Group IV mid-infrared devices for sensing

Abstract

Group IV photonics is a topical research field, with potential applications in diverse areas such as bio-chemical and environmental sensing, security, communications, healthcare and astronomy. Many of these applications require accessing longer wavelengths in what is called the mid-infrared (MIR) region and specifically in the "fingerprint" region, as it contains strong fundamental vibrational transitions of most molecules. The transparency range of the traditional material platform used for years in near-infrared (NIR) silicon photonics (silicon-on-insulator) is not suitable due to its limited wavelength transparency range, therefore new materials need to be explored.

In this project SOI has been used to produce a slot waveguide at 3.8m, a wavelength range in which SiO2 absorption begins to be significant, but with a careful design and a proper selection of a suitable platform (i.e. thicker Si and SiO2 layers), low loss devices can still be produced, as it has been demonstrated extensively. A slot was chosen since it is a waveguide design highly suitable for sensing. The slot waveguide had a propagation loss of 1.4dB/cm and high field confinement in the slot gap. A new platform of suspended silicon with sub-wavelength lateral cladding has also been demonstrated, allowing the use of SOI for the full transparency range of Si. This platform has significant advantages compared to other, suspended solutions, in that a single etch step is required to fabricate the suspended waveguides and the resulting devices are more robust since the suspended region for a comparable device is much thinner, allowing the design of wider devices. This has been demonstrated with the design, fabrication and characterization of waveguides, bends, multimode interferomenters (MMI) and a Mach-Zehnder interferometer (MZI). The waveguides fabricated with this technique achieved a loss as low as 0.82dB/cm.

The Ge-on-Si platform has also been developed with the demonstration of waveguides and MMIs with propagation loss as low as 0.58dB/cm at 3.8m for the former and 0.21dB insertion loss for the latter. Waveguides and MMIs have also been fabricated and characterized in the wavelength range between 7.5m and 9.5m with a minimum loss of 2.5dB/cm, extending the range at which this material has been characterized. Unforeseen losses in Ge have been discovered in this wavelength range, a discussion of the possible sources is included in this thesis. Evanescent field sensing of Thiodiglycol has also been demonstrated, showing a good agreement with a commercial FTIR in the aforementioned range.

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Identifiers

ORCID for Jordi Soler Penadés: orcid.org/0000-0002-1706-8533

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