Limits of applicability of the Kramers-Kronig relations in the presence ofan additional light wave

We analyze the possibility of applying the Kramers-Kronig relations to medi a with substantial spatial diffusion of the permittivity, in which case add itional light waves of the Pekar type emerge. The calculations are done wit hin a broad range of values of the exciton damping constant Gamma with and without a "dead layer" at the crystal surface. We establish the condition i n which the use of the Kramers-Kronig relations in calculating the optical constants of the substance yields correct results. (C) 1998 American Instit ute of Physics. [S1063-7761(98)01610-2].