Novel light response makes material with promise for sensors

A research team led by North Carolina State University has identified and synthesised a material that can be used to create efficient plasmonic devices that respond to light in the mid-infrared (IR) range.

According to NC State, this is the first time that researchers have demonstrated a material that performs efficiently in response to this light range. 

At play is a phenomenon called surface plasmon resonance, where researchers illuminate the interface between a conducting and an insulating material. If the angle, polarisation, and wavelength of the incoming light are just right, electrons in the conductor will oscillate. This oscillation creates an electric field extending into the insulator that can be applied to biomedical sensors, solar cells, and opto-electronic devices.

The wavelength of light that causes these oscillations depends on the nature of the conductive material.

Materials with a high density of free electrons respond to short wavelengths of light, such as those in the ultraviolet range. Materials with lower electron density (such as conventional semiconductors) respond to long wavelengths of light, such as those in the far IR. Until now, however, scientists were unable to identify materials that could support efficient surface plasmon resonance when targeted with wavelengths of light in the mid-IR range.

In a statement, Dr Jon-Paul Maria, corresponding author of a paper on the work said: ‘There are at least three practical reasons for wanting to identify materials that exhibit surface plasmon resonance in response to mid-IR light.

‘First, it could make solar harvesting technology more efficient by taking advantage of the mid-IR wavelengths of light – that light wouldn’t be wasted. Second, it would allow us to develop more sophisticated molecular sensing technology for use in biomedical applications. And third, it would allow us to develop faster, more efficient opto-electronic devices.

‘We’ve now synthesised such a material, and shown that it effectively exhibits low-loss surface plasmon resonance in the mid-IR range.’

Specifically, the research team has ‘doped’ cadmium oxide with a rare earth element called dysprosium, meaning that a tiny amount of dysprosium has been added to cadmium oxide without changing the material’s crystal structure.

This does two things: it first creates free electrons in the material, then increases the mobility of the electrons. Overall, this makes it easier for mid-IR light to induce oscillations in the electrons efficiently.

‘Usually when you dope a material, electron mobility goes down,’ said Dr Maria, a professor of materials science and engineering at NC State. ‘But in this case we found the opposite – more dysprosium doping increases this critical characteristic. In technical terms, our experiments revealed that Dy-doping reduces the number of oxygen vacancies in a CdO crystal. Oxygen vacancies, which correspond to locations where oxygen atoms are missing, are strong electron scatterers and interfere with electron motion. In the most basic terms, by removing these defects, electrons scatter less and become more mobile.’

The paper, ‘Dysprosium doped cadmium oxide: A gateway material for mid-infrared plasmonics,’ was published online Feb. 16 in Nature Materials