Electron spin ensemble strongly coupled to a three-dimensional microwave cavity

We demonstrate the strong coupling between an electron spin ensemble and a three-dimensional cavity in a reflection geometry. We also find that an anticrossing in the cavity/spin spectrum can be observed under conditions that the collective coupling strength g(c) is smaller than the spin linewidth gamma(s) or the cavity linewidth. We identify a ratio of g(c) to gamma(s) (g(c)/gamma(s)>0.64) as a condition to observe a splitting in the cavity frequency. Finally, we confirm that g(c) scales with root N, where N is the number of polarized spins. (C) 2011 American Institute of Physics. [doi:10.1063/1.3601930]