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Optical Method for Producing High-Res, 3D Images of Nanoscale Objects

To design the next generation of optical devices, ranging from efficient solar panels to LEDs to optical transistors, engineers will need a three-dimensional image depicting how light interacts with these objects on the nanoscale. Unfortunately, the smaller the object, the lower the image's resolution in 3D. Now, engineers at Stanford University and the FOM Institute AMOLF, a research laboratory in the Netherlands, have developed a technique that makes it possible to visualize the optical properties of objects that are several thousandths the size of a grain of sand, in 3D and with nanometer-scale resolution. The technique involves a unique combination of two technologies, cathodoluminescence and tomography, enabling the generation of 3D maps of the optical landscape of objects.

Topics:
Electronic Components Electronics & Computers Imaging LEDs Lighting Optics Photonics Power Semiconductors & ICs

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    Transcript

    00:00:00 Stanford University. Have you ever wondered how a CAT scan is able to generate a three-dimensional image of your organs? These images are made using a process called tomography. In this technique, two-dimensional images are taken at several angles. Each angle gives us new information about the object.

    00:00:35 Next, an algorithm can combine all of these 2D images. The end result is a 3D image, or what's called a reconstruction of the original object. Recently, this process was applied to tiny objects much smaller than the human body. Here are images of nanoparticles, where you can see individual atoms in three dimensions. Just like a CAT scan, these nano-scale tomograms provide information about the object structure. But what if we wanted to see how light interacts

    00:01:05 with one of these particles? Take solar cells. To design a better cell, we need to engineer not only the structure of the cell, but also how it interacts with light on the nano scale. We need a tool to visualize these interactions. The technique we use is called cathodoluminescence. If we take a nanoparticle in an electron microscope and shine a beam of electrons at it, it can actually emit light. This light is called the cathodoluminescence.

    00:01:35 Now, if we collect this emitted light, we can map out the optical properties of the object with nanometer-scale resolution. Let's see what this actually looks like. Here is a tiny nanoscale crescent made from a polystyrene sphere coated with a gold shell. If we excite this crescent with an electron beam in the electron microscope, we discover that it emits light in a few different ways. First, we find light emitted from the gold shell.

    00:02:04 We also find that light is emitted by an optical resonance at the top, near the tips of the crescent. These 2D maps show how light interacts with the structure with extremely high resolution, down to only a few tens of nanometers. But these 2D images only tell part of the story. Imagine trying to identify a person just by looking at her shadow. It would be quite a challenge. To push this technique into the third dimension,

    00:02:30 we combine cathodoluminescence with tomography. Remember that tomography requires a tilt series of 2D images of the object taken at different angles. So we tilt the crescent and take 2D cathodoluminescence images at 14 different angles. Then, by using tomographic reconstruction algorithms, we can generate a three-dimensional map of how light interacts with our nanoscale particle. Here's a cathodoluminescence tomogram

    00:02:59 next to a structural tomogram like the one we saw earlier. The cathodoluminescence tomogram shows us what parts of the crescent light up under the electron beam. Using information about the color of light emitted, as well as what part of the particle is emitting, we can make a different kind of 3D reconstruction that helps us visualize this particle in a new way with nanoscale resolution across visible and near-infrared frequencies.

    00:03:27 So to recap, we have combined cathodoluminescence, a technique for probing optical properties of nanoscale systems, with tomography, a 3D imaging technique. This has allowed us to generate the first three-dimensional images of the radiative optical properties of a nanostructure. To learn more about this new techniques, please see our recent article in Nature Nanotechnology. For more, please visit us at stanford.edu.

    00:04:04