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Fabricate planar photonic crystal gradient index lens by laser interference lithography

Four-beam laser interference lithography (LIL) was shown to be a good technology to generate graded-index photonic crystals with a square lattice. Characters of the structure vary regularly with the incident angles and relative thresholds. The finite-difference time-domain method was used to validate the focusing behavior of the resultant structure. High intensity enhancement and sub-wavelength focusing were achieved simultaneously. The results suggest to us that LIL is a new low-cost and high-efficiency way of fabricating planar lenses.