Catching invisible light

When building circuits for faster and more powerful information processing, one key question is how to exploit the speed and data capacity of the optics and compactness of the electronics. Electrons can be confined in spaces far smaller than light waves can; thus, electronics and optics operate on vastly different scales. Bridging the gap between the two would offer major benefits to circuit developers.

Efforts to solve this problem have seeded the field of plasmonics. This growing research area studies how certain materials make use of electron oscillations at the surface of a metal excited by light to produce enhanced electromagnetic waves. When light of the appropriate frequency hits the surface of the metal, electrons begin to resonate; they move away from their equilibrium positions and oscillate at the same frequency as the light. This resonance preserves the optical information, but shrinks it to a scale that would be compatible with electronic devices. The materials most commonly studied for these attributes are noble metals, such as gold and silver, which are good plasmonic materials for the visible wavelength range. However, these metals exhibit high losses of optical information, which makes them unsuitable for low-loss IR applications,¹ such as medical diagnosis and chemical sensing.

To develop materials with a surface plasmon resonance in the IR range, we can degenerately dope quantum dots.² We use a semiconductor heterostructure that contains semimetals with the appropriate free carrier concentration to potentially achieve an IR plasmonic material. There have been many previous studies of semiconductors incorporating different materials; however, the single crystal heterostructured semiconductor/ metal we developed is new.

Figure 1. Artist's concept of nanometer-sized metallic wires and particles embedded in semiconductors. (Reprinted with permission from Peter Allen, University of California, Santa Barbara; UCSB).

We have created a compound semiconductor embedded with nanostructures containing ordered lines of atoms that exhibit the surface plasmon resonance phenomenon. The effect enables the compound to manipulate light energy in the mid-IR range. Key to this technology is the use of erbium, a rare earth metal that can absorb not only visible light, but also longer IR wavelengths, which the human eye cannot detect. Pairing erbium with the element antimony (Sb), we embedded the resulting compound—erbium antimonide (ErSb)—as semi- metallic nanostructures within the semiconducting matrix of gallium antimonide (GaSb).

Figure 2. The ErSb nanostructures, with colors added to highlight detail, as imaged using high-angle annular dark-field scanning transmission electron microscopy. End-view (left) and side-view (right) of ErSb nanowires, in which each dot represents a single atom. (Reprinted with permission from Stephan Kraemer and Trevor Buehl, UCSB.)

ErSb is an ideal material to match with GaSb because of its structural compatibility with its surrounding material, enabling us to embed the nanostructures without interrupting the atomic lattice structure of the semiconducting matrix. This is an important feature because the less flawed the crystal lattice of a semiconductor is, the more reliable is its performance in a device.

The highly conductive nanostructures can also polarize electromagnetic radiation in a broad range, helping to filter and define images with IR and terahertz light signatures. This effect could possibly advance imaging of internal structures for various materials, including the human body, without the risks posed with, for example, x-rays. We recently filed a patent application for a polarizer that can be integrated into IR and terahertz devices.

Our ErSb/GaSb compound semiconductor will help to preserve optical information at the nano-level, harnessing the speed and data capacity of photons and the compactness of electronics for information processing. However, development of instruments that can take full advantage of IR and terahertz wavelength ranges is still an emerging field. We are now exploring possibilities for this technology in thermoelectrics, which studies how temperature differences of a material can create electric voltage, or how differences in electric voltages in a material can create temperature differences.

For further information, including detailed methods of research and specific data, see our publication.³

The author thanks her collaborators, whose contributions are integral to this research: Daniel G. Ouellette, Sascha Preu, Justin D. Watts, Benjamin Zaks, Peter G. Burke, Mark S. Sherwin, and Arthur C. Gossard. The author also acknowledges support from the US Department of Energy through the Center of Energy Efficient Materials (http://ceem.ucsb.edu/), an Energy Frontier Research Center at University of California, Santa Barbara (UCSB), the Defense Advanced Research Projects Agency, and National Science Foundation Materials Research Science and Engineering Centers. She recognizes the contributions from Institute for Terahertz Science and Technology (http://www.itst.ucsb.edu/), and is grateful to Pavithra Rajesh and Sonia Fernandez at UCSB for technical writing.