Mid infrared gas sensing
Many gases are colorless and odorless and therefore are not perceived by the human sense organs. For toxic gases the monitoring in buildings is indispensable, as for example gas leakages can lead to tremendous health damages. But also the detection of nontoxic gases is of utmost importance for many applications as process control or breath analysis for clinical diagnosis. Many gases exhibit absorption lines in specific fingerprint regions in the mid-infrared wavelength range, for example, CO2 shows high absorption between 4.2 and 4.3 µm. Thus an optical signal at the gas specific wavelength range can interact with the gas and its intensity will be decreased. Optical sensors detect the change in intensity in the fingerprint regions and thus get information about the present gas concentration.
In general, an optical sensor consists of three components: 1. a source that emits radiation in the gas specific wavelength range, 2. a gas cell that provides a large interaction length between the light signal and the gas, 3. a detector that is able to detect even small intensity changes.
The overall aim of this project is to develop an optical gas sensor on a silicon chip that offers small size and low power consumption. This would open up the way to low-cost mass-production and also allow the implementation of gas sensors into portable devices like mobile phones.
In contrast to free-path optical sensing, our approach is based on integrated photonics where light will be guided inside the silicon chip. The evanescent field can interact with gases in the environment and can thus be used for optical sensing.
Our current research is based on the following topics:
- Integrated silicon based thermal emitter
- 2D integrating cell
- Integrated silicon based MIR detector
Single mode thermal emitter
We have developed an approach for a single mode thermal emitter that is based on a silicon on insulator (SOI) platform (see Figure 1.(a)). One part of the waveguide can be heated and thus releases broadband thermal radiation into the waveguide. Heating the emission section to T = 1000 K will result in an overall power around 0,5 µW in the waveguide. The width w and height h of the waveguide are adjusted to allow the propagation of a single mode. The length L is chosen to ensure an adequate isolation of the emission section in order to reduce the power consumption. The broadband emitter design can be modified in order to reduce the emission to a small frequency range. Therefore the emission section is embedded into a photonic bandgap cavity as shown in Figure 1.(b). The radiation frequency range depends on the hole dimensions and distances. It can be adjusted to the fingerprint region of a certain gas. For the CO2 specific wavelength range the resonant thermal emitter provides a spectral power density of 3 nW into the single mode waveguide.
Figure 1. (a) Broadband thermal emitter that is based on the heating of a single mode silicon strip waveguide. (b) Resonant thermal emitter design where a photonic bandgap cavity is used to resonantly select a small frequency range. By adjusting the hole dimensions and distances the resonance frequency and bandwidth of thermal radiation can be tuned.
List of publications
Fohrmann, L. S.; Petrov, A. Y.; and Eich, M., "Silicon waveguide based broadband thermal emitter for MIR," in Proceedings of the IEEE 11th International Conference on Group IV Photonics (IEEE, 2014), pp. 185–186.
Fohrmann, L. S.; Petrov, A. Y.; Lang, S.; Jalas, D.; Krauss, T. F.; and Eich, M., “Single mode thermal emission,” Opt. Exp. (to be published).
Prof. Thomas F. Krauss, Department of Physics, University of York, U.K.
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