Nonlinear optical processes in lower dimensional conjugated structures

Abstract

Electron-electron interactions and electron correlation in effectively reduced spatial dimensions markedly determine the unusually large nonlinear optical properties of conjugated organic and polymer structures. Experimental and theoretical studies have been extended to nonresonant nonlinear optical processes originating from real population of π-electron excited states by direct optical pumping. For qua-si-1-D chainlike structures, the nonresonant second-order and third-order susceptibilities increase by orders of magnitude and may even change sign when one-photon, or two-photon, 7r-electron excited states are populated. Thus, for example, with the fundamental frequency at 1.17 eV (λ = 1.06 µm), the microscopic third harmonic susceptibility γxxxx(−3ω;ω,ω,ω) along the 1-D axis of an eight-site chain is 33,237, and–207 × 10−36 esu when the ground state, first one-photon excited state (11Bu), and first two-photon excited state (21Ag) are, respectively, occupied. Compared to the ground state, excited state nonlinear optical properties exhibit new optical resonances and windows from dc to optical frequencies. Thus, by optical pumping into a designated state within an excited state manifold of a 1-D or 2-D structure, second-and third-order optical properties can be controllably selected to greatly enhance their magnitude, or even change their sign, over desired frequency ranges. In turn, the recent development of new nonlinear optical structures can be broadened and expanded to include nonlinear optical and electrooptic properties originating from real population of not only the ground state but also excited electronic states.