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New scientific highlights- by MUST PIs Chergui, Milne, Wörner, Vaníček and Röthlisberger

Exciton control in a room temperature bulk semiconductor with coherent strain pulses

November 29, 2019

Controlling the excitonic optical properties of room temperature semiconductors using time-dependent perturbations is key to future optoelectronic applications. The optical Stark effect in bulk and low-dimensional materials has recently shown exciton shifts below 20 meV. Here, we demonstrate dynamical tuning of the exciton properties by photoinduced coherent acoustic phonons in the cheap and abundant wide-gap semiconductor anatase titanium dioxide (TiO2) in single crystalline form. The giant coupling between the excitons and the photoinduced strain pulses yields a room temperature exciton shift of 30 to 50 meV and a marked modulation of its oscillator strength. An advanced ab initio treatment of the exciton-phonon interaction fully accounts for these results, and shows that the deformation potential coupling underlies the generation and detection of the giant acoustic phonon modulations.

Fig. 1. Characterization of the c-axis exciton in anatase TiO2. Calculated wave function of the c-axis exciton. The isosurface representation shows the electronic configuration when the hole of the considered excitonic pair is localized close to one oxygen atom. The colored region represents the excitonic squared modulus wave function.

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 (Max-Planck Institute, Hamburg) and Pascal Ruello (Université de Le Mans). Publishing in Science Advances, the international team shows, for the first time, control of excitonic properties using acoustic waves. To do this, the researchers launched a high-frequency (hundreds of gigahertz), 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.

See also: EPFL News, AZOoptics,

Reference: Baldini, E., Dominguez, A., Palmieri, T., Cannelli, O., Rubio, A., Ruello, P., and Chergui, M. (2019). Exciton control in a room temperature bulk semiconductor with coherent strain pulses. Sci Adv 5, eaax2937 (10.1126/sciadv.aax2937)

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