Probing ultracold collisional dynamics with frequency-chirped pulses

We demonstrate a new type of a cavity-enhanced optical trap whose standing-wave potential well can be transformed by using phase modulation. When the modulation frequency is matched to the free spectral range of the cavity, the potential well can be changed continuously from a corrugated form of a standing wave to a flattened form of a traveling wave depending on the modulation index. With the phase modulation, more atoms are trapped due to the increase in the trap volume, and there is a pronounced increase by a factor of 1.9 when an optimal modulation is applied. From the increase in number of trapped atoms we also infer that the temperature associated with longitudinal motion is strongly affected by the transformation of the potential well. This technique can be used to study collective behavior of trapped atoms and to manipulate atomic distribution for quantum information processing.