Cross-shaped bandpass filters for the near- and mid-infrared wavelength regions

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Abstract
A photolithographic process has been used to form cross-shaped patterns in 3-μm-thick nickel foils. Patterns with cross arm dimensions in the 10–20-μm range, and with periodicities in the 16–26-μm range, yield self-resonant bandpass filters for wavelengths in the 20–25-μm region. Transmittances as high as 80% were achieved with center wavelength-to-bandwidth ratios (λR/Δλ) of ∼5. We present a simple empirical formula that relates the wavelength of peak transmittance, or resonant frequency, with cross dimensions and periodicity.