A system is disclosed for detecting a nonlinear coherent field induced in a sample. The system includes optics, a modulation system, and a detector system. The optics are for directing a first electromagnetic field at a first frequency .omega..sub.1 and a second electromagnetic field at a second frequency .omega..sub.2 toward a focal volume such that a difference frequency .omega..sub.1 .omega..sub.2 is resonant with a vibrational frequency of a sample in the focal volume. The modulation system is for modulating the difference frequency .omega..sub.1 .omega..sub.2 such that the difference frequency .omega..sub.1 .omega..sub.2 is tuned in and out of the vibrational frequency of the sample of at a modulation frequency. The detector system is for detecting an optical field that is generated through non-linear interaction of .omega..sub.1 and .omega..sub.2 and the sample responsive to the modulation frequency.