Chip-scale Broadband Mid-IR Chemical Sensors Using Silicon Waveguides

Technology #16158

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 FIG. 1A is a perspective diagram of an exemplary air-clad, silicon pedestal waveguide suitable for mid-infrared (mid-IR) sensing with a waveguide width w, a waveguide height h, a pedestal height s, and a pedestal width d.  FIG. 1B is a diagram of the silicon pedestal waveguide of FIG. 1A used to sense organic molecules with an evanescent wave. FIG. 1C is an end-on view of an exemplary air-clad, germanium pedestal waveguide suitable for mid-IR sensing with a waveguide width w, a waveguide height h, a pedestal height s, a pedestal upper base width d1, and a pedestal lower base width d2.  FIG. 1D is a side view of the germanium pedestal waveguide of FIG. 1C.FIG. 2 is a schematic diagram of a chip-based mid-IR sensor that includes a semiconductor pedestal waveguide (e.g., the silicon pedestal waveguide of FIGS. 1A and 1B or the germanium pedestal waveguide of FIGS. 1C and 1D).FIGS. 3A-3F illustrate a process for fabricating silicon pedestal waveguides and splitters.
Professor Lionel Kimerling
Materials Science and Engineering, MIT
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Pao Tai Lin
Materials Processing Center, MIT
Anuradha Agarwal
Materials Processing Center, MIT
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Managed By
Jim Freedman
MIT Technology Licensing Officer - Chemicals, Instruments, Consumer Products
Patent Protection

Methods and apparatus for mid-infrared sensing

US Patent 9,046,650
Chip-scale Mid-Infrared chemical sensors using air-clad pedestal silicon waveguides
Lab Chip, 2013,13, pp. 2161-2166


The Mid-IR pedestal Si waveguide sensor is applicable to broadband Mid-IR scanning, real-time trace chemical detection, concentration monitoring, and identification of organic compounds. It also provides a unique flat form for next generation spectrometer-on-a-chip.

Problem Addressed

Silicon-on-Insulator (SOI) waveguide structures suffer significant optical loss because their silicon dioxide under-cladding layer becomes absorbing 3.7 um. Specifically, conventional UV, visible and NIR sensors use weaker overtones of these fundamental chemical bands, which limits their practicality.


This invention is about 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 wavelengths of 1.3 μm and 6.5 μm, so that its operational spectral range covers most characteristic chemical absorption bands. To extend light transmission beyond the wavelength of  3.7 μm, a spectral region where a typical silicon dioxide under-clad is absorbing, we demonstrate a unique air-clad silicon pedestal waveguide. The sensing mechanism of the proposed 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.


·  Chemically and mechanically robust due to air-clad silicon

·  In situ identification of chemical compositions and concentrations of organic solvents

·  Ultra-compact and portable spectrometer-on-a-chip