ALL-OPTICAL NMR has been achieved by David Awschalom's research group at the University of California, Santa Barbara. Previous versions of nuclear magnetic resonance (NMR) have relied on radio-frequency electromagnetic fields to tip nuclear magnetic moments. This approach (nearing its 50th anniversary with proven success in medical imaging and chemistry), is modified by the UCSB scientists in the following way. They use a laser to excite a bath of electron spins, which then do all the work. As these electron magnetic moments swarm about the nuclei, their number and direction are controlled by the laser in a way that tips nuclear spins. Nuclei are monitored during the process by a second laser beam, making this an entirely optical approach. Demonstrated in the semiconductor GaAs, this fundamentally different alternative to ordinary NMR offers potentially increased resolution because light can be focused more tightly than RF fields. Moreover, because the UCSB strategy exploits electrons as an intermediary, individual electron orbits themselves might be used to obtain atomic-scale focusing. (For background, see Kikkawa and Awschalom, Science, Jan. 21, /pnu/2000/). One aim of this research is to "imprint" electron spin on the nuclear system within integrated "spintronic" devices, where electron spin supplants charge as a source of information.
