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Mid-Infrared QCL Improves Signal Quality of Spectroscopic Measurements

Scientists performed an analytical technique called spectroscopic ellipsometry by using a very bright mid-infrared laser.

Using a quantum cascade laser for infrared spectroscopic ellipsometry allows high-resolution information to be captured in less than one second. This image shows a schematic illustration of the setup [window (W), aperture (A), photo-elastic modulator (PEM), razor blade (RB), sample (S), analyzer unit (AN) lens (L), MCT detector (MCT)]. (Image credit: Alexander Ebner, RECENDT

The new method captures high-resolution spectral data in less than a second and may provide better insights into rapidly changing characteristics of a range of samples ranging from plastics to biological materials.

Spectroscopic ellipsometry measures how the polarization of light varies following the interaction with a sample. When this method is carried out in the infrared portion of the spectrum, it can disclose detailed information about the chemical composition and molecular orientation of a sample.

In Optics Letters, a journal from The Optical Society (OSA), scientists from the Research Center for Non Destructive Testing (RECENDT) GmbH and Johannes Kepler Universität, both in Austria, explained how they integrated a mid-infrared quantum cascade laser (QCL) with a spectroscopic ellipsometry setup. This kind of laser is relatively new and at least 10,000 times brighter than the conventional light sources used for spectroscopic ellipsometry.

The researchers showed that the QCL increasingly enhanced the signal quality of the spectroscopic measurements and reduced the spectral acquisition time from several hours to less than a second, with further enhancements feasible as the new laser technology advances. They have also shown that the method can be used for real-time monitoring of molecular reorientation as a plastic film is expanded.

Our method provides access to sample properties that couldn’t be observed in real-time before. QCL ellipsometry could help improve manufacturing processes and the quality of the resulting product. It might also reveal previously unobservable physical and biological processes that would lead to new scientific discoveries.

Markus Brandstetter, Head of the Research Team, RECENDT

A very bright light source

The mid-infrared QCL that the scientists used showed a brightness level that exceeded more than that of synchrotron sources, which are available only in particular facilities. The brightness of the laser implies that it can be used for mid-infrared spectroscopic ellipsometry of highly absorbing materials or substances, including water-soluble substances. “Because of the high mid-infrared absorbance of water, this has been very difficult or even inconceivable up to now,” said Brandstetter.

It is possible to tune the emission wavelengths of the laser over a broad mid-infrared range that can be matched exactly with mid-infrared detectors available on the market. Another benefit is that it can be used for spectroscopic measurements with cheap and simple optical components like monochromators or interferometers.

The laser we used also offers the possibility of spot sizes that are restricted only by the diffraction-limit of light. This can be exploited for ellipsometric measurements with high spatial resolutions, which will be of interest for both science and industry.

Jakob Kilgus, Member of the Research Team, RECENDT

Making real-time measurements

The scientists tested their new system by comparing it to an instrument, which is considered the gold standard of commercially available infrared spectroscopic ellipsometers. They also carried out real-time measurements of the realignment of molecular chains as a polypropylene film was expanded.

Kilgus continued, "The new setup outperformed the standard acquisition time and signal-to-noise ratio by orders of magnitude. Our measurement of the polypropylene film was only limited by the speed of the stage used to apply the force. Much faster processes could be monitored with the setup."

The scientists emphasized that these are very promising, but initial, results. So, they decided to further enhance the instrument and intend to completely exploit the feasibility of diffraction limited laser spots to obtain hyperspectral mid-infrared ellipsometry images—which would contain the full spectrum for each pixel of the image—with practical acquisition times.

We believe there will be a strong interest in this novel technique and the possibility of developing it for commercial use. The sub-second time resolution combined with the high brightness of the laser will be useful for numerous industrial and scientific applications.

Markus Brandstetter, Head of the Research Team, RECENDT

The researchers used the new technique for real-time monitoring of the molecular reorientation as a polypropylene film was stretched. The method might be useful for real-time monitoring of plastic manufacturing processes, for example. (Video credit: Alexander Ebner, RECENDT)

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