Frequency Combs for Microchip-Size Chemical Sensors

Most lasers have only one color. All the photons it emits have exactly the same wavelength. However, there are also lasers whose light is more compli­cated. If it consists of many different frequencies, with equal intervals in between, it is referred to as a frequency comb. Frequency combs are perfect for detecting a variety of chemical substances. At the Vienna University of Tech­nology, this special type of laser light is now used to enable chemical analysis on tiny spaces. With this new patent-pending technology, frequency combs can be created on a single chip in a very simple and robust manner.

With a new patent-pending technology, frequency combs can be created on a single chip in a very simple and robust manner. (Source: TU Wien)

Frequency combs have been around for years. In 2005, the Nobel Prize for Physics was awarded for this. “The exciting thing about them is that it is relatively easy to build a spectro­meter with two frequency combs,” explains Benedikt Schwarz, who heads the research project. “It is possible to make use of beats between different fre­quencies, similar to those that occur in acous­tics, if you listen to two different tones with similar frequency. We use this new method, because it does not require any moving parts and allows us to develop a minia­ture chemistry lab on a millimetre scale.”

Now, frequency combs are produced with quantum cascade lasers. These semi­conductor structure consist of many different layers. When elec­trical current is sent through the structure, the laser emits light in the infrared range. The proper­ties of the light can be controlled by tuning the geometry of the layer structure. “With the help of an electrical signal of a specific frequency, we can control our quantum cascade lasers and make them emit a series of light fre­quencies, which are all coupled together,” says Johannes Hill­brand.

The pheno­menon is reminis­cent of swings on a rocking frame – instead of pushing indi­vidual swings, one can make the scaffolding wobble at the right frequency, causing all the swings to oscil­late in certain coupled patterns. “The big advantage of our technology is the robustness of the frequency comb,” says Schwarz. Without this technique, the lasers are extremely sensitive to distur­bances, which are unavoi­dable outside the lab – such as temperature fluctuations, or reflec­tions that send some of the light back into the laser. “Our tech­nology can be realized with very little effort and is there­fore perfect for practical appli­cations even in difficult environ­ments. Basically, the compo­nents we need can be found in every mobile phone”, says Schwarz.

The fact that the quantum cascade laser generates a frequency comb in the infrared range is crucial, because many of the most important molecules can best be detected by light in this frequency range. “Various air pollu­tants, but also biomo­lecules, which play an important role in medical diagnostics, absorb very specific infrared light frequencies. This is often referred to as the optical finger­print of the molecule,” explains Hillbrand. “So, when we measure, which infrared fre­quencies are absorbed by a gas sample, we can tell exactly which substances it contains.”

“Because of its robust­ness, our system has a decisive advantage over all other frequency comb tech­nologies: it can be easily minia­turized,” says Benedikt Schwarz. “We do not need lens systems, no moving parts and no optical isolators, the necessary structures are tiny. The entire measuring system can be accom­modated on a chip in millimeter format.”

This results in specta­cular appli­cation ideas: one could place the chip on a drone and measure air pollu­tants. Chips glued to the wall could search for traces of explosive substances in buildings. The chips could be used in medical equipment to detect diseases by ana­lyzing chemicals in the respira­tory air. The new tech­nology has already been patented. “Other research teams are already highly interested in our system. We hope that it will soon be used not only in academic research, but also in everyday appli­cations,” says Schwarz. (Source: TU Vienna)

Reference: J. Hillbrand et al.: Coherent injection locking of quantum cascade laser frequency combs, Nat. Phot., online 10 December 2018; DOI: 10.1038/s41566-018-0320-3

Link: Institute of Solid State Electronics, TU Wien, Vienna, Austria

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