A high-transmission continuously tunable filter based on stacked subwavelength gratings

Abstract

This invention discloses a high-transmission continuously tunable filter based on a stacked subwavelength grating, comprising a substrate layer, an intermediate layer, and a periodic grating array stacked sequentially. The refractive index of the intermediate layer is 1.4–1.5. The periodic grating array comprises multiple periodically arranged straight-line stacked gratings with a linewidth W ranging from 150–180 nm and a period P ranging from 160–400 nm. From bottom to top, the stacked gratings consist of a bottom Ag layer, a BK7 layer, a MgF2 layer, and a top Ag layer, with the MgF2 layer having a thickness H. m With the thickness H of layer BK7 b Satisfying H m / H b =0.55, with the bottom and top Ag layers having equal thicknesses. By adjusting the period P or thickness of the grating array, the tuning range of the filter can cover the target resonance peak across the entire visible light band. Simultaneously, the resonance of multiple resonant modes in this filter can improve transmittance.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano optical device technology, and relates to a transmissive structural color metasurface filter device, specifically a high-transmission continuously tunable filter based on a stacked subwavelength grating. Background Technology

[0002] Optical filters are commonly used optical devices to transmit or reflect light of specific wavelengths to display desired colors. They are widely used in CMOS image sensors, anti-counterfeiting technology, color printing, and nano-display applications. Traditional pigment-colored filters absorb specific wavelengths of light through surface dyes to achieve the desired color display. However, these filters are not resistant to high temperatures or prolonged exposure to ultraviolet radiation and degrade under prolonged illumination, resulting in a significant decline in performance over time. Furthermore, the degradation products are environmentally unfriendly. Structural coloring, on the other hand, is produced through the interaction of the filter's surface microstructure with natural light. This microstructure is small enough to interfere with visible light, typically occurring in optical phenomena such as interference, diffraction, and scattering. Compared to pigment-colored filters, structural coloring is more stable, less prone to fading, and more environmentally friendly, while also offering significant advantages in color gamut, saturation, brightness, and resolution.

[0003] Existing technologies utilize various physical phenomena to design and apply metal and dielectric nanostructures to structural color filters. Common technical solutions include multilayer thin-film Fabry-Perot (FP) cavities and resonant periodic microstructures. However, existing structural color filters typically have two problems: (1) the tuning range of the resonance wavelengths obtained through various resonances cannot cover the entire visible light band; and (2) the transmittance of the resonance peak is not high, which affects its application scenarios. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a high-transmittance continuously tunable filter based on a stacked subwavelength grating. By adjusting the grating period, the tuning range of the resonant wavelength can cover the entire visible light band, while also possessing the advantage of high transmittance.

[0005] A high-transmission continuously tunable filter based on a stacked subwavelength grating comprises a substrate layer, an intermediate layer, and a periodic grating array stacked sequentially. The refractive index of the intermediate layer is 1.4–1.5. The periodic grating array comprises multiple periodically arranged straight-line stacked gratings with a linewidth W ranging from 150–180 nm and a period P ranging from 160–400 nm. From bottom to top, the stacked gratings consist of a bottom Ag layer, a BK7 layer, a MgF2 layer, and a top Ag layer, with the MgF2 layer having a thickness H. m With the thickness H of layer BK7 b Satisfying H m / Hb =0.55, the thickness of the bottom Ag layer and the top Ag layer are equal.

[0006] Preferably, the substrate layer is made of BK7 glass material with a thickness of not less than 200 μm.

[0007] Preferably, the intermediate layer is made of PDMS and has a thickness of 100 nm.

[0008] Preferably, the target visible light wavelength of the filter is tuned by adjusting the grating period P or the thickness.

[0009] Preferably, when the grating period is increased, the target visible light wavelength undergoes a blue shift; when the grating period is decreased, the target visible light wavelength undergoes a red shift.

[0010] Preferably, when the thickness of the BK7 layer and the MgF2 layer are increased proportionally, the target visible light wavelength is red-shifted; when the thickness of the BK7 layer and the MgF2 layer are decreased proportionally, the target visible light wavelength is blue-shifted.

[0011] As a preferred embodiment, when the target visible light band is 400-500nm, the thickness of the top Ag and bottom Ag layers is set to 40nm, the thickness of the MgF2 layer is 66nm, the thickness of the BK7 layer is 120nm, the linewidth W is 150nm, and the period P is 160-360nm.

