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High-sensitivity nonlinear spectroscopy using a frequency-offset pump.

We report the development of a simple method for eliminating coherent background noise and fluctuations from saturation-spectroscopy signals. With this technique, which we believe will also be effective in two-photon and other types of nonlinear spectroscopy, we have routinely observed background-free saturation signals in I2 at 514.5 nm with signal-to-noise ratios within an order of magnitude of the fundamental quantum limit.