χ(3) Processes

This chapter extends the discussions of optical effects to those due to third-order susceptibilities X⁽³⁾. The mechanism for the X⁽³⁾ nonlinearity is attributed to distortions in the electron cloud surrounding an atom or molecule. The chapter investigates X⁽³⁾ processes that are electronic in origin. The corresponding nonlinear phase shifts lead to phenomena such as self-focusing, modulation instability, and solitons. The chapter then introduces the nonlinear polarization for X⁽³⁾ processes, which are proportional to the cube of the input field. It also shows how a X⁽³⁾ material can be used in a resonator geometry to make devices that can have two 240 (or more) stable output intensities for the same input intensity; a phenomenon called optical bistability. Finally, the chapter discusses degenerate four-wave mixing (DFWM) and a technique called z-scan, which is a method to measure the real and imaginary coefficients of the X⁽³⁾nonlinearity of a medium.