Metal-Mesh Linear Variable Bandpass Filters for Far-Infrared Wavelengths

Centrala begrepp

Novel far-infrared linear variable bandpass filters consisting of metal-mesh filters with cross-slots of varying sizes on a silicon substrate have been designed, fabricated, and measured to enable hyperspectral imaging for future far-infrared astronomical observatories.

Sammanfattning

The content describes the design, fabrication, and measurement of metal-mesh linear variable bandpass filters (LVBFs) for far-infrared wavelengths from 24 to 36 μm. Key highlights:

1. The LVBFs consist of a gold film with cross-slots of varying sizes along a silicon substrate, with anti-reflection (AR) coatings. The cross-slot dimensions are scaled linearly along the length of the filter to achieve the desired bandpass wavelength range.

2. Simulations using Ansys HFSS were performed to predict the transmission profiles of the filters, accounting for the metal-mesh geometry, substrate, and AR coatings.

3. Fabrication of the LVBFs was done using electron beam evaporation, direct write laser lithography, and argon ion milling. Measurements of the fabricated cross-slot dimensions showed some deviations from the design.

4. Transmission measurements of the non-AR coated and AR coated LVBFs were performed at room temperature and cryogenic temperatures (5 K) using a Fourier transform infrared spectrometer. The AR coated LVBF showed a high peak transmission of ~80-90% at 5 K.

5. An investigation was also conducted on adding a thin layer of hydrogenated amorphous silicon (a-Si:H) on the metal-mesh to shift the undesired higher-order side bands away from the bandpass peaks. This showed promising results but further optimization is required.

6. The authors discuss plans to further improve the resolving power and address the higher-order side band issue for the LVBFs to meet the requirements of the proposed far-infrared astronomical observatory.

Statistik

The peak transmission of the AR coated LVBF increases from ~80% at 300 K to ~90% at 5 K.
The resolving power (R = λ0/Δλ) of the AR coated LVBF varies from ~4.5 to 3.5 along the length of the filter.
The addition of the a-Si:H layer increased the spectral distance between the bandpass peak and higher-order side bands by 2.4 μm on average compared to the LVBF without a-Si:H.

Citat

"The LVBFs on BEGINS will enable hyperspectral imaging from 25 to 65 µm with a lower limit resolving power of R=7.5. This resolving power is significant, because at R≥7.5 the effects of dust grain size and radiation field intensity from 25 to 65 µm can be separated [1]."
"We measure a high peak transmission of ∼80-90 % for the AR coated LVBF at 5 K and demonstrate that the a-Si:H LVBF is a promising method to address out-of-band high frequency transmission."

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Metal-Mesh Linear Variable Filter for Far-Infrared Wavelengths

by Joanna Perid... på arxiv.org 10-02-2024

https://arxiv.org/pdf/2410.00826.pdf

Djupare frågor

How can the resolving power of the LVBFs be further increased to meet the R≥7.5 requirement for the proposed far-infrared astronomical observatory?

To achieve the required resolving power (R≥7.5) for the linear variable bandpass filters (LVBFs) intended for the proposed far-infrared astronomical observatory, several strategies can be employed.

Narrower Cross-Slot Dimensions: Utilizing UV stepper lithography can enable more precise control over the cross-slot dimensions, allowing for narrower slots. This reduction in cross-slot width (B) increases the ratio of the cross-pitch (g) to the cross-width (B), thereby decreasing the bandwidth and enhancing the resolving power.

Optimized Cross-Slot Geometry: Exploring different cross-slot geometries, such as varying the cross-length (K) and cross-pitch (g) in a more sophisticated manner, can also help in achieving higher resolving power. By optimizing these parameters, the filter can be designed to maintain a constant resolving power across the desired wavelength range.

Stacked Filter Designs: Implementing a stacked filter design, where multiple layers of metal-mesh filters are used, can enhance the overall resolving power. While this may introduce some transmission losses, careful design can mitigate these effects, allowing for improved spectral resolution.

