SPECTROSCOPY: Technique simplifies surface-plasmon spectroscopy

Because the propagation constant of a surface plasmon-an electromagnetic field mode that propagates along a metal/dielectric interface-is very sensitive to optical changes in the dielectric, spectroscopy of surface plasmons can be exploited to determine the properties of thin films and for biosensing applications.

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Because the propagation constant of a surface plasmon-an electromagnetic field mode that propagates along a metal/dielectric interface-is very sensitive to optical changes in the dielectric, spectroscopy of surface plasmons can be exploited to determine the properties of thin films and for biosensing applications. Rather than using a bulky and expensive optical spectrometer to perform wavelength spectroscopy of surface plasmons, researchers at the Academy of Sciences of the Czech Republic (Prague, Czech Republic) have developed a technique that uses a special grating coupler and eliminates the need for a spectrometer.1

The diffractive structure of the grating coupler, referred to as a surface-plasmon-resonance coupler and disperser (SPRCD), simultaneously enables the excitation of surface plasmons and disperses light over a position-sensitive detectowr (PSD).

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A surface-plasmon-resonance coupler and disperser replaces a more expensive, bulky optical-spectrum analyzer for the purpose of performing simpler, more cost-effective surface-plasmon spectroscopy.
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In the experimental setup, a collimated beam of polychromatic light is input to the SPRCD from a dielectric (see figure). A portion of this incident light is coupled to a surface plasmon at the metal-dielectric interface through the second order of diffraction, while the light diffracted from the first order is dispersed onto the PSD. When light is coupled into a surface plasmon, the drop in intensity of the diffracted light is recorded as a dip in the spectrum of diffracted light onto the PSD. By measuring the diffracted coupling wavelength at which this dip occurs, the propagation constant of a surface plasmon can be determined and surface-plasmon spectroscopy is achieved.

The researchers demonstrated that this approach to spectroscopy of surface plasmons could be used for the development of a new optical sensor. This sensor measures changes in the refractive index that occur in the proximity of the metal film supporting a surface plasmon by measuring changes in the coupling wavelength. Theoretical analysis of the SPRCD structure has shown that the sensitivity of the coupling wavelength to refractive index is as high as 610 nm per refractive-index unit (RIU).

Experimental validation of this technique was performed using a collimated light beam from an LED butt-coupled to a 200 µm optical fiber using a planoconvex lens with a 40 mm focal length. This light was polarized by a dichroic polarizer, and in the experiment was incident on a SPRCD structure with period of 1190 nm and depth of 100 nm produced in a layer of photoresist using a holographic technique, coated with a 100 nm layer of gold. The wavelength spectrum revealed a narrow dip at approximately 824 nm, as predicted by the theoretical analysis based on the SPRCD physical parameters.

To demonstrate the potential of the SPRCD-based spectroscopy for sensing, a refractometric experiment was performed that monitored changes in the coupling wavelength as a function of refractive-index changes of a liquid sample placed in a transparent flow cell on top of the SPRCD. In this experiment, changes in the refractive index of 0.002 RIU produced a change in the coupling wavelength of 1.39 nm, demonstrating a refractive-index sensitivity of 695 nm/RIU. Besides working as accurate refractometers, SPRCD-based sensors can be tailored for detection of chemical and biological species. In this configuration, an SPRCD sensor measures refractive-index changes induced by the binding of target molecules to biorecognition elements immobilized on the surface of the sensor.

“We believe that the SPRCD method presents a promising route to development of simple, compact, and low-cost sensing devices,” says researcher Jiri Homola, chairman of the Department of Optical Sensors. “SPRCD-based biosensors can find applications in numerous important sectors, including medical diagnostics, environmental monitoring, food safety, and security.”

Gail Overton

REFERENCE:

1. O. Telezhnikova and J. Homola, Optics Lett. 31(22) 3339 (Nov. 15, 2006).

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