## 3 Future CONERSTONE Platforms
### Germanium-On-Silicon  
MIR group IV photonic devices and systems are well suited to a broad variety of applications including environmental, biological, chemical and pharmaceutical sensing, industrial process control, toxin and contaminant detection, point-of-care diagnostics, communications and astrophysics. The absorption spectra in the fingerprint region (wavelength range 6-25 \(\upmu\)m) are comprised of clearly defined peaks for many molecules [118]. Therefore, this region is particularly appropriate for \begin{table}
\begin{tabular}{c c c} \hline \hline
**Structure** & **O-band (1330 nm)** & **C-band (1550 nm)** \\ \hline Single mode strip waveguide & 1.0 dB/cm (W = 900 nm) & 1.5 dB/cm (W = 1200 nm) \\ Strip waveguide 90\({}^{\circ}\) bend & 0.03 dB (R = 60 \(\upmu\)m) & 0.05 dB (R = 80 \(\upmu\)m) \\ Directional coupler & \textless{}1.0 dB & \textless{}1.0 dB \\ \(1\times 2\) MMI & 0.1 dB & 0.3 dB \\ MZI & 0.4 dB & 0.7 dB \\
4-channel AMMI & \textless{}2.0 dB & \textless{}2.0 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Typical insertion loss of devices demonstrated using e-beam lithography on 300 nm stoichiometric SiN in the O and C telecom bands (TE mode). W = single mode waveguide width. R = bend radius. Figure 14: Si nitride device library: (**a**) grating coupler; (**b**) multimode interferometer; (**c**) directional coupler, (**d**) Mach–Zehnder interferometer; (**e**) angled-multimode interferometer (reprinted from [95], The Optical Society, 2017); (**f**) (de)multiplexer response in the O-band and (**g**) (de)multiplexer response in the C-band. sensing because it can be used to identify molecular composition and quantify concentration, which is possible because the wavelength of each absorption peak corresponds to the vibrational frequency of a molecular bond and the intensity corresponds to concentration. MIR photonic devices can be used for non-destructive characterization of solid, liquid and gas phase substances with minimal sample preparation. The integration of microfluidics and MEMS systems for repeatable and efficient sample handling is straightforward due to the planar geometry of group IV photonic devices. When combined with the economies of scale associated with the fabrication techniques originally developed by the microelectronics industry, there is clear potential for miniaturized and mass producible MIR photonic sensors for many applications. SOI is the most mature group IV photonics material platform. However, SiO\({}_{2}\) is highly absorbing at wavelengths greater than 4 \(\upmu\)m so the SOI platform is inappropriate for longer wavelength operation [119]. Several alternative material platforms have been investigated to extend the usable wavelength range to cover more of the fingerprint region and so enable a greater variety of applications. These include Si on sapphire (SOS) [120; 121; 122], Si on porous Si [123], Si on Si nitride [124], germanium-on-silicon (GOS) in this section [125; 126; 127; 128] and suspended Si [129; 130; 131] (Section 3.3). The chief advantages of germanium-based material platforms for MIR photonics are the wide transparency range (2-15 \(\upmu\)m) and, in comparison with Si, its higher carrier mobility and larger non-linear effects. The first reported germanium-based MIR photonic devices were Ge-core fibers [132] and Ge slab waveguides on ZnS substrates [133]. [PERSON] et al. have demonstrated GOS waveguides with 2.5-3.0 dB/cm loss at wavelength \(\lambda\) = 5.8 \(\upmu\)m [126], and GOS multiplexers, Mach-Zehnder interferometers and thermo-optic modulators at \(\lambda\) = 5.3 \(\upmu\)m [125]. We used two different thicknesses of germanium for device fabrication: 2 \(\upmu\)m and 3 \(\upmu\)m [134]. Initial investigations were performed using 2 \(\upmu\)m Ge layers. The deposition of these layers required less time, which could result in lower cost devices. Latterly, a 3 \(\upmu\)m Ge layer was adopted to allow larger waveguide structures; this platform will become the standard CONERSTONE platform for Ge-on-Si MPW calls.