September 7, 2012
Terahertz (THz) radiation occupies a special place in the electromagnetic spectrum, from the shortest microwaves to the longest infrared rays. “Tera” plus “hertz” signifies that about a trillion waves of THz radiation pass a single point in one second. THz waves, like all electromagnetic waves, travel at the speed of light; that means the length of a THz wave is between 0.1 millimeter and 1.0 millimeter.
THz wavelengths are the natural wavelengths of molecular motions. If we could control THz waves, we could use them to spy on vibrating molecules and watch chemical bonds as they form. This makes THz waves important for the study of scientific problems ranging from energy flow in molecules to quantum control of matter.
If we could easily generate THz fields we could tune them to trigger specific atomic motions – and possibly even induce chemical reactions that can be directly observed by the ultrafast X-ray pulses of SLAC’s Linac Coherent Light Source. THz radiation from space brings us the long, slow songs of cold interstellar dust clouds.
However, we have not made as much progress in putting THz radiation to work as we have with other wavelengths of electromagnetic energy. It exists in a technological no-man's land called the "THz gap" between electronic and photonic technologies.
The usual methods of generating and modulating radio waves and microwaves don't work with THz waves, and the lenses and mirrors that control infrared, visible light and shorter wavelengths can't redirect THz waves. Even if they did, THz waves travel mere meters before they’re absorbed by the water in Earth’s atmosphere.
No THz radios or cell phones are awaiting patents, and telescopes must be located in the highest, driest places on Earth, if not off the Earth entirely.
But THz radiation has important information to give us, and we're learning to listen. Two known sources of THz radiation are synchrotrons like the Stanford Synchrotron Radiation Lightsource and free-electron lasers like LCLS – and other sources are being developed, as well as ways to use them.
This week, SLAC hosted the Frontiers of THz Science workshop to explore ways SLAC can contribute to this effort.