title: Chip-scale Mid-Infrared chemical sensors using air-clad pedestal silicon waveguides.
abstract: Towards a future lab-on-a-chip spectrometer, we demonstrate a compact chip-scale air-clad silicon pedestal waveguide as a Mid-Infrared (Mid-IR) sensor capable of in situ monitoring of organic solvents. The sensor is a planar crystalline silicon waveguide, which is highly transparent, between λ = 1.3 and 6.5 μm, so that its operational spectral range covers most characteristic chemical absorption bands due to bonds such as C-H, N-H, O-H, C-C, N-O, C=O, and C≡N, as opposed to conventional UV, Vis, Near-IR sensors, which use weaker overtones of these fundamental bands. To extend light transmission beyond λ = 3.7 μm, a spectral region where a typical silicon dioxide under-clad is absorbing, we fabricate a unique air-clad silicon pedestal waveguide. The sensing mechanism of our Mid-IR waveguide sensor is based on evanescent wave absorption by functional groups of the surrounding chemical molecules, which selectively absorb specific wavelengths in the mid-IR, depending on the nature of their chemical bonds. From a measurement of the waveguide mode intensities, we demonstrate in situ identification of chemical compositions and concentrations of organic solvents. For instance, we show that when testing at λ = 3.55 μm, the Mid-IR sensor can distinguish hexane from the rest of the tested analytes (methanol, toluene, carbon tetrachloride, ethanol and acetone), since hexane has a strong absorption from the aliphatic C-H stretch at λ = 3.55 μm. Analogously, applying the same technique at λ = 3.3 μm, the Mid-IR sensor is able to determine the concentration of toluene dissolved in carbon tetrachloride, because toluene has a strong absorption at λ = 3.3 μm from the aromatic C-H stretch. With our demonstration of an air-clad silicon pedestal waveguide sensor, we move closer towards the ultimate goal of an ultra-compact portable spectrometer-on-a-chip.
authors.affiliation: Materials Processing Center, Massachusetts Institute of technology, Cambridge, MA 02139, USA. paolin@mit.edu
authors.forename: Pao Tai
authors.initials: PT
authors.lastname: Lin
authors.forename: Vivek
authors.initials: V
authors.lastname: Singh
authors.forename: Juejun
authors.initials: J
authors.lastname: Hu
authors.forename: Kathleen
authors.initials: K
authors.lastname: Richardson
authors.forename: J David
authors.initials: JD
authors.lastname: Musgraves
authors.forename: Igor
authors.initials: I
authors.lastname: Luzinov
authors.forename: Joel
authors.initials: J
authors.lastname: Hensley
authors.forename: Lionel C
authors.initials: LC
authors.lastname: Kimerling
authors.forename: Anu
authors.initials: A
authors.lastname: Agarwal
mesh_terms: D000388:Air; D002251:Carbon Tetrachloride / Q000032:analysis*; D006586:Hexanes / Q000032:analysis*; D056656:Lab-On-A-Chip Devices; D012825:Silicon / Q000737:chemistry*; D013055:Spectrophotometry, Infrared / Q000295:instrumentation; D014050:Toluene / Q000032:analysis*
chemical_list: D006586:Hexanes; D014050:Toluene; D002251:Carbon Tetrachloride; D012825:Silicon