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Posted: May 17, 2010
Speeding up broadband spectroscopy
(Nanowerk News) Spectroscopy, or the comprehensive measurement of light emissions coming from an object, is the cornerstone of many scientific studies. The spectrum of a sample -- whether it comes from a star, a dilute protein solution, or the polluted air of a city street -- consists of the measured frequency of all the light absorbed or emitted by the sample, though sometimes it is difficult to accurately measure all frequencies.
Frequency can be measured quite accurately in the radio portion of the electromagnetic spectrum, where pulsations can be counted directly by electronic circuits. The "frequency comb" approach, introduced a few years ago, has revolutionized spectroscopy by allowing more accurate measurements of frequencies characteristic of infrared, visible, and ultraviolet light. The trick is to convert higher-frequency light into the lower radio frequency range, where the waves can be subjected to detailed measurement.
The word "comb" in the phrase frequency comb refers to the fact that the light being measured can be compared to a laser that emits at light at special frequencies spaced at regular intervals. The spectrum of this laser looks like a comb. This series of light frequencies serves as a sort of "ruler" against which other light signals can be compared.
Birgitta Bernhardt, a graduate student at of the Max Planck Institute for Quantum Optics in Munich, will report on a novel use of two frequency comb devices simultaneously to record broadband spectra, which speeds up the task of recording a spectrum by a factor of one million compared to the traditional Fourier transform spectroscopy. This dual-comb process has been tried before, but not previously for the important mid-infrared region ranging from 2 to 8 µm.
Mid-infrared light is important for the characterization of the structure of matter and for a number of detection problems. "The applications can be found in very different directions," says Bernhardt, "ranging from biomedicine (analysis of breath) to environmental monitoring or analytical chemistry (small traces of environmental and toxic vapors can be detected because of the high sensitivity of the measurement technique), and laboratory astrophysics."