Strong coupling of electron spin ensemble to the eigenmodes of a high-Q dielectric resonator

We have fabricated a compact dielectric resonator set-up on a custom printed-circuit board, which contains a microwave loop antenna coupled to a high-Q cylindrical dielectric resonator. The antenna is designed to operate with a resonant frequency matching that of the dielectric resonator for strong field confinement. We also simulated the combined on-chip system using finite element analysis (HFSS) to visualize the magnetic field profiles and reflection coefficients as a function of frequency. While the internal quality factor is around 8000 for the resonator (at 4.6 GHz), it is deliberately over-coupled to the microwave fields to provide a fast detection bandwidth of about 50 MHz. The setup has been tested using a thin layer of 2,2-diphenyl-1-picrylhydrazyl (DPPH), a radical commonly used for electron spin resonance calibration. A strong electron spin resonance signal was detected from the microwave power reflection at both room temperatures and cryogenic temperatures. We have observed strong coupling of the resonant mode of the electron spin ensemble to different eigenmodes of the dielectric resonator at room temperature. The anti-crossing of the two modes shows a coupling energy of about 50 MHz. Such photon-spin coupling can potentially be useful, as it has been proposed that electron spin ensemble can be used as a medium for quantum memory. Results of the coupling study for different concentrations of the spin ensemble will be reported.