Development of an evanescent optical integrated sensor in the mid-infrared for detection of pollution in groundwater or seawater

Figures & data

Figure 1. Ge–Sb–Se ternary diagram with the two selected compositions (GeSe₂)₉₀(Sb₂Se₃)₁₀ (Ge_28.1Sb_6.3Se_65.6, Se2) and (GeSe₂)₅₀(Sb₂Se₃)₅₀ (Ge_12.5Sb₂₅Se_62.5, Se6) for the bulk glass targets and the sputtered films.

Figure 2. Chemical composition of (GeSe₂)₉₀(Sb₂Se₃)₁₀ (Ge_28.1Sb_6.3Se_65.6, Se2) and (GeSe₂)₅₀(Sb₂Se₃)₅₀ (Ge_12.5Sb₂₅Se_62.5, Se6) sputtered films and bulk glass targets determined by EDS analysis (±1 at.%) compared to theoretical composition.

Figure 3. Refractive index in near- and mid-IR (±0·01) extracted from VASE data of bulk targets and sputtered (Ge_28.1Sb_6.3Se_65.6, Se2) and (Ge_12.5Sb₂₅Se_62.5, Se6) thin films.

Figure 4. (a) SEM image of guiding (Ge_12.5Sb₂₅Se_62.5, Se6) and cladding (Ge_28.1Sb_6.3Se_65.6, Se2) sputtered layer of mid-IR structure, (b) dispersion curves of refractive indices (±0·01) of the sputtered layers, estimated by the analysis of VASE data via Sellmeier model.

Figure 5. (a) Scheme and (b) SEM image of cross-section of chalcogenide ridge waveguide, (c) fundamental mode TM₀₀ intensity profile for the optimal geometrical parameters (width w and height h) of chalcogenide waveguide and evolution of the evanescent power factor η as a function of w, h for single-mode propagation in the detection of any substance dissolved at λ = 7·66 μm.

Figure 6. (a) Near field of propagated light at λ = 7·7 μm and microscopic image of single-mode ridge waveguides in S-shape configuration. (b) Optical propagation loss at λ = 7·7 μm measured by mode profile imaging method for selenide ridge waveguides with height equal to 1·7 μm and width of 10 μm. Each point represents averaged data for 3 experimental measures.

Figure 7. (a) Mid-IR absorbance spectra of toluene at 100 v/v % and water at T = 278 K. (b) Power transmitted response as a function of the waveguide length taking into consideration the strong absorbance of water in the mid-IR. (c) Scheme of waveguide functionalization: a polymer non-absorbing in the mid-IR is deposited as superstrate for detection of pollutants dissolved in water.

Figure 8. Scheme of a mid-IR sensor constituted of a selenide buffer (Ge_28.1Sb_6.3Se_65.6, Se2) and a ridge selenide waveguide (Ge_12.5Sb₂₅Se_62.5, Se6) coated with a polymer allowing molecules detection by evanescent wave spectroscopy.