## 3 Future CONERSTONE Platforms
### Suspended Silicon
We demonstrated straight suspended Si waveguides with loss of 0.82 dB/cm at \(\lambda\) = 3.8 \(\upmu\)m and 3.1 dB/cm at \(\lambda\) = 7.7 \(\upmu\)m compared with GOS, which exhibited loss 0.58 \(\pm\) 0.12 dB/cm at \(\lambda\) = 3.8 \(\upmu\)m and 2.5 dB/cm at \(\lambda\) = 7.575 \(\upmu\)m.  
For fluidic sensing, the sample will occupy the evanescent field both above and below a suspended Si waveguide compared with above but not below a GOS waveguide. This gives an increased light-sample interaction for the suspended Si platform so it could potentially achieve higher sensitivity and lower limits of detection. Suspended Si is more suitable for microfluidic integration than GOS and can implemented, for example, by bonding polydimethylsiloxane flow cells to SiO\({}_{2}\) via surface functionalization with oxygen plasma. The bond requires a surface monolayer of SiO\({}_{2}\) so can be formed using native oxide. Conversely GOS cannot be covalently bonded to common microfluidic materials so would require an additional cladding or a compression-sealed flow cell instead. Suspended Si devices are inherently less robust than their fully supported SOI or GOS counterparts so have a lower limit for high pressure fluidic applications. However, if the SWG structure is not used to provide any flow functionality, careful design of the fluidic circuit should mitigate this disadvantage. The SWG structure limits the size of suspended particles than could be safely transported within liquid samples. A nanoparticle suspension would likely not be influenced by the SWG, surface effects notwithstanding. However, the microparticulate content of biological samples such as cells, blebs and other microvesicles are of a comparable size to the holes in the SWG cladding and would be likely to become trapped, causing flow problems and interfering with the index contrast between the waveguide and the SWG. Figure 16: (**a**) 3D and (**b**) top view and (**c**) cross-sectional schematics of a suspended Si waveguide, which is supported by subwavelength gratings. Suspended Si has been used to implement components and devices for operation at both \(\lambda\) = 3.8 \(\upmu\)m and 7.7 \(\upmu\)m including 90\({}^{\circ}\) bends, S-bends, MMI couplers and grating couplers. Typical device performance metrics are listed in Table 6 for TE polarization. Devices for \(\lambda\) = 3.8 \(\upmu\)m operation were fabricated using an SOI wafer, which is formed of a 500 nm layer of Si on 3 \(\upmu\)m BOX on a Si substrate, which will become the standard CORNERSTONE platform for suspended Si MPW calls. Longer wavelength operation required a thicker 1.4 \(\upmu\)m Si layer on top of the buried oxide, which was epitaxially grown on top of standard 500 nm SOI, but this modified thickness will not form part of the standard CORNERSTONE MPW calls. The future outlook for MIR sensing using the suspended Si platform is broadly similar to that of Ge-on-Si discussed above, albeit with the additional benefits of offering simpler integration with Si MEMS and polydimethylsiloxane (PDMS) microfluidics. One exemplar application would be exploiting the bioanalytical data we have obtained using Fourier-transform infrared spectroscopy (FTIR), including a detailed study of the MIR properties of blood [149]. When combined with comprehensive knowledge of the MIR performance of the GOS and suspended Si platforms, sensitivity can be maximized for blood-based medical diagnostics.