Sound Controls Optical Properties

Illustration of the exciton of Titandioxide interacting with a propagating coherent acoustic wave. (Source: A. Dominguez, MPSD)

One of the main challenges in materials science research is to achieve high tunability of the optical properties of semi­conductors at room temperature. These properties are governed by excitons, which are bound pairs of negative electrons and positive holes in a semi­conductor. Excitons have become increasingly important in opto­electronics and the last years have witnessed a surge in the search for control parameters – temperature, pressure, electric and magnetic fields – that can tune excitonic pro­perties. However, moderately large changes have only been achieved under equi­librium conditions and at low tempera­tures. Significant changes at ambient temperatures, which are important for appli­cations, have so far been lacking.

This has now just been achieved in the lab of Majed Chergui at EPFL within the Lausanne Centre for Ultrafast Science, in colla­boration with the theory groups of Angel Rubio at the Max-Planck Institute in Hamburg and Pascal Ruello from the Université de Le Mans. The inter­national team shows, for the first time, control of excitonic properties using acoustic waves. To do this, the researchers launched a high-frequency, large-amplitude acoustic wave in a material using ultrashort laser pulses. This strategy further allows for the dynamical mani­pulation of the exciton properties at high speed.

This remarkable result was reached on titanium dioxide at room temperature, a cheap and abundant semi­conductor that is used in a wide variety of light-energy conversion technologies such as photo­voltaics, photo­catalysis, and transparent conductive substrates. “Our findings and the complete description we offer open very exciting per­spectives for appli­cations such as cheap acousto-optic devices or in sensor tech­nology for external mechanical strain,” says Majed Chergui. “The use of high-frequency acoustic waves, as those generated by ultrashort laser pulses, as control schemes of excitons pave a new era for acousto-exci­tonics and active-excitonics, analogous to active plasmonics, which exploits the plasmon excitations of metals.”

“These results are just the beginning of what can be explored by launching high-frequency acoustic waves in materials,” adds Edoardo Baldini. “We expect to use them in the future to control the funda­mental inter­actions governing magnetism or trigger novel phase tran­sitions in complex solids”. (Source: EPFL)

Reference: E. Baldini et al.: Exciton control in a room temperature bulk semiconductor with coherent strain pulses, Sci. Adv. 5, eaax2937 (2019); DOI: 10.1126/sciadv.aax2937

Links: Lausanne Centre for Ultrafast Science LACUS, École Polytechnique Fédérale de Lausanne EPFL, Lausanne, Switzerland • Theory Dept., Max Planck Institute for the Structure and Dynamics of Matter, Center for Free Electron Laser Science, Hamburg, Germany

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