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Defense & Security

SPR helps sensor sensitivity

Surface plasmon resonance techniques can improve the sensitivity of sensors based on cantilever probes.
8 September 2008, SPIE Newsroom. DOI: 10.1117/2.2200809.0002
Cantilever bending monitoring via the Surface Plasmon Resonance (SPR) phenomenon

Whether you want night vision equipment that detects very small changes in heat or biological and chemical sensors that can identify very small quantities of harmful agents, sensitive instruments are essential.

Juriy Hastanin and colleagues at the Université de Liège in Belgium have taken up this challenge for detectors based on cantilever probes. Such probes are generally made up of two different materials and work in two basic ways. In one, the materials have different thermal properties. Any slight change in temperature causes them to bend. This might be because they are detecting body heat or because a chemical compound on the end of the probe has reacted with the external agent that it is intended to detect for and the reaction has generated heat. Alternatively, the absorption/adsorption processes induces a mechanical property change, such as the modulus of elasticity, surface stress, or volume dilatation of the sensitive layer. This again causes the cantilever to bend proportionally to the concentration of the molecules.

Figure 1. Monitoring principle.

The probe could have a specific absorber to detect for terahertz or infrared radiation or a particular biological or chemical agent. An array of cantilever probes could then be used to detect for one property or several depending on the absorbers on the probe.

The idea is simple but in practice there are inevitable complications. When the change in temperature is very small, the amount that the probe is bent is also very small -- sometimes too small to be picked up accurately by the interferometers used with this technique today.

Figure 2. (a) Reflectivity of thin metal film, R, vs. incidence angle θ calculated for various thickness of gap between the cantilever and the thin metal film. (b) Reflectivity vs. thickness of gap calculated for incidence angle corresponding to the SPR dip position for the multilayer system “prism/gold/air”.

An alternative, which Juriy Hastanin presented to delegates at the SPIE Europe Security + Defence symposium on 15-18 September and which the group has applied to patent, is to use surface plasmon resonance (SPR) to improve the sensitivity and resolution. This approach, according to Serge Habraken who leads the group working in this area, would enable a change of less than 1nm to be spotted in real time with an improved signal-to-noise ratio over current techniques.

Figure 3. Radiation transducing principle based on thermal bimetallic effect.

SPR is a based on studying what happens when surface plasmons (SPs), which are charge density longitudinal oscillations of the free electron gas on the interface between a metal and a dielectric medium, are excited by TM-polarized light. The SP excitation efficiency and therefore the reflectance of a metallic film measured at a fixed incident angle changes almost proportionally to the cantilever bending.

Figure 4. Surface stress/volume dilatation transducing principle.

The cantilever and the readout process can be physically separated. This enables the technique to be used in harsh environments such as in a nuclear reactor, at high temperatures or in a military or security situation. The array of probes could be inside, for example, a reactor but the readout would be outside in a safe environment.

Despite the advantages, there is a drawback though. The dynamic range is lower than current techniques. However, the researchers believe that this can be balanced again having much better sensitivity and that it will provide benefits in some sensing applications.

Figure 5. Calorimetrical transducing principle.

The researchers are now looking for collaborators to work on microfabrication of the sensor arrays. Habraken said that fabrication techniques are less well-known because the arrays are based mainly on dielectric materials rather than silicon. "We hope to get interest from people with more experience of microfabrication of dielectric materials," he commented.

Siân Harris is a UK-based science and technology journalist.