Study of the near-field modulation property of microwaviness on a KH2PO4 crystal surface

The KH$_2$PO$_4$ crystal is a key component in optical systems of inertialconfinement fusion (ICF). The microwaviness on a KH$_2$PO$_4$ crystalsurface is strongly related to its damage threshold which is a key parameterfor application. To study the laser induced damage mechanism caused bymicrowaviness, in this paper the near-field modulation properties ofmicrowaviness to the incident wave are discussed by the Fourier modalmethod. Research results indicate that the microwaviness on the machinedsurface will distort the incident wave and thus lead to non-uniformdistribution of the light intensity inside the crystal; in a common range ofmicrowaviness amplitude, the light intensity modulation degree increasesabout 0.03 whenever the microwaviness amplitude increases 10~nm; 1order diffraction efficiencies are the key factors responsible for lightintensity modulation inside the crystal; the light intensity modulation isjust around the microwaviness in the form of an evanescent wave, not inside thecrystal when the microwaviness period is below 0.712~$mu $m; light intensitymodulation degree has two extreme points in microwaviness periods of1.064~$mu $m and 1.6~$mu $m, remains unchanged between periods of 3~$mu $m and 150~$mu$m, and descends above the period of 150~$mu $m to 920~$mu $m.