Wide-angle wide-spectral range IMI plasmonic MEMS mirror in the MIR for spectroscopic gas sensing applications

Abstract

Spectroscopy-based gas sensing has occupied a significant role in environmental, industrial and healthcare applications. Two specific concerns of the technology are miniaturization and portability, while maintaining the device performance. The mid-infrared strong absorption characteristics of different gases can offer the required detection limits when used with multi-pass gas cells due to longer interaction lengths. These cells raise the need for wide-spectral range angle-tolerant mirror designs compatible with micromachining technologies. In this work, we investigate the use of silicon-based gold coated deeply-etched optical MEMS mirrors and show that the grain nature of the deposited metal leads to degraded mirror. We study the effect of the grain-to-volume ratio of the gold mirror structure and present a method to boost the mirror performance by using a hybrid wideband insulator-metal-insulator (IMI) plasmonic structure. Different insulators were investigated with different layer thicknesses, taking into consideration the grain nature of the layers. A mirror reflectivity that is about 98% with 10% bandwidth of about 3.1 μm and wide angles of incidence of 0°-70° is obtained. The wide angle feature enables the use of the structure as a curved mirror in multi-pass gas cells used in infrared spectroscopic applications.