T. Geppert, R.B. Wehrspohn, F. Müller and A. Milenin
Many gases (and liquids) exhibit characteristic absorption lines in the mid infrared wavelength region (3-20 µm). So spectroscopic-based sensor techniques play a major role in processing and production measurement (e.g. CO and CO2 measurement in the automotive market) but with the major drawback of high costs for such systems. The aim of this project is to evaluate and develop sensors on the basis of photonic crystals (PCs) for spectroscopic detection of gases. The advantage of such systems over conventional ones could be the possibility of miniaturization, integration, batch processing and thus low-cost versions of such sensor systems. Macroporous silicon is used as photonic crystal material, from which structures for wavelengths in the mid infrared region can be fabricated with great reproducibility.
Theoretical calculations are necessary regarding the structure layout (tailoring of the sensor geometry for the desired wavelength, layout of the coupling regions). Beside the experimental verification of the effects predicted from theory it is the main goal of this project to work out practical solutions for sensors even at this early stage.
Fig. 1. Absorption lines of molecules in the mid infrared spectral range (MIR) and the spectral range of lader sources.
The intensity of light transmitted through a gas filled interaction volume serves as signal for the determination of the gas concentration. In the PC-based scheme the conventional interaction volume usually consisting of a glass tube is replaced by a PC membrane.
Fig. 2. Photonic crystal-based spectroscopic gas sensor (Diss.: T. Geppert).
Fig. 3 illustrates the coupling condition for plane waves (PW) and PC Bloch modes. The mirror symmetry of the Hz (Ez) components of the impinging TE (TM) modes has to be fulfilled by the PC mode under consideration along the direction of incidence (ΓK in the picture, represented by the black dashed arrows). The 3rd TM mode and the 2nd TE mode fulfill this symmetry condition (marked by the green frame), while the 2nd TM and the 3rd TE mode are anti-symmetric.
Fig. 3. Coupling conditions for plane waves (PW) and PC Bloch modes (Diss.: T. Geppert).
Fig. 4. shows SEM micrographs of experimental details of the gas sensor structure. In particular you see: Deep trenches next to macropores realized by PECE. a) Lithographically prestructured n-Si wafer (top view). b) Trench and adjacent macropore array after PECE. c) Side view of the cleaving edge through the macropore array. d) Side view of the cleaving edge perpendicular to the trench. (trench highlighted by the red dotted line) e) Side view of the cleaving edge parallel to the trench. (Diss.: T. Geppert).
See also: Poster from the first DFG evaluation meeting.