A long-wave infrared polarization-independent narrow-band notch filter and a preparation method thereof

Abstract

The application discloses a long-wave infrared polarization-independent narrow-band trap filter and a preparation method thereof, and belongs to the technical field of infrared optical devices. The long-wave infrared polarization-independent narrow-band trap filter comprises periodically arranged two-dimensional grating units, and each of the two-dimensional grating units comprises a substrate, a waveguide layer arranged on the substrate, a grating structure layer arranged on the waveguide layer, a grating anti-reflection layer arranged on the grating structure layer, and a back anti-reflection film arranged on the side of the substrate away from the waveguide layer. The grating structure layer adopts a cross-shaped grating structure, which has the same period Lambda in the x and y directions and is distributed in a 90-degree rotational symmetry. Under the symmetrical structure, TE and TM polarized light experience the same phase matching condition, so that the resonance characteristics are not sensitive to the polarization direction of incident light, and thus polarization-independent filtering is realized.

Description

Technical Field

[0001] This invention belongs to the field of infrared optical device technology, and particularly relates to a long-wave infrared polarization-independent narrowband notch filter and its preparation method. Background Technology

[0002] The long-wave infrared band (8-14 μm) lies within the main transmission window of the atmosphere and coincides with the peak wavelength range of blackbody radiation from typical room-temperature targets. It has applications in all-weather imaging and sensing systems, including thermal imaging, chemical spectroscopy analysis, environmental monitoring, and free-space communication. Achieving high-performance narrowband filtering in this band is crucial for improving the system's signal-to-noise ratio and suppressing background interference.

[0003] Traditional filters achieve the target spectral response by stacking tens or hundreds of layers of dielectric thin films. However, in the long-wave infrared band, the physical thickness of the film corresponding to a quarter wavelength reaches the micrometer level, resulting in excessively long film deposition time. Furthermore, excessively thick films can cause problems such as stress accumulation and poor adhesion.

[0004] Guided-mode resonance (DMR) filters have become a research hotspot due to their compact size and high-efficiency filtering characteristics. DMR filters can generate extremely narrow-bandwidth reflection or transmission peaks over a wide wavelength range, and have broad application prospects in optical communication, microscopic imaging, and spectral analysis.

[0005] However, traditional one-dimensional grating guided-mode resonant filters are highly sensitive to the polarization state of incident light, and their performance degrades significantly in unpolarized or randomly polarized light scenarios, limiting their application in practical systems such as thermal imaging. To overcome the polarization sensitivity problem, CN121500461A discloses a long-wave infrared band 8-12μm narrowband polarization-independent notch filter, which achieves polarization independence by orthogonally cascading two double-layer grating structures. However, this orthogonal cascading scheme suffers from drawbacks such as alignment errors, cascading losses, cumbersome fabrication steps, and complex overall structure.

[0006] Therefore, developing a long-wave infrared narrowband notch filter that is polarization independent, compact in structure, and easy to fabricate has important practical value. Summary of the Invention

[0007] To address the problems of long deposition time, polarization sensitivity, and complex structure of existing long-wave infrared filters, this invention provides a long-wave infrared polarization-independent narrowband notch filter and its fabrication method. This filter is characterized by polarization independence, compact structure, and ease of fabrication. It is particularly suitable for narrowband filtering in the long-wave infrared (8-14 μm) band.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A long-wave infrared polarization-independent narrowband notch filter includes periodically arranged two-dimensional grating units, wherein the two-dimensional grating units include:

[0010] Substrate;

[0011] Waveguide layer disposed on the substrate;

[0012] A grating structure layer is disposed on the waveguide layer, the grating structure layer including an array of cross-shaped grating structures, used to excite surface guided mode resonance to achieve polarization-independent filtering;

[0013] A grating antireflection layer disposed on the grating structure layer; and,

[0014] A back antireflection film disposed on the side of the substrate opposite to the waveguide layer; The grating structure layer is made of the same material as the waveguide layer, and the refractive index of the grating antireflection layer is lower than that of the waveguide layer and the grating structure layer.

