Narrowband midinfrared reflectance filters using guided mode resonance

Area of Science:

• Optics and Photonics
• Spectroscopy
• Materials Science

Background:

• Limited alternatives exist for mid-infrared (IR) spectrometers requiring absorbance of specific vibrational modes.
• A key challenge is discretely accessing narrow spectral positions from broadband sources.

Purpose of the Study:

• To present the theory, design, and fabrication of a new class of mid-IR filters.
• To utilize guided mode resonances (GMR) for narrowband optical reflection.
• To enable selective spectral analysis for IR absorption spectroscopy.

Main Methods:

• Developed a theoretical model using rigorous coupled wave analysis (RCWA).
• Incorporated anomalous dispersion of filter materials in the mid-IR region.
• Designed and fabricated a periodic surface-relief filter configuration.

Main Results:

• Demonstrated proof-of-principle with four filters for the C-H stretching mode (2600-3000 cm(-1)).
• Experimental reflectance spectra closely matched theoretical predictions.
• Observed sensitivity of filter response to the angle of incidence and material dispersion.

Conclusions:

• The developed filters based on GMR provide a viable method for narrowband optical reflection in the mid-IR.
• The theoretical model accurately predicts filter performance.
• These filters are suitable for rational design in IR absorption spectroscopy.