Wavelength variation of the second-order nonlinear coefficients of KNbO 3 , KTiOPO 4 , KTiOAsO 4 , LiNbO 3 , LiIO 3 , - BaB 2 O 4 , KH 2 PO 4 , and LiB 3 O 5 crystals: a test of Miller wavelength scaling