Infrared Absorption Studies Using Metal Micro Arrays

Consider a microwave oven. The door to microwave ovens has a glass window that has a metal screen covering it. The holes in the metal screen are several millimeters in diameter. The reason for this screen is very simple. The microwave radiation that is being generated inside the microwave oven has a frequency of 2450 MHz, which corresponds to about 12 cm or 120 mm.[1] It turns out that electromagnetic radiation can pass through metals as longs as the holes in the metals are larger than the wavelength of the electromagnetic radiation. In the case of our microwave ovens, the holes are smaller than the wavelength of the electromagnetic radiation and therefore the microwave radiation is reflected by the metal screen as if it were a solid sheet of metal.

In the situation where the wavelength of the electromagnetic radiation and the size of the holes in a metal screen are nearly the same, a very interesting phenomenon can occur. When the two sizes are nearly the same, the size of the holes in the metal mesh, what metal the mesh is made out of, and the presence of any other substances adsorbed on the mesh can greatly effect the phenomenon. When several requirements are met, the metal mesh will convert the incident electromagnetic radiation into an excited surface plasmon polariton (SPP). This surface plasmon polariton can then transfer through the mesh to the other side where the mesh converts the excited SPP back to electromagnetic radiation that is emitted from the surface. This effect can result in a larger percent transmission through mesh than the percent open area of the mesh.

SEM micrograph of Ni mesh. — Figure 1. A SEM image of nickel mesh. This mesh is the basis for all my surface plasmon studies.

The study of surface plasmon polaritons in the infrared was the focus of my PhD research and remains of interest to both myself as well as other researchers.

Transmission Properties vs Hole Size

Cu Deposited Mesh — Figure 2. A SEM image of nickel mesh that has be coated with copper via electrochemical deposition.

IR Spectra of Various Meshes — Figure 3. Spectra of the transmission spectra of mesh for meshes with varying hole sizes. The first spectra is of a nickel mesh and the other spectra are nickel meshes coated in copper.

Transmission Properties vs Metal

Transmission vs Angle of Incidence

IR Spectra of Ni Mesh at Different Angles of Incidence — Figure 4. Spectra of the transmission spectra of nickel mesh at various angles of incidence. The top plot has spectra who incidence angle was changed parallel to the hole-to-hole line. The bottom plot has spectra whose incidence angle was change parallel to the diagonal hole-to-hole line.

IR Spectra of Cu Mesh at Various Angles of Incidence — Figure 5. Spectra of the transmission spectra of copper mesh at various angles of incidence. The top plot has spectra who incidence angle was changed parallel to the hole-to-hole line. The bottom plot has spectra whose incidence angle was change parallel to the diagonal hole-to-hole line.

Described in the caption. — Figure 6. Combining all the various spectra in a contour map yielding a dispersion plot.

Described in caption. — Figure 7. Extracting and plotting peak centers in each spectra yields a simpler dispersion diagram where surface plasmon resonance features can be observed.

Absorbances Due to Adsorbed Surface Species

The absorbance spectra of MeOH on the surface of copper mesh. — Figure 8. Absorption spectra of a copper mesh taken after the addition of methanol to the mesh. As time progressed the absorbances change indicating a chemical reaction is taking place.

Kinetic Plot of Methanol Reaction — Figure 9. The extracted absorbance of various identifiable peaks plotted against time. These peaks show the kinetics of converting methanol to the methoxy radical intermediate and the subsequent conversion of methoxy radical into formaldehyde.

References

[1] Hyper Physics, Microwave Ovens (Online). http://hyperphysics.phy-astr.gsu.edu/hbase/waves/mwoven.html [August 18, 2013].