## 3 Future CONERSTONE Platforms
### Germanium-On-Silicon
The 3 \(\upmu\)m thick Ge layer enabled single mode operation at longer wavelengths and allowed the mode to be more completely confined within the waveguide, thereby reducing its interaction with sidewall roughness and threading dislocations at the Ge-Si boundary, which occur due to the lattice mismatch between Si and Ge. GOS devices that were 2 \(\upmu\)m thick were based on rib waveguides using an etch depth of 1.2 \(\upmu\)m and core width of 2.25 \(\upmu\)m, which were optimized for single mode operation, with a measured loss of 3 dB/cm in the region \(\lambda\) = 2-3.8 \(\upmu\)m. The non-linear response of germanium due to TPA was also investigated using GOS waveguides. The TPA non-linearity \(\beta_{\text{TFA}}\) was measured to be greater than \(1\times 10^{3}\) cm/GW in the wavelength range \(\lambda\) = 1.9-2.3 \(\upmu\)m, which is approximately 1000 times greater than that of Si, and corresponds closely to \(\beta_{\text{TFA}}\) measured for bulk Ge [135; 136]. The magnitude and ultrafast nature of the TPA nonlinearity mean this behavior can be used to implement a variety of all-optical functions including modulators, switches, logic gates and pulse shapers. All-optical modulation was demonstrated using GOS waveguides at \(\lambda\) = 2 \(\upmu\)m, where the high \(\beta_{\text{TFA}}\) value enabled high speed cross-absorption modulation (XAM). This experiment used a pump-probe setup where a high-power pump induced an absorption dip on a low-power probe that was too weak to induce TPA itself. Increasing pump power was shown to increase modulation depth, albeit with some roll-off due to pump saturation. A maximum extinction ratio (ER) of 8.1 dB was recorded for a coupled input peak power of 10 W, which is the highest published value for any group IV waveguide device [137; 138]. Alternatively, due to the fact that free carrier effects are significantly stronger in Ge than Si [139], electrically induced modulation based on carrier injection into 1 mm long PIN devices has been demonstrated [140]. A modulation depth of \(>\)35 dB with a 7 V forward bias at \(\lambda\) = 3.8 \(\upmu\)m was demonstrated when operating as an electroabsorption device, and when operating as an electrorefraction device integrated into an MZI, a \(V_{\pi\text{ }}L\) of 0.47 V cm, driven by a 2.5 \(V_{pp}\) RF 60 MHz signal, was demonstrated. 3 \(\upmu\)m thick GOS devices were based on rib waveguides using an etch depth of 1.7 \(\upmu\)m and core width of 2.7 \(\upmu\)m, which were optimized for single mode operation. A scanning electron microscope (SEM) image of a ductile machined end facet of a GOS waveguide is shown in Figure 15. Light was coupled into and out of the devices using surface grating couplers. The etch depth of the grating was kept the same as the waveguide etch depth of 1.7 \(\upmu\)m to reduce the number of required fabrication steps. The period (2 \(\upmu\)m) and duty cycle (0.5) were optimized for TE mode operation using Lumerical. Subsequently, coupling efficiency has been increased and reflections reduced by using inverse taper excitation. Propagation loss was measured as 0.58 \(\pm\) 0.12 dB/cm at \(\lambda\) = 3.8 \(\upmu\)m, which is significantly less than the data published for other GOS devices [125; 126; 127]. A wide variety of devices for building PICs have been demonstrated using the 3 \(\upmu\)m GOS platform (see Table 5), including MMI splitters [141], angled MMIs [142] and cascaded ring and racetrack resonators [143]. Low loss 1 \(\times\) 2 and 2 \(\times\) 2 MMIs are discussed here as an example.