Patterns in photoresist produced by this simple and inexpensive technique can be transferred to the substrate using RIE or to thin films using typical liftoff procedures. To date, the patterns formed by phase-shifting edge lithography have been used to fabricate several structures: single-crystalline nanowires of Si,176 gold, and silica nanowires on planar33 and nonplanar substrates,32 aperture elements for mid-infrared frequency-selective surfaces,177 and single-crystalline nanowires of silicon.176 In combination with etching, physical vapor deposition, and liftoff, phase-shifting photolithography has been employed by Odom and co-workers for the fabrication of mesoscale pyramids with plasmonic properties,178,179 or combined with controlled undercutting to produce other periodic arrays of structures.103

11.1.3 Femtosecond Laser Writing with a Phase Mask
The small coherence length of the fs laser poses some constraints on the writing of FBGs with a phase mask. First, it is not possible to write FBGs with a standard tunable Talbot interferometer by scanning (see Figs. 3.5, 3.7–3.8), because the coherence length is too short to give a uniform visibility over the length of the grating more than a few microns long. The different wavelength components of the fs pulse diffract at different angles and therefore do not overlap at the same point at the fiber. Second, tuning the interferometer increases the walkoff between the wavelength components and thus cannot produce good visibility fringes. However, the phase mask may be used to scan a grating into a fiber by placing it directly in front of the fiber (see Fig. 3.12 and Fig. 11.8).