[0015] The grating structure layer has the same period in the x and y directions and has 90° rotational symmetry.

[0016] The grating structure layer has a period of 2.6 to 3.2 μm, a height of 0.4 to 0.6 μm, and a duty cycle of 0.25 to 0.65.

[0017] The waveguide layer has a thickness of 0.8-1.1 μm, and the grating antireflection layer has a thickness of 0.8-1.2 μm.

[0018] The waveguide layer and the grating structure layer are made of Ge, and the grating antireflection layer is made of ZnSe.

[0019] The substrate material is ZnSe or ZnS.

[0020] The back antireflection film is a multilayer film structure, which includes one or more combinations of Ge layer, YbF3 layer and ZnSe layer. The back antireflection film is used to achieve antireflection effect in the 8-14μm long-wave infrared band and reduce the reflection loss on the back of the substrate.

[0021] The above-mentioned method for fabricating a long-wave infrared polarization-independent narrowband notch filter includes the steps of depositing a back antireflection film on one side of a substrate and sequentially depositing a waveguide layer, a grating structure layer, and a grating antireflection layer on the other side of the substrate.

[0022] Specifically, depositing a back-side antireflection film on one side of the substrate involves using a thin film deposition method to deposit a back-side antireflection film on the back side of the substrate.

[0023] Depositing a waveguide layer, a grating structure layer, and a grating antireflection layer sequentially on the other side of the substrate includes the following steps:

[0024] A Ge thin film and a ZnSe thin film are sequentially deposited on the front side of a substrate using a thin film deposition method; wherein the Ge thin film is used to form a Ge waveguide layer and a Ge grating structure layer, and the ZnSe thin film is used to form a grating antireflection layer;

[0025] Photoresist was spin-coated onto the ZnSe thin film and then baked.

[0026] A grating mask is formed on the photoresist by laser direct writing lithography and development;

[0027] ZnSe and Ge thin films were etched in one step using an etching method to obtain a grating antireflection layer and a grating structure layer;

[0028] The photoresist on the grating was removed by stripping and then post-processed.

[0029] The beneficial effects of this invention are as follows:

[0030] (1) The grating structure layer of the present invention adopts a cross-shaped grating structure, which has the same period Λ in the x and y directions and is distributed in a 90° rotational symmetry. Under this symmetrical structure, the TE (transverse electric mode) and TM (transverse magnetic mode) polarized light undergo the same phase matching condition, making the resonance characteristics insensitive to the polarization direction of the incident light, thereby realizing polarization-independent filtering.

[0031] (2) Compared with the traditional scheme of orthogonally cascading two one-dimensional gratings, the present invention integrates the orthogonal grating structure into a single cross-shaped grating layer, eliminating the need for cascading alignment and fundamentally avoiding cascading alignment errors and cascading losses.

[0032] (3) The present invention can simultaneously excite the fundamental mode and the higher-order mode dual resonance in a single cross-shaped grating layer under the same polarization state, providing additional design freedom for narrowband notch filtering and flexibly meeting the needs of different long-wave infrared application scenarios.

[0033] (4) By adjusting the grating period, duty cycle, waveguide layer thickness, grating structure layer thickness and grating antireflection layer thickness, the present invention can achieve the control of center wavelength and half width at half maximum (WWHM). Attached Figure Description

[0034] Figure 1 This is a three-dimensional structural schematic diagram of the long-wave infrared polarization-independent narrowband notch filter of the present invention;

[0035] Figure 2 This is a three-dimensional structural schematic diagram of the two-dimensional grating unit in the long-wave infrared polarization-independent narrowband notch filter of the present invention;

[0036] Figure 3 This is the transmission spectrum of the long-wave infrared polarization-independent narrowband notch filter prepared in Example 1 of the present invention;

[0037] Figure 4 This is the transmission spectrum of long-wave infrared polarization-independent narrowband notch filters with different grating periods in this invention;

[0038] Figure 5 The transmission spectra of long-wave infrared polarization-independent narrowband notch filters with different duty cycles in this invention are shown.

