Nonlinear optics and spectroscopy

The development of masers and lasers has been reviewed in the 1964 Nobel lectures by Townes (1) and by Basov (2) and Prokhorov (3). They have sketched the evolution of the laser from their predecessors, the microwave beam and solid state masers. Lasers are sources of coherent light, characterized by a high degree of monochromaticity, high directionality and high intensity or brightness. To illustrate this last property, consider a small ruby laser with an active volume of one 1 cc. In the Q-switched mode it can emit about l0 18 photons at 694 nm wavelength in about l0-8 sec. Because the beam is diffraction limited, it can readily be focused onto an area of l0-6c m2, about ten optical wavelengths in diameter. The resulting peak flux density is l0 13 watts/cm*. Whereas 0.1 Joule is a small amount of energy, equal to that consumed by a 100 watt light bulb, or to the heat produced by a human body, each one-thousandth of a second, the power flux density of 10 terawatts/cm2 is awesome. It can be grasped by noting that the total power produced by all electric generating stations on earth is about one terawatt. (The affix "tera " is derived from the Gree