Miniaturized reflectors enlarge uses of remote infrared spectroscopy

Spectroscopy is a powerful technique to perform non-destructive testing along with qualitative and quantitative analysis of various compounds. In particular, infrared spectroscopy has numerous applications in many industries, including pharmaceutical, medical, health care, chemical, defense, and security, as well as in many other fields of technology, engineering, and science. Highly reflective mirrors are the critical part of traditional laboratory-suited bench-top spectroscopic systems. The excessive size, weight, power, and cost requirements of traditional bench-top laboratory spectroscopy systems make them highly unattractive to pursue the remote gathering of real-time data.

Optical remote imaging and sensing are essential and in high demand for many applications and industries. In order to achieve lightweight field-portable spectroscopic systems for optical remote imaging and sensing, it requires the application of microelectromechanical systems (MEMS). Fabry-Pérot interferometers (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 SPIE’s Journal of Optical Microsystems gives further insights into the MEMS-based implementation of spectroscopic systems. In this study, thermally evaporated Ge and BaF₂ thin films were investigated and employed in DBR structures for their application in the MEMS toward the development of portable micro-spectrometers. Thin-film deposition and fabrication processes were optimized to achieve mechanical and optical properties that provide flat suspended structures with uniform thickness and maximum reflectivity.

Ge-BaF₂-Ge solid-material DBRs fabricated on a Si-substrate coated with a BaF₂ spacer layer that match the predicted simulation performance. Freestanding Ge thin-film based, Ge-Air-Ge air-gap DBRs, in which air rather than BaF₂ served as the low refractive index layer, are also presented, exhibiting layer flatness at the level of 10-20 nm across lateral DBR dimensions of several hundred micrometers. Measured DBR reflectance was found to be ≥90% over the entire wavelength range of the mid-wave infrared band and for the long-wave infrared band up to a wavelength of 11 µm. The DBR experimental results extend the current MEMS approach towards the fabrication of FPIs for spectroscopic sensing and imaging applications. The expected characteristics of FPIs based on the reflectance response of the fabricated DBRs are also predicted via optical simulations.

According to Prof. Lorenzo Faraone, head of the Microelectronics Research Group at the University of Western Australia, Australia, "Due to their extremely low size, weight, and power (SWaP), the MEMS-based DBRs developed in this study make possible the realization of an on-chip microspectrometer technology that can readily be field-deployable in numerous applications requiring mechanical robustness, such as robotic vehicles, airborne unmanned aerial vehicles (UAVs), etc."

These lightweight and small-size devices are being seen as futuristic solutions enabling applications in systems where minimizing SWaP requirements is of most critical importance. This is particularly relevant to remote infrared imaging and spectroscopic sensing for target identification and space situation awareness.

Read the open-access article by G. S. Gill et al., "Ge/BaF₂ thin-films for surface micromachined mid-wave and long-wave infrared reflectors," J. Optical Microsystems 2(1), 011002 (4 January 2022) doi: 10.1117/1.JOM.2.1.011002.