Ultrafast terahertz-driven ionic response in thin film ferroelectric oxides [electronic resource]
- Frank Chen.
- Physical description
- 1 online resource.
Also available at
At the library
All items must be viewed on site
Request items at least 2 days before you visit to allow retrieval from off-site storage. You can request at most 5 items per day.
|3781 2016 C||In-library use|
- Chen, Frank.
- Lindenberg, Aaron Michael, primary advisor.
- Fan, Shanhui, 1972- advisor.
- Wang, Shan X., advisor.
- Stanford University. Department of Electrical Engineering.
- Ferroelectric materials comprise non-centrosymmetric unit cells with permanent electric dipole moments switchable by electric fields, exhibiting strong coupling between polarization, strain, and electronic degrees of freedom. The dynamics of the ferroelectric polarization underlies their functionality but these processes, and the speed limits determining how fast the polarization can change, remain largely unknown. In particular, the ability to all-optically generate significant modulations or reorientations in the ferroelectric polarization represents a key step towards terahertz-frequency information storage technologies, actuators, and optical modulators. From a more general perspective, the use of light to modulate the functional properties of ferroelectrics holds promise for both directing these degrees of freedom and for elucidating their fundamental properties. Previous theoretical predictions indicate that large-amplitude polarization modulations can be achieved on hundreds of femtosecond time-scales, and studies with hundreds of picosecond to nanosecond time-resolution using electrical bias pulses have provided evidence for large amplitude modulations or switching of the polarization. Here we use terahertz pulses as an all-optical bias to apply sub-picosecond duration electric fields to BiFeO3 and BaTiO3 thin films. We show that these generate large amplitude changes in the ferroelectric polarization. In BiFeO3 we used the second harmonic light generated by the thin film as a structural probe and observed modulations in the intensity of second harmonic light corresponding to on-off ratios of 220x, gateable on few hundred femtosecond time-scales. These effects are enhanced through the use of rare-earth-doping to position the sample at a morphotropic phase boundary where the electromechanical and nonlinear-optical responses are enhanced but where the dynamical response of these materials has not previously been explored. In BaTiO3, we probed the atomic-scale response by femtosecond x-ray scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. These measurements define the fastest speeds on which nanoscale ferroelectric devices can be manipulated by electric fields and open up novel opportunities for dynamic optomechanical strain engineering without activation of electronic degrees of freedom.
- Publication date
- Submitted to the Department of Electrical Engineering.
- Thesis (Ph.D.)--Stanford University, 2016.
Browse related items
Start at call number: