Cascaded self-induced holography can produce submicron gratings
Interference patterns produced by selected diffractive orders from cascaded gratings can yield high-resolution gratings for various optoelectronic applications. Yu-Hwa Lo and his research group at Cornell University (Ithaca, NY) have fabricated gratings with submicron periods starting with lithographically produced masks. Leader Chih-Hsien Lin says the experimental setup consists of a laser-illuminated glass substrate bearing diffraction gratings on the upper and lower surfaces. The first-order diffracted beams produced by light on the upper grating propagate through the glass plate to the lower grating. The second-order beams from the lower grating then interfere to produce a high-resolution pattern. A central opaque region on the glass plate acts as a spatial filter, creating an image plane where only the second-order diffracted beams from the lower grating interfere to form a pattern with submicron periodicity. The method yields a grating with a period smaller than that of the mask: two cascaded gratings yield a fourfold reduction; four cascaded gratings yield an eightfold reduction.
The group exposed a photoresist-coated indium phosphide substrate by illuminating a glass mask having 2-µm-period gratings with 457.9-nm argon-ion laser light, and a combination of ion milling and wet chemical etch transferred the resulting 0.5-µm pattern to the substrate. By varying the grating period within a mask, the researchers also have fabricated a four-part grating for a four-channel, wavelength-division-multiplexing, distributed-feedback laser array with grating periods varying between channels by 6 Å or less, corresponding to a wavelength spacing of 2 nm.