Technologies for Room-Temperature Mid-Infrared Photodetection using Graphene

Abstract

Mid-infrared light is used for thermal imaging, relying on thermal radiation, and chemical analysis, based on vibrational absorption spectra. Ironically, these applications require detector materials and architectures with resilience to thermal noise and infrared-transparent optical materials with minimal vibrational absorption, restricting the mid-infrared material toolbox. 2D materials, which promise to combine high crystallinity with inexpensive and low-temperature processing paradigms, may alleviate some of the material compatibility issues that complicate the design of advanced mid-infrared systems beyond photodetectors and imagers. Graphene is a particularly promising 2D material whose photoresponse has been shown to range from visible to terahertz wavelengths and enjoys fairly mature synthesis and processing technology. Thus, in this thesis, I demonstrate two different mid-infrared systems with novel features enabled by graphene as the optically active material. First, I introduce a multispectral imager concept based on metasurfaces composed of differently-sized, graphene-loaded slot antennas. Here, the tight juxtaposition of sub-wavelength antennas allows broadband transfer and wavelength-sorting of incident mid-IR light into graphene patches with a theoretical efficiency of up to ∼ 58%. I develop a compact circuit model which accurately predicts the absorption spectra of these slot antennas, and demonstrate an electroplating process for fabricating such metasurfaces. This research paves the way towards CMOS-integrable mid-infrared spectral imagers. Second, I demonstrate a chalcogenide glass-on-CaF₂ platform accommodating waveguide-integrated split-gate photothermoelectric graphene photodetectors. These devices achieve waveguide-integrated photodetection at a record-long wavelength of 5.2 µm with a Johnson noise-limited noise-equivalent power of 1.1 nW/Hz¹⸍² . They also feature fast response, with no fall-off in photoresponse up to 𝑓 = 1 MHz and a predicted 3-dB bandwidth of 𝑓₃ subscript d subscript B > 1 GHz. The demonstrated platform can be readily extended to longer wavelengths and opens the door to distributed gas sensing and portable dual-comb spectroscopy applications. Taken together, these results demonstrate the ability of graphene to enable novel mid-infrared microsystems with unique features and capabilities.