Electron spin hyperpolarization via radiative cooling

Electron spin resonance (ESR) spectroscopy is widely employed for the detection and characterization of paramagnetic species and their magnetic and chemical environment. In ESR the spins precessing around an applied static... [ view full abstract ]

Electron spin resonance (ESR) spectroscopy is widely employed for the detection and characterization of paramagnetic species and their magnetic and chemical environment. In ESR the spins precessing around an applied static magnetic field are first excited by microwaves and subsequently emit a signal into an inductively coupled resonant cavity. A high degree of polarization is essential to maximize the signal. Here, we are interested in increasing the polarization beyond thermal equilibrium. In the present work we give a proof of principle of a new universal hyperpolarization scheme based on the coupling of the spins to a colder electromagnetic bath via Purcell-enhanced radiative relaxation.

For spins in solids, radiative relaxation is completely negligible. The dominant process is relaxation to phonons. However, by coupling the spin ensemble to a high Q resonant cavity with a small mode volume, the spontaneous emission rate can be enhanced up to the point where the radiative relaxation dominates. This enhancement is called Purcell effect. In our experiment we demonstrate Purcell enhanced thermalization of a spin ensemble to a colder radiation bath, while the crystal lattice remains at higher temperature.

The spin system under study is an ensemble of bismuth donors implanted into a host silicon crystal. The spins are inductively coupled to a high Q superconducting niobium resonator. The silicon crystal is installed at the 800 mK stage of a dilution cryostat while the resonator is coupled via a switch either to a 10 mK or to a 800 mK thermal source. When the switch is connected to the colder black body, the electronic spins thermalize to 10 mK via radiative relaxation while the silicon crystal remains at 800 mK.

An increase of the ESR signal by a factor 2 compared to the case without hyperpolarization is observed. An independent proof of the radiative cooling is given by the increase of the spin Purcell relaxation time by the same factor. The magnitude of the observed effect is smaller than the predicted factor of 4.5. This is caused by losses and reflections in the circuit between the resonator and the cold thermal source.