Material Selection: Investigating alternative materials with higher conductivity for the metal-mesh, such as silver or specialized alloys, could reduce resistive losses and improve transmission efficiency, thereby contributing to higher resolving power.

Advanced Coating Techniques: Utilizing anti-reflection (AR) coatings with optimized thickness profiles can further enhance transmission and, consequently, the resolving power. The coatings should be designed to minimize reflections at the specific wavelengths of interest.

By implementing these strategies, the LVBFs can be tailored to meet the stringent requirements of the far-infrared astronomical observatory, ensuring high spectral resolution and effective performance in scientific observations.

What other metal-mesh geometries or filter designs could be explored to more effectively suppress the higher-order side bands without significantly reducing the bandpass transmission?

To effectively suppress higher-order side bands while maintaining high bandpass transmission in metal-mesh filters, several alternative geometries and designs can be explored:

Double-Metal Layer Structures: Implementing a double-layer metal-mesh configuration can help in reducing higher-order side bands. By placing a second layer of metal-mesh with a different cross-pitch or geometry, the interaction between the layers can create destructive interference for unwanted frequencies, effectively suppressing side bands.

Gradient Cross-Slot Dimensions: Designing filters with a gradient in cross-slot dimensions along the length of the filter can help in shifting the higher-order side bands further away from the desired bandpass peaks. This approach allows for a tailored response that can minimize overlap between the bandpass and side bands.

Hybrid Filter Designs: Combining metal-mesh filters with other types of filters, such as dielectric or photonic crystal filters, can create a hybrid design that leverages the strengths of both technologies. This can lead to improved suppression of higher-order side bands while preserving the desired transmission characteristics.

Non-Periodic Structures: Exploring non-periodic or quasi-periodic structures can disrupt the regular diffraction patterns that lead to higher-order side bands. By introducing randomness or varying the geometry in a controlled manner, the filter can be designed to minimize unwanted transmission at higher frequencies.

Advanced Simulation Techniques: Utilizing advanced computational modeling and simulation techniques can help in predicting the behavior of various geometries and designs. This can lead to the identification of optimal configurations that effectively suppress higher-order side bands without compromising transmission.

By investigating these alternative geometries and designs, researchers can develop more effective metal-mesh filters that meet the stringent requirements of far-infrared applications while minimizing the impact of higher-order side bands.

What are the potential applications of these metal-mesh LVBFs beyond far-infrared astronomy, and how could the design and fabrication be adapted for those use cases?

Metal-mesh linear variable bandpass filters (LVBFs) have a wide range of potential applications beyond far-infrared astronomy, including:

Remote Sensing: In environmental monitoring and remote sensing applications, LVBFs can be used to analyze spectral signatures of various materials, aiding in the detection of pollutants, vegetation health, and land use changes. The design can be adapted to target specific spectral ranges relevant to these applications.

Medical Imaging: LVBFs can be utilized in medical imaging technologies, such as hyperspectral imaging systems, to enhance the contrast and resolution of images. By tailoring the filter design to specific wavelengths associated with biological tissues, improved diagnostic capabilities can be achieved.

Industrial Process Monitoring: In industrial settings, LVBFs can be employed for real-time monitoring of chemical processes, ensuring quality control by analyzing the spectral characteristics of products. The fabrication process can be adapted to create filters that withstand harsh industrial environments.

Optical Sensors: LVBFs can be integrated into optical sensors for various applications, including security and surveillance systems. By customizing the filter design to specific wavelengths of interest, enhanced detection capabilities can be achieved.

Telecommunications: In fiber-optic communication systems, LVBFs can be used to filter specific wavelengths for wavelength division multiplexing (WDM) applications. The design can be optimized for high transmission efficiency and low insertion loss.

To adapt the design and fabrication of metal-mesh LVBFs for these applications, considerations such as material selection, environmental durability, and specific spectral response requirements must be taken into account. Additionally, employing advanced fabrication techniques, such as nanoimprinting or 3D printing, can enable the production of complex geometries tailored to the unique demands of each application. By leveraging the versatility of metal-mesh LVBFs, a wide array of industries can benefit from enhanced spectral filtering capabilities.