"Nuclear Spin Polarization Transfer with a Single Radio-Frequency Field in Optically Pumped Indium Phosphide", C.A. Michal and R. Tycko, Phys. Rev. Lett. 81, 3988 (1998).
"Optical Pumping of Dipolar Order in a Coupled Nuclear Spin System", R. Tycko, Mol. Phys. 95, 1169 (1998).
"Stray-field NMR Imaging and Wavelength Dependence of Optically-Pumped Nuclear Spin Polarization in InP", C.A. Michal and R. Tycko, Phys. Rev. B 60, 8672 (1999).
"Optical Pumping in Indium Phosphide: 31P NMR Measurements and Potential for Signal Enhancement in Biological Solid State NMR", R. Tycko, Solid State Nucl. Magn. Reson. 11, 1 (1998).
Optical pumping (i.e., continuous excitation of electronic interband transitions) generates extremely large nuclear spin polarizations, and hence extremely large NMR signals, in semiconductors (GaAs, InP) at temperatures below approximately 20 K. The optical pumping mechanism in semiconductors is only partially understood. The large nuclear spin polarizations generated by optical pumping may be useful in enhancing the sensitivity of biological solid state NMR measurements, if the polarization can be transferred from spins in the semiconductor to biological molecules on the semiconductor surface. For such transferred optical pumping (TOP) experiments, spin-1/2 nuclei are preferred, hence our interest in indium phosphide.
In the course of investigating optical pumping in InP, we noticed an unprecedented photophysical effect. 31P NMR signals excited by a long, weak rf pulse after an optical pumping period were stronger than signals excited by a short p/2 pulse. Normally, a short p/2 pulse should create the largest possible transverse nuclear magnetization from spin polarization and therefore create the largest signals. Subsequent double-resonance experiments showed that the 31P NMR signals after a long, weak pulse originated from optically-pumped dipolar order among 115In spins. This result is the first indication that optical pumping can generate nuclear spin dipolar order, in addition to nuclear spin polarization. It is also the first case in which polarization transfer from one spin species (115In) to another (31P) is accomplished with a single radio-frequency field.