[0012] When the target visible light band is 500-600nm, the thickness of the top and bottom Ag layers is set to 80nm, the thickness of the MgF2 layer is 99nm, the thickness of the BK7 layer is 180nm, the linewidth W is 150nm, and the period P is 160-360nm.

[0013] When the target visible light band is 600-700nm, the thickness of the top and bottom Ag layers is set to 80nm, the thickness of the MgF2 layer is 143nm, the thickness of the BK7 layer is 260nm, the linewidth W is 150nm, and the period P is 160-360nm.

[0014] When the target visible light band is 700-900nm, the thickness of the top Ag and bottom Ag layers is set to 80nm, the thickness of the MgF2 layer is 220nm, the thickness of the BK7 layer is 400nm, the linewidth W is 180nm, and the period P is 200-400nm.

[0015] The present invention has the following beneficial effects:

[0016] The filter structure designed in this invention boasts higher transmittance and a tuning range covering the target resonance peak across the entire visible light band. This is attributed to the fact that the stacked grating structure does not rely on a single resonance mode, but rather on multiple resonance modes. When incident light strikes the silver layer surface, surface plasmon resonance is generated, exciting surface plasmons. The simple FP resonant cavity formed by MgF3 and BK7 further enhances the resonance, resulting in higher transmittance. The bottom Ag layer of the plasmon waveguide mode selectively couples the incident light, thus achieving highly efficient color filtering characteristics. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the filter structure in the embodiment;

[0018] Figure 2 This is a schematic diagram of the vertical cross-section of the filter in the embodiment;

[0019] Figure 3 This is a diagram showing the magnetic field energy distribution in the filter under normal incidence.

[0020] Figure 4 The transmission spectra of the filters in the embodiments are shown under different structural parameters.

[0021] Figure 5 The transmission spectrum of the filter in this embodiment is when the target wavelength is 630 nm.

[0022] Figure 6 This is a scan pattern showing that the target wavelength is continuously tunable in the range of 700–900 nm in the embodiment. Detailed Implementation

[0023] The present invention will be further explained below with reference to the accompanying drawings;

[0024] like Figure 1 , Figure 2 As shown, a high-transmission continuously tunable filter based on a stacked subwavelength grating includes a substrate layer 3, an intermediate layer 2, and a periodic grating array stacked sequentially. The substrate layer 3 is made of BK7 glass, and its thickness is greater than that of the intermediate layer 2 and the periodic grating array. The periodic grating array includes multiple periodically arranged straight-line stacked gratings 1, which, from bottom to top, consist of a bottom Ag layer, a BK7 layer, a MgF2 layer, and a top Ag layer. The linewidth W ranges from 150 to 180 nm, and the period P ranges from 160 to 400 nm.

[0025] Figure 3This is a magnetic field energy distribution diagram of a single straight-line grating in a grating array. Three resonance modes are clearly visible. Surface plasmon resonance occurs when incident light strikes the surface of the top Ag layer, exciting surface plasmons. The simple FP resonant cavity formed by the MgF2 layer and BK7 in the middle further strengthens the resonance, resulting in high transmittance. Due to the presence of the plasmon waveguide mode, the bottom Ag layer selectively couples the incident light, thus achieving efficient color filtering characteristics.

[0026] The target visible light wavelength of the filter, i.e., the position of the resonance peak in the transmission spectrum, is mainly determined by changing the thickness H3 of the MgF2 layer, the thickness H2 of the BK7 layer, and the period P of the grating array in the stacked grating. Simulations using the finite-difference time-domain (FDTD) method reveal that increasing the grating period P results in a blue shift of the target visible light wavelength; decreasing the grating period P results in a red shift. Under the premise that the optical thickness of the MgF2 layer satisfies the λ / 4 film system and the optical thickness H2 of the BK7 layer satisfies the λ / 2 film system, proportionally increasing the thicknesses of the BK7 and MgF2 layers results in a red shift of the target visible light wavelength; proportionally decreasing the thicknesses of the BK7 and MgF2 layers results in a blue shift of the target visible light wavelength.

