Techniques
Time- & Angle- Resolved Photoemission Spectroscopy
We explore unoccupied states and the non-equilibrium electrodynamics of quantum materials using our laser-based time- & angle- resolved photoemission spectroscopy (tr-ARPES) setup. Our tr-ARPES setup is comprised of an electronic deflector-equipped hemispheric analyzer (DA30, Scienta Omicron) and a pulsed laser source (Pharos, Light Conversion). In the deflector mode a full cone angular scans in reciprocal space can be performed at a fixed position without rotating and moving the sample position.
The manipulator is connected to a closed-cycle He compressor that can maintain the sample temperature down to 6 K for a desired period with high stability. We use 206 nm (6 eV) as a probe which is generated as the 5th harmonic of the fundamental beam (1030 nm) for high momentum resolution study of the electronic band structure. Two separate pump paths can cover mid-IR to visible wavelength beams from the OPA (optical parametric amplifier). The pump beam is spatially and temporally overlapped with the probe beam for time-resolved studies of light-induced, non-equilibrium phases of matter.
Terahertz Time-Domain Spectroscopy and Polarimetry
Terahertz (THz) is part of the far infrared electromagnetic radiation with energy scale of few to tens of meV. Many interesting collective excitations and features in condensed matter systems, such as antiferromagnetic resonance and superconducting energy gaps, lie within this energy range.
In the FM lab, we generate single-cycled THz pulses by sending short pulses (~160fs) of laser onto an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal. The emitted THz pulses interact with our sample of interest through a transmission geometry, and are subsequently focused onto a CdTe detection crystal. The THz pulses are read out by electro-optic sampling which allows us to measure the THz electric field directly. Since the measured signal is proportional to the strength of the THz electric field rather than the intensity, we can directly extract the THz complex transmission and complex conductivity of our sample.
The setup can also perform THz emission spectroscopy, where the sample directly serves as a THz emitter. In this measurement scheme, we can relate the emitted THz signal to the photo-induced transient charge current inside the sample.
We use a closed-cycle helium cryostat to allow measurements between 4 K and 325 K.
Optical pump-Terahertz probe
In our setup, nonequilibrium electrodynamics in condensed matter systems can be probed by shining a beam of visible to mid-infrared pump pulses onto the sample prior to the THz pulse. The optical pump photo-excites the sample above its ground state, altering its optical properties such as refractive index and complex conductivity. By varying the time delay between pump pulse and THz pulse, we take “snapshots” of sample’s electrodynamic response to the optical pump at subsequent times with a time resolution of better than 200fs. Processes such as excitation and relaxation of quasiparticles, light-induced phases of matter can be studied in this fashion.
Terahertz polarimetry
Material systems with non-zero Hall conductivity alter the polarization and ellipticity of the transmitted light, known as Faraday rotation (or Kerr rotation in reflection geometry). With our THz polarimetry setup, we are capable of accessing the polarization information of the THz signal with sub-mrad precision.
Terahertz Pump / Magneto-Optical Kerr Effect (MOKE) Probe
In this setup we generate strong THz pulses on the order of 100 kV/cm using optical rectification of tilted wave-front laser pulses in a LiNbO3 crystal. This THz pulse is focused onto a sample to act as a 'pump' to resonantly excite collective modes and alter the magnetization of a sample. The resulting transient change in the magnetic order can then be probed by measuring the polarization rotation (Faraday or Kerr) of a time delayed IR pulse. This setup can also be adapted for non-linear and multidimensional THz spectroscopy to measure the second and third order THz conductivity and susceptibility of a sample down to a temperature of 4 K.
