A team led by scientists from North Carolina State University has identified and synthesized a material that can be utilized to develop plasmonic devices able to respond to light in the mid-infrared (IR) range, the first time a material has been shown to perform efficiently in response to this light range. The advance could lead to various applications, including in high-speed computers, solar energy optoelectronic devices and biomedical devices such as sensors.

The researchers, whose work on dysprosium-doped cadmium oxide as a gateway material for mid-IR plasmonics was published in Nature Materials [Sachet et al. Nat. Mater. (2015) DOI: 10.1038/nmat4203], used the phenomenon of surface plasmon resonance, where the interface between a conducting and insulating material is illuminated. Given a specific angle, polarization and wavelength of the incoming light, electrons in the conductor begin to oscillate. This creates an intense electric field extending into the insulator.

The wavelength of light that results in the oscillations is dependent on the type of conductive material. Those with a higher density of free electrons (for example, metals) respond to short wavelengths of light, such in the ultraviolet range. Materials with lower electron (for example, conventional semiconductors) respond to long wavelengths of light, such as those in the far-IR. Although plasmonic materials for ultraviolet–visible light and near-IR wavelengths have already been identified, the mid-IR range has remained a challenge, as few systems can achieve sub-wavelength optical confinement with low loss in this range.

They doped cadmium oxide with a rare earth element called dysprosium – adding a small amount of dysprosium to cadmium oxide without changing its crystal structure. This has the effect of creating free electrons in the material, and also increasing the mobility of the electrons, making it easier for mid-IR light to induce oscillations in the electrons efficiently. Although when a material is doped, electron mobility tends to decrease, here the team demonstrated the opposite. On a basic level, by removing these defects, electrons scatter less and are more mobile.

There are many useful, practical reasons for identifying materials that exhibit surface plasmon resonance in response to mid-IR light, including their ability to make solar harvesting technology more efficient as IR light would not be squandered. Also, these materials could allow for the development of more sophisticated molecular sensing technology for biomedical applications, and are hoped will lead to faster and more efficient optoelectronic devices.