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 l0c m, 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/cm 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 Greek ‘C&@CY.~ = monstrosity, not from the Latin “terra”!) Indeed, from Poynting’s vector it follows that the light amplitude at the focal spot would reach l0 volts/cm, comparable to the electric field internal to the atoms and molecules responsible for the binding of valence electrons. These are literally pulled out of their orbits in multiphoton tunneling processes, and any material will be converted to a highly ionized dense plasma at these flux densities. It is clear that the familiar notion of a linear optical response with a constant index of refraction, i.e., an induced polarization proportional to the amplitude of the light field, should be dropped already at much less extreme intensities. There is a nonlinearity in the constitutive relationship which may be expanded in terms of a power series in the electric field components.