Photonic sponges

Semiconductor fabrication techniques permit the feature size of Si-based devices to reach into the deep submicron regime [i]. Additionally, Si can be anodized electrochemically or chemically (e.g., in an HF-containing electrolyte) to produce a sponge-like porous layer of silicon, with pore dimensions that range from several microns in width to only a few nanometers [ii]. These properties of Si make it a useful substrate for fabricating sensor platforms, photonic devices and fuel cell electrodes [iii]. 

Figure 7.14 Depth profiles of the 3-to-2 photon ratio for plain silica Xerogel and versions that include tantalum (blue upward triangles) or iron (green downward triangles). In the plain and tantalum case, positronium can move far through the open sponge like network of channels. This is reduced dramatically for the case of embedded iron. The line stems from a fit to extract the escape depth of 880 nm.

Barthelemy P, Ghulinyan M, Gaburro Z, Toninelli C, Pavesi L, Wiersma DS (2007) Optical switching by capillary condensation. Nat Photonics 1(3) 172-175 Billat S, Thonissen M, ArensFischer R, Berger MG, Kruger M, Luth H (1997) Influence of etch stops on the microstructure of porous silicon layers. Thin Solid Films 297(l-2) 22-25 Bisi O, Ossicini S, Pavesi L (2000) Porous silicon a quantum sponge structure for silicon based optoelectronics. Surf Sci Rep 38(1-3) 1-126... 

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