System Bits: Sept. 29

Light detector on a chip for portable sensors
The invention by Vanderbilt University and Ohio University researchers of the first integrated circularly polarized light detector on a silicon chip could open the door for development of small, portable sensors to expand the use of polarized light in drug screening, surveillance, optical communications, quantum computing, and other applications.

The chip in the hand does the same job as the conventional circularly polarized light detector on the right. (Source: Vanderbilt University)

Although it is largely invisible to human vision, the polarization state of light can provide a lot of valuable information, but the traditional way of detecting it requires several optical elements that are quite bulky and difficult to miniaturize, explained Jason Valentine, Vanderbilt assistant professor of mechanical engineering who directed the team. He said they managed to get around this limitation by the use of ‘metamaterials’ that have been engineered to have properties that are not found in nature.

The metamaterial that the researchers developed to detect polarized light consists of silver nanowires laid down in a sub-microscopic zig-zag pattern on an extremely thin sheet of acrylic fixed to an optically thick silver plate. This metamaterial is attached to the bottom of a silicon wafer with the nanowire side up.

The nanowires generate a cloud of free-flowing electrons that produce “plasmon” density waves that efficiently absorb energy from photons that pass through the silicon wafer. The absorption process creates “hot” or energetic electrons that shoot up into the wafer where they generate a detectable electrical current.

Variance spectroscopy advances nanoparticle analysis 
In an example of the ‘less is more,’ approach, Rice University researchers developed a method to analyze carbon nanotubes in solution using variance spectroscopy to zoom in on small regions in dilute nanotube solutions to take quick spectral snapshots.

By analyzing the composition of nanotubes in each snapshot and comparing the similarities and differences over a few thousand snapshots, the researchers said they gain new information about the types, numbers and properties of the nanoparticles in the solution.

Rice chemist Bruce Weisman — a pioneer in the field of spectroscopy who led the discovery and interpretation of near-infrared fluorescence from semiconducting carbon nanotubes — expects variance spectroscopy to become a valuable tool for researchers who study nanoscale materials.

He said variance spectroscopy could help characterize nanotube samples in the ongoing drive to sort and separate specific types for electronic and optical applications.

A covariance matrix produced with a new technique at Rice University maps fluorescence signals from various species of single-walled carbon nanotubes that are beginning to aggregate in a sample. The matrix allows researchers to know which types of nanotubes (identified by their fluorescence spectra) have aggregated and in what amounts, in this case after four hours in solution. (Source: Weisman Lab/Rice University)