Mid-infrared silicon photonics for sensing applications
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Mid-infrared silicon photonics for sensing applications Goran Z. Mashanovich1, Milan M. Milosevic2, Milos Nedeljkovic1, David Cox2 , Vittorio M. N. Passaro3, Harold M. H. Chong1 and Richard Soref4 1 School of Electronics and Computer Science, University of Southampton, Southampton, UK* 2 Advanced Technology Institute, University of Surrey, Guildford, Surrey, UK 3 Dipartimento di Elettrotecnica ed Elettronica, Politecnico di Bari, 70125 Bari, Italy 4 Physics and Engineering Departments, University of Massachusetts at Boston, Boston, MA 02125 USA ABSTRACT The mid-infrared wavelength region offers a plethora of possible applications ranging from sensing, medical diagnostics and free space communications, to thermal imaging and IR countermeasures. Hence group IV mid-infrared photonics is attracting more research interest lately. Sensing is an especially attractive area as fundamental vibrations of many important gases are found in the 3 to 14 μm spectral region. To realise group IV photonic mid-infrared sensors several serious challenges need to be overcome. The first challenge is to find suitable material platforms for the mid-infrared. In this paper we present experimental results for passive midinfrared photonic devices realised in silicon-on-insulator (SOI), silicon-on-sapphire (SOS), and silicon on porous silicon (SiPSi). Although silicon dioxide is lossy in most parts of the midinfrared, we have shown that it has potential to be used in the 3-4 μm region. We have characterized SOI waveguides with < 1 dB/cm propagation loss. We have also designed and fabricated SOI passive devices such as MMIs and ring resonators. For longer wavelengths SOS or SiPSi structures could be used. An important active device for long wavelength group IV photonics will be an optical modulator. We present relationships for the free-carrier induced electro-refraction and electro-absorption in silicon in the mid-infrared wavelength range. Electroabsorption modulation is calculated from impurity-doping spectra taken from the literature, and a Kramers-Kronig analysis of these spectra is used to predict electro-refraction modulation. We have examined the wavelength dependence of electro-refraction and electro-absorption, and found that the predictions suggest longer-wave modulator designs will in many cases be different than those used in the telecom range. INTRODUCTION Mid-infrared group IV photonics has attracted increasing attention in the last few years, driven mainly by the lure of possible applications such as chemical-bio-physico sensing, freespace communications, thermal imaging and infrared countermeasures. Using group IV materials such as silicon and germanium in the mid-infrared is attractive because many of the techniques developed for design, fabrication and testing of telecoms wavelength silicon photonics, which has flourished in recent years, should be readily transferable to longer wavelengths. Potentially the most desirable aspect of this approach is that in future there could be chip-level integration of a mid-infrared system a
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