[0039] Figure 6 Different waveguide layer thicknesses in this invention Transmission spectrum of a long-wave infrared polarization-independent narrowband notch filter;

[0040] Figure 7 Different grating structure layer heights in this invention Transmission spectrum of a long-wave infrared polarization-independent narrowband notch filter;

[0041] Figure 8 Different ZnSe grating antireflection layer thicknesses in this invention Transmission spectrum of a long-wave infrared polarization-independent narrowband notch filter;

[0042] Figure 9 This is a flowchart illustrating the fabrication method of the long-wave infrared polarization-independent narrowband notch filter of the present invention.

[0043] The figures are labeled as follows: 1-backside antireflective coating, 2-substrate, 3-waveguide layer, 4-grating structure layer, 5-grating antireflective layer. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0045] Example 1:

[0046] See Figure 1 and Figure 2 This embodiment provides a long-wave infrared polarization-independent narrowband notch filter, which comprises periodically arranged two-dimensional grating units. Specifically, the two-dimensional grating unit includes, from bottom to top, a back antireflection film 1, a substrate 2, a waveguide layer 3, a grating structure layer 4, and a grating antireflection layer 5. The substrate 2 is a polished substrate.

[0047] In this embodiment, the substrate 2 is made of ZnSe or ZnS and has a thickness of 2mm. The substrate 2 is double-sided polished and has good light transmittance in the long-wave infrared 8-14μm band.

[0048] See Figure 2 The waveguide layer 3 is disposed on the front side of the substrate 2, and the material of the waveguide layer 3 is Ge, with a thickness of [missing information]. =1.0μm, used to adjust the center position of the filter stopband and the full width at half maximum (FWHM).

[0049] See Figure 2 The grating structure layer 4 is disposed on the waveguide layer 3, and the material of the grating structure layer 4 is Ge, the same as that of the waveguide layer 3. The grating structure layer 4 includes an array of cross-shaped grating structures, that is, the grating structure layer 4 is arranged in a two-dimensional periodic pattern, its cross-section is cross-shaped, and it has 90° rotational symmetry.

[0050] In this embodiment, the grating period Λ of the grating structure layer 4 is 3.2 μm, and the grating height is... =0.4μm, duty cycle =0.45.

[0051] See Figure 2 The grating antireflection layer 5 is disposed on the grating structure layer 4, and the material of the grating antireflection layer 5 is ZnSe, and its thickness is... =1.0μm, used to improve sideband transmittance.

[0052] See Figure 2 The back-side antireflective film 1 is disposed on the back side of the substrate 2. The back-side antireflective film 1 is a multilayer film structure. Preferably, the multilayer film consists of the following layers deposited sequentially: a Ge layer with a thickness of 70 nm; a ZnSe layer with a thickness of 757 nm; a Ge layer with a thickness of 136 nm; a ZnSe layer with a thickness of 1462 nm; a YbF3 layer with a thickness of 1106 nm; and a ZnSe layer with a thickness of 152 nm. This is used to achieve antireflection in the 8-14 μm long-wave infrared band, reducing reflection loss on the back side of the substrate.

[0053] The principle of the filter in this embodiment is as follows: When incident light is perpendicularly incident on the front side of the filter, the light passes sequentially through the grating antireflection layer 5 and the grating structure layer 4 on the front side, and enters the waveguide layer 3. The grating antireflection layer 5 is used to reduce the reflection loss at the air-grating interface, allowing more incident light to couple into the grating structure layer 4 and improving the sideband transmittance. The incident light diffracts on the surface of the cross-shaped grating structure layer 4, producing diffracted light of different orders. Among them, the diffracted light that meets the phase matching condition resonates and couples with the guided mode in the waveguide layer 3, forming a guided mode resonance effect. At this resonance wavelength, the incident light energy is strongly confined in the waveguide layer 3, forming a narrow and deep transmission valley (i.e., a notch) in the transmission spectrum, thereby achieving filtering of light of a specific wavelength.

