Observation of Modulation-Induced Feshbach Resonance

In this Letter, we observe a novel resonant mechanism, namely, the modulation-induced Feshbach resonance. By applying a far-detuned laser to the cesium D_{2} transition with intensity modulation, we periodically modulate the energy levels of atomic collisional states. This periodic modulation connects the free-scattering states to shallow molecular states. At specific frequencies, significant atom loss is observed, which corresponds to the resonant coupling between these two types of states. This precisely corresponds to a form of Feshbach resonance, yet in the frequency domain rather than the magnetic-field domain. Using this method, we can directly scan the energy spectrum of molecular bound states without synthesizing any molecules. In addition to these bound states, we can also probe the molecular states embedded in the continuum, which are typically very difficult to detect by the conventional methods based on molecular synthesis. Moreover, by using a far-detuned laser instead of a magnetic field, it enables spatially dependent control over atomic interactions, coupling multiple levels simultaneously and inducing new Feshbach resonances for those atoms that do not have conventional magnetic resonances. Therefore, we believe that this new resonant mechanism offers new opportunities for controlling atomic and molecular interactions in quantum simulations.