Waveguide platforms for mid-infrared on-chip spectrometer applications.

Mourgelas, Vasileios (2023) Waveguide platforms for mid-infrared on-chip spectrometer applications. University of Southampton, Doctoral Thesis, 156pp.

Record type: Thesis (Doctoral)

Abstract

On-chip spectroscopy has been recently attracting considerable interest and has seen significant development in the past few years due to the increasing demand for ultraportable/integratable, efficient and cost-effective devices that can be employed on-site for chemical analysis of specimens. Spectrometers operating in the mid-infrared (MIR) region can provide very useful information due to the inherent chemical selectivity in this spectral range, thus making it suitable in many applications such as environmental analysis and biomedical diagnostics. There is a variety of Mid-IR spectrometers available on the market, however, such instruments are not very accessible and are usually lab-use only limited. Ultra-portable MIR spectrometer devices are relatively scarce since their design and fabrication tends to be complicated. In this thesis, platforms for realisation of on-chip thermo-optic spectrometers are discussed which incorporate two different approaches. The first approach utilises a IIIV semiconductor material platform and the second a chalcogenide glass-based platform. The semiconductor platform comprised of In_0.49Ga_0.51P/GaAs optical layers on GaAs substrate and the chalcogenide platform comprised of Ge₃₃As₁₂Se₅₅ (IG2) and Ge₃₀As₁₃Se₃₂Te₂₅ (IG3) layers on Si substrate. An In_0.49Ga_0.51P/GaAs thermo-optic spectrometer on-chip configuration was modelled for the estimation of optical losses and thermal performance. The optical losses introduced by the metallic heater absorption and radiation into the substrate, were estimated to be less than 0.06 dB/cm for cladding thickness of 5 µm and the bend losses were negligible for bends of radii of 200 µm and above. These losses combined with the very low ∼0.5 dB/cm reported and estimated propagation loss for GaAs waveguides, make up a very efficient optical platform for this application. The thermal modelling revealed that the required temperature excursion to achieve a spectral resolution of 10 vi cm^-1 (∼83 K) is reached within 10-12 ms at a cost of a few tens of Watts and with very acceptable crosstalk between the tuning and the reference MZI arms. Fabricated In_0.49Ga_0.51P/GaAs waveguides showed clear guiding between λ= 2.8 µm and 8.5 µm and the estimated propagation losses were found to be around 2.53 and 0.53 dB/cm for λ= 3.5 µm and 7.7 µm, respectively. IG2 and IG3 films were optically characterised using ellipsometry (λ= 192-1690 nm) and prism coupling (Metricon, λ= 1553 nm). The estimated propagation loss of the films was found to be between 2.3-4.26 dB/cm at λ= 1553 nm. Si micromaching eliminated the need for any post processing of the chalcogenide layers through the pattering of pedestals on the Si substrate. Propagation losses lower than 0.1 dB/cm were obtained, in the long-wave IR band, from dry etched pedestal waveguides, which are amongst the lowest ever reported. Thermal Fabry-Perot measurements of the IG2/IG3 waveguides revealed efficient thermo-optic tunability and the effective thermo-optic coefficient was found to be 1.1067 x 10^-4 °C -1 at λ = 1630 nm, which is comparable to that of commonly used mid-IR thermo-optic materials, such as Si and GaAs and of the same order as the 2 x 10^-4 °C ^-1 estimated for the InGaP/GaAs platform. The results presented in this thesis manifest the great potential of two material platforms that can be utilised in long-wave IR applications such as on-chip spectrometers and sensors, due to their excellent properties and ease of fabrication. More notably, the presented results on the pedestal platforms, demonstrate, that it is possible to overcome the issues that accompany the fabrication of chalcogenide waveguides and thus solely exploit their excellent properties in a wide range of applications.