Principal Investigator Juejun Hu
In this report, we demonstrate the first on-chip spectroscopic chemical sensor with a monolithically inte- grated supercontinuum (SC) light source. Unlike traditional broadband, blackbody sources used in benchtop Infrared Radiation (IR) spectrophotometers waveguide SC sources feature high spatial coherency essential for efficient light coupling and manipulation on a photonic chip. Compared to tunable lasers, SC offers superior bandwidth coverage. The broadband nature of SC facil- itates access to wavelengths that are difficult to cover using semiconductor lasers, and thereby, significantly expands the identifiable molecule repertoire of spec- troscopic sensors. In our experiment, we use chalcogenide glass (ChG) as the waveguide material for both SC generation and evanescent wave sensing. ChGs are known for its broadband infrared transparency, large Kerr nonlinearity, and low two-photon absorption (TPA), ideal characteristics for the application.
400 nm thick Ge22Sb18Se60 (GeSbSe) films were thermally evaporated onto 4” silicon wafers with 3 μm thermal oxide as an under cladding from GeSbSe glass powders. GeSbSe waveguides with varying length were fabricated using our previously established protocols. In the process, a 350-nm-thick ZEP resist layer was spun onto the substrate followed by exposure on an Elionix ELS-F125 tool at a beam current of 10 nA. The resist pattern was then developed by immersing in ZED-N50 developer for one minute. Reactive ion etching was performed in a PlasmaTherm etcher to transfer the resist pattern to the glass layer. The etching process used a gas mixture of CHF3 and CF4 at 3:1 ratio and 5 mTorr total pressure. The incident Radio Frequency (RF) power was fixed at 200 W.
Finally, the device was immersed in N-Methyl-2- pyrrolidone (NMP) overnight to remove the ZEP resist and complete device fabrication. The waveguides assume a zigzag geometry with lengths up to 21 mm. Figure 1a plots the SC spectra in GeSbSe waveguides with the different lengths and the optimal dimensions (W = 0.95 μm, H = 0.4 µm). As indicated in the figures below, the SC bandwidth extends to over half an octave, albeit with decreased total output power when the waveguide length increases to 21 mm. In the sensing experiment, the GeSbSe waveguide was immersed in carbon tetrachloride (CCl4) solutions containing varying concentrations of chloroform (CHCl3). The CCl4 solvent is optically transparent across the near- IR, whereas the C-H bond in chloroform leads to an overtone absorption peak centering at 1695 nm, a wavelength outside the standard telecommunication bands. SC spectra near the chloroform absorption peak obtained with GeSbSe waveguides of different lengths are presented. The data were normalized to the background (collected in pure CCl4).