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Surface plasmons sense carbon dioxide

20 Jan 2009

Optics.org speaks to the team pioneering a surface plasmon resonance sensor that works at mid-infrared wavelengths.

Gas sensing and spectroscopic analysis of biological samples look set to be the initial beneficiaries of what is claimed to be the first surface plasmon resonance (SPR) sensor to operate in the mid-infrared.

The team already believes that the sensitivity of its proof-of-principle carbon dioxide sensor is five times higher than a similar SPR sensor operating in the visible and is keen to push the technology further and study biological samples. (Optics Express 17 293)

"SPR sensors have been developed in the visible and near-infrared due to the availability of efficient light sources and detectors," Sylvain Herminjard of EPFL in Switzerland told optics.org. "One way to increase the sensitivity is to use wavelengths close to the absorption peaks of the measured medium. For gases, this means moving to the mid-infrared, which was the main motivation behind this research."

Surface plasmons (SPs) are charge-density oscillations localized at the interface between a metallic film, known as the active layer, and a dielectric surface. SPR refers to the optical excitation of these oscillations and is a polarization dependent phenomenon.

"SPR is not a transmission-based measurement," explained Herminjard. "SPs are used as a probe to track the variations of the refractive index of the measured medium. This method is extremely interesting for spectroscopy in water-based samples where the mid-infrared absorption is very strong and standard absorption spectroscopy starts to be problematic."

The crucial components in any SPR sensor are a polarized light source, a sensing device with an active metallic layer and a suitable detector to track the variations of the refractive index of the measured medium. "The metal layer is the most important part of the system as this is where SPR occurs," commented Herminjard.

The team used a 4.4 micron quantum cascade laser (QCL) that emitted a strongly TM-polarised beam. Light from the QCL is fired into a calcium fluoride prism, which is coated with titanium-gold active layer. The reflected beam exits the prism where it is focused into an MCT detector.

Carbon dioxide and nitrogen mixtures were used as test samples and the researchers say that due to the absorption of carbon dioxide at 4.4 microns, the sensitivity of this configuration is five times higher than a similar SPR sensor operating in the visible.

According to Herminjard, the next step is to use liquids samples in order to perform selective SPR spectroscopy on biological samples. "We also want to use detector arrays for faster SPR measurements and deconvolution algorithms in order to decrease the loss of contrast due to the detector size," he concluded.

Sylvain Herminjard works at EPFL's Institute of Microengineering, however, this work was performed when the Institute was associated with the University of Neuchatel. This study, done in collaboration with Hach Ultra Analytics, was funded by the Swiss Innovation Promotion Agency (KTI/CTI).

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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