2, on which a high-refractive-index dielectric grating layer 1 is disposed. Let the thickness of the dielectric grating layer 1 be h, the width of the grating strips be w, and the grating period be Λ. In this invention, the finite-difference time-domain (FDTD) method is used to design the geometry of a strongly dispersive dielectric grating guided-mode resonant optical filter, calculate the reflectivity spectrum of the device under different geometric parameters, and calculate the band structure of the dielectric grating under different geometric parameters. To accurately calculate the band structure of the subwavelength dielectric grating on the MgF2 substrate, multiple magnetic dipoles are randomly distributed within a single grating period to excite all potential optical modes. To ensure comprehensive capture of the field information of these modes, multiple time monitors are placed at different positions within the grating period Λ. During this process, the wave vector K... x The determination of depends on the application of Bloch boundary conditions. Specifically, Bloch boundary conditions replicate the field on one boundary of the simulation region and re-inject the field on the opposite boundary in a manner that includes a specific phase factor. This phase factor includes the wave vector K. x Information was collected from various time monitors to accurately identify the resonant frequencies. At frequencies where optical modes exist, the field is significantly enhanced due to destructive interference, exhibiting high intensity. Conversely, at frequencies where no optical modes exist, the field decays rapidly, resulting in relatively low intensity. Based on this principle, we determined and calculated the frequency-wave vector curve by selecting frequencies and wave vectors with field strengths exceeding a preset tolerance. Figure 2This diagram illustrates the band structure of a subwavelength dielectric grating. The horizontal axis represents the normalized wave vector of the optical mode, and the vertical axis represents the normalized frequency of the optical mode. Two band structure curves can be seen on the upper and lower sides of the band gap. This patent uses the upper band optical mode to design the filter. It can be observed that the frequency of this optical mode near the band edge remains almost constant with the change in wave vector. We call the region where the frequency changes slowly with the wave vector the high-dispersion spectral region (also known as the strong-dispersion region). Therefore, when the filter operates in this spectral region, its resonant frequency changes slowly with the incident angle. That is, we can utilize the strong dispersion characteristics of the subwavelength dielectric grating's band edge to design a narrowband filter that is insensitive to angle.

[0025] Example 1

[0026] The band structure diagrams of subwavelength dielectric gratings with different grating thicknesses were calculated using the finite-difference time-domain method, as follows: Figure 3 As shown in (a), in this embodiment, the thickness of the high refractive index grating layer... h The wavelength varies from 420 nm to 520 nm. The grating period Λ is 772 nm, and the width of the grating strips... w The refractive index of the grating is 420 nm. n It is 3.2. It can be seen that at each grating thickness... h Under these conditions, as the optical mode wave vector increases, the frequency changes first slowly and then rapidly. Conversely, as the grating thickness decreases, the rate of frequency change gradually decreases. Figure 3 (b) illustrates the wavelength variation of a subwavelength dielectric grating optical resonant filter with the incident light angle under the same structural parameters. It can be seen that with increasing incident angle, the wavelength of the filter's reflectivity peak first undergoes a slow blue shift followed by a rapid blue shift. The degree of blue shift of the reflectivity peak gradually increases with increasing grating thickness. This phenomenon indicates that the variation of the filter's reflectivity wavelength with the incident light angle is determined by the change in the optical mode frequency with the wave vector. Figure 3 (c) shows the reflectance spectra of the subwavelength dielectric grating optical resonant filter under different incident angles. The thickness of the grating is shown in (c). h The wavelength is 460 nm. It can be seen that when the incident angle is less than 4°, the center wavelength of the filter reflectance peak remains almost unchanged. As the incident angle continues to increase, the center wavelength of the filter reflectance peak rapidly blue-shifts. Furthermore, the linewidth of the reflectance spectrum also increases with the increase of the incident angle. In summary, reducing the thickness of the filter grating layer can increase the filter's angle tolerance.

[0027] Example 2

[0028] The band structure diagrams of subwavelength dielectric gratings with different grating stripe widths were calculated using the finite-difference time-domain method, as follows: Figure 4As shown in (a). In this embodiment, the width of the grating strip... w The grating wavelength varies from 340 nm to 460 nm. The grating period Λ is 772 nm, and the grating thickness... h The refractive index of the grating is 460 nm. n It is 3.46. This can be seen in the width of each raster strip. w Under these conditions, near the Γ point, the frequency of the optical mode changes slowly and then rapidly with the increase of the wave vector. And with the increase of the grating strip width... w As the value decreases, the degree of frequency change gradually decreases. Figure 4 (b) illustrates the wavelength variation of the subwavelength dielectric grating optical resonant filter with the incident light angle under the same structural parameters. It can be seen that when the incident light tilt angle is small, the wavelength of the filter's reflectivity peak slowly blue-shifts with increasing angle. As the grating strip width increases... w As the angle of incidence increases, the blue shift of the reflectivity peak gradually increases. This phenomenon indicates that the change in the reflectivity wavelength of the filter with the incident light angle is determined by the change in the frequency of the optical mode with the wave vector. The frequency region near the Γ point corresponds to the wavelength region when the tilt angle is close to vertical. This region is defined as the high dispersion region, so the filter will exhibit angle insensitivity when operating in this region. Figure 4 (c) shows the reflectance spectra of the subwavelength dielectric grating optical resonant filter under different incident angles. The width of the grating strips is shown in the figure. w The wavelength is 380 nm. It can be seen that when the incident angle is less than 4°, the center wavelength of the filter reflectivity peak remains almost unchanged. However, as the incident angle continues to increase, the center wavelength of the filter reflectivity peak rapidly blue-shifts. In summary, reducing the width of the filter grating... w This can increase the filter's angle tolerance.

