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As mentioned previously, even if an isotropic index n = -1 can be defined for the lens, it is not the case for the surface impedance which varies with incidence. As shown for the reference lens, the spot size mainly results from three contributions, three beams, at -45°, 0° and 45°, issued from the input interface which spread in the lens during their propagation, are negatively refracted at the second interface and form the final image. Quantitatively, the final spot width strongly depends on the diverging character of these light beams within the lens which tends to degrade resolution performances.

26 can be extracted for the fundamental guided mode. 15. 16. 15(a) Two-dimensional Dielectric Photonic Crystals 29 The “membrane approach” is more common in silicon technology. Ideally, it consists of a Si slab, properly dimensioned, surrounded by air on both sides. It is obviously the highest index contrast achievable… However, at infrared or telecommunications wavelengths, the resulting thickness of the Si slab (the “membrane”) is such that it becomes inconceivable to fabricate large enough surfaces to fulfill the size requirement of usable prototypes (a few mm2  the reasoning is also true for III-V semiconductor membranes).

In the second step, a 300 nm-thick positive resist (ZEP520A) is deposited by spin coating throughout the wafer (g). E-beam lithography is used to define the photonic crystal pattern (h), and after development (i), the SiO2 mask is etched to define the final hard mask (j). 18. Schematic top view of a photonic crystal-based device including strip circuitry in silicon technology and SEM views of fabricated samples (courtesy of IEF) After a second cleaning process to remove any trace of resist (k), the silicon film is finally etched through a SF6/O2 anisotropic process (l) to transfer the desired pattern in silicon.