The effects of coherent interference between resonant and nonresonant signals and the effects of absorption on multiresonant four-wave mixing spectra are investigated both theoretically and experimentally in azulene-doped naphthalene crystals at 2 K. Both effects can strongly alter line shapes and intensities. Line splittings and negative spectral features are demonstrated. Coherent interference, although measurable in this system, is weak. Absorption, however, plays an important role, and its effects on phase matching, peak shapes, and peak intensities are predicted theoretically and are confirmed by the experimental measurements. A new expression for the four-wave mixing is derived in terms of sample absorption coefficients and absorption cross sections, and general conditions for maximum efficiency are determined.
Roger J. Carlson and John C. Wright, "Absorption and Coherent Interference Effects in Multiply Resonant Four-Wave Mixing Spectroscopy," Appl. Spectrosc. 43, 1195-1208 (1989)