Figure 1.

Ray optics analogy for the three-pulse stimulated photon echo experiment. The rays represent the phase evolution of the quantum system. (a) After the initial pulse (τ = 0), the rays fan out with slopes determined by the value of the offset from the mean frequency of individual members of the inhomogeneous distribution. The second pulse (first lens) collimates the rays by converting the superposition into a population state. The third pulse (second lens) refocuses the rays by converting the population state to the Hermitian conjugate of the first superposition state. The echo intensity is proportional to the square of the field amplitude and thus depends on the amount of constructive interference generated by the third (rephasing) pulse. For a wide inhomogeneous distribution, the constructive interference is restricted to a very short time interval at around t = τ, depicted by the sharpness of focus produced by the second lens. (b) The disruption of the smooth phase evolution during the population period, T, and during the two coherence periods (τ and t) leads to a loss of refocusing ability. Thus, by recording the photon echo as a function of the population period, the fluctuations in the inhomogeneous distribution (“spectral diffusion”) can be followed.