Frequency-Comb-Assisted Terahertz Quantum Cascade Laser Spectroscopy
Published April 9, 2014
Trace-gas
sensing with high sensitivity and precision in the terahertz regime can be
important in environmental monitoring, security, and astrophysics, as well as
in tests of fundamental physics. Now, as reported in Physical Review X, a research team has
performed the first terahertz spectroscopic measurements using a so-called
frequency comb—a technique that allows frequency measurements with extremely
high accuracy. As a proof-of-principle, the team measured a rotational
transition in a gas molecule (methanol) to a precision of 4 parts in one billion, 10 times better than the
previous record. The result is also twice as precise as the theoretically
predicted frequency, suggesting the technique could help refine theoretical
models.
Saverio
Bartalini of the Italian National Institute of Optics (INO-CNR) and the
European Laboratory for Non-linear Spectroscopy (LENS) and his colleagues have
taken a terahertz system they previously developed and used it for spectroscopy.
The researchers focused near-infrared laser pulses into a nonlinear crystal to
produce a terahertz comb—a single beam containing thousands of discrete and
closely spaced frequencies of light. The comb is referenced to a cesium atomic
clock. To provide enough intensity for spectroscopy, they “phase locked” a
quantum cascade laser to one of the comb’s “teeth.” The result is an
ultrastable source with which they can measure the absorption of a gas sample
as they slowly vary the laser frequency. With some simple improvements, the
authors believe they can further boost their measurement precision by a factor
of 100. – David Ehrenstein
