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Nanostructures can greatly increase the sensitivity of optical sensors – provided that the geometry meets certain conditions and corresponds to the wavelength of the incident light. Indeed, the electromagnetic field of the light can be greatly amplified or reduced by the local nanostructure. The group of young researchers HZB "Nano-SIPPE" led by Professor Christiane Becker works on the development of this type of nanostructures. Computer simulations are an important tool for this. Dr. Carlo Barth of the Nano-SIPPE team has now identified the major models of field distribution in a nanostructure using machine learning and thus very well explained the experimental results for the first time.
Quantum dots on nanostructures
The photonic nanostructures discussed in this paper consist of a silicon layer with a regular hole pattern covered with so-called quantum dots made of lead sulfide. Excited with a laser, the quantum dots close to local field amplifications emit much more light than on an unordered surface. This makes it possible to demonstrate empirically how laser light interacts with the nanostructure.
Ten different models discovered by machine learning
In order to systematically record what happens when individual parameters of the nanostructure change, Barth calculates the three-dimensional distribution of the electric field for each set of parameters with the help of software developed by the Zuse Institute from Berlin. Barth then had these huge amounts of data analyzed by other computer programs based on machine learning. "The computer has gone through some 45,000 data records and has grouped them into a dozen different models," he says. Finally, Barth and Becker succeeded in identifying three basic models among which the fields are amplified in various specific areas of nanoholes.
Outlook: Detection of unique molecules, eg cancer markers
This optimizes photonic crystal membranes based on excitation amplification for virtually any application. Indeed, some biomolecules accumulate preferentially along the edges of the holes, while others prefer the trays between the holes, depending on the application. With correct geometry and correct excitation of light, the maximum amplification of the electric field can be generated exactly at the binding sites of the desired molecules. This would increase the sensitivity of optical sensors for cancer markers at the level of individual molecules, for example.
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Material provided by Helmholtz-Zentrum Berlin for Materialien und Energie. Note: Content can be changed for style and length.
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