[0054] Under normal incidence conditions, according to the two-dimensional grating equation, the (th) , The propagation constant components of the order diffracted wave in the x and y directions are as follows: , , in, and They are ( , The propagation constant of the first-order diffracted wave. and Let denoted by , and Λ be the diffraction order along the x-axis and y-axis, respectively, where Λ is the grating period. The total propagation constant of the diffracted wave is denoted by . satisfy: When the propagation constant of the waveguide mode ( (where m is the waveguide mode) satisfies the phase-matching condition Resonance can be achieved at that time.

[0055] From the above equation, it can be seen that different diffraction orders ( , (Corresponding to different total propagation constants) This allows for the excitation of waveguide modes of different orders. Specifically, first-order diffraction ( , The total propagation constant for (±1, 0) or (0, ±1) is: Corresponding to the fundamental mode resonance, the total propagation constant of the second-order diffraction (p, q) = (±1, ±1) is: This corresponds to higher-order mode resonance.

[0056] because Higher-order mode resonances occur at shorter wavelengths. Therefore, in situations such as... Figure 3Two resonance peaks can be observed in the transmission spectrum shown: the main valley on the long-wavelength side corresponds to the fundamental mode (first-order resonance), and the secondary valley on the short-wavelength side corresponds to the higher-order mode (second-order resonance). In this embodiment, the main valley located at 10.35 μm in the transmission spectrum corresponds to the first-order resonance (fundamental mode), and the secondary valley located at 9.36 μm corresponds to the second-order resonance (higher-order mode).

[0057] Because the cross-shaped grating has the same period Λ in the x and y directions and the diffraction pattern has fourfold symmetry, TE and TM polarized light undergo the same phase-matching condition. Therefore, the resonance is insensitive to the polarization direction of the incident light, achieving polarization-independent characteristics. By optimizing the grating period and waveguide parameters, effective separation of resonance peaks of different orders can be achieved.

[0058] See Figure 9 The method for preparing the above-mentioned long-wave infrared polarization-independent narrowband notch filter includes the following steps:

[0059] S1: Substrate pretreatment: Select a double-sided polished substrate with a thickness of 1-3 mm, and wipe the substrate surface with a mixture of ether and anhydrous ethanol in sequence;

[0060] S2: Deposit a back-side anti-reflection film on the back side of the substrate using a thin film deposition method;

[0061] S3: Ge thin film and ZnSe thin film are sequentially deposited on the front side of the substrate using a thin film deposition method; wherein, the Ge thin film is used to form a Ge waveguide layer and a Ge grating structure layer, and the ZnSe thin film is used to form a ZnSe grating antireflection layer;

[0062] S4: Spin-coat photoresist onto the ZnSe thin film and bake it;

[0063] S5: A grating mask is formed on the photoresist by laser direct writing lithography and development;

[0064] S6: Using an etching method, ZnSe thin film and Ge thin film are etched in one step to process ZnSe grating antireflection layer and Ge grating structure layer;

[0065] S7: Remove the photoresist on the grating using a stripping method and perform post-processing to obtain a long-wave infrared polarization-independent narrowband notch filter.

[0066] Specifically, the thin film deposition method in this embodiment can be electron beam evaporation or resistance evaporation, with a background vacuum degree of <6×10⁻⁶. -4 Baking temperature: 160-200℃; baking time: 1-2 hours. Ge film deposition rate: 0.5-0.6 nm / s; ZnSe film deposition rate: 0.6-1.2 nm / s; YbF3 film deposition rate: 0.6-1.0 nm / s.

[0067] Steps S3 to S7 above are steps of depositing a waveguide layer, a grating structure layer and a grating antireflection layer sequentially on the front side of the substrate. It should be noted that the order of steps S2 (i.e., the step of depositing the back antireflection film) and steps S3 to S7 (the steps of depositing the waveguide layer, the grating structure layer and the grating antireflection layer) can be reversed. That is, the waveguide layer, the grating structure layer and the grating antireflection layer can be deposited on the front side first, and then the back antireflection film can be deposited on the back side.

[0068] Example 2:

[0069] In this embodiment, the influence of key structural parameters on the transmission spectrum is examined. Specifically, based on Embodiment 1, this embodiment keeps other parameters constant and changes the grating period Λ and the duty cycle of the grating structure layer 4, respectively. Thickness of waveguide layer 3 Height of grating structure layer 4 and the thickness of the grating antireflection layer 5 The influence of each parameter on the transmission spectrum was analyzed.

