Metamaterial-Based Mid-Infrared Gas Sensing

Abstract

In recent years, air pollution finally has been recognized as a global, pressing issue. The resulting increase in environmental awareness leads to a tremendous rise in demand for low-cost CO2 sensors, which are key components of air quality monitoring and "smart home" applications. In order to access these high-volume, price-sensitive markets, technologies that offer low cost (<$10), sensitive (<50 ppm) and compact (mm-sized) CO2 detection are required. Among the large number of different gas sensing schemes, optical absorption sensors are known for their high selectivity, fast response time, and long-term stability. Furthermore, for certain gases such as CO2, optical absorption sensing is the only reliable detection method currently available. Within the field of optical gas sensors, non-dispersive infrared (NDIR) sensing at mid-infrared (mid-IR) wavelengths is a compact and relatively low-cost measurement principle. Owing to its simplicity, it is one of the commercially most relevant optical gas sensing schemes to date. Commercial NDIR CO2 sensors offer sensitivities on the order of 30 ppm in centimeter-scaled systems at prices around $50 - $100. In order to prevail in above-mentioned markets, mid-IR gas sensors must substantially scale down in both size and cost without compromising performance. Unfortunately, the simple scaling of NDIR sensors by sheer size reduction approaches an intrinsic limitation resulting from their use of discrete sensor components and dielectric interference filters. In this thesis, an all-metamaterial optical gas sensing concept has been developed that overcomes the integration limit of conventional NDIR sensors. Key to its design are metamaterial perfect absorbers (MPAs), which serve as optical filter elements that are integrated into the membranes of on-chip thermal emitters and detectors. Combining MPAs on the emitter and the detector side cascades their individual filter functions, yielding a combined narrowband resonance that is matched to the absorption band of the target gas, in this case CO2. The MPAs' angle-independent filter characteristics allow for a non-resonant cavity design that "folds" the required cm-long absorption path into a mm-sized cuboid cavity, thereby reducing the absorption volume by a factor of 30 when compared to conventional cavity designs. The all-metamaterial gas sensor exhibits a decrease in energy consumption by 80% when compared to commercial solutions without compromising performance (CO2 sensitivity 22 ppm, humidity cross sensitivity 1.2 ppm/%rH). The sensor architecture developed in this thesis offers a viable path toward compact and low-cost mid-infrared gas sensors without trade-offs in sensitivity or robustness. This cumulative dissertation is structured as follows: Chapter 1 serves as an introduction to this thesis. After motivating the research, it provides an overview on the state of the art in non-dispersive optical gas sensing. The chapter concludes with the vision of an all-metamaterial optical gas sensor. Chapter 2 summarizes the theoretical backgrounds relevant to the interdisciplinary field of metamaterial optical gas sensing. First, an introduction to absorption spectroscopy and non-dispersive infrared gas sensing is given. The principles of thermal emission and detection are established, followed by an overview on electromagnetics, plasmonics, and the concept of metamaterials. The chapter concludes with an introduction to the theory of integrating spheres. Chapter 3 demonstrates an on-chip thermal light source exhibiting narrowband and efficient mid-infrared emission. The light source's spectral properties are tailored by metamaterial perfect absorbers, which are integrated into the emitter membrane. Employed in a gas sensing setup, the metamaterial light source leads to a 5-fold increase in relative sensitivity when compared to a conventional blackbody emitter. Chapter 4 presents a CMOS-compatible metamaterial thermal detector with a narrowband absorption resonance at 4.29 μm. The high selectivity of the device leads to a 6.5-fold reduction in humidity cross sensitivity when employed in a gas sensing setup. The metamaterial's potential for highest integration densities is showcased by the realization of a dual-band detector on a single thermopile membrane. Chapter 5 demonstrates for the first time an all-metamaterial optical CO2 sensor. By advantageously combining metamaterial thermal emitters and detectors with an efficient non-resonant cavity, the sensor's absorption volume could be decreased by a factor of 30 when compared to conventional non-dispersive infrared gas sensors. The all-metamaterial sensor performs at par with much larger commercial devices, while consuming 80% less energy per measurement. Chapter 6 summarizes the findings of this thesis and gives an outlook on future research.