Experimental Techniques

Pump Probe Spectroscopy

Pump probe spectroscopy is the simplest experimental technique used to study ultrafast electronic dynamics. In this technique, an ultrashort laser pulse is split into two portions; a stronger beam (pump) is used to excite the sample generating a non-equilibrium state and a weaker beam (probe) is used to monitor the pump-induced changes in the optical constants (such as reflectivity or transmission) of the sample. Measuring the changes in the optical constants as a function of time delay between the arrival of pump and probe pulses yields information about the relaxation of electronic states in the sample. Depending on the specific electronic excitations being studied, wavelengths of the pump and probe beams can either be the same (degenerate pump-probe) or different (non-degenerate).

Transient Grating Spectroscopy

Pump probe spectroscopy described above is well suited for measuring the lifetime of electronic excitations with femtosecond time resolution. In order to measure propagation of these excitations in real space, we use transient grating spectroscopy. In this technique, a pair of femtosecond pulses is interfered on the sample to generate a sinusoidal intensity modulation, that in turn induces a density grating of photoexcitations. Because the index of refraction depends on the local excitation density, a periodic modulation of the index of refraction is formed. The period of this pattern in real space can be changed either by changing the wavelength of the laser or the angle between the two beams. An incident probe pulse on this pattern is therefore both reflected and diffracted. Measuring the time evolution of both the reflected and diffracted waves enables us to track the propagation of these excitations in real space.

Ultrafast Electron Diffraction

Direct determination of structural dynamics requires the ability of measuring atomic motions with angstrom scale spatial resolution. Conventional ultrafast optical spectroscopy based on measuring transient changes in optical constants is sensitive to dynamics of electronic excitations but can provide only indirect information about structural dynamics. The spatial resolution in these techniques is also limited to micron scales due to diffraction limit.

Ultrafast electron diffraction (UED) can directly couple to structural dynamics and provide sub-angstrom spatial resolution together with sub-picosecond temporal resolution. The principle of UED is similar to pump probe spectroscopy. An ultrafast laser pulse is split into two; the first part of the laser pulse is directly focused on to the sample to create a non-equilibrium state. To probe the induced structural change, the second part is frequency tripled and focused on to a photocathode generating an ultrafast electron packet via photoelectric effect. These electrons are then accelerated through a high voltage (typically through 30 keV, de Broglie wavelength = 0.07 Å) and diffracted from the sample.

The relative arrival time of the probing electron packet and the initiating laser pulse at the sample can be changed by changing the relative optical path-lengths of the two laser beams. Recording the diffraction pattern of the electron packet as a function of this time delay provides both the equilibrium structure and a movie of the structural evolution with sub-Angstrom spatial resolution (reaching ~0.001 Å level) and sub-picosecond temporal resolution.

Materials

Quantum Materials

In conventional materials, kinetic energy of the electrons is much higher than the interaction energy between them. Therefore, simple theories ignoring electron-electron interactions or treating it as a small perturbation can be used to describe the properties of these systems very accurately. Materials in which the interaction energy between the electrons is much higher than the electron kinetic energy are known as quantum materials or strongly correlated electron systems. These materials exhibit fascinating properties such as high temperature superconductivity or collosomagnetoresistance which can not be explained by conventional theories using a perturbative approach to treat interactions.

Cuprate superconductors are classical examples of quantum materials. The undoped parent cuprates have one electron per copper site, so the band is half filled. The band theory would predict them to be metals; yet they are anti ferromagnetic Mott insulators! The reason is that the energy cost of doubly occupying a copper site (U ~ 10 eV) due to strong coulomb repulsions is much bigger than the kinetic energy gained by hopping (t ~ 1 eV). So the electrons antiferromagnetically align themselves and localize at copper sites. Cuprates have several interesting features. Firstly, different degrees of freedoms (i.e. charge, spin and lattice) are strongly coupled in these systems and interplay between them results in many of their fascinating properties. Secondly, when these materials are doped chemically they display a complex phase diagram and can have multiple competing phases. Understanding the nature of coupling between different degrees of freedoms and dynamics of phase transitions between different competing phases are crucial in understanding the physics behind these materials.

We use the experimental techniques described above to study quantum materials. These novel time-resolved techniques can provide information that is not accessible to conventional techniques.