S. D. Gedney, "An anistropic PML absorbing media for the FDTD simulation of fields in lossy and dispersive media," electromagnetics, vol. 16, pp. 399 - 415, 1996
Unfortunately I do not have that paper. But have you had a look at some of Gedney's later
papers on the CPML. For dispersive media CPML is easier to set up than PML.