Coherent Transients

Abstract

Chapter 8 on saturation spectroscopy treats the interaction of two and three-level systems with simple time varying fields consisting of the Fourier superposition of two or three cw modes. The response of the medium depends heavily on the decay times of the medium and thereby provides ways of measuring these times. Similarly Chap. 10 considers time-varying fields in cavities, typically given by the superposition of modes. In this chapter we consider the response of atoms to time varying fields consisting of either short pulses or step functions that prepare the medium in special ways. In such situations, we find it convenient to follow the time-varying atomic (or molecular) response using the coupled Maxwell-Bloch equations developed in Secs. 4-3 and 5-1 without a modal decomposition of the field. Since the induced atomic polarization is initially coherent, the situations we consider are categorized as “coherent transients.” As the induced polarization decays due to various line broadening mechanisms, the coherent response is lost. The time taken for this decay is typically directly related to the dipole (T ₂) and population (T ₁) decay times as well as to the width of the inhomogeneous broadening. Hence coherent transients provide alternative ways to measure these basic decay times. While saturation spectroscopy works in the frequency domain, coherent transients work in the time domain. Some media are equally easily studied in either domain, but often one domain is more convenient than the other.