[0027] The thicknesses of the top and bottom Ag layers are set to 40 nm, the MgF2 layer to 66 nm, the BK7 layer to 120 nm, the linewidth W to 150 nm, the period P to 200–360 nm, and the target visible light band to 400–500 nm. Alternatively, the thicknesses of the top and bottom Ag layers are set to 80 nm, the MgF2 layer to 99 nm, the BK7 layer to 180 nm, the linewidth W to 150 nm, the period P to 200–360 nm, and the target visible light band to 500–600 nm. Finally, the thicknesses of the top and bottom Ag layers are set to 80 nm, the MgF2 layer to 143 nm, the BK7 layer to 260 nm, the linewidth W to 150 nm, the period P to 200–360 nm, and the target visible light band to 600–700 nm. The thicknesses of the top and bottom Ag layers are set to 80 nm, the MgF2 layer to 220 nm, and the BK7 layer to 400 nm. The linewidth W is 180 nm, the period P is 200–400 nm, and the target visible light band is 700–900 nm. The corresponding transmission spectrum is as follows: Figure 4 As shown. When the target's visible wavelength is 630 nm, the transmission peak can reach up to 92%, as... Figure 5 As shown.

[0028] The thicknesses of the top and bottom Ag layers were set to 80 nm, the MgF2 layer to 220 nm, and the BK7 layer to 400 nm. The linewidth W was 180 nm, and the period P started at 200 nm and gradually increased in 1 nm increments. The resonance peak position scan is shown in the figure. Figure 6 As shown, the resonance peak is continuously tunable in the 700–900 nm band.

Claims

1. A high-transmission continuously tunable filter based on a stacked subwavelength grating, comprising a substrate layer, an intermediate layer, and a periodic grating array stacked sequentially, characterized in that: The refractive index of the intermediate layer is 1.4–1.5; the periodic grating array comprises multiple periodically arranged straight-line stacked gratings with a grating period P of 160–400 nm; the linewidth W of the stacked gratings is 150–180 nm, and from bottom to top are a bottom Ag layer, a BK7 layer, a MgF2 layer, and a top Ag layer, with a thickness H of the MgF2 layer. m With the thickness H of layer BK7 b Satisfying H m / H b =0.55, the thickness of the bottom Ag layer and the top Ag layer are equal.

2. The high-transmission continuously tunable filter based on a stacked subwavelength grating as described in claim 1, characterized in that: The intermediate layer is made of PDMS and has a thickness of 100 nm.

3. A high-transmission continuously tunable filter based on a stacked subwavelength grating as described in claim 1 or 2, characterized in that: The base layer is made of BK7 glass and has a thickness greater than that of the intermediate layer and the periodic grating array.

4. The high-transmission continuously tunable filter based on a stacked subwavelength grating as described in claim 3, characterized in that: The thickness of the substrate layer is not less than 200 μm.

5. The high-transmission continuously tunable filter based on a stacked subwavelength grating as described in claim 1, characterized in that: The target visible light wavelength of the filter is tuned by adjusting the grating period P or the thickness.

6. A high-transmission continuously tunable filter based on a stacked subwavelength grating as described in claim 1 or 5, characterized in that: Increasing the grating period causes a blue shift in the target's visible light wavelength; decreasing the grating period causes a red shift in the target's visible light wavelength.

7. A high-transmission continuously tunable filter based on a stacked subwavelength grating as described in claim 1 or 5, characterized in that: Increasing the thickness of the BK7 and MgF2 layers proportionally causes a red shift in the target's visible light wavelength; decreasing the thickness of the BK7 and MgF2 layers proportionally causes a blue shift in the target's visible light wavelength.

8. A high-transmission continuously tunable filter based on a stacked subwavelength grating as described in claim 1 or 5, characterized in that: When the target visible light band is 400-500nm, the thickness of the top and bottom Ag layers is set to 40nm, the thickness of the MgF2 layer is 66nm, the thickness of the BK7 layer is 120nm, the linewidth W is 150nm, and the period P is 160-360nm. When the target visible light band is 500-600nm, the thickness of the top and bottom Ag layers is set to 80nm, the thickness of the MgF2 layer is 99nm, the thickness of the BK7 layer is 180nm, the linewidth W is 150nm, and the period P is 160-360nm. When the target visible light band is 600-700nm, the thickness of the top and bottom Ag layers is set to 80nm, the thickness of the MgF2 layer is 143nm, the thickness of the BK7 layer is 260nm, the linewidth W is 150nm, and the period P is 160-360nm. When the target visible light band is 700-900nm, the thickness of the top Ag and bottom Ag layers is set to 80nm, the thickness of the MgF2 layer is 220nm, the thickness of the BK7 layer is 400nm, the linewidth W is 180nm, and the period P is 200-400nm.

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