Abstract
The mid-infrared (mid-IR) wavelength range hosts unique vibrational and rotational resonances of a broad variety of substances that can be used to unambiguously detect the molecular composition in a non-intrusive way. Mid-IR photonic-integrated circuits (PICs) are thus expected to have a major impact in many applications. Still, new challenges are posed by the large spectral width required to simultaneously identify many substances using the same photonic circuit. Ge-rich graded SiGe waveguides have been proposed as a broadband platform approach for mid-IR PICs. In this work, ultra-broadband waveguides are experimentally demonstrated within unprecedented wavelength range, efficiently guiding light from 5 to 11 µm. Interestingly, losses from 0.5 to 1.2 dB/cm are obtained between 5.1 and 8 µm wavelength, and values below 3 dB/cm are measured from 9.5 to 11.2 µm wavelength. An increase of propagation losses is seen between 8 and 9.5 µm; however, values stay below 4.6 dB/cm in the entire wavelength range. A detailed analysis of propagation losses is reported, supported by secondary ion mass spectrometry measurement, and different contributions are analyzed: silicon substrate absorption, oxygen impurities, free carrier absorption by residual doping, sidewall roughness and multiphonon absorption. Finally, Mach-Zehnder interferometers are characterized, and wideband operation is experimentally obtained from 5.5 to 10.5 µm wavelength.
Highlights
The mid-infrared wavelength regime, spanning from 2 to 20 μm, has recently gained a great significance in the Photonic Integrated Circuit (PIC) community due to its vast number of applications [1]
The use of mid-IR PICs has been proposed as a powerful solution to overcome several major existing drawbacks in other important areas such as thermal imaging [8] or optical communications [9,10]
It is experimentally demonstrated that Ge-rich graded SiGe waveguides can be used in an unprecedented wavelength range, from 5 to 11 μm wavelength, which is a unique feature of the graded index profile in the SiGe waveguide
Summary
The mid-infrared (mid-IR) wavelength regime, spanning from 2 to 20 μm, has recently gained a great significance in the Photonic Integrated Circuit (PIC) community due to its vast number of applications [1]. New challenges arise due to the wide spectral range of mid-IR applications, such as the development of ultrawideband photonic circuits to analyze simultaneously many substances using the same photonic circuit In this context, Ge-rich graded SiGe waveguides have been proposed as a broadband platform approach for mid-IR PIC and propagation losses in the range of 2-3 dB/cm, from 5.5 to 8.5 μm have been previously reported [22]. The experimental characterization of asymmetric Mach-Zehnder interferometers (MZIs) has been carried out, showing clear interference patterns from 5.5 to 10.5 μm wavelength These results confirm the wide bandwidth operation of integrated devices based on the Ge-rich SiGe platform
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