Strong pulsed excitations using circularly polarized fields for ultra-low field NMR.
A pulse, which is produced by a single coil and thereby has a linear polarization, cannot coherently drive nuclear spins if the pulse is stronger than the static field B0. The inaccuracy of the pulse, which arises from the failure of the rotating wave approximation, has been an obstacle in adopting multiple pulse techniques in ultra-low field NMR where B0 is less than a few μT. Here, we show that such a limitation can be overcome by applying pulses of circular polarization using two orthogonal coils. The sinusoidal nutation of the nuclear spins was experimentally obtained, which indicates that coherent and precise controls of the nuclear spins can be achieved with circularly polarized pulses. Additional demonstration of the Carl-Purcell-Meiboom-Gill sequence verifies the feasibility of adopting multiple pulse sequences to ultra-low field NMR studies.
Center for Brain and Cognition Measurement, Korea Research Institute of Standards and Science, Daejeon 350-340, Republic of Korea. Electronic address: jhshim@kriss.re.kr.
Jeong Hyun
JH
Shim
Center for Brain and Cognition Measurement, Korea Research Institute of Standards and Science, Daejeon 350-340, Republic of Korea.
Seong-Joo
SJ
Lee
Center for Brain and Cognition Measurement, Korea Research Institute of Standards and Science, Daejeon 350-340, Republic of Korea.
Kwon-Kyu
KK
Yu
Center for Brain and Cognition Measurement, Korea Research Institute of Standards and Science, Daejeon 350-340, Republic of Korea.
Seong-Min
SM
Hwang
Center for Brain and Cognition Measurement, Korea Research Institute of Standards and Science, Daejeon 350-340, Republic of Korea.
Kiwoong
K
Kim
Bloch–Siegert effect; Circular polarization; Nutation; SQUID detection; Ultra-low field NMR