Sharper Images of Quantum Dots

Whereas the image taken with a normal microscope is blurry (left), the new method (right) clearly shows four quantum dots as bright yellow spots. (Source: U. Basel)

Physicists have developed a technique based on optical micro­scopy that can be used to create images of atoms on the nano­scale. In particular, the new method  – developed at the University of Bochum, the Univer­sity of Basel’s Department of Physics and the Swiss Nanoscience Institute – allows the imaging of quantum dots in a semi­conductor chip. To generate images of structures measuring just a few nano­meters across, researchers use lasers of various wave­lengths to trigger fluores­cence in molecules in part of the substance while suppres­sing it in the surrounding areas. This allows them to image structures such as dye molecules, which are just a few nano­meters in size. The develop­ment of this basic method (Stimu­lated Emission Depletion, STED) was honored with the Nobel Prize in Chemistry 2014.

Timo Kaldewey, from Richard War­burton’s team at the University of Basel’s Depart­ment of Physics and Swiss Nano­science Institute, has now worked with colleagues at Ruhr-Uni­versity Bochum to develop a similar technique that allows the imaging of nano­scale objects, particularly a quantum mecha­nical two-level system. The physicists studied what are known as quantum dots, artificial atoms in a semi­conductor, which the new method was able to image as bright spots. The scien­tists excited the atoms with a pulsed laser, which changes its color during each pulse. As a result, the atom’s fluorescence is switched on and off.

Whereas the STED method only works by occupying at least four different energy levels in response to the laser exci­tation, the new method from Basel also works with atoms that have just two energy states. Two-state systems of this kind constitute impor­tant model systems for quantum mechanics. Unlike STED micro­scopy, the new method also releases no heat. “This is a huge advantage, as any heat released can destroy the molecules you’re examining,” explains Richard War­burton. “Our nano­scope is suitable for all objects with two energy levels, such as real atoms, cold molecules, quantum dots, or color centers.” (Source: U. Basel)

Reference: T. Kaldewey et al.: Far-field nanoscopy on a semiconductor quantum dot via a rapid-adiabatic-passage-based switch, Nat. Phot., online 22 January 2018; DOI: 10.1038/s41566-017-0079-y

Link: Applied Solid State Physics, Ruhr-Univ. Bochum, Bochum, Germany • Dept. of Physics, Univ, of Basel, Basel, Switzerland

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