We live in a nonlinear world. While linearization beautifies physics, nonlinearity provides excitement in physics. In recent years, significant advances have been made in the field of nonlinear optics and optical computing. These areas have grown rapidly in the last decade, so that today these have emerged as important disciplines in their own right. Optics is already preferred for many applications owing to its wide bandwidth and autonomy from electromagnetic interference. Since fiber optic cable has low absorption, they can transmit signals for several kilometers, but the signal must be amplified. It is more efficient if the light beam is amplified directly by a laser beam in a suitable medium. Such a process has come into the realm of nonlinear optics. The science that deals with the interactions of a laser beam with an optical medium is called nonlinear optics and the medium itself is called a non-linear optical (NLO) material.
Since the invention of the first laser 40 years ago, the frequency conversion of laser radiation in nonlinear optical crystals has become an important technique widely used in quantum electronics and laser physics for solving various scientific and engineering problems. The fundamental physics of the wave light interactions in nonlinear optical crystals is now largely understood.