Figure 1. a. Disordered and Ordered Domain wall patterns. b. CPSHG measurement setup.c. CPSHG patterns for single-domain BiFeO3 thin films.d. CPSHG patterns for stripe-domain BiFeO3 thin films. e. PFM and SHG mapping (with q = 90° and j = 45°) for the stripe-domain BiFeO3. f. Electric polarization vectors across the Néel-type domain wall.
Optical second harmonic generation (SHG) is a sensitive and powerful technique to probe various symmetry breaking, especially ferro-electric/magnetic domains in artificial thin films or heterostructures. As a typical example of breaking state at boundaries, domain wall has been recently demonstrated to possess diverse and intriguing properties that differ from those of the domain entities. SHG investigation on domain walls has also attracted increasing interests in recent years, including nonlinear imaging of domain walls, optical tracking of domain wall dynamics, and revealing domain wall types. Nonetheless, due to the tiny volume of domain wall, its SHG contribution is commonly weak and neglected in many cases, unless no signal can be detected from domain entities, which suggests the observed SHG behavior is often related to the ordering in domains rather than at their boundaries. Consequently, exploration of domain wall types, such as Ising-, Néel-, or Bloch-type through optical method remains challenging for many materials.
What is discovered?
We have observed a co-polarized SHG effect from the well-aligned domain wall arrays in BiFeO3thin films. This SHG component is highly dependent on the local orientation of the wall arrays, and its polarization feature reveals the Néel-like evolution of electric polar vectors in the wall region.
What is important?
Néel-like domain walls are revealed based on the analysis of SHG polarization. It is proved that the co-polarized polarization feature can be extended from R-phase films to T- or mixed-phase films, which allows us to resolve the complex phase mixtures by a simple 90° rotation of the polarizer and the analyzer. This provides an optical readout approach that can be applied in all-optics storage devices encoded by phase states, as the mixed-phase BFO is an ideal candidate for the technological application of non-volatile memories whose ferroic orders can be tailored by light. It is expected that the combination of CPSHG measurement and spatial scanning in this work can highlight the importance of SHG polarization state in analyzing the symmetry breaking in different cases, and provide an extendable method for studying other complicate material systems with nanoscale phase-mixtures.
Why did we need WHMFC?
This study was based on the ultrafast Ti:Sapphire lasers in the magneto-optical station of the pulsed high magnetic field facility. The fast spectrometer, optical fibers and optical filters are all from this lab.
Who did the research?
Han Gao1, Chao Chen2, Lu You3, Fei Sun2, Chengliang Lu4, Sijie Yang5, Guofu Zhou2, Deyang Chen2*, Yibo Han1*, and Jun-Ming Liu6
1Wuhan National High Magnetic Field Center and School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China.
2Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China.
3Jiangsu Key Laboratory of Thin Films, School of Physical Science and Technology, Soochow University, Suzhou 215006, P. R. China.
4School of Physics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China.
5State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, P. R. China
6Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China.
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