Microwave EPR self-induced transparency

Abstract

The waveguide propagation of a microwave magnetic field pulse at electron paramagnetic resonance (EPR) with a free radical sample of electron spins distributed along the guide displays the phenomenon of self-induced transparency (SIT). Predictions of plane-wave optical SIT are modified by summation of the magnetic-moment-microwave-field coupling over the transverse waveguide mode for specific sample geometries. Mode modified pulse area theorems are presented. Observed output pulse areas and shapes are compared with computer predictions. After pulse propagation over several absorption lengths a spontaneous zero-area pulse propagation effect is observed for a short weak input pulse with spectral width comparable to or larger than the spin-resonance linewidth. Alternating 180\ifmmode^\circ\else\textdegree\fi{} phase shifts in the pulse carrier wave, accounting for zero area, signify magnetic dipole transitions which alternate between coherent absorption and emission to some extent, and therefore anomalous pulse propagation is observed. For pulses applied off-resonance, linear and nonlinear shifts of the average carrier frequency occur toward the center of the spin resonance.