Transmission blazed-binary gratings for visible light operation: performances and interferometric characterization

We present experimental results obtained for blazed-binary gratings with periods between 1.6lambda and 114.7lambda blazed at lambda = 633 nm. These gratings are composed of subwavelength pillars with pulse-width modulation within the periods. For small periods up to 3lambda, the measured efficiencies agree well with the theoretical predictions. But for large periods, the discrepancy between the theoretical and experimental efficiencies is significant, reaching almost 20% for the grating with the largest period. In order to identify the origin of this discrepancy, a Mach-Zehnder interferometer dedicated to the characterization of small-size gratings (approximate to400 x 400 mum(2)) is set up. The interferometric characterization combined with scanning-electron-microscope observations reveal that the, 20% discrepancy is due to a deficiency of the fabrication process for the smallest pillars and to an overall fabrication of pillars with actual widths smaller than their nominal values. These two fabrication errors can reasonably be expected to be eliminated in future work.