Researchers have developed a fully in-situ, optoelectronic mid-infrared bandpass filter using thermo-optic silicon metasurfaces. Their filter design offers precise, compact, and stable spectral tuning without mechanical elements.
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The study, published in Laser & Photonics Reviews, revealed a silicon-on-sapphire (SoS) metasurface design that harnesses the thermo-optic effect in zero-contrast gratings (ZCGs) to achieve narrowband, tunable filtering in the mid-infrared (MIR) range. This newly developed filter holds potential for miniaturized sensing, imaging, and spectroscopy by eliminating the need for mechanically adjustable components.
Rethinking MIR Filtering
Bandpass filters are central to chemical sensing, particularly in non-dispersive infrared (NDIR) systems that rely on detecting specific gas absorption lines.
Traditional tunable MIR filters use distributed Bagg reflectors that require accurate deposition of alternating material layers, are bandwidth limited to one free spectral range, require wedges spacers to create varying cavity thickness, and can only achieve tuning through mechanical means. This complicates the fabrication process, limits spectral range and introduces mechanical reliability issues.
In contrast, the metasurfaces presented in this study, ultra-thin, nanostructured layers capable of finely controlling light, provide a scalable alternative. By incorporating a tuning modality, metasurfaces can reliably produce hyperspectral images and molecular fingerprints.
Tuning optoelectronically rather than mechanically results in devices that are more integrable, stable and miniaturizable. In particular, thermo-optic tuning has garnered considerable interest, using temperature-induced changes in refractive index in materials like crystalline silicon, which offers a strong thermo-optic response in the MIR.
Sapphire substrates complement this system with their high thermal conductivity and low MIR absorption.
Filter Design and Fabrication
The research team designed their filters around ZCGs, which consist of a grating and waveguide layer with a minimal difference in refractive index. These structures support resonant phenomena such as quasi-bound states in the continuum (qBICs) and evanescent cavity modes, resulting in sharp transmission peaks within broader stopbands.
They developed one-dimensional (1D) and two-dimensional (2D) ZCG filters, using transmission-mode designs thermally tuned with the thermo-optic effect in silicon. The designs were optimized using particle swarm algorithms and rigorous coupled wave analysis (RCWA) simulations in MATLAB, focusing on etch depth, period, fill factor, and wavelength targeting.
The filters were made using SoS wafers with a ~1.05 µm silicon layer. Grating patterns were created on the wafers using electron beam lithography, followed by precise etching using coupled plasma-reactive ion methods with an aluminum oxide etch mask. This method achieved depths of approximately 1,000 nm.
The team then analyzed the filters using scanning electron microscopy (SEM) and used spectral measurements to fine-tune the results against simulations.
Precision Tuning via Temperature
The thermal-optic tuning of the filters were tested using a Fourier transform infrared (FTIR) set up, which included a blackbody source, polarizer, iris, temperature-controlled stage and mercury cadmium telluride detector.
The ZCG filters were then subjected to temperature of -180 °C to 600 °C. Results from the tests revealed that the 1D ZCG showed a redshift from 3486 nm at 25 °C to 3586 nm at 600 °C, and the 2D design shifted from 3421 nm at -180 °C to 3562 nm at 600 °C. These results demonstrated their respective tuning ranges of 100 nm and 141 nm.
The results obtained significantly exceeded the full-width-at-half-maximum (FWHM) values of 25 nm (1D) and 29 nm (2D), showing the high spectral resolution of the ZCG filters.
Angular response testing demonstrated symmetry-protected qBIC resonances, with broadening at higher angles. The experimental results from the study closely followed their MATLAB simulations, with slight deviations in some cases due to fabrication tolerances.
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Toward Real-World Applications
These optoelectronic, thermo-optic-tunable ZCG filters present a promising solution for compact and robust MIR systems.
They’re especially well-suited for applications such as gas sensing, environmental monitoring, molecular fingerprinting, hyperspectral imaging, and optical communication—particularly in devices where space is limited and mechanical reliability is critical.
By pairing the ZCG filters with broadband light sources and detectors, temperature-controlled spectrometers can be built without moving parts. The researchers also propose integrating on-chip microheaters made from materials like indium tin oxide or graphene, to enable fast, energy-efficient tuning in real-world devices.
Future Directions
This work published in this study establishes a strong foundation for scalable, MEMS-free MIR filters compatible with standard semiconductor manufacturing. With their narrow line widths and fine tunability, they represent a practical step forward for portable and field-ready photonic sensors.
Next steps will focus on microheater integration to support real-time spectral tuning, broadening their utility in domains such as bio-diagnostics, industrial sensing, and environmental analysis.
Journal Reference
Russell, B, J. et al. Thermo-Optically Tunable Mid-Infrared Bandpass Filters Comprising Ultra-Thin Silicon-on-Sapphire Metasurfaces. Laser & Photonics Reviews, 2025. DOI: 10.1002/lpor.202400853, https://onlinelibrary.wiley.com/doi/10.1002/lpor.202400853
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