Microwave cavity resonance spectroscopy of ultracold plasmas

We present an in situ study of the electron dynamics of an ultracold Rb85 plasma using microwave cavity resonance spectroscopy with submicrosecond temporal resolution and vastly improved sensitivity. We observe a three times faster plasma decay than predicted by a self-similar Gaussian expansion model, which might be attributed to collisional microwave heating and plasma-wall interactions. The demonstrated sensitivity enables nondestructive probing of other (nearly) ionized media of fundamental importance as well, such as cold Rydberg gases and antihydrogen plasmas.