Fast Photorefractivity and Energy and Momentum Transfer in Dense, Two-Level Media
B. A. Samson, Wojciech Gawlik
polymers, etc.). \-Vit.hin (\. t.wo-Ievel model we dellloll sll'aL<' creat.ion of t.he p()Luia.t.ioll gratillg that. is phase- shifted tda.iv 10 lite illeident fi,·It!. Tit pltase sitifl is duc 10 I.lte nonlinearity arising fr0 1ll lite local-field effects. Precisely I he sall rca.lttI'(' of I.ite phase-shined polari,alioll respons(, is cltaraclerist.ic for Ihe photomftacliv(' 1I1<'<lia. We int.crprcl, therefore, this effecl. as the IOeILl jidd-ind1tced lJ/lOtorcjmciilJ ily. It is nol,cwortity I.hal it can occur in any two-level mcdilltn and, most interestingly, it is a vcry fasl. phenomenon with response time not limiled hy (,harge l'cal'l'angement in st.!ldard phot.ordract.ivc materials. The appropriate pha.,� shift. between incident field and polarization grating r esul ts in unidirectional energy jmolllcntufli ,'xchange bciween the coupled beams and possible ampliGcation of one of them. We take the net light Geld in t.he IIwoium as omposed of two wave s The small frequency shift < « -: (: beillg t.he lincwidth) causes the movement of the i,t· t.erference pattern which eliminates all uncertailli(.y in a spatial distribution of the atomi(' variables whose dependence on IIIl' lighl. intensit.y is bistable under conditions of lOB. Thi, purpose can be achieved also by a slow f(·la.l.ive motion of the medium with resped (0 (.h(' ill tel-ference pattern and the polarization grating (2). Similarly as in Ref.(2J, we find t.he appcarence of a phase-shifted contribution to the polarizat.ion grating that is absent for a low· density medium. This phase-shifted component is responsible for the unidirectional energy flow determined by the sign of E, exactly in the same way as in the standard phol.orcfracl.iv ity.
•Jan 1, 1990•0 citations