Instrumentation
Α. Β. Harvey Chemical Diagnostics Branch Naval Research Laboratory Washington, D.C. 20375
Coherent Anti-Stokes Raman Spectroscopy
Nearly two decades ago the discov ery of the optical maser or the laser, as it is now called, ushered in a new era. T h e laser introduced scores of ex citing fields of endeavor that affect physics, chemistry, biology, and engi neering. One such field is nonlinear optics, which arose almost immediate ly with the advent of the laser and grew very rapidly in the years to fol low (/). As we shall soon see, the fields of nonlinear optics and nonlinear spectroscopy are dependent on lasers because of the high electric field strengths available from these intense sources. In Figure 1 we depict how a medium is polarized or how a dipole moment is induced in a medium in the pres ence of high electric fields of a laser beam. In its most simplistic mathe matical form, the polarization can be expressed as an expansion in the field strength as follows: Ρ = aE + bE2 + cE3 + . . .
(1)
where Ρ is the polarization, Ε is the field strength, a is the linear coeffi cient, and b, c, etc., are the nonlinear coefficients. It is readily seen t h a t if a > b > c > . . . , then at low fields, terms in Equation 1 beyond the first are negligibly small, and we are left with the linear term t h a t characterizes ordinary linear optics. However, at greater and greater field strengths the higher-order or nonlinear terms begin to contribute significantly to the over all polarization. Let us now consider the first nonlinear term, which is de pendent on the square of the electric field strength: P
