Ultrafast kinetics of photo-induced phase transitions:

The understanding of equilibrium phase transitions caused by spontaneous symmetry breaking is one of the hallmark achievements of twentieth century physics. These transitions are marked by a diverging correlation length and correlation time of equilibrium fluctuations at the transition temperature, Tc. Much less is understood about non-equilibrium transitions though they are of great importance, perhaps even to the evolution of the early universe. To understand non-equilibrium phase transitions in solids, it is necessary to capture, depending on the system examined, the evolution of the electronic, magnetic or lattice  structure after a strong perturbation. Because these dynamics often evolve on femto- to picosecond timescales, it is necessary to use the shortest perturbations that are technologically available today – ultrashort light pulses. We thus concentrate on photo-induced phase transitions, a particular subset of non-equilibrium  phase transitions, because they offer insights that we would not otherwise be able to gather. As part of this research theme, we study model systems, carefully selected materials that exhibit the requisite simplicity such that the dynamics after photo-excitation can hopefully be understood. A major part of this research  theme also concentrates on inducing phases of matter with light.

Light-induced symmetry breaking in insulators:

In a bona-fide insulator, light is not absorbed by the material system -- light simply passes right through. Passing light through transparent compounds has been studied for centuries, from Newton's observation of light dispersion through a prism to Ibn Sahl/Snell law of refraction. Famously, Faraday showed that when light passes through a transparent magnetic insulator, the polarization of light rotates upon passage, an observation that linked light to magnetism. Historically, such studies have focused on how light is affected by the insulating system. A much less frequently asked question is the converse: how does the material system respond as light propagates through it? Although naively one might not expect much to happen, it turns out that the electrons in the material system can exhibit rich behavior as light propagates through. We are interested specifically in studying broken symmetries of the electronic subsystem, where the lattice and spin subsystems remain unperturbed.

Coulomb energy in high temperature superconductors and correlated electron systems:

The main question asked here is: to what extent does the Coulomb interaction between electrons determine the ground state and its corresponding properties in strongly correlated electron systems? The lab seeks to develop the spectroscopic tools to quantitatively measure the Coulomb energy change across phase transitions in order to place various constraints on how solids save energy. This is particularly pertinent in the field of unconventional superconductivity, where there is still much debate about where the energy is saved as a material undergoes a change from its “normal” to superconducting phase. The lab is in the process of designing transmission electron energy loss spectroscopy instrumentation to perform such experiments..

Ultrafast Electron Diffraction

Ultrafast Electron Diffraction (UED) is a type of pump-probe experiment. In these investigations, we use a laser pump pulse to excite a sample into a non-equilibrium state, and a subsequent electron probe pulse to capture snapshots of the crystal structure at different stages of the excitation. When the electron pulse hits the sample, it scatters from the material and forms a diffraction pattern, which is then collected on a phosphor screen and recorded by a camera. By varying the time delay between the initial pump pulse and the subsequent probe pulse, we can track the relaxation of a lattice as it returns back to equilibrium. The series of images are strung together to create a movie that shows the evolution of a photo-excited crystal structure through time.

(Left) Schematic of our UED experiment. (Middle) A gif showing the pump pulse (red) exciting the sample prior to the arrival of the electron probe pulse (blue). (Right) A diffraction image taken by our experiment.

Second Harmonic Generation

Second harmonic generation (SHG) is a nonlinear optical phenomenon in which two photons of frequency omega are converted to a single photon with frequency 2*omega: red light turns into blue light. In our experiment, we measure the frequency-doubled light produced in condensed matter systems to study their electronic, crystallographic, and magnetic properties. 

By varying the light polarization, SHG becomes a highly sensitive probe of a material's point group symmetry. We utilize this to detect subtle changes in the symmetries of various equilibrium and non-equilibrium phases. Moreover, by tuning the wavelength of the light we can obtain detailed information about specific resonances. 

We are particularly interested in using these techniques to study systems away from equilibrium. Because we use a pulsed laser, we can measure the SHG response as a function of time after a perturbation is applied to the system by another laser pulse. This experiment thus constitutes a powerful method for investigating light-matter interactions. 

Condensed matter systems that we are interested in studying with SHG include (but are not limited to): Mott insulators, multiferroics, charge density waves, and Floquet systems.

(Left) A decrease in the SHG signal after excitation with an intense laser pulse. (Right) A rotational anisotropy plot showing the SHG signal's dependence on the polarization angle of the probe.