Laser terahertz emission microscopy (LTEM) is being used to characterise the performance of solar cells, integrated circuits and other systems and materials. Laser pulses illuminating a sample material cause the emission of terahertz radiation, which carries important information about the sample's electrical properties.
"This is a well-known tool for studying essentially any material that absorbs light, but it's never been possible to use it at the nanoscale," said Daniel Mittleman, a professor in Brown's School of Engineering and corresponding author of a paper describing the work. "Our work has improved the resolution of the technique so it can be used to characterise individual nanostructures."
LTEM measurements are usually performed with resolution of a few tens of microns, but this new technique enables measurements down to a resolution of 20 nanometers, roughly 1,000 times the resolution previously possible using traditional LTEM techniques.
The research was led by Pernille Klarskov, a postdoctoral researcher in Mittleman's lab, with Hyewon Kim and Vicki Colvin from Brown's Department of Chemistry.
The team has adapted for terahertz radiation a technique already used to improve the resolution of infrared microscopes. The technique uses a metal pin, tapered down to a sharpened tip only a few tens of nanometers across, that hovers just above a sample to be imaged. When the sample is illuminated, a tiny portion of the light is captured directly beneath the tip, which enables imaging resolution roughly equal to the size of the tip. By moving the tip around, it's possible to create ultra-high-resolution images of an entire sample.
Klarskov was able to show that the same technique could be used to increase the resolution of terahertz emission as well. For their study, she and her colleagues were able to image an individual gold nanorod with 20-nanometer resolution using terahertz emission.
The researchers believe their new technique could be broadly useful in characterising the electrical properties of materials in unprecedented detail.
"Terahertz emission has been used to study lots of different materials -- semiconductors, superconductors, wide-band-gap insulators, integrated circuits and others," Mittleman said. "Being able to do this down to the level of individual nanostructures is a big deal."