  The third order optical response of bosonic excitons is calculated using the
Green's function solution of the Nonlinear Exciton Equations (NEE) which
establish a quasiparticle-scattering mechanism for optical nonlinearities. Both
time ordered and non ordered forms of the response function which represent
time and frequency domain techniques, respectively, are derived. New components
of the response tensor are predicted for isotropic ensembles of periodic chiral
structures to first order in the optical wavevector. The nonlocal nonlinear
response function is calculated in momentum space, where the finite
exciton-exciton interaction length greatly reduces the computational effort.
Applications are made to coupled anharmonic vibrations in the amide I infrared
band of peptides. Chirally-sensitive and non sensitive signals for alpha
helices and antiparallel beta sheets are compared.
