How to Take Detailed 3D Images of Nano-Objects

April 10, 2015 Updated: April 12, 2015

A new technique could make it possible to capture high-resolution, 3D images of how light interacts with objects on the nanoscale.

Until now, that’s been a challenge with traditional imaging techniques due to the physics of light: the smaller the object, the lower the image’s resolution in 3D.

The new method uniquely combines two technologies—cathodoluminescence and tomography—to make 3D maps of the optical landscape of objects, says study lead author Ashwin Atre, a graduate student in the lab group of Jennifer Dionne, an assistant professor of materials science and engineering at Stanford University. 

The target object in this proof-of-principle experiment was a gold-coated crescent 250 nanometers in diameter–several hundred times as thin as a human hair.

To study the optical properties of the crescent, they first imaged it using a modified scanning electron microscope. As the focused electron beam passed through the object, it excited the crescent energetically, causing it to emit photons, a process known as cathodoluminescence.

Both the intensity and the wavelength of the emitted photons depended on which part of the object the electron beam excited, Atre says. For instance, the gold shell at the base of the object emitted photons of shorter wavelengths than when the beam passed near the gap at the tips of the crescent.

From 2D to 3D

By scanning the beam back and forth over the object, the engineers created a 2D image of these optical properties. Each pixel in this image also contained information about the wavelength of emitted photons across visible and near-infrared wavelengths.

This 2D cathodoluminescence spectral imaging technique, pioneered by the AMOLF team, revealed the characteristic ways in which light interacts with this nanometer-scale object.

“Interpreting a 2D image, however, can be quite limiting,” Atre says. “It’s like trying to recognize a person by their shadow. We really wanted to improve upon that with our work.”

To push the technique into the third dimension, the engineers tilted the nanocrescent and rescanned it, collecting 2D emission data at a number of angles, each providing greater specificity to the location of the optical signal.

By using tomography to combine this tilt-series of 2D images, similar to how 2D x-ray images of a human body are stitched together to produce a 3D CT image, Atre and his colleagues created a 3D map of the object’s optical properties.

This experimental map reveals sources of light emission in the structure with a spatial resolution on the order of 10 nanometers.

Leds and Solar Panels

For decades, techniques to image light-matter interactions with sub-diffraction-limited resolution have been limited to 2D.

“This work could enable a new era of 3D optical imaging with nanometer-scale spatial and spectral resolution,” says Dionne, who is an affiliate of the Stanford Institute for Materials and Energy Sciences at SLAC.

The technique can be used to probe many systems in which light is emitted upon electron excitation.

“It has applications for testing various types of engineered and natural materials,” Atre says. “For instance, it could be used in manufacturing LEDs to optimize the way light is emitted, or in solar panels to improve the absorption of light by the active materials.”

The technique could even be modified for imaging biological systems without the need for fluorescent labels.

In addition to Atre and Dionne, researchers from DIPC in Spain and the FOM Institute AMOLF in the Netherlands collaborate on the research, which is detailed in Nature Nanotechnology.

Source: Bjorn Carey and Leslie Willoughby via Stanford University. Republished from under Creative Commons License 4.0.

* Image of 3d white person via Shutterstock