[0070] (1) The influence of the grating period Λ is discussed. Specifically, the grating period Λ is adjusted within the range of 2.6 to 3.2 μm, with specific grating periods Λ of 2.6 μm, 2.8 μm, and 3.0 μm. The transmission spectra of long-wave infrared polarization-independent narrowband notch filters with different grating periods Λ are shown in the figure. Figure 4 As shown. Figure 4 The results show that the center wavelengths of both stopbands redshift with increasing period. The center of the stopband of the base film can be tuned in the range of 8 to 10 μm, and the center of the stopband of the higher-order film can be tuned in the range of 8 to 11 μm.

[0071] (2) Discussion of duty cycle The impact. Specifically, adjust the duty cycle within the range of 0.25 to 0.65. That is, adjusting the duty cycle The values ​​are 0.25 and 0.65, respectively. The transmission spectra of long-wave infrared polarization-independent narrowband notch filters with different duty cycles are shown below. Figure 5 As shown. Figure 5 The results show that the two stopband centers redshift as the duty cycle increases, while the full width at half maximum (FWHM) narrows.

[0072] (3) Discussion of the thickness of waveguide layer 3 The effect of this. Specifically, adjusting the waveguide layer thickness within the range of 0.8 to 1.1 μm. That is, adjusting the thickness of waveguide layer 3 The thicknesses are 0.8 μm, 0.9 μm, and 1.1 μm, respectively. (Different waveguide layer thicknesses) The transmission spectrum of a long-wave infrared polarization-independent narrowband notch filter is shown below. Figure 6 As shown. Figure 6 The results show that the center wavelengths of both stopbands increase with the thickness of waveguide layer 3. The increase in wavelength causes a shift towards longer wavelengths.

[0073] (4) Discuss the height of grating structure layer 4 The impact. Specifically, the height of grating structure layer 4. The height of the grating structure layer 4 is adjusted by varying the height within the range of 0.4 to 0.6 μm. The thicknesses are 0.5 μm and 0.6 μm, respectively. (Different grating structure layer heights) The transmission spectrum of a long-wave infrared polarization-independent narrowband notch filter is shown below. Figure 7 As shown. Figure 7 The results show that the center wavelengths of both stopbands shift towards longer wavelengths as the height of the grating structure layer increases, while the full width at half maximum (FWHM) also increases.

[0074] (5) Discuss the thickness of the grating antireflection layer 5 The impact of this. Specifically, the height of the grating antireflection layer 5. The height of the grating antireflection layer 5 is adjusted by varying the height within the range of 0 to 1.2 μm. The thicknesses are 0, 0.8 μm, and 1.2 μm, respectively. Different grating antireflection layer heights. The transmission spectrum of a long-wave infrared polarization-independent narrowband notch filter is shown below. Figure 8 As shown. Figure 8 The results show that as the thickness of the grating antireflection layer decreases, the sideband transmittance decreases, the stopband center corresponding to the higher-order film shifts towards shorter wavelengths, while the stopband center corresponding to the base film shows no significant shift. The thickness of the grating antireflection layer 5... When the value is 0, the minimum sideband transmittance of the filter is 58%, the center wavelength and full width at half maximum (FWHM) of the base film stopband are 10.3 μm and 134 nm, respectively, and the FWHM of the higher-order film is 9.08 μm and 92 nm. The thickness of the grating antireflection layer 5... When the thickness is 1.0 μm (Example 1), the sideband transmittance of the filter is greater than 80%, the center wavelength and full width at half maximum (FWHM) of the base film stopband are 10.35 μm and 140 nm, respectively, and the FWHM of the higher-order film is 9.36 μm and 45 nm. The thickness of the grating antireflection layer 5... Within the range of 0.8–1.2 μm, the shift between the two stopband centers is not significant. The results indicate that the introduction of the single-layer grating antireflection layer 5 effectively narrows the stopband width, significantly improves the sideband transmittance, and shifts the stopband center wavelength towards longer wavelengths.

