## 3 Future CONERSTONE Platforms
### Suspended Silicon  
In photonics Si is most commonly encountered in the SOI platform for optical communications at the telecoms wavelengths of 1.31 \(\upmu\)m or 1.55 \(\upmu\)m. Si is a viable mid-infrared material given that its usable transparency window extends up to wavelengths of 8 \(\upmu\)m; the factor limiting the wavelength range of SOI devices is the absorption of the buried silica layer above \(\lambda=4\)\(\upmu\)m.  
Air-clad Si structures offer an alternative to germanium-based material platforms (see Section 3.2) for extending the usable wavelength range beyond that of SOI by elevating a Si waveguide so the propagating mode does not interact with the substrate material. Such structures are implemented in one of two ways: pedestal-type structures [145], which are supported by a thin rib of Si, or suspended structures, where the region under the waveguide is completely removed and the waveguide is supported by lateral bars or membranes [146; 147; 148]. Such air-clad platforms avoid using a new material platform and thereby allow the full range of established SOI fabrication techniques to be used. The major advantage of laterally suspended Si structures is that the suspending structure can be fabricated using lithographically patterned etching. Material underneath the waveguide must be removed using an isotropic etch, whether this is partial (as for pedestal structures) or complete (as for suspended Si devices). This limits the degree of longitudinal patterning that can be achieved underneath the waveguide, for example in the pedestal. Conversely, the lateral supports of a suspended Si can be periodically structured to form a grating [148]. This enables the mode confinement in the waveguide to be optimized so that the sample-light interaction and therefore sensitivity can be adjusted for a particular application. Three techniques have been used to suspend Si waveguides. First, Si membrane waveguides have been fabricated using an array of holes far from a rib waveguide core to expose the buried oxide (BOX) to hydrofluoric (HF) acid and undercut the Si rib [146]. Second, suspended Si membrane rib waveguides were fusion-bonded to a Si substrate, which had prepatterned air trenches, to provide more support to the waveguide and improve stability [147]. Third, we have demonstrated a method where a subwavelength grating is used to suspend a waveguide core [148]. The grating is etched through the entire upper Si layer to expose the BOX for removal with HF acid. A schematic of this structure is shown in Figure 16.  
The subwavelength grating (SWG) supporting structure performs several functions, each with its own constraints. The SWG cladding must be strong and stiff enough to physically support the central waveguide structure without sagging or breaking. The periodicity must be such that it forms a subwavelength structure at mid-infrared wavelengths to suppress any propagation outside of the waveguide. The lateral refractive index can be controlled to optimize mode confinement, which is achieved by varying the longitudinal proportion of Si to air gap (i.e., \(L_{\text{Si}}\):\(L_{\text{hole}}\) in Figure 16b). A higher fraction of Si provides a lower index contrast with the waveguide. Finally, the gaps between the supporting bars allow the circulation of HF acid to selectively etch SiO\({}_{2}\) under the Si to form the undercut. The suspended Si platform shares the potential for the same variety of mid-infrared applications as the GOS platform, discussed in detail in Section 3.2. The choice of which platform is more suitable is application specific. GOS provides a wider transparency region so is better suited to sensing applications that require access to the longer wavelength part of the fingerprint region. The two platforms have comparable loss although GOS is generally slightly lower.