Development of a Free-Electron Laser for Ultrafast Electron Magnetic Resonance

Technical Abstract

Like nuclear magnetic resonance (NMR), EPR becomes much more powerful at high magnetic fields and frequencies, and in a pulsed rather than continuous wave (cw) modality. The major bottleneck for high-field, high-frequency pulsed EPR has been the absence of electromagnetic sources capable of high frequency (>100 GHz), high power (>1 kW), high long-term frequency-stability, and pulse-programmability. Supported by a previous MRI grant and an award from the W. M. Keck Foundation, the world's first FEL-powered pulsed EPR spectrometer has been demonstrated at UC Santa Barbara. The most dramatic achievement is extremely rapid spin manipulation-spin ½ electrons have been rotated by 90 degrees in 6 ns at 240 GHz, two orders of magnitude faster than the next fastest 240 GHz spectrometer in the world, which is based on a solid-state source.

The major research instrumentation to be developed is a free-electron laser (FEL) that is optimized for electron paramagnetic resonance (EPR) at frequencies between 240 and 500 GHz (corresponding to magnetic fields between 8.5 and 18 T). This development heavily leverages 25 years of infrastructure, investment, institutional commitment, and expertise at UC Santa Barbara. The existing 6 MV electrostatic accelerator will be upgraded and a new free-electron laser (undulator + cavity) will be built. Together, these improvements will increase the peak power available at 240 GHz from 300 W to >10 kW, the repetition rate from 1 Hz to >10 Hz, and also greatly improve the long-term stability and reliability of the system. The new FEL will bring times for 90-degree rotations of spin ½ electrons below 1 ns, enabling resolution of extremely rapid spin relaxation processes. Data acquisition times for pulsed EPR will be reduced by at least a factor of 1000. The new FEL and associated EPR spectrometer will be made available to a national and international user community, and enable transformative studies in materials science, physics, chemistry and molecular biology.

Nontechnical Abstract

The world's brightest source of tunable terahertz radiation will be developed to manipulate electron spins faster than has ever been possible. This ultrafast spin manipulation will enable pathbreaking studies with applications ranging from development of inexpensive solar cells to understanding how protein molecules fit together and move to regulate the flow of energy, information and matter in living organisms.

Electrons and atomic nuclei both have a property called spin, which makes them behave like (very tiny) magnets. In nuclear magnetic resonance (NMR), which is the basis for magnetic resonance imaging (MRI), a strong external magnetic field aligns nuclear spins, while powerful pulses of radio-frequency electromagnetic radiation manipulate nuclei to discover otherwise invisible information about neighboring atoms. Electron paramagnetic resonance (EPR), in a fashion similar to NMR, uses an external magnetic field to align electron spins (rather than nuclear spins). Typically, pulses of microwave-frequency electromagnetic radiation manipulate these electrons to learn about local environments over larger neighborhoods. EPR becomes even more powerful when extremely high-frequency terahertz radiation is used. The free-electron lasers (FELs) at the University of California at Santa Barbara (UCSB) are famous as the world's brightest sources of tunable terahertz radiation. Recently, researchers at UCSB demonstrated that one of the UCSB FELs could be used to rotate electron spins 50 times faster than ever before at .25 terahertz. This project will fund the construction of an even more powerful FEL. The new FEL, which will be used by scientists from all over the nation and world, will be 100 times more powerful than the existing one, and will pulse ten times faster, enabling at least 1000 times more rapid acquisition of experimental data. The EPR spectrometer powered by this new FEL will create an unprecedented capability to observe the structure and ultrafast dynamics of molecules, materials and devices at nanometer length scales.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Charles E. Bouldin
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University of California Santa Barbara
Santa Barbara
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