Nonlinear narrow optical resonances induced by laser radiation

Abstract

THE ATTAINMENT of narrow and frequency stable resonances in the absorption or emission spectra of any substance in various regions of the electromagnetic spectrum is of great importance in physics. Each discovery in this direction increases considerably the accuracy of many physical experiments and gives rise to numerous applications in different fields of science. Two classic examples can be used as illustrations. The decade 194@50 saw the development of the use of atomic and molecular beams for observing narrow resonances in the radiofrequency region. In one example of this (Ramsey ) (181 two spatially separated electromagnetic fields interacting with an atomic beam of cesium were used to measure the frequencies of the transitions between the hyperfine levels of the ground state of Cs. Resonances with a width Av = 50Hz at a frequency v0 = 9.3 GHz (Bechler et ~1.‘~~‘) were obtained. This corresponds to a relative width Av/u, = 5 x 10e9 or a quality factor Q of 2 x 108. Such narrow radiofrequency resonances have provided the basis of atomic frequency standards and for the time scale which has now been internationally adopted. Extremely narrow resonances in a higher frequency region, in nuclear transitions, have been observed by Mossbauer. (176) For example, in a y-ray transition in 67Zn with a quantum energy of 93 keV it is possible to obtain a resonance with a quality factor of 2 x 10”. The narrow resonances of recoilless nuclear transitions in a crystal lattice give at the present time the highest resolution of any physical experiments, of the order of 10-i’. Until recently the resonance width in the optical region of the spectrum has been no better than 10-6. In the last few years a number of effects have been discovered in the resonant nonlinear interaction of a light field with atgmic and molecular gases. These effects have produced a narrowing of the optical resonance width of at least 103-lo4 times. To illustrate this, Fig. 1 gives some values of the relative width for the sharpest resonances of quantum transitions in the microwave, optical and y-ray regions of the spectrum. These discoveries gave impetus to the development of nonlinear optical spectroscopy with superhigh resolution and high precision. The purpose of this paper is to present successively and from one single point of view the resonant effects which occur in the saturated gas absorption of laser radiation, which underlie the methods for obtaining narrow nonlinear resonances in the optical region. As is well known, the motion of the gas particles shifts the spectral line centre of the emission a(w) of an individual particle (atom or molecule) by a value which depends on the component of the atomic velocity u in the direction of observation n: