Microwave cavity resonance spectroscopy of ultracold plasmas

We present an in situ study of the electron dynamics of an ultracold ${}^{85}\mathrm{Rb}$ 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.

Figure 1

Experimental setup showing the microwave cavity with diffraction grating chip.

Figure 2

Blue curve: measured microwave cavity reflection voltage $V_r$ as a function of applied frequency $\omega$, with curve fit according to Eq. (3). Red curve: measured and calculated change in reflection voltage $\mathrm{\Delta }{V}_r$ right after the moment the ionization pulse creates the plasma.

Figure 3

Top: microwave cavity reflection voltage $V_r$ as a function of time $t$ for different applied frequencies to the cavity. Bottom: photodiode signal $V_{\text{pd}}$, showing that at $t=t_0=10\mu \mathrm{s}$ the ionization pulse enters the cavity and creates the plasma.

Figure 4

Field-averaged electron density as a function of time.