High-Sensitivity Metal-Dielectric Surface Plasmon Micro-Sensors

PI: Dr. Wan Kuang, Electrical and Computer Engineering, Boise State University

Many applications exist for sensors capable of providing real-time analysis, for instance, detection of toxic industrial pollutants and monitoring for substances of national security interests. Processing of samples through chemical labs results in significant time delays.

In this program, we will make use of metal-dielectric surface plasmon resonant cavity to accomplish simultaneous and instantaneous analysis. Surface plasmon is a group of excited electrons (Ritchie, 1957), behaving like a single electrical entity at the metal-dielectric interface as a result of the resonant interaction between the surface charge oscillation and the electromagnetic field. The appeal of surface plasmon excitation for sensors arises from the large electromagnetic field enhancement at the metal surface, which leads to increased light-matter interactions. Any slight change in the chemical composition of the environment within the range of the plasmon field causes a change in the wavelength of light that resonates with the plasmon.

The application of surface plasmon in biological and chemical sensors is not new; Liedberg et. al. (1995) has demonstrated high sensitivity and accuracy surface plasmon sensor. However due to the use of a prism as an input coupler, they are bulky and fragile and are intended for laboratory use only. Since each sensor can detect only a single substance, it is extremely time-consuming and thus impractical to be deployed for airport security or other situations where simultaneous analysis of multiple substances is required. To overcome this limitation, we will replace the prism coupler with a properly engineered diffractive optical element. The optical element serves to reduce the device footprint as well as to enhance its sensitivity. As a result, miniaturized sensor cells, each of which is responsible for the detection of a unique material, can be integrated onto a single sensor chip.
Fig. 2 shows a schematic for the proposed surface plasmon micro-sensor. A thin metal film is deposited on a silica or silicon substrate. A nanometer-scale periodic structure is patterned on the metal film with e-beam lithography and dry etching to serve as an equivalent prism. Fig. 3 shows a scanning electron micrograph of device surface. Continuous efforts are made to improve fabrication process and reduce the surface roughness. The sensor is illuminated from the bottom (silica) side at a normal angle. The reflection off the patterned metal film is fed to an infrared detector. At a certain wavelength when the phase matching condition is satisfied for surface plasmon polaritons at the metal-air interface, optical power is absorbed by the electrons, which leads to a decrease of intensity for the reflected light. The presence of an absorbed sample alters the width, position, and the height of the peaks in both reflection and transmission spectrum.

Bo Liedberg, Claes Nylander, and Ingemar Lundstrom, Biosensing with surface plasmon resonance-how it all started, Biosensors and Bioelectronics, 10:1–9, 1995.

R. H. Ritchie. Plasma losses by fast electrons in thin films, Physical Review, 106(5):874–881, 1957.