Coherent Optical Phononics

Abstract

In my thesis, I have strengthened the understanding of the vibrational dynamics of materials in response to stimulation by intense laser pulses. Vibrations of the crystal structure can be used to manipulate a material's properties in the electronic ground state on timescales less than a picosecond by changing the average ionic lattice that the electrons experience. In order to leverage this technique, the quantized vibrations of the lattice, called optical phonons, have to be excited coherently by intense pulses in the terahertz and mid-infrared spectral region. Excitation mechanisms for optical phonons have been investigated theoretically on a phenomenological level for several decades, but due to the high computational costs, a quantitative description within the density functional theory framework has only become possible in recent years. I further developed and used this toolkit to explain experimental discoveries that were not previously understood and to predict new phenomena arising from coherent optical phonon excitation. Here, I present three major research projects that I have completed on this topic: In the first project, I proposed a novel nonlinear phonon coupling mechanism that enhances the controllability over structural distortions induced in the crystal lattice. In the second project, I created a general framework that captures the different stimulated Raman scattering mechanisms for the excitation of Raman-active phonons in insulators. In particular, I proposed a novel scattering mechanism that significantly increases the efficiency of the excitation over established methods. In the third project I showed that magnetism arises from temporally varying electric polarization. I have demonstrated this phenomenon with the example of circularly polarized phonons that produce a magnetic moment, which interacts with an external magnetic field or the magnetic order of the material itself. I conclude the thesis by giving a perspective for future research directions. I suggest that new discoveries in the field will be led by experiments carried out at free-electron laser facilities and by developments in the first-principles description of coupled electron- and spin-phonon dynamics. Understanding the time-dependent physics of phonons may in turn lead to new paradigms in designing materials with greater functionality in the static case.