THEORY OF NONLINEAR-OPTICAL EXPERIMENTS WITH HARMONIC-OSCILLATORS

We develop a theory of photon-echo phenomena in harmonic vibrational m odes. Although classical harmonic oscillators cannot produce any nonli near optical signal in the linear response limit, we demonstrate that quantum harmonic oscillators that are coupled to any physically reason able bath can give rise to novel nonlinear optical behavior, even in t he perturbative limit. We show that photon echoes in high-frequency vi brational modes are strongly affected by both population relaxation an d pure dephasing. The time dependence of the echo signal is shown to b e highly sensitive to the amount of inhomogeneous broadening in the vi brational line. As an example, we develop the simple model of populati on relaxation resulting from linear coupling to the bath and pure deph asing resulting from quadratic coupling to the bath. Counter to the cl assical picture, echo signal is present when the only coupling to the bath is linear, but absent when the only coupling is quadratic. (C) 19 95 American Institute of Physics.