Ultrafast Anisotropic Optical Response and Coherent Acoustic Phonon Generation in Polycrystalline BaTiO₃-BiFeO₃

Abstract

<jats:title>Abstract</jats:title> <jats:p>We optically study the as-yet little explored multiferroic material, BaTiO<jats:sub>3</jats:sub>-BiFeO<jats:sub>3</jats:sub> (BTO-BFO), that has demonstrated enhanced magnetic properties, a higher DC resistance in comparison to BFO, and improved magnetoelectric coupling. Our studies include: ultrafast time resolved differential reflection, optically induced birefringence, and second-harmonic nano-imaging of the ferroic order. We observe a strong sensitivity to pump/probe polarizations, photo-induced ferroelectric poling on a picosecond timescale, as well as the generation of photo-induced coherent acoustic phonons with a frequency of <jats:inline-formula> <jats:alternatives> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/ehs-2015-0028_inline1.png" /> <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mo>∼</m:mo> <m:mn>11</m:mn> </m:math> <jats:tex-math>$ \sim 11$</jats:tex-math> </jats:alternatives> </jats:inline-formula> GHz. The second-harmonic generation nano-imaging reveals disordered but distinct ferroelectric domain order, percolating even across grain boundaries in the poly-crystalline thin film.</jats:p>