Free-space Interconnects: Diffractive lens contains tiny features

Arrays of lithographically fabricated diffractive elements show promise for coupling light emitters to optical detectors in free-space optical interconnects. One method of creating diffractive elements is to imitate a phase-varying refractive surface by creating subwavelength binary features of varying width etched into a flat surface; for many applications, performance of such diffractive elements improves when the features are made smaller. Researchers at the US Army Research Laboratory (ARL; Adelphi, MD), the California Institute of Technology (Pasadena, CA), and the University of Delaware (Newark, DE) have fabricated and tested a diffractive lens having binary features less than 60 nm wide, making possible high-performance diffractive elements whose performance can approach that of the ideal 2pi continuous-phase Fresnel zone plate.

Each Fresnel zone is made up of many narrow, deeply etched circular structures, thus imitating a smoothly phase-varying refractive material. To make the lens, the researchers coat a fused-silica substrate with a 150-nm layer of gold, sandwiching a 20-nm layer of silicon nitride between the substrate and the gold to improve adhesion. A 70-nm layer of poly(methyl methacrylate) used as photoresist permits patterning by electron-beam lithography. Developing and subsequent ion milling with argon gas creates a patterned gold mask. The substrate is then reactive-ion-etched to a depth of 400 nm. Although the minimum feature width was not directly measured, scaling assumptions give a minimum feature width of approximately 48 nm.

The feature width and depth are a result of a balance between the aspect ratio achievable by the fabrication process, the depth required to give adequate phase range, and the width required for the continuous phase effect. The resulting lens is 36 µm in diameter, has a focal length of 65 µm, and, when focusing 600-nm light, produces a 3-µm-diameter focal spot that agrees with the predicted lens response.