Subpicosecond Optical Stress Generation in Multiferroic BiFeO₃

Abstract

Optical excitation leads to ultrafast stress generation in the prototypical multiferroic BiFeO 3 . The time scales of stress generation are set by the dynamics of the population of excited electronic states and the coupling of the electronic configuration to the structure. X-ray free-electron laser diffraction reveals high-wavevector subpicosecond-time scale stress generation following ultraviolet excitation of a BiFeO 3 thin film. Stress generation includes a fast component with a 1/e rise time with an upper limit of 300 fs and longer-rise time components extending to 1.5 ps. The contributions of the fast and delayed components vary as a function of optical fluence, with a reduced a fast-component contribution at high fluence. The results provide insight into stress-generation mechanisms linked to the population of excited electrons and point to new directions in the application of nanoscale multiferroics and related ferroic complex oxides. The fast component of the stress indicates that structural parameters and properties of ferroelectric thin film materials can be optically modulated with 3 dB bandwidths of at least 0.5 THz.

Temporal and spatial tracking of ultrafast light-induced strain and polarization modulation in a ferroelectric thin film

“…First, to the best of our knowledge, the longitudinal strain level achieved in this study is among the largest photo-induced longitudinal strain levels ever reported in BiFeO 3 film. Similarly, high strain levels have been reported recently but only in multidomain BiFeO 3 film ( 15 ). This efficient photo-induced strain is likely connected to efficient generation mechanisms of acoustic waves that combine light-induced thermal (ultrafast thermal expansion) and nonthermal (deformation potential mechanism, inverse piezoelectric effect) processes, as already discussed in the literature for the rhombohedral BiFeO 3 ( 9 – 11 , 15 , 18 ).…”

“…Similarly, high strain levels have been reported recently but only in multidomain BiFeO 3 film ( 15 ). This efficient photo-induced strain is likely connected to efficient generation mechanisms of acoustic waves that combine light-induced thermal (ultrafast thermal expansion) and nonthermal (deformation potential mechanism, inverse piezoelectric effect) processes, as already discussed in the literature for the rhombohedral BiFeO 3 ( 9 – 11 , 15 , 18 ). Second, it is the first observation and evaluation of ultrafast light-induced shear strain component in ferroelectric thin films.…”

“…In the present work, we first of all overcome the limitation of the multidomain complexity by fabricating a high-quality epitaxial single-domain BiFeO 3 film through strain and domain engineering ( 24 – 27 ). This archetypal system is a prerequisite for the understanding of the photo-induced response beyond bulk materials ( 11 ) or multidomains states ( 10 , 15 , 28 ). Second, we combine time-resolved optical and time-resolved x-ray diffraction (XRD) experiments to determine the complete light-induced ultrafast deformation of the unit cell in a BiFeO 3 thin film (Fig.…”

Strong Electron‐Phonon Coupling Mediates Carrier Transport in BiFeO3

“…The main recognizable feature is the only peak at 30.3 GHz, which is associated with the longitudinal acoustic (LA) phonon mode in BiFeO 3 thin films. [22][23][24]…”

“…[15][16][17] These couplings can be readily manipulated by optical excitations, making multiferroics an outstanding platform for the investigation of electron-phonon coupling. The bismuth ferrite (BiFeO 3 ), the most widely studied room-temperature multiferroic, [18,19] exhibits unique physical properties arising from the inherent couplings and exchange interactions such as magnetoelectric coupling, [20,21] photoinduced mechanical strain, [22][23][24] and magnon sidebands. [25,26] Moreover, unlike most multiferroics, the BiFeO 3 exhibits strong photoresponse, [27,28] photovoltaic effect, [28] and domain wall conductivity, [29] making it a potential material for optoelectronic and photovoltaic device applications.…”