Optical-pumping effects on Raman-heterodyne-detected multipulse rf nuclear-spin-echo decay

Raman-heterodyne-detected multipulse rf nuclear-spin-echo decay of Pr³⁺ in YAlO₃ is measured with continuous-detection light throughout the spin-locking pulse train and with the light on during a short period centered at the echo time only for every sixth echo. The echo decay shows three components: an initial fast decay ({}400 {μ}s), an intermediate decay (5.4±{}0.6 ms), and a slower decay of 22.5±{}1.7 ms for gated echoes. For cw light, the decay is described by the same initial fast decay, but a single decay at 7.6±{}0.6 ms is observed at longer times. The difference in the observed echo decay is attributed to optical pumping effects. A rate-equation model is constructed, consisting of three ground-state levels and three excited-state levels. It includes radiative decay, spin-lattice relaxation, the optical pumping and rf pulses, and rf coherence loss due to excited-state occupancy. The model shows three decay times, fast ({}400 {μ}s), intermediate (6.08±{}0.07 ms), and slow (21.0±{}0.1 ms) decay for gated echoes. The last two rates are affected by the strength and duration of the optical field. For continuous light, the calculated echo is seen to decay at {}400 {μ}s, 2.04±{}0.07 ms, and 3.43±{}0.02 ms and then increases at a much slower rate (100 ms). This is only an apparent increase, because significant signal cancellation occurs in the sum of the six ion groups contributing to the signal. If a reduced light level and faster spin-lattice relaxation rates are assumed, the calculated echo-decay times for cw light are 2.44±{}0.03 and 7.37±{}0.04 ms. This corresponds to a homogeneous linewidth increase of 2.5 times and a spin-lattice relaxation rate increase of 2.5 times equivalent to crystal heating by the cw detecting light of 0.35 K over that of the gated light.