Coherent Anti-Stokes Raman Spectroscopy - ACS Publications

the laser, as it is now called, ushered in a new era. The laser int roduced scores of ex- citing fields of endeavor that affect physics, chemistry, bi...
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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