[0029] Example 3

[0030] The band structure diagrams of subwavelength dielectric gratings with different refractive indices were calculated using the finite-difference time-domain method, as shown below. Figure 5 As shown in (a). In this embodiment, the refractive index of the grating... n The value varies from 2.8 to 3.4. The grating period Λ is 772 nm, and the width of the grating strips... w The grating thickness is 420 nm. h The value is 460 nm. It can be observed that each bandgap curve in the high-dispersion region exhibits a characteristic where the frequency changes slowly initially and then rapidly with increasing optical mode wave vector. Furthermore, with increasing refractive index... n As the frequency increases, the degree of change gradually decreases. Figure 5(b) shows the wavelength variation of the subwavelength dielectric grating optical resonant filter with the incident angle of the incident light under the same structural parameters. It can be seen that as the incident angle increases, the wavelength of the filter's reflectivity peak first undergoes a slow blue shift followed by a rapid blue shift. The degree of blue shift of the reflectivity peak gradually increases with increasing refractive index contrast. Figure 5 (c) shows the reflectance spectra of the subwavelength dielectric grating optical resonant filter under different incident angles. The refractive index of the grating is shown in (c). n The value is 3.2. It can be seen that when the incident angle is less than 4°, the center wavelength of the filter's reflectivity peak remains almost unchanged. As the incident angle continues to increase, the center wavelength of the filter's reflectivity peak rapidly blue-shifts. Furthermore, the linewidth of the reflectivity spectrum also increases with the increase of the incident angle. In summary, the use of high-refractive-index materials can increase the angle tolerance of the filter.

[0031] The present invention discloses an angle-insensitive, strongly dispersive subwavelength dielectric guided-mode resonant optical filter device, comprising a transparent substrate layer 2 and a dielectric grating layer 1. The wavelength of the strongly dispersive subwavelength grating filter is insensitive to the incident angle. Reducing the grating thickness and linewidth of the filter can further reduce the angular sensitivity. This is because the change in filter wavelength with the incident angle is essentially controlled by the intrinsic dispersion of the optical resonant mode. The frequencies near the band edge change slowly with the wave vector of the optical resonant mode, which is called the high-dispersion spectral region. The response wavelength of a filter operating in the high-dispersion spectral region is insensitive to the incident angle. The optical filter of the present invention has advantages such as simple structure, insensitivity to angle, and compatibility with traditional integrated circuit fabrication processes, and has wide applications in optical communication, hyperspectral and multispectral imaging, and sensing.

Claims

1. An angle-insensitive strongly dispersive dielectric optical grating guided-mode resonance filter, characterized in that it comprises, from bottom to top, a substrate layer and a subwavelength dielectric grating layer; the substrate layer is made of transparent dielectric material, the subwavelength dielectric grating layer is a grating structure arranged periodically, and the refractive index of the material of the subwavelength dielectric grating layer is higher than that of the material of the substrate layer; in operation, a strongly dispersive optical mode is excited from the subwavelength dielectric grating layer, and the filter operates in a strongly dispersive spectral frequency region, i.e. a frequency region in which the resonance frequency-propagation constant curve is close to the edge of the forbidden band, and in this case the response wavelength of the filter changes little with the angle of the incident angle, i.e. the filter is angle-insensitive.

2. The angle-insensitive strongly dispersive dielectric optical grating resonator filter according to claim 1, characterized in that, The angle tolerance of the filter is increased by reducing the thickness of the subwavelength dielectric grating layer.

3. The angle-insensitive strongly dispersive dielectric optical grating resonator filter according to claim 1, characterized in that, The angle tolerance of the filter is increased by reducing the width of the subwavelength dielectric grating layer.

4. The angle-insensitive, strongly dispersive dielectric optical grating resonator filter of claim 1, wherein, The angle tolerance of the filter is increased by using a material with a higher refractive index for the subwavelength dielectric grating layer.

5. The angle-insensitive, strongly dispersive dielectric optical grating resonator filter of claim 1, wherein, The substrate layer is made of MgF2 substrate; the refractive index of the material of the dielectric grating layer is greater than 2.

8.

6. The angle-insensitive strongly dispersive dielectric optical grating resonator filter according to claim 1, characterized in that, The thickness of the dielectric grating layer is between 420 nm and 460 nm.

7. The angle-insensitive strongly dispersive dielectric optical grating resonator filter according to claim 1, characterized in that, The width of the grating bar is between 340 nm and 380 nm.

8. The angle-insensitive strongly dispersive dielectric optical grating resonator filter according to claim 1, characterized in that, The device is polarization-dependent, the electric field direction of the incident light is along the grating bar direction, and the magnetic field direction is along the periodic arrangement direction of the grating; the light source is a plane wave light source.

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