[0075] In summary, by changing the grating period Λ and the duty cycle... The thickness of waveguide layer 3, the height of grating structure layer 4, and the thickness of ZnSe grating antireflection layer 5 are adjusted to meet the matching conditions with the incident light wave vector, thus enabling the selection of different wavelengths.

[0076] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention. The above embodiments are provided only for the purpose of describing the present invention and are not intended to limit the present invention. Parts not described in detail in this specification are well-known in the art and are not intended to limit the scope of the present invention. The scope of the present invention is defined by the appended claims. All equivalent substitutions and modifications made without departing from the spirit and principle of the present invention should be covered within the scope of the present invention.

Claims

1. A long-wave infrared polarization-independent narrowband notch filter, characterized in that, It includes periodically arranged two-dimensional grating units, wherein the two-dimensional grating units include: Substrate; Waveguide layer disposed on the substrate; A grating structure layer is disposed on the waveguide layer, the grating structure layer including an array of cross-shaped grating structures, used to excite surface guided mode resonance to achieve polarization-independent filtering; A grating antireflection layer disposed on the grating structure layer; and A back antireflection film disposed on the side of the substrate opposite to the waveguide layer; The grating structure layer is made of the same material as the waveguide layer, and the refractive index of the grating antireflection layer is lower than that of the waveguide layer and the grating structure layer.

2. The long-wave infrared polarization-independent narrowband notch filter according to claim 1, characterized in that, The grating structure layer has the same period in the x and y directions, and the grating structure layer has 90° rotational symmetry.

3. The long-wave infrared polarization-independent narrowband notch filter according to claim 2, characterized in that, The period of the grating structure layer is 2.6 to 3.2 μm, the height of the grating structure layer is 0.4 to 0.6 μm, and the duty cycle of the grating structure layer is 0.25 to 0.

65.

4. The long-wave infrared polarization-independent narrowband notch filter according to claim 1, characterized in that, The thickness of the waveguide layer is 0.8-1.1 μm, and the thickness of the grating antireflection layer is 0.8-1.2 μm.

5. The long-wave infrared polarization-independent narrowband notch filter according to claim 1, characterized in that, The waveguide layer and the grating structure layer are made of Ge, and the grating antireflection layer is made of ZnSe.

6. The long-wave infrared polarization-independent narrowband notch filter according to claim 1, characterized in that, The substrate material is ZnSe or ZnS.

7. The long-wave infrared polarization-independent narrowband notch filter according to claim 1, characterized in that, The back antireflection film is a multilayer film structure, which includes one or more combinations of Ge layer, YbF3 layer and ZnSe layer. The back antireflection film is used to achieve antireflection effect in the 8-14μm long-wave infrared band and reduce the reflection loss on the back of the substrate.

8. The method for preparing the long-wave infrared polarization-independent narrowband notch filter according to any one of claims 1 to 7, characterized in that, The fabrication method includes the steps of depositing a back antireflection film on one side of the substrate and sequentially depositing a waveguide layer, a grating structure layer and a grating antireflection layer on the other side of the substrate.

9. The method for preparing a long-wave infrared polarization-independent narrowband notch filter according to claim 8, characterized in that, Depositing a back antireflection film on one side of a substrate specifically involves using a thin film deposition method to deposit a back antireflection film on the back side of the substrate. Depositing a waveguide layer, a grating structure layer, and a grating antireflection layer sequentially on the other side of the substrate includes the following steps: A Ge thin film and a ZnSe thin film are sequentially deposited on the front side of a substrate using a thin film deposition method; wherein the Ge thin film is used to form a Ge waveguide layer and a Ge grating structure layer, and the ZnSe thin film is used to form a grating antireflection layer; Photoresist was spin-coated onto the ZnSe thin film and then baked. A grating mask is formed on the photoresist by laser direct writing lithography and development; ZnSe and Ge thin films were etched in one step using an etching method to obtain a grating antireflection layer and a grating structure layer; The photoresist on the grating was removed by stripping and then post-processed.

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