Constraining New Forces in the Casimir Regime Using the Isoelectronic Technique

We report the first isoelectronic differential force measurements between an Au-coated probe and two Au-coated films, made out of Au and Ge. These measurements, performed at submicron separations using soft microelectromechanical torsional oscillators, eliminate the need for a detailed understanding of the probe-film Casimir interaction. The observed differential signal is directly converted into limits on the parameters {alpha} and {lambda} which characterize Yukawa-like deviations from Newtonian gravity. We find {alpha} < or approx. 10{sup 12} for {lambda}{approx}200 nm, an improvement of {approx}10 over previous limits.