Synopsis: Terahertz Combs Get Fine Teeth
Frequency combs—light sources whose spectra are made of a series of discreet, equally spaced frequencies—can be used as rulers that measure the light emitted by atoms or molecules with extraordinarily high precision. Most frequency combs work in the visible or infrared, but terahertz combs would allow more precise measurements of rotational and vibrational resonances of molecules and materials. A team led by Geoffrey Blake at the California Institute of Technology, Pasadena, has now demonstrated a terahertz comb that features greater bandwidth and better frequency precision than current technologies.
The most common way to make a frequency comb is through so called “mode-locked” lasers. Such lasers emit a train of short pulses, whose spectrum is a frequency comb. The authors started with an infrared mode-locked laser and used it to excite currents in an “antenna,” which emitted lower frequency terahertz pulses. A second infrared laser detected the electric field of the terahertz pulses by “sensing” how they modified the index of refraction of a crystal in which the two beams co-propagated. Although this approach is not new, the authors found new ways to stabilize the frequencies of the two lasers and minimize noise. As a result, they were able to achieve, over a spectral range extending up to 2.4 terahertz, a frequency precision of a few parts per billion—over two orders of magnitude better than existing schemes for this spectral region.
With their setup, Blake and his co-workers determined the frequency of several rovibrational transitions of water vapor with a precision that was limited only by the molecules’ motion. The researchers plan to use the setup to measure precise reference spectra of molecules, which will help interpret astrophysical spectra measured by the Hershel Space Observatory and the Atacama Large Millimeter Array.
This research is published in Physical Review Letters.