Tiny infrared spectral filters for remote thermal sensing and imaging

Optical remote imaging and sensing are essential and in high demand for many applications and industries. More importantly, high-performance wavelength discrimination in the long-wave infrared region (LWIR) of the electromagnetic spectrum (wavelength range from 8 µm to 12 µm) is highly attractive for both civilian and military applications. Of particular importance is to achieve on-chip fully passive thermal imaging with spectral selectivity, allowing remote target recognition without requiring any light source for illumination since radiation in the LWIR spectral band is emitted by objects simply due to their room temperature.

Microelectromechanical systems (MEMS) enable the development of lightweight field-portable spectroscopic systems for optical remote imaging and sensing by hybridizing MEMS-based Fabry-Pérot interferometers (FPIs) with either single-point infrared detectors or focal plane imaging arrays. FPIs provide a spectrometer architecture that is compatible with thin-film surface-micromachined MEMS. These FPIs consist of two mirrors, which in a MEMS implementation generally consist of a pair of distributed Bragg reflectors (DBRs), separated by an optical cavity.

A new study in the SPIE Journal of Optical Microsystems gives further insights into the MEMS-based implementation of spectroscopic systems. A proof-of-concept for large-area narrowband MEMS-based fixed cavity FPIs operating in the LWIR region has been reported for their application towards the development of portable micro-spectrometers. This work reports for the first time on the use of low-index BaF₂ thin films in combination with Ge high-index thin films for such applications.

Furthermore, extremely flat and stress-free ~3 µm thick free-standing distributed Bragg reflectors (DBRs) were realised using thick lift-off process of a tri-layer structure fabricated using Ge and BaF₂ optical layers. A peak-to-peak flatness was achieved for free-standing surface micromachined structures at the level of 10 – 20 nm across large spatial dimensions of several hundred micrometers. The fabricated FPIs are shown to have a linewidth of approximately 110 nm and a suitable peak transmittance value of ~50%, which meets the requirements for their utilisation in tuneable MEMS-based LWIR spectroscopic sensing and imaging applications requiring spectral discrimination with narrow linewidth.

An optical microscopic image of a released α-series FPI. The crack propagation from the sharp corners of notches is due to long-time exposure to the oxygen plasma. Credit: Gill et al. doi 10.1117/1.JOM.2.2.023502

According to Prof. Mariusz Martyniuk, a member of the Microelectronics Research Group at the University of Western Australia, "These miniaturised on-chip lightweight and small size devices are being seen as futuristic solutions towards simple and low-cost miniature spectroscopic remote systems operating in the very important thermal infrared emission band of the electromagnetic spectrum, where minimising weight, size and power requirements is of most critical importance.”

The demonstrated on-chip microspectrometer technology can readily be field-deployable in numerous applications requiring mechanical robustness such as robotic vehicles, airborne unmanned aerial vehicles (UAVs), etc. This can be particularly relevant to remote infrared imaging and spectroscopic sensing for target identification and space situation awareness.

Read the Gold Open Access article by G. S. Gill et al., "Large-area narrowband Fabry-Pérot interferometers for LWIR spectral sensing," J. Optical Microsystems, 2(2) 023502 (2022). doi 10.1117/1.JOM.2.2.023502