Guilin Sun
Ansys Employee

As you know, infinite period grating creates diffraction at isolated diffraction angles. So your result is correct.

As for the published result, please check the paper again. I suspect that its incidence is a cone beam, such as from a microscope objective. because of broad incident angles, it may induce continuous  angular distribution. From your grating analysis, the 1st diffraction has diffraciton angle of 30deg. therefore, I guess the incident beam would have about 30 deg cone. Please check.