In the first successful experiment of its
type at SLAC National Accelerator Laboratory’s Linac Coherent Light Source,
scientists used terahertz frequencies of light to change the magnetic state of
a sample and then measured those changes with ultrafast pulses from a powerful
X-ray laser.
Invisible to human eyes, terahertz
describes a band of frequencies between microwave and infrared light. These
frequencies are alluring to scientists because they can be used to control and
study magnetic and electric states in materials, and have been applied to
fields ranging from data storage to biological imaging and explosives
detection. They provide an atomic-scale window into fundamental processes such
as magnetism, molecular motion and protein vibrations.
But observations in the terahertz range
were until recently largely out of reach for scientists, said Matthias Hoffman,
a SLAC scientist specializing in terahertz laser research who worked on the
latest experiments. “There were no real efficient sources and detectors.
It was relatively difficult to do science” at terahertz frequencies, he
said.
The experiment in July (2012) involved a
technique called pump-probe in which one laser, the “pump,” is used
to stimulate changes in the sample—in this case a material with exotic magnetic
properties—while the X-ray laser probes these changes.
A team led by Urs Staub of the Paul
Scherrer Institute in Switzerland and Steven Johnson of the Institute for
Quantum Electronics at ETH Zurich in Switzerland generated the terahertz laser
pulses by aiming an infrared laser beam at a specialized crystal. The passage
through the crystal changed the frequency of light from near-infrared to
terahertz light. The terahertz pulses then hit a sample, and the researchers
measured changes in the sample using closely synchronized pulses from the LCLS
X-ray laser.
The crystal they used was a special type of
thin organic crystal known by the acronym DAST, grown by a private company in
Switzerland. DAST crystals tend to be more fragile and susceptible to damage
than some non-organic crystals designed for terahertz conversion, Hoffmann
noted. He is working with the LCLS laser group to develop better sources of
intense terahertz pulses as a regular option for LCLS users conducting
pump-probe experiments.
Hoffman said another possible way to
generate terahertz frequencies is with the electron beams that power advanced
synchrotrons and LCLS. “This can produce even higher pulse energies,”
he said, but has proven more challenging for use in experiments.