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Tracking electrons inside quantum dots

Nanotechnology could help bring such advances as quantum computing, ultradense data storage, and quantum cryptography.

But even the most sophisticated methods for exploring material properties and dynamics falter when applied at the nanoscale. Current techniques either have good spatial resolution or an ultrafast time resolution, but not both.

Now a machine from researchers at the EPFL (Ecole Polytechnique Federale de Lausanne) in France can track electrons moving inside a single nanoobject, at a time scale of just 10 picoseconds and at a spatial resolution of 50 nm.

How? They replace the electron-gun filament on an offtheshelf electron microscope with a 20-nm-thick gold photocathode. An ultraviolet mode-locked laser illuminates the gold, generating an electron beam that pulses at 80 MHz. Each pulse contains fewer than 10 electrons. The electrons excite the sample, causing it to emit light.

Spectroscopic information is collected and analyzed to recreate the surface morphology and to trace the path electrons follow through the sample.

The researchers tested their machine on 2- m-high pyramidal quantum dots. When the electron beam impacts the pyramid, the electrons diffuse towards the closest nanostructure. From there, diffusion continues until electrons reach their lowest energy state, in this case, at the quantum dot topping the pyramid.

The wide energy range of the electron beam can excite materials that won't luminesce with laser techniques, explains researcher Deveaud-Pledran.

"With a laser, you can't get a short enough wavelength to excite diamond or silicon, for example. This machine will."

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