Electron-Spin Echoes at 604 GHz Using Far Infrared Lasers

We have observed electron-spin echoes at 604 GHz in a magnetic field of 21.5 T. This frequency and magnetic field are more than 4 times larger than frequencies and fields at which electron-spin echoes have been reported before. We have used far infrared lasers for the excitation pulses and have employed a heterodyne detection system with a Schottky diode as mixer element and a far infrared laser as local oscillator. For a relatively concentrated nitroxide free-radical sample, we show that the spin-memory time is longer at higher frequencies due to the increased spin polarization at high magnetic fields.