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Engineers produce high-resolution, 3D images of nanoscale objects

13 April 2015

The technique, called cathodoluminescence tomography, could lead to high-efficiency solar cells and LEDs, or improve the visualisation of biological systems.

Engineers at Stanford and the FOM Institute AMOLF, in the Netherlands, have developed a way to visualize the optical properties of nanoscale objects, in 3D and with nanometre-scale resolution

To design the next generation of optical devices, ranging from efficient solar panels to LEDs to optical transistors, engineers will need a three-dimensional (3D) image depicting how light interacts with these objects at the nanoscale. Unfortunately, the physics of light sets limits for traditional imaging techniques: the smaller the object, the lower the image's resolution in 3D.

Now, engineers at Stanford and the FOM Institute AMOLF, a research laboratory in the Netherlands, have developed a technique that makes it possible to visualise the optical properties of objects that are several thousandths the size of a grain of sand, in 3D and with nanometre-scale resolution.

The technique involves a combination of two technologies: cathodoluminescence and tomography, enabling the generation of 3D maps of the optical landscape of objects.

The target object in this proof-of-principle experiment was a gold-coated crescent 250 nanometres in diameter. To study the optical properties of the crescent, the researchers 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. 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.

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 reveals the characteristic ways in which light interacts with this nanometre-scale object.

"Interpreting a 2D image, however, can be quite limiting," says Ashwin Atre, a graduate student who was involved in this work. "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 re-scanned 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 of the order of 10 nanometres.

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 Jennifer Dionne, an affiliate of the Stanford Institute for Materials and Energy Sciences at the Stanford Linear Accelerator Centre (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 optimise 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.

The research is detailed in the current issue of Nature Nanotechnology.

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