Optics & Photonics News

Team Builds Mid-Infrared
Lumped Nanocircuit

Researchers at the University of Pennsylvania (U.S.A.) have built a working 2-D optical nanocircuit designed out of “lumped” elements analogous to the familiar building blocks of electronic circuitry (Nature Mater. 11, 208).

The team led by University of Pennsylvania electrical and systems engineering professor Nader Engheta created tiny rectangular rods of silicon nitride, between 175 and 325 nm thick, and arranged them in an array with 75-nm gap spacing. These parameters were much smaller than the overall length of the rods, 200 µm, and the wavelengths of the mid-infrared signal sent through the circuit, 8 to 14 µm.

Through the array, the researchers sent signals with polarization either parallel with or perpendicular to the rods. They found that the array acted like a parallel combination of lumped impedances in the case of parallel polarization and like a serial circuit of lumped elements in the case of perpendicular polarization.

Engheta has been working on the theory behind lumped optical circuit elements for several years (OPN, June

2006). Standard electric circuit theory uses lumped quantities such as inductance, capacitance and resistance to describe electrical currents and voltages. In today’s modern technology, electronic circuits are often smaller than the wavelengths of the radio frequency and microwave signals passing through them.

What happens when both the physical circuits and the wavelengths of the signals shrink even further? According to Engheta’s theory, nanoscale structures can still behave like capacitors, inductors and resistors if the electromagnetic permittivity of the materials in the structures falls within certain ranges. Engheta’s team showed that the experimental data from their nine different nanorod arrays was in excellent agreement with the results predicted by their theory.

The researchers made the tiny array out of silicon nitride because of its intrinsic resonance around 12 µm. He

Mid-infrared lumped nanocircuit made of rectangular rods of silicon nitride, between 175 nm and 325 nm thick.

said the most challenging aspect of the experimental work was fabricating the high-aspect-ratio nanorods so that they would be suspended by themselves in a 600 × 600 µm window without any supporting substrate.

Engheta and his colleagues call this nanoscale circuit theory “ meta-tronics.” According to the authors, other materials may be used in functional nanocircuits to manipulate optical displacement currents for information processing and data handling applications.

Next, the U. Penn. group will experiment with other materials, such as transparent conducting oxides, that would behave like lumped circuit elements in the near-infrared and possibly even the visible spectrum. Also, they will test more complex circuits and structures.

—Patricia Daukantas

Stimulating Specific Neurons with Light

A new, noninvasive method to control neurons uses the unique optical and electrical properties of quantum dots (QDs). Researchers at the University of Washington (U.S.A.) showed that neurons and other cells changed their behavior in specific ways when nearby QDs were optically excited (Biomed. Opt. Express 3, 447). The new method provides a tool that could target particular cell types, which would help researchers investigating brain function, including research into Parkinson’s disease, Alzheimer’s and depression.

Electrical impulses can activate neurons, but using electrodes for delivering the impulses turns on thousands or millions of them all at once. This makes it impossible to tease out the behavior of an individual or specific type of cell. Optical methods of triggering neurons are attractive because light is noninvasive and reduces other changes in the environment. The University of Washington team led by Fred Rieke and Lih Y. Lin used QDs to make cells photosensitive without genetically or chemically changing them.

QD electrons have limited energy bands because they are so small—only a few microns in diameter, which is roughly the thickness of a cell membrane. This results in precisely defined absorption and emission wavelengths, as well as magnetic fields created by electrons moving between the few allowed energy levels. Also, QDs can be attached to proteins designed to seek out receptors on the surface of specific cell types. These characteristics can turn QDs into a kind of beacon, making it possible for researchers to identify and precisely activate cells.