Abstract:

Ultrafast light-matter interactions can synthesize novel metastable states of matter. For example, when nanoscale phase mixtures of polar vortices and a1/a2 ferroelectric domains in (PbTiO₃)/(SrTiO₃) (PTO/STO) superlattices are driven by a 400 nm wavelength optical excitation with sufficient fluence, a metastable 3D ordered phase or “supercrystal” is created. It remains unknown how this photoinduced metastable state appears, grows, and is affected by the initial heterogeneity in real space. To gain microscopic insight, we used the recently enabled laser-pump, nano x-ray diffraction imaging capability with 25 nm spatial resolution to perform in situ hard x-ray diffraction with controlled optical excitation at Sector 26 of the Advanced Photon Source (APS).

We demonstrated that varying the fluence and the exposure time plays an important role in the creation and manipulation of novel nanostructures and creates crystallographically unique species that have not been reported before. Within 10 seconds of 3 mJ/cm² of 343 nm laser exposure at a repetition rate of 54 kHz, a unique, metastable, “intermediate” crystallographic species appears preferentially in the a₁/a₂ domains and grows isotropically. However, upon further laser exposure, novel “superstripe” species appear, which convert between each other and the “intermediate” species until the system reaches a steady state upon 1000 s of laser exposure. Because 343 nm light excites both PTO and STO layers while 400 nm light only excites the PTO layer, the distinct intermediate phase can arise from different charge screening dynamics at the superlattice interface. Our work of imaging nanoscale phase evolution shows how new light-induced ferroelectric phases with unique optical and electronic properties form and transform in real space. This will lead to new ways of optically controlling phase transformations in emerging ferroelectric technologies. If time permits, I will also propose how similar techniques could be used at high energy x-ray diffraction microscopy beamlines.

Location - Hybrid:

431/C010 or Teams
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