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Mid-IR Plasmonic Materials

Experimental and Theoretical Background


A field of research that has been dominated by noble metals

Surface plasmons have been known in some form or other since an article by Wood in 1902 that first observed the effect of light coupling into a grating on gold metal. By the 1960s it was understood that light could be coupled into noble metals, mainly gold and silver, using attenuated total reflection methods. Under these conditions if the light has a particular angle and energy, the electromagnetic wave can couple into the conductor and drive the conduction electrons leading to near total extinction. Because the condition for this coupling is quite specific, it was recognized that the ATR geometry would be appropriate for the design of a sensor.

Attenuated Total Reflection geometry and Surface Plasmon Resonance/ pdf

Noble metals as the materials of choice for 100 years/ pdf

In 2002, we published a paper that suggested the same methods that work for noble metals could be applied to conducting metal oxides, such as indium tin oxide (ITO), doped zinc oxide (X:ZnO) and others. We published numerous studies on ITO showing that materials properties could be systematically altered and thereby one could study the surface plasmon resonance (SPR) in a manner that was not possible on noble metals. If one can systematically change the charge carrier density and mobility of a material one can tune the position and width of the surface plasmon polariton (SPP)

Studies of ITO thin films as SPR materials/ pdf

Conducting metal oxides (CMOs), such as ITO, ZnO, CdO and others are degenerate semi-conductors. They are free electron conductors and follow the Drude model for conductors.

The Drude free electron model/ pdf

The limitations of ITO and X:ZnO are clearly that the mobility of these materials is never very high. The process of doping creates lattice imperfections and these act to scattering conducting electrons. If these were the only CMOs available this field might have been a curiostiy and nothing more since the broad spectral features of these materials was not competitive with noble metals. However, after a search of materials, CMOs with much higher mobility were identified.

Doped ZnO as a second generation CMO for mid-IR SPR/ pdf

In 2015, in a collaboration with our long-term collaborator Dr. Maria and his graduate student Edward Sachet we reached a breakthrough. Doped cadmium oxide (CdO:X) was shown to have sufficiently high mobility to rival the noble metals (well Au at least). Although initially dysprosium (Dy) was used as a dopant, subsequent research has shown that many alternative ions can be used (Y, Eu etc.).

Doped CdO as a gateway material for mid-IR SPR/ pdf

Since the SPP of CdO is in the mid-IR it is in the range of molecular vibrations. We have tested the interaction of molecular vibrations with CdO using gaseous N2O. The enhancement of the N2O stretching rovibrational transition provides a test of the magnitude of emerging fields from the SPP of CMO. These interactions are difficult to measure in noble metals. CMOs such as CdO:X provide a test of the role of emerging fields in the enhancement of spectroscopic signals. The findings of surface enhanced infrared reflection absorption (SEIRA) spectra have ramifications for all surface-enhanced spectroscopy.

Surface enhanced infrared reflection absorption (SEIRA)/ pdf

One approach to presenting the information that has been obtained by studying surface plasmons resonance (SPR) on conducting metal oxides (CMOs) is to use the graphics of the angle vs. energy plots to illustrate the concepts of SPR without using equations. The seminar entitled the Dao of Surface Plasmon Resonance uses this approach to describe the important issues in the phenomenon without using any mathematical equations.

The Dao of Surface Plasmons/ pdf