29.5 A Portable 14GHz Dual-Mode Pulse and Continuous-Wave Electron Paramagnetic Resonance Spectrometer Using a Subharmonic Direct Conversion Receiver

Electron paramagnetic resonance (EPR) is a powerful spectroscopic technique for direct detection and characterization of free radicals, with high sensitivity and specificity suited for a wide range of uses, including biomedical applications such as site-directed spin labeling and potential applications in quantum technology such as spin qubit control. Unpaired electrons within a static magnetic field B0 undergo splitting in energy levels (Zeeman effect), and energy transitions may be excited by applying an oscillating field B1 with a frequency $\omega_{0}$ determined by B0. In continuous-wave (CW) experiments, B0 or $\omega_{0}$ is swept, typically with frequency or field modulation to enhance sensitivity. In pulse experiments, B1 pulses are applied to manipulate the electron spins, and the transient evolution of the magnetization is detected. These methods are complementary: CW EPR offers greater sensitivity while pulse EPR enables more detailed characterization of the coherence and relaxation of the spin system.