Laminar versus trapezoidal grating profiles: AFM measurements and efficiency simulations

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Abstract
Laminar groove profiles promise a high suppression factor for the even orders of reflection gratings. For normal incidence mounts the theoretical suppression of the second order should be nearly perfect if the groove to land ratio of 1 : 1 is matched. Experimental experience with laminar gratings demonstrates that the suppression of the second order is not as good as expected from calculations. Once we understood that the behaviour of the second order contribution cannot be explained in terms of a mismatch of the groove to land ratio alone, we started to investigate the characteristics of laminar groove profiles in more detail by means of atomic force microscopy. The main discrepancy between the idealised laminar profile and the measured profiles is the limited aspect angle of the edges of the grooves: the profiles are rather trapezoidal than laminar. Simulating the diffraction efficiency of the measured trapezoidal profiles revealed that the shallow aspect angle has a significant influence on the higher order diffraction efficiencies. Optimising groove profile parameters under assumption of a realistic aspect angle yielded essentially different values than the same procedure for idealised laminar profiles.

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