Speaker
Description
Ultrafast nonlinear terahertz (THz) spectroscopy is applied at the TeraFERMI beamline of the Italian FERMI free electron laser (FEL), where intense (multi MV/cm peak field), broadband (up to 4 THz) and ultrashort (< ps) THz pulses are generated by coherent transition radiation from the relativistic electron bunches of the FEL [1].
The beamline allows addressing the THz nonlinear properties of materials, ranging from quantum materials to bio-chemical samples, by measuring the single(two)-color THz pump-THz(infrared) probe response.
Here, we show an example on titanium dioxide, a largely available and naturally occurring functional oxide, and a benchmark among photocatalytic materials.
Oxygen vacancies in titanium dioxide have a strong impact on its photoactivity, by introducing band-gap states and consequently extending the photoresponse from the UV to the visible light region [2].
The time-resolved THz pump/infrared probe measurements allow achieving control on the oxygen defect state in TiO2-x, by modulating the infrared response associated to photoexcitation of oxygen vacancies under the intense terahertz short pulses delivered by TeraFERMI. The experiment explores the polaron dynamics around the oxygen vacancies, by unveiling the conversion of deeply localized defect states into long-lived metastable states, with a lifetime in the nanosecond range [3].
This approach is completely new, being based on a pump-probe experiment on titanium dioxide where a sub-gap terahertz pump beam is used for the first time. Our new finding impacts the exploitation of oxygen deficient titanium dioxide, a functional material largely usable for solar energy conversion, as well as in novel opto-electronic applications.
[1] P. Di Pietro et al., Synchrotron Radiation News 30, 4, 36 (2017).
[2] P. Orgiani et al., Phys. Rev. Applied 13, 044011 (2020).
[3] P. Di Pietro et al., Phys. Rev. Research 7, 023011 (2025).
| Scientific Topics | Solid State Physics |
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