Associate Professor Elbert Chia

Background - The power of ultrafast optical spectroscopy (UOS) lies in its ability to temporally resolve phenomena at the fundamental timescales of atomic and electronic motion, and unravelling the competing degrees of freedom. In condensed matter physics, sub-picosecond temporal resolution combined with spectral selectivity enables studies of electronic, spin and lattice dynamics, and more importantly, the coupling between these degrees of freedom. In recent years, femtosecond time-resolved spectroscopy has been shown to present an excellent experimental alternative for studying temperature-dependent changes of the low-lying electronic structure of superconductors and other strongly correlated electron systems including charge density waves, manganites, or heavy fermions. In these experiments, a femtosecond laser pump pulse excites quasiparticles. These high-energy quasiparticles rapidly thermalize (within 10s of femtoseconds) via electron-electron collisions, reaching states near the Fermi energy. The subsequent relaxation dynamics are strongly affected by the low-energy electronic structure in these materials. The dynamics are extracted either by time-resolved measurements of the photoinduced changes in reflectivity (ΔR/R) or transmission (ΔT/T) at optical frequencies, or by directly measuring conductivity dynamics, with the probe wavelength in the terahertz (far-infrared) range.

Technique- The figure on the right depicts, in general terms, an UOS experiment. By changing the relative delay of the pump and probe pulses it is possible to temporally measure the subsequent relaxation dynamics. In UOS the pump and the probe pulses need to be derived from the same pulse, typically at 1.5 eV, to insure ultrafast time resolution but nonlinear optics can be used to produce pulses with very different frequencies, including subpicosecond, broadband terahertz (THz) pulses with frequencies from ~0.1-10 THz. Coherent detection offers the ability to measure both the amplitude and phase of the THz pulse after passing through a sample, providing direct access to the complex permittivity and conductivity of the sample over the measured frequency range without Kramers-Kronig analysis. When combined with a synchronized optical pump pulse, optical-pump THz-probe spectroscopy (OPTP), in which the transmitted THz pulse is detected at a certain time delay after optical excitation of the sample provides a powerful variant of UOS, enabling optically induced changes in the complex conductivity, σ_real(ω) + i σ_imag(ω) to be measured with sub-picosecond time resolution. For superconductors, σ_imag(ω) reveals a 1/ω dependence whose amplitude is proportional to the superconducting condensate at a given time while σ_real(ω) is proportional to the quasiparticle population.

Equipment -We now have a (a) 80 MHz Ti:sapphire oscillator (KM Labs Griffin), (b) 250 kHz amplifier (Coherent RegA) with visible and mid-infrared OPAs, and (c) 1 kHz/3.5 mJ amplified laser system (Coherent Legend-Elite). We also have optical cryostats from Janis (ST-500) and Cryo Industries (XE-102) that go down to 4 kelvin. We hope to purchase a 7 tesla superconducting magnet in the near future. Our all-optical pump-probe system looks like this.

Strongly Correlated Electronic Systems (SCES) - We plan to study the ultrafast quasiparticle dynamics of SCES such as high-T_c cuprate superconductors and other unconventional superconductors; multiferroic single crystals, thin films and hetrostructures; and heavy fermions, using samples from collaborators overseas as well as from within the Department (PAP). From these studies we hope to determine the temperature dependence of the charge gap that opens up below a transition temperature, and how these "hot" electrons couple to the different degress of freedom in the material, such as phonons. We also hope to collaborate with colleagues from outside the Department, to study nanocomposites, chemical and biological systems.