Breaking symmetry with light: ultrafast optical control of structural order in solids

Thesis

This thesis explores how light can be used to induce and control symmetry-breaking phenomena in solids through resonant excitation of lattice vibrations. Within the framework of nonlinear phononics, the coherent excitation of infrared-active phonon modes by intense terahertz pulses transiently reshapes the crystal potential energy landscape, providing a means for dynamic and selective control of material functionalities. Two case studies are presented. The first demonstrates the light-induced emergence of chirality in the non-chiral crystal BPO4. By selectively exciting infrared phonons with linearly polarised light, it is shown that the system can be driven into a transient chiral state on ultrafast timescales. The second example focuses on the ferroaxial material RbFe(MoO4)2, where a conjugate axial field associated with circular phonon excitation is experimentally identified and shown to couple directly to the ferroaxial order parameter. Together, these results highlight the effectiveness of light as a symmetry-selective control parameter for lattice-driven phenomena. The symmetry-based approaches developed in this work are general and are expected to be applicable beyond the specific materials studied. The coherent excitation of phonon modes in strongly correlated systems, together with the use of light fields featuring tailored or complex polarisation profiles, promises a rich landscape of unexplored opportunities for non-equilibrium phase control and the discovery of emergent phenomena beyond equilibrium limits.

Authors: Zeng, Z

• DOI: 10.5287/ora-vjemk4yzg
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: light-induced phase transitions; non-equilibrium phenomena; light-matter interaction; ultrafast dynamics
• Subjects: Nonlinear optics; Condensed matter; Solid state physics