19 February 2008 Fabrication of variable effective refractive index artificial media
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Optical properties of periodic structures formed at the sub-wavelength scale differ significantly from those of the bulk materials in which these structures are formed. Prior research has shown that periodic structures at the subwavelength scale possess a polarization sensitive artificial effective refractive index. This effective index is dependent upon both the duty cycle for a constant period and the period to wavelength ratio. Artificial diffractive structures have been formed in structured media by spatially varying the duty cycle dependent refractive index variation. In this paper we describe a novel technique for the patterning and fabrication of sub-wavelength structures with the effective refractive index spatially varying across the optic using a combination of additive lithography and analog optics technology that our group has previously developed. A two dimensional grating was formed in the resist by delivering a partial exposure dose and superimposed with an analog intensity profile generated from a phase mask to saturate the resist exposure. The exposure was tailored such that the point of least intensity will still completely expose the photoresist in any of the holes in the array. The local size of the opening created upon development is dependent upon the amount of controlled over-exposure. The optic was then transferred into the desired substrate by dry etching. The exposure process is studied by modeling and diffractive and refractive structures with analog phase functions are demonstrated. The optical response of the fabricated structures as a function of duty cycle variation is analyzed by numerical modeling.
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Pradeep Srinivasan, Pradeep Srinivasan, Zachary Roth, Zachary Roth, Eric G. Johnson, Eric G. Johnson, } "Fabrication of variable effective refractive index artificial media", Proc. SPIE 6883, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics, 68830J (19 February 2008); doi: 10.1117/12.764188; https://doi.org/10.1117/12.764188


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